Frequently Asked Dehumidification Questions - DXair

Author: CC

Mar. 03, 2026

12

0

0

Frequently Asked Dehumidification Questions - DXair

Indoor pools and spas, due to the nature of containing a large body of water, have a much higher humidity level than any other area of commercial or residential buildings. This is due to evaporation of the pool water at set design temperatures and levels of humidity within the space. Dehumidification is designed first and foremost to prevent condensation within the space. As the moisture reaches a 100% humidity saturation level within the pool room, humidity and moisture—if not controlled—will deteriorate the building and is destructive to dry wall, wood, metal, insulation, windows, doors, skylights, interior structural members, and equipment. Additionally, this can foster an uncomfortable and unhealthy environment for all by creating mold and mildew issues.

AirTS contains other products and information you need, so please check it out.

A properly designed indoor pool dehumidification system that includes the dehumidifier, a source to heat your space, and cooling, will control the environment and prevent the early deterioration of the structure by reducing the pool room humidity levels to the standards required by ASHRAE (50-60% RH). While dehumidifying the indoor pool room environment, the system will also provide the necessary heating, cooling and additional free heat resources to the natatorium.

^ Back to top

The signs include an uncomfortable space, the presence of mold, corrosion, rust, wet and/or saturated drywall, ceilings caving in, and windows dripping to name a few.

^ Back to top

One traditional approach to dealing with all this humid air is to simply open all the doors and windows in the pool room and let Mother Nature dehumidify with drier outdoor air. This passive approach might work on the days when the outdoor air is at the same temperature as that in the pool room and with a lower relative humidity but these conditions rarely exist. So you get the same results as indicated below, with the added unpleasant high cost of pool heating; because the pool heat energy is literally “thrown out the window.”

The second “method” is to install what is called a “waste ventilation system” (see Waste Ventilation Design Guideline for additional information) with exhaust fans to remove the humid air and install make up air units to bring in fresh air; and either heat or cool it to the desired temperature. This “conditioned” air is then circulated around the pool area in hopes that it will be dry enough to absorb the excess moisture and be vented out thru exhaust fans. These types of waste ventilation systems were in play over 30 years; they didn’t work then and will not work today.

Operating costs of these waste ventilation systems are the highest of any available alternative. They cannot work when the outdoor air has the same or higher relative humidity (RH) and moisture content than the indoor air, and the outdoor air temperature is not at the same as the indoor pool 365 days/year. Both must be the exact same temperature with the same amount of humidity 365 days/per year (an impossibility). With this type of system, you do not recover the heat from the system and reject that “free” heat back to pool water to help keep it up to temperature (called pool heat recovery). Typically there may not be the proper ductwork because this may be sized according to a cooling unit (tonnage) and not dehumidification system.

The first most important factor of any system is to ensure no condensation forms anywhere within the structure and maintain the environment at the design conditions for the space to prevent deterioration of the structure. You cannot prevent condensation in any structure without true dehumidification and the proper air delivery/ductwork system sized for the environment.

The answer is also clearly demonstrated in the appalling image of a “health club” of all places. By relying on an exhaust fan/waste ventilation system instead of proper dehumidification, you can see both the structural deterioration and mold and mildew accumulation in this pool room which forced it to be shut down. This of course was caused by excessive humidity.

^ Back to top

DXair builds systems for residential, commercial, hotels/hospitality properties, club houses, schools, rehab/therapy , swim schools, diving schools, community centers, elderly communities and resorts, athletic facilities, diving schools, canine swimming schools, veterinarian therapy pools, spa rooms, mikvahs and more. All of the principles remain the same for each project. They will vary by voltages required.

^ Back to top

DXair builds what is called packaged pool heat recovery systems. We offer a variety of equipment from 2-60 tons. Our LV Series is 1-6 tons in vertical, horizontal or counterflow configurations. Our EC Series is our 6-25 ton dual compressor units, with vertical or horizontal configurations, and the MC Series is our 30-60 ton modular units for larger commercial applications. These units go to 10 tons on single phase.

The secondary line is our Stainless Steel (SS Series) product line in vertical units only. These units go to 9 tons in single phase. Take Apart Construction (TAC) is also available.

^ Back to top

System costs are determined by the size of the unit and the options you choose.

^ Back to top

A complete packaged heat recovery pool dehumidification system is included in our pricing. As a manufacturer our “engineering” includes includes our dehumidifier sized according to the building loads (sensible calculations/heat loss and heat gain in BTU’s) you provide, the heating of the space based upon your choice of electric, gas, propane, hot water coil or geothermal, assistance with sizing, design and layout of your duct work/air delivery system for our system. We will size each system based on all loads provided, the air turnover rate required, supply air and return air CFM.

We will also design for heat recovery/pool heat reclaim if this option can be used. We will provide guidelines for building materials, mechanical space, insulation, windows, lighting, vapor barriers, outside air, and negative pressure. We will provide information regarding the materials used for your air delivery system. We recommend each customer research all of the resources available to them in the design stages for the building, design stages for the dehumidification system and utilizing ASHRAE HVAC Applications Manuals, ASHRAE Manuals for all building requirements, ACCA Manuals, SPS Manuals and the dehumidification manufacturer’s recommendations.

DXair is not a licensed mechanical engineering firm, not an architectural firm and is not licensed for mechanical installations. Therefore, DXair does not provide nor calculate any building loads (heat loss/heat gain/sensible calculations) for the space. This is the responsibility of the customer/end user, mechanical engineering firm, builder, or architectural firm. All proposals will be considered estimates only without the engineering calculations and the pool room data required.

^ Back to top

DXair is the manufacturer of these systems. We are not licensed builder/engineering firm, or mechanical installation contractor. A licensed mechanical contractor will handle the installation of the system. DXair will provide all specifications and information to your contractor. DXair does have a large network of local reps you may wish to contact.

^ Back to top

There are generally four components to all dehumidification systems:
  1. The dehumidifier
  2. The heat source for the space (electric duct heater, gas/propane duct furnace, hot water coil)
  3. The outdoor cooling unit for cooling
  4. The ductwork or the air delivery system dedicated to the pool room

All dehumidification systems are installed and attached to a forced air system of ductwork that is designed for your pool room based on system sizing, air flow/air turnover rates, outside air, and negative pressure.

A forced air system of ductwork (much like your home furnace system), is installed, but unlike your home or office, this ductwork is recommended to be installed in one continuous loop around the pool room. Duct work can be installed overhead or underground depending upon your design. The register, or diffusers as they are called in a natatorium, are then strategically installed to ensure that a flow of warm air is moved across any surfaces that are prone to condensation (i.e. glass, windows, sliding doors, skylights). You will have a supply air duct and a return air duct (like your home).

The dehumidification process begins by pulling warm moist air into the return air (duct) grill and into the dehumidifier. This air is passed over cold evaporator coils which pulls the moisture from this air and cools it at the same time. The moisture goes to a condensate drain in the dehumidification cycle. The air then passes through what is called a hot gas reheat HGR coil in the unit. This HGR is designed to begin reheating the air that has just been dehumidified.

The air passed through the HGR coil then goes through the supply air. Once the air moves into the supply air duct, the thermostat used with the system will indicate if this air needs to be heated secondarily so that it is returned at the heat setting for the pool room. In the supply air duct is the secondary component. That component will now come into play if the air still requires a few more degrees of heat. It will heat that air to match the pool room temperature setting (set at the stat).

EXAMPLE: A pool at 82° water temp and 84° room temp, cooling temperature should be set to 86° and relative humidity (RH) should be 50-60%. If the room warms up past your set point, the cooling will automatically begin at the setting of 86 degrees.

For more information, including a diagram of the process, check out this Pool/Spa Humidity Control article…

^ Back to top

Like an HVAC system for your home or office, certain information will be required that will help provide us with the best approach to your indoor pool room project. No two projects or designs are the same, although the principles are the same. Some of the information for new construction that will be required is:

  • The total square feet of the pool room
  • Air temperature to be maintained
  • The total square feet of the pool surface area and the temperature of water to be maintained
  • The total square feet of the spa/hot tub area and the temperature of water to be maintained
  • The ceiling height of the space (average and actual)
  • Any other water features that will create evaporation
  • Type of structure and what it will be used for: (commercial, residential, rehab/therapy, etc.
  • New or existing structure
  • Pool chemistry (chlorine/bromine/salt, etc.)
  • The construction of the building (glass and frame, standard construction, concrete block etc.)
  • How the room will be heated (gas/propane/electric/hot water coil tied to boiler/geothermal open or closed loop)
  • Will cooling be used?
  • The building loads (these loads are generally handled by a mechanical contractor/mechanical engineering firm)

The building loads on the above list consist of the total heat loss for the space in BTU’s and total heat gain (for cooling side) in BTU’s. DXair is a manufacturer, we are not a licensed mechanical engineering firm and we do not provide the total loads. The final loads will allow us to size the heating peripheral for the room, and what size the condenser/fluid cooler will need to be.

Remember, southern and northern climates are very different. Where northern climates require more heating, southern climates will have a much higher cooling load, and these climate based calculations are important to the overall sizing of the unit.

^ Back to top

We take the size of the total square feet of the pool room and with the ceiling height, we determine the air flow requirements or CFM for your supply air. This allows us to size the blower for your dehumidification system. With these calculations, we will provide the outside air requirements (brining in outside air per ASHRAE Guidelines) and we will also size a negative pressure fan for the space based on these calculations.

We take the size of the pool and/or spa (and other water features if included), and calculate the evaporation rate of the water based on your design temperatures you want to maintain. Once the evaporation rate is calculated – this equates to the “tonnage” and compressor size for the dehumidifier to ensure that it can dehumidify your space. You now have the tonnage and CFM as the first part of the equation for dehumidification.

With your building loads and your specific construction, we determine what size your auxiliary space heating needs to be in BTU’s to ensure we meet the heat loss of the space at your coldest climate times. We determine what your cooling loads are based on warmest climate times and use these loads to ensure we size the cooling unit properly for your project.

These will cover your complete dehumidification system: heating and cooling of the space to control your specific environment.

^ Back to top

Ductwork is sized for the proper supply air CFM and return air CFM. This is accomplished by taking your total square feet and ceiling height and with a series of calculations determining the air turnover rate and CFM needed for this ductwork. We utilize ACCA Manuals, SPS Manuals and ASHRAE Manuals as our guidelines for our design.

Your duct working can be installed over head or underground. If it is underground duct it will be PCD (coated) to protect against the earth’s natural moisture and it will be installed at a slight pitch back towards a floor drain.

Your return air would be at the highest point in the room.

If overhead ducting, this duct can be galvanized metal painted, or aluminum. ASHRAE does not recommend stainless steel ductwork in a pool room due to corrosion factors. Fabric duct can be used but greater consideration must be used with fabric ductwork due to sagging and “cinching” issues in the duct.

Return air with overhead ducting is located more towards the floor level.

Return and supply air is not installed at the same level to prevent short circuiting of your air flow.

^ Back to top

Yes, however, we generally do not recommend fabric duct in larger pool rooms due to air flow issues. Fabric is porous, therefore you lose some air velocity because it is fabric. Metal duct is sized with certain parameters and fabric ductwork must be sized by the fabric duct manufacturer and it must meet the total CFM requirements.

In general, we’ve seen the fabric duct “cinched”, pinched at corners and/or sagging, creating serious issues with air flow in a pool room. Do your homework and ensure that if you use fabric duct, that it is done properly as DXair will take no responsibility for this type of ductwork and/or poor air flow within the structure. Galvanized metal painted or aluminum ductwork is more often used in these structures. Underground PCD can be used if going underground ducting.

^ Back to top

Your mechanical engineer/contractor etc. will work with DXair to determine the best approach to ductwork. Many times there are vaulted ceilings from 12-30 ft. in pool rooms. Ceiling areas are the first place for stratification to occur in a pool room. If ductwork cannot reach the ceiling areas, or cannot reach skylights that may be installed in these areas, then ceiling fans installed BLOWING UPWARD and strategically placed to move air flow will be the alternative to ductwork in that area.

^ Back to top

Yes, ductwork is required whether you have glass surface areas in the structure or if it is four conditioned walls. You must have ductwork to move air flow in the space at the proper air turnovers to dehumidify, and to heat the space and cool it if you chose cooling. So yes, the dehumidifier is the heart of the system, but without arteries (ductwork) the system is rendered useless and ineffective.

^ Back to top

For more Dehumidifying Heating and Cooling Unit information, please contact us. We will provide professional answers.

Pool rooms must be maintained separately from any other living or office/work space. Most codes do not permit mixing these two environments due to humidity, moisture and chemicals that should not be permitted to enter these areas that are not protected from the effects of these high humidity envelopes. You do not use the same vapor barriers or negative pressure as required in a pool room. The pool room does not share the HVAC system or ductwork with any other part of the building or home. We recommend that all areas not related to the indoor pool be kept closed and maintained separately.

Generally the rooms are enclosed by glass or walls away from other areas of the home or office. For example, you will not go into an LA FITNESS and find exercise equipment or offices inside the pool room. Same for your home… We want to protect those areas that are not maintained at the same temps or humidity levels and do not share spaces with chemicals.

^ Back to top

There is misconception in our industry that if you maintain a pool cover, that a system can be a smaller size. Keep in mind, you have chosen your water and your air temperatures and how the natatorium will be used (residential, commercial with a number of people, etc.). A cover provides the following benefits:

  • Covering the pool stops evaporation of water; hence the dehumidifier does not have to run to control the humidity in the space and reduces your cost of operation
  • Covering the pool keeps heat in the pool and it reduces your cost of pool heating
  • Pool chemicals evaporate with an open pool so a cover reduces the chemical replacement costs
  • When pool covers can be used they provide additional safety for small children and pets

Downsizing a unit means that when the cover comes off, you still have the same water and air temperatures your design was based on…but you are using a smaller system to try and do the job. Therefore downsizing a system may now not cover that load any longer. If that is case, the unit can continue to run at a higher cost and still not properly condition the space.

^ Back to top

No, these units are not designed to provide the total pool water heating requirements for an indoor pool. All of our units provide what is called HEAT RECOVERY or POOL HEAT RECLAIM, which means when these systems are running, there is excess heat generated. Via our design, we reject that heat back to the space and to pool water to help maintain these temperatures. Once those are satisfied, the excess heat is rejected to your outdoor cooling unit.

Talk to your pool company regarding the purchase of a properly sized pool heater. This will always be your primary source of pool heat.

Other manufacturers may offer 100% pool heating; however we have chosen not to do so as our experience has shown that this feature shortens the life span of compressors (as they must carry the load to heat a pool). If a compressor or component is “down” and needs to be serviced or replaced, the customer now has no pool heating and the pool is losing heat during this time. A pool heater is much more efficient and at much less cost.

Other areas of concern are pool heat being lost to high water tables, ground voids, and un-insulated pool shells. Years ago we built units with this option, but hands on experience has taught us that the pool heat recovery feature is the better choice and we discontinued the 100% pool heating many years ago.

^ Back to top

There are many companies that design and build swimming pool enclosures who tell clients that they do not need a dehumidification system and the heat from the pool will heat the room. There is nothing further from the truth!

Heating up an indoor pool outside of the normal range of temperatures creates increased moisture, evaporation and humidity levels when water is warmer than air. This eventually leads to deterioration and is uncomfortable in any room. Dehumidification may not be sized to handle the increased loads and this potentially leads to higher operating costs, building deterioration, damage, rust, corrosion, mold and mildew within the structure. Turning up the heat of pool water to provide heat to your space is never recommended.

^ Back to top

Cooling is always optional with every system, however it can be determined by your building loads, the type of structure you build, insulation, how much glass is involved, your geographics (southern vs. northern climates), etc. Generally speaking, cooling may be needed. Cooling is at the discretion of each customer, but an informed choice based on your situation is recommended. Most commercial projects will use cooling, residential projects will vary with that option.

Can we install this system outside? Or do we need a mechanical space dedicated for this system?

DXair does not build any rooftop or outside pool dehumidifiers. All of our systems are built with the smallest footprint available to be placed inside a small mechanical room. This is much less than the cost of outdoor equipment, the equipment is more easily and reliably maintained, and provides for a longer life span when not subjected to weather conditions. Your outdoor cooling unit for cooling will be installed outside at ground level or above the DXair unit.

^ Back to top

A licensed mechanical contracting firm will install the system and ductwork and the peripherals. They may subcontract the plumbing and/or electrical out to other companies they work with that are licensed to handle these areas. Generally the contractor will do most of the work. No owner/homeowner is permitted to install these systems due to the laws governing refrigeration based equipment. We require a mechanical contractor, licensing information, and the contractor also signs off on the start-up and commissioning for warranty to begin. We may be able to refer an existing contractor we have used or the client may need to research local contractors who understand pool dehumidification systems.

^ Back to top

Some of our oldest systems are more than 20 years old. The average life span of a DXair dehumidifier, if the proper maintenance and preventive maintenance is accomplished, pool chemistry is maintained properly (extremely important!), is 15-20+ years.

^ Back to top

For the property owner simple maintenance is required: CLEAN YOUR FILTERS and/or change them out! Dirty Filters means you are restricting the air flow coming into the return air and the dehumidifier can’t do its job. Check the unit once a week to see if there are any leaks, drips, or any red lights on the unit indicating a potential problem. Keep your condensate line and drains/traps free of dirt and debris. Check your drain pan and ensure it is free from dirt and debris.

Anything else you will call your service contractor. Do not turn off valves or controls at any time. A preventive maintenance/maintenance check schedule should be set up for every 6 months to maintain your warranty and all records maintained for warranty documentation.

^ Back to top

ASHRAE Manuals provided a number of alternatives for water and air depending upon the use of the facility. Generally somewhere between 80° and 84° water is pretty standard in our industry. Once the water temperature is set, the air MUST be maintained 2° above your air temperature when NO POOL COVER is used. Maximum air temperature is 86° for comfort levels and heating costs per ASHRAE. DXair will discuss the specifics of your project with you to help determine the best settings for your needs.

For humidity, ASHRAE recommends 50-60% relative humidity (RH) to maintain the integrity of the structure and comfort for patrons.

^ Back to top

All clients can choose their settings. Many physical therapy and rehab facilities, diving schools, baby swim/teaching schools can have water anywhere from 84° to 92° and their air temperatures at 78° to 84°. For these types of pools, the units must be upsized to a larger system to cover the evaporation rate. It is imperative to know exactly where air and water will be maintained for each customer so that we provide the correctly designed system.

^ Back to top

Many customers believe they can fluctuate the temperatures up and down depending upon how they want to use the space. This is not recommended because going up in temps may mean your system was not sized for the increased loads, and the unit may run extensively and not keep up. This does more harm than good and it can wreak havoc in a pool room that you are working to maintain to prevent condensation and moisture.

If you provide a range of temperatures you may want to maintain, we will design with the highest temperatures in mind first and allow the customer to make the final decision. The higher the water and the warmer the air, the more costs of operation.

^ Back to top

Radiant floor heating heats objects, not air. There is not enough BTU’s in radiant floor to meet the total heating loads of the building. Secondarily, we cannot take that heat and put into the duct system to move warm air across any surfaces that can condensate to prevent condensation, nor provide even distribution of air throughout the space. Therefore a secondary or auxiliary source of heating is required.

^ Back to top

DXair does not recommend these types of installations. If it is a retrofit installation there may not be any other alternative. Generally, these spaces are not conducive to being conditioned spaces, cannot meet the specs for clearances required for equipment, nor can the equipment be checked on a regular basis. These spaces have a degree of difficulty for mechanical contractors to install ductwork, plumbing/piping and the heating equipment. If equipment needs to be replaced it can be difficult as well. Any space used must be a conditioned space.

^ Back to top

There will be a few items that are required that are not used in your home or office; these are:

  • Vapor Barriers are required per ASHRAE HVAC Applications Manuals-Natatoriums in all pool rooms. This has two options:
    1. Polyethylene sheeting – (8, 10 or 12 mil.) over your insulation and just before you put your finished surfaces on (walls and ceilings that share the pool or warm side of the pool room). This barrier protects water from penetrating your structure materials and prevents this moisture from moving into the interior part of the structure where it will deteriorate the building materials.
    2. Vapor Barrier or a Vapor Retardant Paint – If existing structure, vapor barrier paint is typically used. You will want to choose the paint that is applicable to the particular building materials you have in place. This may require contacting specialty paint stores.
  • Negative Pressure Fan for the pool room. This small fan, sized by DXair, once installed, will prevent moisture migration into the structure and will also prevent the pool room environment from travelling into homes/offices or other areas not related to pool room.
  • Note that Vapor Barriers and Negative Pressure fans are mandated by ASHRAE for all natatoriums.

The rest of the building materials are standard to any home build or office build. Drywall, stucco, wood, etc. can be used in any natatorium. Any number of windows, sliding glass doors, skylights can be used in any natatorium.

Glass note – All windows should be double pane low E glass; single pane glass cannot be used in pool room environments because due to the low insulative quality, these windows will condense when warm meets cold.

Lighting note – top hat recessed can lights are not recommended as this type of lighting “punches” holes in your vapor barrier. This defeats the purpose of your barrier and holes can allow moisture migration. We recommend surface mounting all lighting.

^ Back to top

With existing buildings and no system in place, we will review what is in place that you use to condition the room. Photos are always helpful of existing pool rooms. We will review any ductwork that may be in place or determine where ductwork can now be installed and that we can move air flow to glass areas and prevent stratification in the building. We will review mechanical space to ensure we have space for a system. If you have no room inside the building for a system; we may recommend you to another company that builds outdoor and rooftop equipment.

^ Back to top

With an existing dehumidification system (if not our older models), we will work with each client to determine what you have in place (model number/brand) and if there are specs available for that unit at that time. We will review what you are heating with, cooling unit and location, ductwork and mechanical space allocated for the existing system. We will review the cooling side to determine what is being used for cooling, if it’s R22 or R410A, what the line set sizing is and distance/height on the current condenser and location.

We will request the same information that we do for new construction in the sq.ft. of the pool room, sq.ft. of pool and/or spa, and your ceiling height. We will ask for photos of the room. We will require similar data for new construction and temperatures of water and air within the space. We use our Retrofit Quote Form for these applications. That will provide us with a great deal of information to begin to approach your project properly.

Building loads may need to be performed to ensure the heating and cooling is sized properly. Once all of this information is obtained for existing buildings with and without dehumidification systems, we will provide a proposal for replacement of the existing system. If the old system has been undersized or oversized, we will also provide a quotation for the accurately sized system.

^ Back to top

Yes there are several companies who build pool dehumidification systems. however, no two companies build alike, nor do all companies size their systems the same way. We utilize the ASHRAE HVAC Applications Manuals, SPS Manuals, ACCA Manuals and other engineering information available in our industry.

Competitors have been typically undersized for these applications in a few ways:

  • They “lock down” the customers’ water temperature at 80° and air at 82° and 60% RH. That helps keep the size of the system and cost down to do so. However, what we have found in over 30 years in this industry is that most customers do not keep these temperatures—they want it warmer. And at the time of an original system design, no one asked the customer what they wanted. This leads to systems that are undersized, cannot keep up and the customer does not understand why. That customer may now be keeping water at 84° to 88° and the air at 75-86°, causing additional evaporation and moisture damage to the structure. This happens too many times in this industry.
  • Therefore one of the most critical aspects of retrofitting any project or starting on a new project is to ASK the CUSTOMER what temperatures they want to maintain and how they perceive using the pool.
  • Elderly communities want it warmer—and we find they are also locked into a 80° water/82° air which is too cold for them, so they continually fluctuate temperatures in a pool room. Fluctuating temperatures after system is installed is very problematic for any pool room.
  • Another area that competitors are remiss in is the number of air turnovers used in these projects. ASHRAE Manuals are “recommended guidelines” only. Therefore when you have, for example, a residential property, the recommended guidelines are 4 to 6 air turnovers per hour (via ductwork). However, some of the residential pool rooms are larger than hotels and other projects. They no longer build the small rooms we were accustomed to back in the ’s. From hands on experience we’ve learned that to properly condition these spaces, a better air turnover is required; therefore we use the ASHRAE higher end of the scale for air turnovers. This will be between 6 and 8 and we’ll always start with 8 to calculate the CFM for supply air.

^ Back to top

  1. First all of our systems are built in the USA, not Canada as several manufactures are located in Canada. The costs of our systems are lower due to reduction in customs/broker/freight charges when equipment is shipped from Canada. Also when parts are required, our parts are stocked at factory unless on an older unit which may need to be ordered.
  2. We provide a much smaller footprint than others and with vertical, horizontal and counterflow, and with Take Apart Construction (TAC) we can accommodate a variety of projects where space is a concern for new and/or replacement systems.
  3. We ensure that flexibility is provided in all system sizing for water and air temperatures/RH. We do not lock the customer into settings they may not use.
  4. We build a diverse product line; one line offers complete Bronze Glow Coating interior/exterior of units for protection, and the secondary line is built in Stainless Steel with Electro Fin Coated coils.
  5. We build a much simpler machine, with less complicated controls and less complicate design. This helps with contractors who install and service the units once in place.
  6. We build in the R410A Standards—not all manufacturers do.
  7. We build units with 3 or 4 Row FIELD CLEANABLE EVAPORATOR COILS. This is a huge advantage in this industry because competitors who build with 8 row deep coils must have larger blowers to move air thru these coils, and 8 row coils are at best, very difficult to clean. Our design of coils allows for easy cleaning and maintaining and our systems operate at much lower head pressures which equates to longer life spans of compressors and components in these system.
  8. We build up to 10 tons in single phase in our EC line and up to 18 tons in single phase in our Stainless Steel TAC units.
  9. We build the only true GEOTHERMAL pool dehumidifier today and can be used with open or closed loop for those who want to reduce their carbon footprint.
  10. Our standard units also offer a reduced refrigerant management system by utilizing fluid coolers for cooling vs. a refrigeration based dx condenser. This alleviates refrigerant to the outdoor unit, the loop is filled with glycol and water (the equivalent of an Anti-freeze solution), and alleviates all copper plumbing to the outdoor unit. You have no distance or height restrictions with these units. Another advantage to these fluid coolers: if you need to replace a dehumidifier down the road, this unit doesn’t necessarily need replacement as it is not refrigerant based.
  11. Some competitors refuse to provide a warranty unless you use their controls and they monitor your system at their factory. We do not require your unit to be monitored by us on line – you get the full warranty no matter what controls you may want to use!

^ Back to top

This web site information, content, and guidelines are subject to change without notice.

Please us if you would like to see a specific question and answer included in our FAQ’s. Check back often as DXair updates these FAQ’s!

The company is the world’s best Side Mounting Gas Heating Unit supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

Magical Reheat Dehumidification: How It Works

Nerd Level 5/10 - I'll go slow but this is a bit technical. It's one of the most important concepts I teach though, it's often the key to a healthy home. Part 2 is about what it costs, read that once you've read this post. Link at the bottom.

A Dry Home Is A Healthy and Comfortable Home 

Reheat dehumidification is a fairly obscure topic, it's common in commercial HVAC but not residential. In all my years of house whispering, humidity control has been by far the most challenging part, especially in older homes that don't have a plastic vapor barrier under the basement floor, or homes with vented crawlspaces. 

Keeping a house dry during humid seasons is critical to indoor comfort, reducing thermostat wars between spouses,  reducing the growth of nasty stuff like mold, mildew, and rot, and keeping the house and indoor air quality healthy. I target 45-55% relative humidity (RH) which is 45-55F dew point in cooling season. 

In humid climates where the grass stays green without having to water it* there are typically several months per year where it's humid outside, but not warm enough to turn on the air conditioner or have it run very long. Sadly, the days you'd like to open the windows when it's 70-75F/20-24C out are often these times. I don't make the rules, physics does, I don't like this either!

If you have noticed your house feeling sticky in spring or fall, or noticing where the dog peed last year, humidity is probably getting too high in your home. If you break 60% RH in your home, the odds are high something bad is growing somewhere in your home, often inside walls, crawlspaces, or attics where you can't see it (but you still breathe it.)

As air conditioners have gotten more and more efficient, they are actually getting significantly worse at dehumidification. 

Myself and fellow building science nerds have noticed a significant bump in mold and humidity complaints in the last five years, in fact I did a presentation in called The Coming Mold Explosion. 

So what's the solution? HVAC systems that have excellent dehumidification capabilities, in particular those with "reheat dehumidification". I view it as real world magic like air conditioning and flight: even though I understand how it works, it still seems like magic!

Let's take a look at what reheat dehumidification looks like. First, we need to look inside your air conditioner.  The Air Handler Unit (AHU) from a heat pump, the indoor coil that heats or cools the air is on the top shaped like a letter A. Below that is the fan or air handler. We set this unit up for downflow, the air is blowing down into the ducts in the crawlspace.
​The First Nerdy Part: The Indoor Coil

This is a picture of the air handler unit (AHU) now installed in my house, AHU is the name for the indoor part of a heat pump (a heat pump is an AC that can heat and cool where an AC can only cool.) If you have a furnace and AC, the coil sits on top of your furnace and the furnace unit has the fan in it. 

On the bottom is the fan, the air handler itself. Above that is the indoor coil. This unit is set up for downflow operation, where the air blows from the top to the bottom and into duct work in the crawlspace. If you have a basement your system is almost certainly setup as an upflow unit, and in an attic or crawlspace as a horizontal flow unit. 

​In this case it's an "A coil" because it's shaped like a letter A, some are flat (aka a slab coil), some are shaped like a letter N (you guessed it, an N coil.) 

When the indoor coil gets cold, it removes heat from the air inside your home so it can pump it outside. 

​When the coil gets cold and it's humid inside, the coil will get wet with condensation. I don't have a good picture of that, but here's condensation on the bottom of another air handler that is in a very humid crawlspace (since fixed.)
Hitting Set Point Too Fast to Dehumidify

When your air conditioner is running, it's removing both heat and humidity from the air inside your house. 

An AC needs to run for a while, 5-15 minutes, before the coil gets cold enough to dehumidify. Until then it is just removing heat from the house, which you think of as cooling.

This is problematic on mild days because the air conditioner will remove heat too quickly so the thermostat is "satisfied" and the AC shuts off before the coil gets cold enough to start dehumidifying. This leaves your house cold and wet which is a recipe for high humidity, mold, and discomfort. 

A Partial Solution: Variable Speed AC

If an air conditioner is too big to run long enough to dehumidify, one option is to make it smaller. You can choose a smaller size in a typical single stage on/off model, but the real key is to buy a variable speed AC or heat pump that can turn down to a very low output.

The best variable speed ACs or heat pumps can turn down to about 25% of their full capacity. A 2 ton/24,000 btu AC or heat pump can turn down to 6,000 btus. For reference the smallest window air conditioners are 5,000 btus and this is for an entire house. As small as this is, it still typically leaves several months where it still shuts off because it's not warm enough to run the AC for long before the thermostat satisfies.

The other key is that variable speed units need to turn down low while also running a cold coil so that condensation forms and they dehumidify. Only a few systems do this well. That's for another article. 

The problem is that on mild days in the 70-80F range, even very low outputs will often cool the house before dehumidifying it enough. 

The Actual Solution: Cool and Dry the Air, Then Reheat It Reheat dehumidification is where you put humid room temperature air through an air conditioner coil which removes the heat and humidity, then you heat the air back up so instead of cold dry air coming out you get room temperature dry air. Here's why I had to tell you about the coil inside your air conditioner. It's the first step in reheat dehumidification. It removes heat and humidity from air going through it, making the air cold and dry. 

But we've already established that this is bad, it overcools a house without dehumidifying it, and bad stuff can start happening. 

To fix this, we need to "reheat" that cold dry air so that it is room temperature dry air. 

We can do this two ways, by running it through a second coil with the now warm refrigerant in it often called "hot gas reheat", or by using electric resistance that looks a lot like the coils inside a toaster but larger that's called "electric reheat". 

In commercial systems, the second coil/hot gas is quite common, but in residential systems this adds a lot of cost, complexity, and risk of early equipment failure. Lennox has a product called Humiditrol that does this, but the two times I mentioned it to contractors they practically ran away screaming afraid of the potential failure risk. 

​By the way, traditional dehumidifiers work with "hot gas" reheat. The first coil runs cold and cools and dehumidifies the air, then the warm refrigerant goes through a second coil which adds the heat back. The air comes out above room temperature because of the heat the compressor and fan create. 


​Back to home HVAC systems, that leaves us the second option, "electric reheat", which technically most heat pumps are capable of. 

Electric reheat dehumidification is when you run the air conditioner (which takes out heat and humidity) at the same time as the resistance backup heat strips (which adds the heat back in.) To repeat, you put in room temperature humid air and you get room temperature dry air out the other side of the system so that you dry the house without cooling it.

I know it sounds crazy to run the air conditioner and the backup heat strips at the same time, but it uses less energy than you might think when done correctly.

In the next article I'll show data from multiple client homes and one of our AirBnbs, as well as comparing that use to three other homes we own that do not have reheat where we can look at the usage of the dehumidifiers.

Most Heat Pump Only Systems Have the Capacity to Do Reheat Dehumidification This is what backup electric resistance heat strips look like when they are on. This is a large setup, 15,000 watts aka 15 kw. It's better to run far smaller backup strips, preferably with 2 or 3 stages, not just on/off.
You can set up a standard heat pump system for reheat by running a Honeywell Prestige thermostat in commercial mode, but note that it will likely use a painfully high amount of electricity because the heat strips will be running for 15 minutes or so before the coil gets cold enough to dehumidify. And you'll want to be sure you have the airflow settings right so the coil can get cold. It takes a good technician with holistic understanding of how homes work and how to set up the equipment to get this right. I'd love to say that's easy to find, but it seems to be under 1% of HVAC techs in my experience. 

My equipment preference, shockingly, is variable speed equipment with 2 or 3 stages of backup heat so that they can run low and slow while dehumidifying which is both more efficient and more effective. More in a minute.

Let's look at a few other factors about reheat and alternative methods to see why I've settled on using it as the first line of defense in my projects. 

Superior Longevity

I've been called crazy for my near obsession with reheat dehumidification, but there's a really important reason why: it basically never breaks. 

I've done energy audits on homes with ancient 40-50 year old heat pumps and while the heat pump may not work, the resistance backup strips worked every time. As long as the fan turns on to flow air over them and prevent them from melting themselves, they almost never break. 

Another Solution: Whole Home Ventilating Dehumidifiers A Santa Fe Ultra 155 whole home ventilating dehumidifier. I had one of these in our last house, it's one of the nicest and most efficient models on the market.
I skipped one potential solution to high humidity on purpose because I wanted to teach you what reheat dehumidification was first. 

There's a lovely product called a whole home ventilating dehumidifier. They pretty much self define, it's a large dehumidifier that can serve the whole home and also brings in outdoor air aka mechanical ventilation. I've used them on a number of my House Whispering projects including one on our last personal home which was right on the Cuyahoga River outside Cleveland Ohio.

My friend Ken Gehring aka Teddy Bear invented these and also coined the phrase "green grass climate" I used earlier. 

While I quite like ventilating dehumidifiers and have used them in a number of client homes, they have two main drawbacks: they are expensive (typically $4-10K installed) and they have shorter lives than a well installed home HVAC system which should last 15-20 years. 

Santa Fe has the best built units and a generous 6 year warranty, the longest in the business. From my own experience and from talking to fellow HVAC pros who install them, they typically have 5-10 year lives. 

This means that if you install one with a new HVAC system, you will almost certainly have to buy two of them in the life of your new HVAC system, that's $8,000-20,000 total. 

Instead, I'd rather you spend that money upgrading from a basic single stage HVAC system to a higher end fully communicating variable speed heat pump with reheat dehumidification. (Say that 10 times fast...) Then one product can do all the work for you, and like I said earlier the resistance heat strips tend to outlast the units. 

My goal is to make it so the HVAC system with reheat dehumidification handles the majority of dehumidification work and all you need to pick up the slack is a basic $250 portable dehumidifier from a big box store. 

Where I use ventilating dehumidifiers is in client homes that have a newer AC that the client does not want to replace again, or occasionally when a house needs an unusually large amount of dehumidification. They are a useful tool in those situations. 

You Can't Do Reheat Dehumidification with a Furnace

I've skipped one other important piece about reheat dehumidification: you can't do it with a typical home furnace. The reason is that you have to add the heat (reheat) to the cold dry air after the air conditioner coil cools and dehumidifies it.  
In the typical home furnace and air conditioner system, the furnace comes before the AC coil, so you can't reheat the air**, you can only feed very hot air into the AC coil which is very likely to break the system.

Therefore home HVAC systems with a furnace can't do reheat dehumidification whether it's paired with a one way AC or a two way AC heat pump (aka hybrid or dual fuel system). 

What's the Best Way to Do Reheat Dehumidification?

Remember how I talked about the best partial solution to good dehumidification is a variable speed AC or heat pump that can run a cold coil at low 25% capacity to provide the best comfort without reheat?

The best reheat dehumidification is done with that nice system running a cold coil at low capacity matched with as small a resistance backup heat strip as possible. 

This means the system will run for a while at low capacity, sucking as much humidity out of the air as possible. 

What's the Worst Way to Do Reheat Dehumidification?
Basic single stage ACs or heat pumps are bad for reheat dehumidification - they use a lot of energy to do the job and usually have poor dehumidification performance. While you can do reheat with a basic single stage AC or heat pump with electric resistance backup heat strips, it tends to burn far more energy. 

Remember how I said that an AC needs to run for 5-15 minutes to get the coil cold enough to dehumidify well? If you don't want to overcool the home, you need to run the resistance backup the whole time the AC is on. 

The resistance backup is running while the system is not dehumidifying, so you are wasting energy. Plus the AC is very oversized when it's 70-80F outdoors, so it will cool the house faster than you want, using more energy still. 

Ideally you also want to get the AC coil cold fast and run as little resistance backup as possible. More on that in a second. 

What's the Worst System for Reheat Dehumidification?

Many (frankly almost all) AC and heat pump systems are really bad at dehumidification. They focus on cooling over dehumidification. I'll write about this in depth another article, but it's called the Sensible Heat Ratio. An AC can do 65% dehumidification and 35% cooling which is a 0.35 Sensible Heat Ratio (SHR), or it can do 1% dehumidification and 99% cooling, a 0.99 SHR. 

The 0.99 SHR system does almost no dehumidification, even when it's running correctly. 

Most ACs used to be in the 0.7-0.8 SHR range, meaning they did 20-30% dehumidification which is about right to keep a house dry if the AC is sized as small as possible to the house. 

The trouble is that efficiency standards are driving basic single stage minimum efficiency systems into the 0.85-0.95 SHR range. Even if they are running perfectly these systems don't dehumidify well. 

Sadly, efficiency standards have made it where if you want good dehumidification you either have to buy a separate dehumidifier or a "communicating" HVAC system. 

Communicating Systems or Bust

In a communicating system the air handler (indoor unit), the outdoor unit, and the thermostat are constantly talking to each other and making small adjustments in how the system is running to deliver efficiency, comfort, and dehumidification. 

A "communicating" HVAC system is fully variable speed. It can vary the speed of the air handler fan and the speed of the compressor that actually cools (or heats) the house. 

Most importantly, communicating systems can run the indoor coil cold for the best dehumidification and at 25% capacity so it runs a long time and also maximizes dehumidification.  

The challenge is that only two systems are both communicating and do reheat dehumidification: Carrier Infinity and Trane XV heat pump only systems. Each one has multiple brands that are the same products with different badges. 

Carrier Infinity Heat Pumps

While I have two close HVAC contractor friends that swear by Trane XV heat pump systems with reheat, Tim Portman and Stephen Rardon, I have only used Carrier Infinity VNA8 and Carrier Infinity GreenSpeed VNA4 heat pumps. I absolutely love them and finally bought one personally last year. 

Not only do the regular dehumidification and reheat dehumidification functions work very well, but Carrier has the only 3 stage resistance backup heat strips. There is a 3/6/9 kw model (3,000, 6,000, or 9,000 watts) and a 5/10/15 kw model (the glowing photo earlier is the 5/10/15 model running at 15 kw.)

The 3/6/9 kw model of heat strips is an amazing match with a two ton heat pump because it will run at 3 kw and the system at 25% capacity. 25% capacity is 6,000 btus, remember that a small window AC is 5,000 btus. That makes it low and slow, exactly what we want for maximum dehumidification and minimum energy use, something I'll show data on in the next article. 

I should say that Carrier is not a sponsor in any way, in fact I've had zero technical support from them in 10 years of using their products, I've either figured things out myself or learned from other HVAC pros. I can consult on setting your Carrier Infinity system up properly if desired. 

Some Real World Results This is the relative humidity data from the Haven Indoor Air Quality Monitor in Mothmanor, our AirBnb with a Carrier Infinity VNA8 heat pump system with reheat. It also has a dehumidifier in the basement, I'll show energy use next time. I've noticed that the relative humidity runs a bit lower than what the Haven says, 5-10%. 

It's been a dry year here in the New River Gorge National Park, but the temperatures have stayed higher along with humidity through October. 

Here's a chart from Game House AirBnb that has a Daikin Fit heat pump system without reheat, it has a small dehumidifier as well.  Note how the humidity levels are much less steady. The energy use is comparable to Mothmanor which I'll show next time. 

These homes are quite comparable in size, Mothmanor is 620 square feet with a full basement, the Game House is 800 square feet with a full basement. 
​​​
Wrapping Up and What's Coming Up I warned you this post was kinda nerdy, but hopefully it makes sense now how your AC coil needs to be cold to dehumidify well, run as long as possible at as low a capacity as possible, and for the best dehumidification your system needs to be able to reheat the cold dry air that comes off the AC coil. 

To do this, it's best to run as small an AC as possible that is also communicating, which means it can run a cold coil at very low capacity. Only the Carrier Infinity and Trane XV heat pump only systems are capable of this, as well as their other brands like Bryant and American Standard respectively. 

​Done right, reheat dehumidification doesn't use a ton of energy, in fact I'll show how it's the same or less than running a dehumidifier. Like I said at the beginning, I view it like real world magic. 

Here's a comment on this article from cleantech entrepreneur KC Boyce in Atlanta about his Carrier Infinity system and my help setting up/commissioning it:

Comments

Please Join Us to post.

0

0/2000

Guest Posts

If you are interested in sending in a Guest Blogger Submission,welcome to write for us.

Your Name: (required)

Your Email: (required)

Subject:

Your Message: (required)

0/2000