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I have a greenhouse with insulating glass on the roof and three sides. One side connects to the house via a double pane sliding door. The calculated heat loss for this area at an outside temperature of 0 degrees and an inside temperature of 65 degrees is 15,285 BtuH.

This greenhouse is operational (not inhabited) and is set to maintain 65 degrees during the day and 55 degrees at night, particularly for the orchid growing cycles. On sunny winter days, temperatures can rise to between 75 and 80 degrees due to solar gain, requiring minimal heating.

The heating system consists of 90 feet of copper/aluminum fin tube installed in a separate zone. This fin tube is located inside a ditch below the finished floor level, remaining open to the space above it, allowing heat to radiate into the greenhouse through the joist spaces at one end.

Currently, the greenhouse struggles to maintain 65 degrees during cloudy days or 55 degrees overnight when outdoor temperatures drop to single digits. This issue did not occur with my previous boiler, which operated with a supply temperature of 180 degrees. Recently, I have been forced to leave the door to the house open during cold nights just to keep the greenhouse temperature above 50 degrees.

After switching to gas heat and installing an ES-2 boiler, I’ve recorded readings indicating the greenhouse's return temperature is approximately 9 degrees lower than the supply temperature. This consistent delta reflects supply temperatures ranging from the 140s to 160s. Therefore, it suggests the fin tube is delivering between 24,000 to 44,000 BtuH. My calculations are as follows:

(90 ft. of fin tube) x (310 BtuH/ft.) x (0.85 factor for H2O @ 140) = 23,715 BtuH

and

(490 constant for H2O @ 140) x (10 gpm flow) x (9 degrees delta T) = 44,100 BtuH, assuming a flow rate of 10 gpm, as the greenhouse piping is located only about 4 feet above the Taco 007 pump on the return line.

Clearly, something is amiss as those outputs far exceed the calculated requirement. For the entire house, actual heat losses measured from gas consumption are about 85% of initial calculations, leading me to believe that my figures are reasonably accurate.

Questions to Consider

Here are the questions I’m pondering:

• Which calculation should I prioritize?
• Is the water temperature adequate?
• Is the flow rate too high, inhibiting the fin tube's capacity to release adequate heat into the space?

Possible Solutions

Please provide feedback on whether the following solutions are viable or suggest alternatives:

• Install floor vents at the closed ends of the joist cavities to enhance air circulation over the fin tube.
• Reduce the flow by partially closing either the supply or return valves.
• Repipe to access supply water for this zone ahead of the bypass (expected to be about 14 degrees warmer).

Thanks for any assistance you can provide.

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