We have a vacation house that has a (south-southeast facing) greenhouse off of one of the bedrooms. On a sunny day in winter, the greenhouse can easily reach 80F or more. When we are there, we open the sliding door to the greenhouse in the morning and allow it to heat the room, which is very effective if we remember to close the door when it starts to cool down in the evening. Since we only use the house part time, I'd like to find a way to recover some of this heat when we're away (and the room thermostat is set to 52F). I've been considering putting a length of radiant baseboard along the wall above the door (about 8-10' of wall available) where the greenhouse would be the warmest, and then looping it thru a similar length in the room and creating a control circuit that would circulate antifreeze thru the system when the greenhouse temp exceeds the indoor temp by some threshold. I know the system would need an expansion tank... Would this scheme capture a meaningful amount of heat?

The home is very well insulated and at a 52F setting, the heat (radiant hot water) doesn't run much -- averaging less than 60 minutes/day of heat calls during Jan-Feb this year, so it seems like a small amount of added heat could make a noticeable difference.

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    This sort of thing is usually better done by directly moving warmed air using ducts/dampers, so you don't have the added steps of transfer heat to pipe, pump water through pipe, transfer heat out of pipe. You'd want well-insulated dampers, but it might be worth considering .vs. the added complexity and inefficiency of multiple additional steps of heat transfer. – Ecnerwal Jul 31 '20 at 2:57
  • Yeah, that was my first thought, but there isn't really an elegant way to do that given the very small amount of wall surface that isn't sliding door. But I'll give it some more thought, because it probably is most efficient - a solar powered fan might work well and fits the usage cycle well, too. – TomG Jul 31 '20 at 19:34

Pumping heat from the greenhouse into the room could be a feasible thing to do. Recall that the amount of heat transferred from one thing to another is proportional to the difference in temperature between the things. You've got four transfers in the proposed system (greenhouse air -> radiator -> water -> radiator -> indoor air) -- it might be a struggle to get good performance from that many transfers with only about 30 degrees F across the whole thing.

Instead, consider exchanging the air directly. Maybe you can conceive some blower door insert to place in the sliding door opening. You can get a damper "off the shelf" disguised as a clothes dryer vent, for instance. Place a damper up high to admit warm air into the room and another down low to admit cool air into the greenhouse. It might prove interesting to see how much heat transfers by thermosyphon alone, ie without any fan forcing the air to circulate.

  • Thanks, the relatively small delta was my first concern, but I can make a much cleaner install with only two runs of pipe through the wall, so I was hoping for some encouragement on the warm, water direction. – TomG Jul 31 '20 at 19:37

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