# Using greenhouse heat



## Guest (Feb 20, 2009)

It's 32 degrees here, wind chill 24. I just walked out to the greenhouse, it's 66 out there, no heat going at all. Just sunlight. I have considered putting an greenhouse lean-to on the south side of the house. Couple things, if I were put a higher area in the greenhouse, I assume heat would rise and accumulate there. Then, if said higher area were at and enclosing the back door, I could open the back door, and have a nice source of daytime heat. I am aware that in the summer, I'd need ventilation, and have plans for that. But, my question is, if I allow the heat buildup in the greenhouse to go into the house, will heat still build up in the greenhouse enough to make a difference? or is the slow buildup the greater part of the heat, and letting it escape into the house will negate the usefulness of the attached greenhouse(as a heat generator)? What I don't want to do is go through the trouble and find that there is no advantage to having the greenhouse attached to the house as opposed to having it a hundred feet away. 
Cross posting in gardening forum.


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## genebo (Sep 12, 2004)

By building a greenhouse addition you're building a large version of a Trombe wall. It works.

http://en.wikipedia.org/wiki/Trombe_wall

Genebo


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## SolarGary (Sep 8, 2005)

Hi,
The greenhouse will work most efficiently as a house heater if you do this:

- Face it south with as much glazed area as possible.

- Keep the thermal mass of the greenhouse low -- you want it to heat quickly when the sun comes on it, not store heat in its own thermal mass.

- Insulate the floor with something so that the sun shining on the floor does not get absorbed by the ground. This could be straw, garden bark, AstroTurf -- anything with some R value.

- While the open door is OK, using a fan is better -- ideally have two openings - one up high to bring hot air in (best with a fan), one down low to let cool house air back out to the GH.

- The GH will be more efficient as a solar collector if you don't let the air get hot, but move it into the house when its warm enough to heat, but not hot. The hotter you let the air in the GH get, the greater the losses out the GH glazing to the outside.

- You can control the fan with the same kind of thermostat switch that is used on attic vent fans -- the ones with an adjustable temperature on point are the best. HD sells just the thermostat switches as replacements for $15ish. Put the thermal switch near the top of the GH space, so it turns the fan on when the air gets up to 90F ish.


Sunspaces/Greenhouses used in this way are just about the most cost effective solar collector there is. Payback can be less than 1 heating season.

Some links:
http://www.builditsolar.com/Experimental/ShurcliffPart1/PolySpace.htm

http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm
(there are some house heating GH's scattered in on this page)

http://www.builditsolar.com/Projects/SpaceHeating/MiniPolySunspace.htm

http://www.builditsolar.com/Projects/frugal.htm

Gary


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## cowboy joe (Sep 14, 2003)

I have a similiar set up with the greenhouse. I rigged up two 12VDC fans off of a small solar panel. They only need to run when the sun is out so you really don't need a battery. The fans turn on with either a thermostat (old, bimetallic strip type salvage when I upgraded to a programmable) or via a bypass switch. The fans draw the hot air off the top of the greenhouse and returns cool air from the house at floor level. 

Still working on a bit more automation as the door between the house and the greenhouse is closed during the day when I'm at work meaning the heat builds in the greenhouse rather than going into the house. Thinking of adding thermatic vents (http://www.charleysgreenhouse.com/i...=3236&s=automatic&cid1=1010&cid2=-99&cid3=-99) which open & close by themselves.


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## wendle (Feb 22, 2006)

My south facing door was a cold area in the winter and hot in the summer until I added the lean to greenhouse. Even if the sun isn't out it acts as a barrier from the cold outside. When the sun is out it is a heater. The dogs go through that way to do their business, so less tracking in the house. I have wire mesh around the bottom so in the summer I can open it up to let air through. I plan on installing screens eventually. Last year I used a shade over the top which turned it into a cool area instead of hot. Those silver reflective shades used for cars work great for smaller areas or over windows in the summer.


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## cowboy joe (Sep 14, 2003)

Good point Wendle. A shade is a must in the summer time even here in the northeast. Heck, the temps go above 120 on a sunny day in the winter. Can't imagine how high they would go without shade in the summer. Just add a pile of rocks & water and it could double as a sauna in the summer without the shade.


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## Guest (Mar 6, 2009)

I had thought to make a 2 ft wide area across the top of the greenhouse where it attached to the house, of several sections that were removable. would this work for the summer heat problem? I would incorporate them in the framework so that it would be easy to lift out and replace, with a latching system. My initial idea was a hinged section that I could open and close easily, but feel that being open for so long could be a problem with wind.


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## wendle (Feb 22, 2006)

Any areas left uncovered from the sun will potentially create heat. If your shade isn't enough you can always add more.


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## DMD Farms (Jun 8, 2008)

Add a solar chimney with insulated door, shade the greenhouse and you can draw cool northern air through the house and out the chimney for some old time cooling. I try to add the northern vent low and a high vent near the chimney.


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## woodsy (Oct 13, 2008)

.........


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## CamM (Dec 6, 2008)

I'd think humidity would be a problem, like your house may be warm in the winter, but a muggy warm with all the transpiration from the plants. I've read about the trombe wall like genebo brought up, and it seems like a good idea.


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## sulix (Jan 28, 2003)

This has been an interesting post. Thank you. A lot of good information.


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## cowboy joe (Sep 14, 2003)

CamM said:


> I'd think humidity would be a problem, like your house may be warm in the winter, but a muggy warm with all the transpiration from the plants. I've read about the trombe wall like genebo brought up, and it seems like a good idea.


Humidity is sometimes a problem in the winter if I over water. I've learned over the years that a very light watering in the morning, every other day in the winter, is the best way to go as their is less humidity and that which there is stays in the greenhouse with the door closed. A lot will depend on how / when you vent your greenhouse.


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## Guest (Apr 3, 2009)

Well, I've started anyway. Putting the corner posts in and framing up in my spare time. Gathering windows from the window maker, when they replace windows, they bring the old windows to their shop and stack them up for a couple days, if anybody wants them they're free. My last greenhouse was built with all storm windows, but this time I'm just finding regular wooden windows. Once I get it framed up, I'll put in raised beds and 18" or so of good topsoil in the boxes. I definitely want to be able to open most of the windows, I may well put them on hinges instead of the standard tracks. 
I was wondering about an exhaust fan on a thermostat. Maybe that would take care of most of the moisture problem. If I open a section at the top of the greenhouse leanto for the hot air to escape, it shouldn't be any hotter than it is before the greenhouse. So, do I really need to shade it then?


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