I have not heard of anyone using in-ground radiant heat for a GH. I suspect that is because the volume of soil (and its thermal mass) would take a huge amount of heat to warm up, before it starts radiating to the air and plants above it. And if you are using gas or electricity to get that soil heated, your utility bills would be equally huge. It is generally cheaper and quicker to heat the air as needed (although even that tends to be pricey in very cold climates).

A few things you might want to consider along these lines:

1. Look into the SHCS for GH heat. It is a heat capture system which uses the soil as the heat sink. Similar in some ways to a radiant heating system, but with some improved efficiencies due to a condensation cycle. If you are in an area that has sunny winters it might work well for you (though you will still likely need some supplemental heat as it will not heat a GH alone thru the winter in Z3).

2. Even if you don't go with the SHCS, there are some pointers that they and other cold weather GH growers have found that might be useful for you:

SHCS suggests NOT putting insulation at the bottom of the GH (below the soil). Even though it might be easy to do while it is all dug out, not enough heat is lost down there. One is better off putting that insulation around the perimeter of the GH soil from ground level down a couple of feet (or adding to the insulation if there was some already there).

You might want to read some of Elliot Coleman's GH books. He has worked out several techniques for growing in cold winters, and they do not rely on external heat (or at least not much).

Have you considered winter light levels? In addition to temps, the lower level of winter sunlight can be a major limit on plant growth until days lengthen.

Might help to have a fairly good plan for what you want to grow over the winter season, and how much growth you want to get. Many folks get by with minimal growth over the height of winter and are just bringing plant through until spring causes those plants to really start growing again. Some others just close their GH down for winter. And some add the resources to keep the GH's actively growing and productive over winter. All are possible, but IMO they would call for variations in the GH design to best accomplish their goals.

Steve333, thank you for your helpful comments. I will take your advice and check out the books. I have a local friend whose greenhouse is heated with a radiant under floor system successfully, but she does not grow in winter months. I have also spoken to a gentleman from maryland who put a system under his dome greenhouse to heat his tropical oasis/hot tub room. I do recognize that supplemental light may be needed for winter crops, but the thought of fresh greens and other cold tolerant crops is so appealing. I will continue to investigate my options. Certainly an LP heater is cheaper and less hassle to install than what I have in mind, but I am not convinced it will save in energy usage and bills. Part of my goal is to provide a warm, plant-filled room into which I can escape our long cold winters. I do appreciate your input and welcome any additional thoughts.

A lot will depend upon your location/climate.

In my area (8300' in the Rockies) we generally get abundant sun. On many days it can be in the teens or single digits outside, and the GH warms up to 70F or more, due solely to the sun. On a 30-40F sunny day it was hitting >100F in the GH before I put in some heat storage. Of course it will cool right back down again after sunset if you don't have heat storage or supplemental heat. And there are always a few stretches without sun in a winter, where supplemental heat would be needed to keep temps above freezing, if that is required for your growing system. In a typical winter, days sunny enough to heat the GH might happen 80% of the time here. Using a SHCS storage system, I am estimating (GH is not fully functional yet) that I can get ~90%+ of my heating costs from the sun. But I am not planning on growing orchids or other tropicals either.

It can be a very different situation other places, where the climate is different. Many years ago I was living in the NE. The sunny winter day count was pretty much the reverse of where I am now. This design would not perform as well heating-wise there for obvious reasons (although there are people using SHCS GHs in cloudy climates). That said, there are GH designs for cold climates that could perform well thermally, the pit GH design (and its variations) is one that comes to mind.

Another possibility is to attach your GH to your residence. The GH can supply surplus heat during the day to the house and the house can give that back at night. Again how well this works depends upon your climate, land and house location, but designs like this can be very effective (and make getting to the GH in winter very easy).

There are several possible different design goals for a winter GH. If you only want some winter greens and other cold tolerant plants, and maybe only in a section of the GH instead of the entire thing, you could design a GH that did that without the costs of heating the whole thing all winter. You could likely still enjoy a warm sunroom on sunny days but not be paying to heat that area on cold nights, with some well planned partitioning.

I'm sure others will chime in here. There are quite a few possibilities. Whichever design you choose, you might want to get a good estimate of heating costs, just so that you don't have any unpleasant surprises later on.

Steve333, thanks again. All sage advice. Our climate is not the greatest for solar in the winter. We have overcast skies for a week or more, adding to the desire for some greenery to brighten the spirits! Not helpful for solar gain. I like your idea of partitioning in the winter to get the advantages of a limited growing area which is still comfortable but less costly to maintain warm. I downloaded the Coleman book last night and will see what advice I can glean. I am firm on wanting to garden in raised beds, to take the strain off my body. My current gardens are all raised and my knees thank me! As for attaching to our home, I briefly considered it, but I would like to keep the option of a small airtight woodstove in the GH and don't want the fire risk to our house. And part of the joy for me is the oasis separate from our home into which i can lose myself. Anyway, this is a work in progress and all of these ideas open my mind to new possibilities before I start digging and pounding nails.

I love this forum and I appreciate all of the help and useful discussions archived here.

Kim

Slack Hollow Farm in Argyle NY (zone 4) uses radiant hot water pipe buried in the soil to heat their greenhouses. I have personally spoken to them about it and they are very happy with it. Many plants are with put up with cooler temps if the root zone is kept warm. We have a 34x96 greenhouse in South Dakota (zone 4) with a shcs in in and are very happy with how it is working.

Just for clarity, is the pipe in the soil under the greenhouse or in raised beds in the Slack Hollow greenhouse? Can you tell me more about your shcs? My current plan is radiant heat under the flooring to maintain steady temps, and hot air circulated through the raised beds as excess accumulates on sunny days.

Here is a diagram of my shcs. This is buried in the soil under my greenhouse. There is about 18 inches of soil over top of the top layer of pipe. Slack Hollow does not use raised beds. They are more commercial growers so going for efficiency of growing space.

Thanks for the photo. Very helpful. At what height do you take warm air from the interior of your greenhouse to circulate down through this? And at what height are the return ducts? Can you also tell me what the diameter of the ducts in this diagram represent?
I answered my own question about Slack Hollow when I visited their website. Nice place.
thanks so much for your help-

As of right now the intake height id 2 ft. Exhaust is also. I plan on extending the intakes up toward the peak. The intakes are 24" the horizontal manifolds are 18 and the tubing is 4".

Here is a link to the pictures of the install of the SHCS.

Here is a link that might be useful: SHCS Pictures

Have you talked to a professional?

I can not imagine pipes in soil or sand heating a greenhouse above.

I heat my 6' X 12' greenhouse with hot water piped through an insulated cement slab floor. I supplement with an electric space heater. I am in zone 5. I grow in containers.

My system is attached to my shop/garage which is also heated with the same heating system. It is rather expensive for me to heat in zone 5. Can't imagine trying in zone 3.

tdscpa, I have discussed my plans with several professionals and 2 greenhouse owners who currently heat their structures with in floor heat. My greenhouse plans hinge on heavily insulated 6" walls and roof everywhere there is not glazing, and tight seals to avoid air leaks. One of the greenhouse owners is in Connecticut and has a dome structure. He owns his radiant floor heating company and advised me to use 3 layers of coils to heat adequately, but this was when I was considering building an uninsulated domed greenhouse. The second owner is my friend who lives 20 miles from here. Her greenhouse is well insulated and sealed, and is heated strictly by solar and one level of coils 6" below the wood slat floor set in sand, and not insulated underneath. Our winter has been quite bitter this year and they have maintained 60 degrees. I cannot attest to their heating bills but they find it tolerable. I am still adjusting my plans and am so happy to hear from other peoples' experiences, especially when they challenge me to rethink my ideas. I still think this can work, even if I have to compromise on temperatures during deep winter.

how did your greenhouse work out. I am also thinking of a greenhouse with in floor heating but was thinking that the heating would be in the concrete floor