Wacky Idea?

[Eric Johnson]

I do some of my best thinking in the shower and with wood-heated DHW these days, I get more time to think.

So this morning, while pondering yet another future heating system upgrade, it occurred to me that I ought to be able not only to heat our greenhouse on cold days efficiently with infloor radiant, but also cool the floor on hot day. That's the biggest problem with greenhouses in the summer--they get too hot and fry the plants. There are all kinds of ways to cool things off--from automatic roof vents (got that) to fans (don't got that) to moveable shades (had that). But I'm thinking cool water pumped through the pex embedded in the slab might work pretty well. And, if done right, the heat removed could actually be stored and possibly used either for reheating the floor at night, or to preheat domestic water going into the hot water heater. I could even pump it (either directly or through a heat exchanger) back into the bottom of my hot water storage tank if the delta T is high enough.

So that's a rough sketch. I'm thinking some kind of a tall, narrow tank for mucho stratification would be the best storage reservoir, and a reverse flow arrangement, depending on what I'm trying to accomplish.

Any thoughts on this idea? Am I suffering from early-morning pie-in-the-sky wishful thinking?

 

[kuribo]

One thing that may be an issue for you is condensation....If the radiant water drops the slab below the dewpoint temp, water will condense from the air onto your slab....

 

[begreen]

I suspect it might not work all that well. It seems the cold will stay right at floor level. If you have a really abundant source of cold water then it might work better with a fan driven heat exchanger. FWIW, I think a greenhouse screen cover over the glass might be more effective. At least they work pretty well out here.

I'm hoping to get my greenhouse built this summer, so I will be in the same boat soon. Been thinking a lot about the same issues. One thought was to create an earth buried thermal siphon by burying long culvert tubes feeding uphill into the greenhouse. The theory being it will create a drafting chimney effect pulling cooled air through the pipes up and out of the top vents at the peak of the greenhouse.

 

[mikeyny]

Eric, if you have a water source, like a pond or stream you could draw the cool water off the bottom and run it through your floor and then back into the source again. A spring fed pond works great for this. Geothermal systems are used all over the world. It is a very effective way to heat or cool almost anything.
Mike

 

[Tony H]

The geothermal system would work for cooling but in many areas (mine) they will not issue permits to allow system installation for fear of ground water contamination. My thought would be to drive a well point to make a small pond and exchange to the inside the plenum exchanger in my warm air furnace.
Run hot water in the winter and cool in the summer with seperate inlet and outlets valved then run the water back to the pond. This would make the condensation problem managable with some pipe insulation and the existing drain for the AC coil.
Anyone have an idea how much and how cool of water would it take to make it workable? My wife would just love for me to dig up the yard again this summer

 

[Nofossil]

Interesting idea....

I agree that cooling the floor is probably marginal in terms of reducing the air temp, but if you do it anyway,

If you have a tank positioned above slab level to coold the slab during the day, you'll have a problem getting the heat to thermosiphon back into the slab at night. Have to have the tank on a vertical track with a counterweight - up during the day, down at night.

How about louvers just below the glass that are built on small copper pipes? You could adjust the angle to block some percentage of sunlight, and get solar hot water in the bargain.

 

[Burn-1]

Eric what you are looking for is called a valence convector. Google it and you'll find they're a pretty good option for greenhouses.

 

[Eric Johnson]

Thanks everyone for the great observations and suggestions. I like 'em all. Gives me some more stuff to think about in the shower.