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Old 01-21-2008, 01:37 AM   #1 (permalink)
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Wood stove radiant heat!??!

Hello All!

I have a 1400 square foot house in Wisconsin, USA. It is a one and a half story, in which the living room and kitchen is just one room with a vaulted ceiling and the upstairs on the other half of the house is an upstairs bedroom.

I have a small cast iron wood stove in the living room. The floor is hardwood, with tile in the bathroom.

I love the woodstove and the heat from it, but would like to get some of the heat into the rest of the house, especially into the cold floor.

My house sits over a clean cement and block crawlspace. Most people in this part of the country have full basements. I couldn't because of hight ground water levels - I am just a block or two from a lake.

I am considering the idea of some sort of modified hydronic heat. If I can run heat from my woodstove to some hot water pipes, then run those under the floor of the kitchen, hall, and bathroom and back, then I can spread out the heat from the stove and have nice warm floors.

I have only ever done really basic plumbing, but am not afraid to experiment.

I am generally thinking that I need some sort of heat exchanging device on the back of the woodstove, maybe an old car radiator? Then a small fluid pump to pump the water through the system, and copper pipe, or maybe PEX to run under the floor.

I would like to keep this really simple - use water, not antifreeze, and not have an expansion tank. I would also like to keep it as affordable as possible.

Does anyone have any suggestions or links to information on something similar to this?

Thanks in advance,

-Ben

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Old 01-21-2008, 01:46 AM   #2 (permalink)
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Can you run air ducts? I would think those with fans would be the easiest way to distribute heat.
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Old 01-21-2008, 02:03 AM   #3 (permalink)
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I have a central natural gas furnace. That uses the crawl space as the main cold air return.

Right now, if I just turn on the fan feature on the furnace, it does even out the temperature in the house a bit, but it is moving a lot of air and is rather loud.

I love how the woodstove makes nice quiet heat, other than the pleasant crackling sounds, which I like.

I would also like to get the heat as directly into the floor as I can, so fluid pipes in the floor seems like the way to go.

I would also like to get some thermal mass going in my system. The woodstove is pretty small, so it heats up and cools down pretty fast. If I had a drum of hot water in the crawlspace, it could potentially give off heat for hours after the fire is out.
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Old 01-21-2008, 03:26 AM   #4 (permalink)
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It would keep more heat around longer if you can a storage system to distribute the hot water from, but it's too much of a pain IMO. Surely a quiet circulatory fan would be cheaper than another set of pipes running through the floorboard? Unless the liquid system was way cheaper than another fan and better insulation I wouldn't bother, but that's just my two cents.
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Old 01-21-2008, 09:57 AM   #5 (permalink)
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I'd do a lot of surfing, or buy a book on in floor heating. I'm sure it wouldn't be complicated to do. But, those resources will at least tell you what not to do and that'll save you a boat load of problems.
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Old 01-22-2008, 11:42 PM   #6 (permalink)
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Wow, Wisconsin. Milwaukee, no less. I envy your real winters (don't get much of that in E. TN) and cheese curds. I lurves me some cheese curds.

Is your crawlspace insulated? I'm assuming it must be, since it's part of the air return plenum. That's good. We're learning that crawlspaces pretty much everywhere should be considered part of the house envelope and should be sealed, insulated, and conditioned along with the rest of the house. Knowing that, I can make this recommendation: the radiator idea is workable. You don't have to have antifreeze running through the loop, since the crawlspace is conditioned space: it should never go below freezing.

The bad news: an actual radiator isn't your best choice for getting heat out of the woodstove and into your coil. What you want is a big, flat, full-contact plate with coils running through it either attached directly to the woodstove, or coils inside the woodstove itself. I'm not positive how you would go about doing this, but one way to start is to construct your coil according to the size of the woodstove surface you intend to use, form up around the perimeter of that surface, and then pour a monolithic slab of stucco mix directly on the woodstove. You will need to do this when the weather is warm so you can orient the woodstove, and you will need to provide a means of attaching the slab to the woodstove, since masonry products don't generally adhere to metals well. Once that's done, it's a simple matter of running the coil to wherever you want the radiant heat.

Warning: concrete and stucco stick to aluminum pretty well, but I don't recommend you rely on it for attachment if that's what your stove is made of (and I don't know of any that are) - the differences of thermal expansion would probably crack your collector plate pretty badly, or cause stove warpage.

Get a plain ol' box fan and set that near the stove, set on Low. It won't make a lot of noise, but it will get a lot of heat off the stove and around the house. Another fan that blows cool air along the floor from a cold part of the house toward the stove will tend to push warm air along the ceiling toward where that fan is: also very good. I haven't had much luck getting that to work in my house from the second level, though, and mine is also a 1.5 story.

Good luck - there's lots of interesting things to do with your situation, some of which you can do immediately for really low outlay and surprising results.
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Old 01-23-2008, 12:22 AM   #7 (permalink)
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With an open system (no expansion tank) you'd need a non-ferrous pump (SS or bronze) and some open area so that the water can expand and contract without bursting pipes.

Better to use a small expansion tank... although if you could do a simple gravity loop then you could skip everything (pump included) but you'd have to provide a continuous piped path up and then back down for the water. It could handle smaller laterals but no dips or they'd form thermal traps.
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Old 01-23-2008, 12:33 AM   #8 (permalink)
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Thanks elhigh,

Yep, the crawlspace is an insulated, integrated part of the building. I wish there was insulation beneith the concrete slab too, but the only people who usually do that are the passive solar folks. Also there are some pretty severe drainage issues here to with me being so close to a lake.

My brother joked with me a while back about just pouring concrete over the top of the stove. I laughed it off, buy what you are suggesting sounds like a fair idea.

I do have a heat shield that goes on the back of the stove. It is just plain thin sheet steel, designed so that you can legally have the stove fairly close to the wall. I could possibly use that as the base for a concrete and copper tube heat exchanger or at least a template for one.

The design of the stove is not a good one to try to fit heat tubes through the firebox. It does however, give me the idea of maybe wrapping copper tubing around the chimney pipe, right where it exits the stove.

That part is always super hot. I can not imagine a copper tube wrap causing enough temperature drop to cause any creosote problems.

I have seen designs for DIY heat exchangers between hot water going down your shower drain and cold water coming in to your house. This would be the same idea. Reclaim some of the lost heat and use it elsewhere.

I saw a great photo once of an owner built house. There was 3-foot diameter concrete sewer tube running vertically straight up the middle of the house. It was the main pillar support and the woodstove metal chimney ran right up the middle. The space between the chimney and concrete was filled with tons and tons of sand. The support column was a warm concrete radiator all winter.
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Old 01-23-2008, 12:39 AM   #9 (permalink)
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Hi Who,

I guess thermosiphoning works great, but the main disadvantage is the storage tank needs to be above the heat source.

Unless I want a 55 gallon drum on stilts in my livingroom, that isn't going to work.

I was thinking that a water tank in the crawl space with some sort of partially open top would work for both thermal mass and pressure control.

Then all I need is a pump and some tubing to run to the places I want warmed.

Thanks for the comment on the "non-ferrus" pump. I remembered you have to do something a little different for an open loop system, but I couldn't remember what.
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Old 01-23-2008, 02:49 PM   #10 (permalink)
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I like to draw house designs for fun; a big solid masonry chimney is a typical defining feature of most of my designs. Like your sewer pipe example, mine is for structural and thermal service.

I don't see any major headaches with wrapping tubing around the vent pipe - you might have to clean your chimney an extra time or two per heating season, but that's about it.

The heat shield on the back of the heater would be completely replaced by the heat exchanger - while the heat exchanger is pulling heat from the stove, the plate would probably be safe to touch - hot, like a teapot: you can touch it, but not for long. And it wouldn't set anything on fire.

Try laying a lot of Styro slab insulation directly on your crawlspace floor. It ain't cheap, but once it's paid for, it's paid for. The dollars that go into heating it go right back out again, soaked into the ground under it.

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