Wood stove coils

Sorry if this seems a little off topic, but you guys know metals better than anyone, so here I am.
My basement slab has pex tubes in it. I would like to put some coils in the chimney of my wood stove on order to capture a little more heat. There will be an intermediate LP fired 80 gallon HW heater in order to moderate temperature of water going into pex tubing. I want to keep water near 120F and do not want to chance overheating pex tubes in slab. They are a bitch to change out. I will most likely be running automotive type glycol thru systerm.
I am thinking to use copper tubing for coils. I think the coils should probably be plated with nickle (or even chrome) to resist corrosion. Other materials, stanless, monel, inconel are also a possibility. I'm looking for advice on coil material that would be reasonably priced.
I realize there is the chance of creosote buildup in the stack and the danger of steam explosions and so forth. I intend to monitor the stack by inspection and run a pressure relief valve and low water alarm. that will shut off air to stove.
thanks, Bob

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My guess is that ordinary copper will be the lowest cost as long as you plan for replacement and do not count your labor as part of the cost.. The automotive type antifreeze will keep the corrosion down on the inside of the tubing. The outside will not be all that hot as the liquid on the inside will keep the outside relatively cool. And when they do start to leak, you can get some money back at your local scrap yard. Maybe go with the first foot or so of stove pipe with the copper tubing on the outside of the stove pipe.
Dan

In article
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Be very, very careful!!! A friend of mine who lived off grid put a set of coils in his low air wood stove to pre heat the water going into the propane heated water heater. It blew the over temp valve. And as I understand it on new water heaters this is not just a simple reset.
CP

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I doubt that you will have problems with overheating as long as you have positive circulation. I have had good service from a very small Grundfos circulating pump that only draws ~ 1amp of power. IMHO, This type of system works best as a closed system, you will also require an expansion tank and an air purger. I operate my system at ~5 psi and seldom need to add water.
If you loose circulation the system will overheat and things can get exciting very quickly. Some people suggest routing the steam exhausting from a low pressure relief valve into the firebox to quickly cool the fire in the event of overheating. I do not rely on the relief valve on the top of the hot water heater.
I have used plain water in my system for almost 30 years with a self fabricated steel low temp & pressure boiler and mostly steel piping. I have had no problems with corrosion. I suspect this is due to the water being largely deoxygenated.
Good luck, YMMV

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Eh.
If you are running your woodstove properly you're only going to cause yourself problems. A woodstove needs to maintain a flue temperature. If you maintain that temp and then try to scavenge heat out of that you're going to drop the temp too low.

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I have to somewhat agree. If the flue temp gets too low there will be no draft (worst case) or creosote will build up very fast. To counter this you could run the stove hotter than usual. Not really a good idea either and it certainly cuts into your overall savings if you use more wood. Also hotter firebox temps will shorten the life of the stove too. (rock) (hard place)
Other than the above concerns I thought of using a 4 ft section of double wall stainless flue pipe instead of the coil. You would need someone to seal it up and add an intake (low end) and outlet (upper end) to attach the rest of your loop to. This would give you a waterjacket around the inner (likely 6" flue) to heat the water in rather than some copper tubing strapped to the outside of the pipe. Still gotta feed it and outlet to the rest of the loop but I bet it would be more efficient than the coils.
Just my opinion and it does require a fairly competent welder to build/adapt such a thing.
Good luck with your efforts.

Ok, thanks for all the good info. I'll run a closed system with a some type of sight glass up on the first floor by the fireplace to monitor water level. I can get a relief valve in the system somewhere. I will have thermocouples to monitor water temperatures and stack temperatures and so forth. I'm aware of what can happen when we turn a hot water system into a steam plant. I think they did that in Cherynobl maybe. I'm going to run bare copper tube.
I'm not sure if I will have probs pulling heat out of the flu or not. I will be running outside air into the box with a blower and I intend to actually control the combustion rate. I am hopeful that I can get this thing to work.
House is in process now. Pouring concrete next week. House is on a hilltop in NH on 40 acres, so firewood is free. It's the electricity that concerns me. Power company wants a pile of money to connect me so I tole em no thanks. PV panels & Diesel back up for this winter. Doing a wind survey over the winter to see if a small <1kW turbine makes sense.
Thanks again. I'll report back when hardware is up and running.

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An efficient woodstove, all on it's own, can economically heat an average sized home. Of course, for varying values of "average sized homes".
You are pursuing two economies, wood heat and hot water.
If had my druthers and was in a similar situation I would build a large mass stove. Using native rock or brick.
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Afa grid power, a brother had fifty acres (in upper Ohio, and flat) and power was already run on poles, to barn, machine shed, rented out building that was previously converted from an airplane hanger and his house. The total distance was over a quarter mile.
In two and half days we cut, with a trencher, we ditched the entire run. Electric co. set a transformer near the first three buildings that were clustered together and ran the line to his house.

On Thursday, September 27, 2012 1:27:46 PM UTC-4, Sano wrote:
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Yes, all good points. We are well over a 1/4 mile to road and elec company wants 17k to string wire. For 17k I can do a really nice system and never pay another electric bill. I am a power conversion engineer, so all of t he electronics to get the system up will be home-brew.
A large masonary heater is a possibility for the future. We poured a gigan tic pad in basement and will frame floors for a equally large pair of stac ks. Probably go with a traditional fireplace on the main floor and a masona ry heater with a brick Apizza oven in the basement. Have to replenish walle t, and still need to get the place move in ready by Dec 21. End of Mayan c alander bash that night.
The principal behind a masonary heater is to run a hot fire in the heater o nce or twice a day and add some thermal inertia to the gigantic mass a litt le at a time. The large mass of masonary will give off some heat even when the firebox is cold. Similar strategy with the wood stove. One or two fire s in the woodstove in the basement and see if we can add some energy into t he basement slab a little at a time. Only economy we are chasing is warm. Hot water is no concern at this point. Probably use liquid sodium or glyc ol to transfer heat from stove to slabs. Slabs in basement and first floor have total weight of over 35 tons. So no real need to build a masonary he ater - other than they look kinda cool and doo a good job of cooking bread.
We have 40 very boney (New Hampshire) acres and pulled out a few hundred to ns of granite doing the driveway and foundation excavation. I don't expect I'll ever have to buy another stone as long as I live.
regards, Bob

wrote in
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A large masonary heater is a possibility for the future. We poured a gigantic pad in basement and will frame floors for a equally large pair of stacks. Probably go with a traditional fireplace on the main floor and a masonary heater with a brick Apizza oven in the basement. Have to replenish wallet, and still need to get the place move in ready by Dec 21. End of Mayan calander bash that night.
The principal behind a masonary heater is to run a hot fire in the heater once or twice a day and add some thermal inertia to -the gigantic mass a little at a time. The large mass of masonary will give off some heat even when the firebox is cold. Similar -strategy with the wood stove. One or two fires in the woodstove in the basement and see if we can add some energy into the basement slab a little at a time. Only economy we are chasing is warm. Hot water is no concern at this point. Probably use liquid sodium or glycol to transfer heat from stove to slabs. Slabs in basement and first floor have total weight of over 35 tons. So no real need to build a masonary heater - other than they look kinda cool and doo a good job of cooking bread.
We have 40 very boney (New Hampshire) acres and pulled out a few hundred tons of granite doing the driveway and foundation excavation. I don't expect I'll ever have to buy another stone as long as I live.
regards, Bob -----------------------------------
I heat with a clone of a Jotul 118 in the basement and passive air circulation up the stairs, using the basement (and machine tools) as the thermal mass. Although the stove warms quickly and air flow is unimpeded there is still about an hour's lag between lighting the fire and detecting a change of 0.1F upstairs. Likewise the reading doesn't drop for at least an hour after the stove cools.
I put a remote thermocouple readout of the stove temperature in the kitchen to know when to feed it. It has two channels, one for the woodstove and another to show when a kettle of stew boils. The sensor is in a thick washer I put under the lid knob. An unpowered analog meter has proven good enough although the t/c's location on the stove reaches only 150C.
These panel jacks fit the rectangular cutouts in an audio/visual wall plate with only a little whittling: http://www.omega.com/ppt/pptsc.asp?ref=MPJ&Nav=temg11
jsw