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Country Discussion Topics
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Hot Water from Wood Furnace?
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CAH    Posted 12-05-2003 at 11:09:53       [Reply]  [No Email]
Has anyone made their own coil for heating water with a wood stove/furnace. If so, did you use copper, black pipe, SS?. Mine will not go into the firebox but rather between the firebox and the metal jacket around the box. I will be using a 30gal storage tank with a thermosyphon system so water should always be moving as it gets heated. I will use a relieve valve for safety. Will a copper coil overheat with this system? Any ideas?

don    Posted 11-05-2006 at 16:09:32       [Reply]  [Send Email]
i have seen many sytems the one close to the one i am building puts a coil in my furnace and pipes from the drain of the electric hot water tank through the coil then to a tee in the supply line to the house.

problem i had i think there was air in the coil witch super heated the coil making steam then it poped the joints in the kteck lines because of the air i think it would not circulate the water from tank the only pressure valve is on the hot water tank none in line witch i think i will install witch will help get air out of the coil

if there is any one that thinks i have other probs please let me know

deadcarp    Posted 12-05-2003 at 15:53:18       [Reply]  [No Email]
CAH, where you're looking at high pressure (up to about 70 psi) i'd consider a zero-pressure heating coil(well, maybe 10 psi - whatever the circulating pump creates). in other words, create a separate low-pressure hydronic system that really only carries heating water, not potable house water. that way your plumbing and storage tank doesn't need to take pressure and have sealed tops etc, and since the same water basically stays in there so your lime buildup or potential flooding-from-leaks problems are limited. in that case yep the coiled copper will behave fine, as long as it doesn't boil over too often. make sure there's a good pressure-relief vent to outside somewhere near the steam source, and i gotta tell ya, it will surprise you how fast you can boil over a 30-gallon tank. our hydronic system is in central mn, we have an outdoor boiler and over 200 gallons of storage and i could use another 100 gallons. the trouble is - wood likes to burn efficiently at 800-900 degrees and water becomes steam at 212, so the heat often has to get distributed rather quickly.
if you used straight antifreeze intsead, you could raise the boiling point to maybe 240 degrees, under pressure to maybe 275, straight oil might take it to 325 safely. by the time you consider all that, a bigger tank looks cheaper.
here's an online sketch of my boiler, and i recently put a sketch of a pipe-in-a-pipe in the photo gallery. the rest of our heating system needs its own sketch but i'll have to draw one. what you're proposing is quite do-able and interesting. hope these ideas help. don't be afraid to holler for details. :)

deadcarp-sketch    Posted 12-05-2003 at 18:00:24       [Reply]  [No Email]
it's not very clear but here's a sketch of our setup - best i can do for now :)

Lazy Al    Posted 12-05-2003 at 13:44:00       [Reply]  [No Email]
I personally would use black pipe right next to the furnace . Then copper when you get away a little. Over a period of time the pipe next to the furnace will lime up and not let the water circulate . So put unions in so you can change that part. Some times water won't go down into the tank so you might have to take out the top element and go in there If your using an old electric hot water tank .

CAH    Posted 12-05-2003 at 15:02:54       [Reply]  [No Email]
I understand about taking out the top element but what do you mean "go in there"? Do you mean take the heated water through that hole into the old water heater? Thanks.

Al    Posted 12-05-2003 at 17:30:08       [Reply]  [No Email]
As you know in normal hot water operation is the cold water is piped down to the bottom of the water heater . If I was doing what you are planning I would hook up the water heater like you would a normal water heater, Then come out of the drain pipe and thru your heating system and go into the top element hole . with a steady upward flow of pipe from the drain pipe thru the furnace part then into the upper element hole reduced down to your pipe size .
Is this going to be your only water heater ?
If it is, remember on an electric hot water heater the top elements heats the top half then it shuts off an allows the bottom element heat the bottom half. So your bottom element does most of the work . You would have to rewire the stats .
Be best to use two tanks

TimV    Posted 12-05-2003 at 11:31:51       [Reply]  [No Email]
CAH: I've done this myself to heat the water for my homemade hot tub, but I used my oil furnace, not a wood furnace. I coiled the (1/2" copper) pipe into two loops and put them in the furnace ductwork. It worked fine, with the only problem being you had to run the furnace full-time in order to get the water up to 104 F. This was fine for winter, but precluded running the system for most of the rest of the year. I ended up replacing this with a dedicated propane hot water heater, which has worked fine for the last 2 years. One other note--be SURE that your system is properly designed and cannot get over-pressure. A friend (and a better-than-average engineer) had his home-built system rupture while his wife was sleeping on the couch 3 feet away. If she had been sitting up on the couch, she would have been killed--debris was embedded in the wall 20' away. Luckily, the end of the couch took the brunt of the explosion, leaving her with only minor bruises.

CAH    Posted 12-05-2003 at 13:54:50       [Reply]  [No Email]
My system is only to pre-heat the water so my regular gas hot water heater will only need to take the temp. up from a much higher water temp. thus lowering my gas bill for the water heater. I do not plan to utilize the system when the wood furnace is not being used, 6 months out of the year. Since I have the furnace burning I figured I might as well get the most out of it. Did your friend have a pressure relief valve on his system?

Lazy Al    Posted 12-06-2003 at 06:41:00       [Reply]  [No Email]
The way my brother in law has his set up is the cold water runs in the tank by his furnace and out of that to his regular tank and is set up the way I described . His electric water heater doesn't ever come on all winter . Then in the summer the tank by the furance will temper the water before it goes into the electric one .
Works great ,is simple to do and all it cost is an old water heater and some pipe .

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