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Author Topic: water heating with woodstove  (Read 11467 times)

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Offline Handy Andy

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water heating with woodstove
« on: June 14, 2010, 11:18:10 PM »
  I'm thinking of putting a tank for heating water beside my woodstove.  The tank would be plumbed so as to be in line before my gas water heater.  Wondered what kind of tank I could get, and does it need to have a relief valve, or would the one already on the water heater take care of that?  My thinking is, the water is colder in winter coming into the house, and I could preheat the water so the water heater doesn't have to work as hard.  Saving money in the process.  The woodstove is maybe 14' from the water heater.  I can insulate the pipe between the tanks to keep heat loss down.  My woodstove is the biggest model from Vermont Castings, and is located in the basement of the house.  I built the flue out of 8" clay tile, with the chimney built of haydite 4" block , and the gap between filled with sand.  Have a cleanout just above the basement floor, so it is easy to clean out what falls off flue.
My name's Jim, I like wood.

Offline Gary_C

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Re: water heating with woodstove
« Reply #1 on: June 15, 2010, 01:02:23 AM »
Anything you do to preheat the water going to your hot water heater will save some gas. The only question is how much you will save and what it will cost.

Most of your heat gain in the water will come from radiation energy from the wood stove so it's best to maximize the surface area exposed to the stove and paint the exposed surfaces flat black. You could use a tank, a coil of pipe or tubing, or a hot water radiator.

And no, don't insulate the line from your heat exchanger to the water heater. That line could also be a part of the heat exchanger. However you may have to insulate or bypass the system when the stove is not running to prevent condensation on exterior surfaces.

As far as a relief valve, yes you maybe should use one depending on how far away your heat exchanger is from the stove and how hot the water may get in your preheater.

I can't say for sure, but I suspect you will not save enough if you have to buy all the parts. However if you had some of the components on hand, it could save you some money.
Never take life seriously. Nobody gets out alive anyway.

Offline SwampDonkey

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Re: water heating with woodstove
« Reply #2 on: June 15, 2010, 02:54:47 AM »
We had one hooked up to the furnace to use in winter. The tank didn't last very long as I recall and with installation there was no savings. Electric here isn't that expensive when you aren't using it as a heat source. This time of year I pay around $75 a month includes lights, hot water and clothes dryer. The largest bill last winter was $99 with a furnace fan being the only thing extra running. The phone bill is almost as much, maybe I'll cut that service. :D
Move'n on.

Offline Handy Andy

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Re: water heating with woodstove
« Reply #3 on: June 15, 2010, 11:39:08 PM »
  We heat with propane, and it was kindof pricy this last winter.  And the woodstove is a little big for the house, so figured I had a little extra capacity.  I saw one fixed up the way I described,  and the guy had his gas turned off on his water heater.  He used a galvanized tank looked about 30 gallons, sitting behind his wood stove, and pretty much heated everything with wood.
My name's Jim, I like wood.

Offline SwampDonkey

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Re: water heating with woodstove
« Reply #4 on: June 16, 2010, 05:04:01 AM »
Electric heat has become cheaper than oil now that it is $0.85 or more a litre. I knew one day that all that oil, gas and propane would be too expensive for the average home. Oh yeah, natural gas is way higher here because we subsidize it's distribution to the bigger markets. I knew that was going to happen to, it happens with gasoline and oil already. They just discovered the largest natural gas shale deposit on the continent, won't do NB'ers no good. First off, it will be outsiders that work it and they don't spend money here. The tax man will get royalties, but that's mostly Federal. Just look at what Danny Williams has been fighting for in NFLD.
Move'n on.

Offline Woodcarver

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Re: water heating with woodstove
« Reply #5 on: June 17, 2010, 11:17:22 AM »
We use a worn out hot water heater as the storage tank in our system.  A V shaped stainless steel pipe in the firebox of our furnace heats the water.  We have a small expansion tank connected to the line to the storage tank.  We still blow the pressure relief valve on the storage tank occasionally.  Typically happens during a January cold spell when we are firing the furnace frequently.

I guess in theory the pressure relief valve on the operational hot water heater should take care of the problem, but being a belt and suspenders type guy I wouldn't use a storage tank that doesn't have a relief valve.
Just an old dog learning new tricks.......Woodcarver

Offline Holmes

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Re: water heating with woodstove
« Reply #6 on: June 17, 2010, 09:33:48 PM »
 Trying to pre heat water to your water heaters is a great idea but any tank that  is being warmed by another heat source must have a temperature pressure relief valve . Heated water under pressure can be lethal by explosion. Each tank must have its own relief valve. Just for all to know. Holmes
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Offline WH_Conley

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Re: water heating with woodstove
« Reply #7 on: June 17, 2010, 09:39:38 PM »
Standing a tank alone, a, "tempering tank" is pretty passive. Just don't tie it into an active system.
Bill

Offline bandmiller2

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Re: water heating with woodstove
« Reply #8 on: June 22, 2010, 09:07:25 PM »
Mayby an old cast iron radiator painted black and put behind the wood stove would work better than a tank,more surface. Frank C.
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Offline StephenRice

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Re: water heating with woodstove
« Reply #9 on: September 21, 2010, 12:55:57 AM »
You can easily hook up a line loop to your existing hot water heater without any other tanks. 

There are several ways of hooking it up, but here is one suggestion.

I might suggest first installing a one-way check valve in the cold water line after the cold water line to the water heater has split off from the main house supply line so that the hot water does not back up the cold water supply line (not necessary, but helpful).

Most regular home water heaters have at least two holes on the top that are blocked off with a plug.  Usually in the center of the water heater is a 3/4" thread plug that actually has a long sacrificial zinc anode attached to the end of it.  Leave that anode in the water heater.  It prevents the heater tank and elements from rusting out.  The other hole with a 3/4" inch plug is usually just that, a plug on an unused threaded hole.  Attach your first pipe fitting to that hole to draw the water out to go to your wood stove or heater.  (If you cannot find one of those empty holes on top of your water heater, then use the expansion valve hole with an inline fitting to reinstall the expansion valve inline next to or above the water heater with a "T" fitting.)

Somewhere near to this point, install a small water circulation pump rated for hot water.  You will have to provide electrical service to this point with a switch installed wherever it is convenient to turn on only when using the wood stove or heater.

From the circulation pump, run the water line to the inlet side of your wood stove heat exchanger (such as a 1/2" coil of copper or, better yet, stainless steel, but NOT with soldered joints for obvious reasons) where it enters the stove.

From the exit point of your wood stove heat exchanger, run a line back to bottom of the water heater installing a 3/4" T-fitting and pipe nipple at the point where the drain line spigot comes out of the bottom of the water heater.  Reinstall the drain spigot into the open spot on the T-fitting.

Wahlahhhh... you are done.  The thermostat on the water heater will only allow the gas burner or electric heating elements on the water heater to come on when there is not enough hot water from the wood stove.
 
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

Offline John Mc

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Re: water heating with woodstove
« Reply #10 on: September 21, 2010, 08:59:36 AM »
One problem with putting a coil in the firebox is that even heating domestic hot water (which is much less of an overall demand than home heating water) will pull a lot of heat out of the firebox, dropping the combustion temperature. This makes your burn much less efficient, and makes for a fire that generates a lot more creosote.

Even wrapping a few coils around the exhaust stack can cause problems by cooling the exhaust smoke, causing condensation (and creosote deposits) on the walls of the chimney.

Just passively heating a radiator set next to the wood stove should not be a problem. This might let you preheat a "radiator load" while the water is siting in the radiator, but don't expect a huge temperature rise when the water is actually flowing through the radiator... probably not enough surface area and dwell time to really get it hot while flowing water through it.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Offline StephenRice

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Re: water heating with woodstove
« Reply #11 on: September 21, 2010, 11:28:14 AM »
One problem with putting a coil in the firebox is that even heating domestic hot water (which is much less of an overall demand than home heating water) will pull a lot of heat out of the firebox, dropping the combustion temperature. This makes your burn much less efficient, and makes for a fire that generates a lot more creosote.

Even wrapping a few coils around the exhaust stack can cause problems by cooling the exhaust smoke, causing condensation (and creosote deposits) on the walls of the chimney.

Just passively heating a radiator set next to the wood stove should not be a problem. This might let you preheat a "radiator load" while the water is siting in the radiator, but don't expect a huge temperature rise when the water is actually flowing through the radiator... probably not enough surface area and dwell time to really get it hot while flowing water through it.

Well, I was not trying to design the heat exchanger for him, only trying to describe how to hook up his water heater to the wood stove.  Obviously, each installation would have to be looked at closely to make sure that it did not cool down the fire too much, but I might also add that I was looking at some old cooking stoves that had a heat exchanger in them hot water line hookup to heat household water.  So, if some stoves already have this feature, then hopefully it is not all that bad.  Besides, isn't extracting as much heat as possible before it escapes the chimney the name of the game in terms of efficiency? 

Think about those popular stove pipe heat exchangers with the electric fan that people use all of time above their wood stoves.  They are obviously extracting heat from the exhaust stream to add to the room.  Fireplaces often have heat exchanger plenums as well.  They are extracting heat as well.

Obviously, if you have a stove that is undersized already or does not do a good enough job of burning a hot enough fire AND you are in a high wind situation, you may encounter some difficulties.  You might just have to adjust by burning a little hotter fire and maybe that would require burning a little more wood.  But, for people that want to spend less on wood than on electricity or gas, it would probably be worth it to them. 

I would have an operational suggestion for anyone wanting to heat their water this way...

Turn on the circulation pump and open ball valves in the morning a half hour or more before taking showers. (When you get up in the morning to stoke up the fire and refill the wood stove.)  If they want the fire to last longer at night when they go to bed and they are not using much hot water, then they simply shut off the recirculation pump (and possibly close a ball valve) and the heat exchanger stops drawing off heat.  That way they can turn the damper down in the stove to get a cooler and longer lasting fire without having to worry about a heat exchanger drawing off too much heat.  I would also insulate the water heater with a fiberglass blanket to keep the water in the heater warm as long as possible and put a timer on the electric water heater to prevent it from operating at night and wasting electricity.

The rest of the time during the day, they can keep the fire hotter and not worry about excessive heat loss to the exchanger.  If no one will be home during the day, then they can go through the night time procedure again to shut down the hot water production when no one is home.  That will save wood and electricity.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

Offline SwampDonkey

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Re: water heating with woodstove
« Reply #12 on: September 21, 2010, 05:39:42 PM »
We have had different gadgets here to heat water, electric turns out is the cheapest and best option in my case. The others never even paid for themselves by recouping electric cost. My biggest electric bill in this old house is $120 in January. Last year that was down $20 because it was a warmer winter. I still can figure that out for the life of me. Does it cost more to turn an electric motor in a heated basement when it's -30 outside than it is at 0 outside. I mean I still have to heat the place and the motor runs as much whether it's severe cold or 0 out. I think they pad KW hours in the cold. :D
Move'n on.

Offline StephenRice

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Re: water heating with woodstove
« Reply #13 on: September 21, 2010, 08:54:33 PM »
It sounds like you either do not use electric for everything or you have really cheap electricity or both.  But, I do know that most power companies charge more for the same electricity during high usage times than when they do during off-peak times.  That may have something to do with your bill.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

Offline whiskers

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Re: water heating with woodstove
« Reply #14 on: September 21, 2010, 10:14:58 PM »
 





This is a copy of a production unit by Hearthstove that I built in the late 70's. It has 40 inches of schedule 80 "3/4 black pipe plumbed inside the firebox just under the top plate. The 40 gal electric water heater ( with timer) had been moved to a cupola like  box on the roof above the room behind the stove. The original plumbing had been extended from the crawl space below through the void between the studs to the roof passing behind the stove and reconnected to the water heater. The schedule 80 was then placed in parallel with the w/h by teeing into both cold and hot water lines. The elevated tank eliminated the need for a circulation pump. The stove provided 100% of the heat for the home of just over 1700 sq ft. and for several months helped considerably with hot water for our family of five. One cold day with a roaring fire  wife turns on the kitchen sink hot water and gets steam along with the hissing sound. So ended the hot water project and she was not amused. The relief valve had not vented, no injuries or damage. I blanked the tees and removed the external pipe. Later we sold that house with the stove and w/h still in place.
These pictures are Polaroid scans from early in the project. The room is 17 x  21, cathedral ceiling with exposed ridge beam, SYP board and batten exterior that the squirrels were climbing on a couple of days before I nailed it up. The stove is on a stone hearth opposite the door. The last snow in the neighborhood would be on the North side roof above it. No other pictures so it must not have happened.
Just to add this thought, water heaters can be dangerous. Relief valves and thermostats  fail causing serious explosions that can destroy a house and severely burn, maim or kill. My next water heating project will be similar but solar assisted.
many irons in the fire.........

Offline StephenRice

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Re: water heating with woodstove
« Reply #15 on: September 21, 2010, 10:39:50 PM »
That looks really nice, whiskers.  I would really like to set up some solar water heating panels as well, but I think that it would be great to incorporate those with the wood stove heating as well in a combination system.  The solar would work great during the summer and the wood stove in the winter.  You could isolate each system from the other with ball valves and drain whichever system you were not using at the time to increase efficiency.  That would also prevent the solar collector water lines from freezing and bursting in the winter when it gets really cold.

As far as the dependability of pressure relief valves and the possibility of one failing, I would simply suggest to install a second valve inline with the first one as insurance. If one failed, surely the second one would work as a backup.
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

Offline whiskers

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Re: water heating with woodstove
« Reply #16 on: September 21, 2010, 11:20:16 PM »
Steven, There was no pressure gauge on mine and I don't know if the pressure ever approached the opening pressure of the relief valve. A directional flow control check valve and plumbing directly to the water heater would have prevented the episode with mine though there was no convincing one certain "wet hen" that it would work. The second relief valve is sound thinking.
many irons in the fire.........

Offline StephenRice

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Re: water heating with woodstove
« Reply #17 on: September 21, 2010, 11:35:45 PM »
ROFLMBO!  You just know the wet hen rules the roost!
"Pure gold fears no fire!" - (Ancient Chinese proverb)  What do you fear?

Offline whiskers

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Re: water heating with woodstove
« Reply #18 on: September 22, 2010, 12:00:59 AM »
I ain't 'fraid of that big ole woman!
many irons in the fire.........

Offline SwampDonkey

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Re: water heating with woodstove
« Reply #19 on: September 22, 2010, 02:33:26 AM »
It sounds like you either do not use electric for everything or you have really cheap electricity or both.  But, I do know that most power companies charge more for the same electricity during high usage times than when they do during off-peak times.  That may have something to do with your bill.

We pay a little more for rural service (service fee which is only a flat fee not tied to power use, a couple bucks more), but the rate actually drops if you use more power (over 1300 Kwh) on that portion over 1300, but I don't even consume that. I wouldn't burn much more than 600 kwh in winter and 450 maybe in summer or less. My rate is $0.0985. I could save the $6 rent on the hot water heater if I chose by putting in my own, they're only $125 plus install.
Move'n on.

Offline John Mc

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Re: water heating with woodstove
« Reply #20 on: September 28, 2010, 09:36:28 PM »
isn't extracting as much heat as possible before it escapes the chimney the name of the game in terms of efficiency? 

The name of the game in terms of efficiency starts with burning the fire at the appropriate temperature. Some of the gasses given off when burning wood will not ignite if the temperature is not high enough (unfortunately, I don't remember the ideal temps). If that's the case, the gasses go up your chimney unburnt, or condense on the chimney walls. Failure to burn these gasses gives up a LOT of BTUs that you could otherwise burn.

For a good web site with information on the subject, check out www.woodheat.org Their site has a lot of good info, as well as links to other sites with good technical background.

John
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow


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