# Wood Fired water heater,expansion tank?



## greg273 (Aug 5, 2003)

I'm getting my hot water system together, my first wood-fired system, by the way, and I was wondering about the neccesity of some sort of expansion tank.I'm planning on using an old 30gallon water heater as a holding tank, mounted above the wood furnace. (Mounted at a level above the fire in order to produce a self-circulating thermosiphon, if my physics are correct) 
I'm guessing it would be POSSIBLE, but probably not advisable, to simply rely on a pressure-relief valve, and pretty much expect it to blow off some steam now and then. But it seems that some sort of small auxillary holding tank could be used to better effect. 
Are pressure-relief valves able to withstand repeated opening/closing cycles? The water in this system is rainwater, so mineral deposits wouldn't be an issue.
I plan to send the heated water to the fixtures via a tempering valve, to keep the temp safe for use.
And, when building the coil, I'm wondering what would be better, to solder 90's together, or use a pipe bender. It would seem that one could get more copper in a limited space using the sweat-fitted elbows connecting straight pipes. I'm thinking about 24" wide by about 48" tall for the coil.
Thanks for any advice or comments anyone might have.




Greg


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## Ross (May 9, 2002)

If its a closed system (not open anywhere to the atmosphere or the household pressure tank in the system) then you need an expansion tank. Your TPR valve is a saftey not an operating device. Leave it to do it's job and do test it periodically. Is this for potable water or heating or both? Likely violates every building code and plumbing code written, but then the heat exchanger system I've seen offered for sale for my outdoor wood boiler to heat potable water violates the plumbing code here too!


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## MELOC (Sep 26, 2005)

if you can disregard everything i stated in another recent post that makes me seem like a bumbling idiot...

i have a wood/oil fired eshland furnace. there is a hot water coil in the boiler. i use it to pre-heat water before it enters the electric water heater. i never connected it in such a way as to allow it to recirculate. i probably will but am trying to find a way to incorporate other means of heating water as well. anyway...my system is gravity fed from a spring with 20+ feet of head. i have no pressure tank or pump. i have never had any issues with pressure. 

i guess if i allowed the water to circulate as you will, my water heater would climb to the "water jacket" temperture maintained by the furnace. i guess if the water in my water heater climbed to 160 i may need one.


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## rambler (Jan 20, 2004)

MELOC said:


> if you can disregard everything i stated in another recent post that makes me seem like a bumbling idiot...
> 
> i have a wood/oil fired eshland furnace. there is a hot water coil in the boiler. i use it to pre-heat water before it enters the electric water heater. i never connected it in such a way as to allow it to recirculate. i probably will but am trying to find a way to incorporate other means of heating water as well. anyway...my system is gravity fed from a spring with 20+ feet of head. i have no pressure tank or pump. i have never had any issues with pressure.
> 
> i guess if i allowed the water to circulate as you will, my water heater would climb to the "water jacket" temperture maintained by the furnace. i guess if the water in my water heater climbed to 160 i may need one.


If you have _no_ check valve in your gravity fed system - as pressure builds, water simpley goes back up the pipe to the spring.

If you wre feeding the system with a pump, or had a check valve, as the water heats, it expands, & needs to expand to somewhere. Perhaps the pump's expansion/pressure tank will take the extra pressure - tho one would at times go above the normal pressure design of your tank. AND be careful of any check valves along the way. You understand, you can start with 50 gallons of 50 degree water, and in a closed system you could heat it to 180+ degrees. I'm not an engineer, but look up the % of expansion you will create, and _something_ has to give. Hopefully it will always be a pocket of air designed for this purpose in an expansion tank......


As someone mentioned, potable water & any of these heating setups are not supposed to be co-mingled. You should, to be to code, most likely have heat exchangers out of everything to heat a tank of water somewhere. Which of course becomes complex, $$$$, and less efficient.


Tarm is one of the top notch wood boiler makers, and they prefer a hot water storage tank on their systems. You might want to track down their web site & explore how they connect the wood boiler, the storage tank, & home hot water all together into a system. While you will fall over at the sticker shock, it is informative to look over their products & setups.

Here it is. They upgraded their installation section a bit, & more photos too. 

http://www.woodboilers.com/

--->Paul


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## MELOC (Sep 26, 2005)

perhaps since i don't recirculate the water and perhaps because my house needs alot of heat and maybe because i only use the furnace in the winter and cold part of spring time, i shed enough heat from the non-insulated pre-heat line to not have alot of heat building. maybe my need for hot water is tremendous and i use much of what i make. also, being a spring fed system, it runs year round at the outside faucets to prevent freezing and to keep the water "fresh". maybe that relieves the pressure.

perhaps someday i will have a system that is a bit more conventional.


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## Ross (May 9, 2002)

Ok if I'm reading this right you want to add a coil to the wood heat source to heat water going to a tank for household use. I'd just use copper tubing (not pipe) which is soft enough to be formed into the coil you want. Heck it comes in a coil! Every bend adds a restriction (head pressure) so a soft corner is better. If your hot water tank is connected to the household pressure tank (and with no check valves as Paul says) then you're fine without the expansion tank. Just don't use the TPR valve as an entry point by moving it further out on a T fitting. You lose your sensor position and it won't be reliable. Ever see a house after a water heater explodes? Water flashing to steam expands something like 1400 times it's original volume. So 30 gallons of water (it's cubic volume) becomes 42,000 "gallons" of steam, instantly!


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## greg273 (Aug 5, 2003)

Ross, 

With this copper tubing, how will I bend it without kinking the stuff? I seem to recall putting sand in the tubing would work, but its been awhile. 
How does a conventional batch-type water heater deal with expansion issues? Does the pressure tank take up the expanded volume? It would seem that it would, to a point. If so, then leaving out the check valve would be prudent. Hmm, looks like I need to do some math to figure out exactly how much this heated water is going to expand.

Man , a diagram of what i'm thinking here would be helpful...

What would constitute a homemade expansion tank? How to keep the air from entering the system? If a small cylinder was fixed to the top of the recycled water heater tank, any air in the system would stay there, right? And provide a cushion for the expanded water? Does this sound reasonable?

Maybe I'll just put a pressure gauge on the tank and watch it like a hawk the first time, and see what happens. The pressure tank next to the pump should absorb enough of the expanded water to keep it safe. Seems like thats the only way I can really learn, by doing. (and hopefully not injuring myself or anyone else!)


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## Ross (May 9, 2002)

Temperature is your issue solid fuels like wood burn hot, plenty hot, too darned hot for some systems. You would want to be certain that your water tank has no restrictions back to the pressure tank, (like check valves) and that the presure tank was working correctly with the right pressure so there is room to expand. A small secondary pressure tank acting as an expansion tank isn't going to hurt the system. TPR valves are set to blow off at 200 degrees or 12 degrees lower than water's boiling point so if you test it regularly and replace as they fail, you should be fine. Copper tubing bends quite easily, I use the slip over springs to bend the tubing, it's faster than filling the tubing with sand! Which would work I guess. "L" Tubing is made to be bent, it's light years better than "L" pipe in all applications, even if it's not supposed to be.


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## joken (Dec 25, 2005)

Greg, Residential water heater expansion is into the city water system assuming someone has not installed a check valve. The same principle holds true with a well. Typically hot water boilers have a relief valve and a expansion tank. Water is fed into the system via a pressure reducing valve that is set at a pressure equivalent to the altitude you need to lift the water i.e.two story house. The PRV is probably not necessary if you are watchfull of your system. One thing you may consider that is very old technology is an open system. Simply install a pipe to a point above the highest part of your system. Old hot water heating systems had a pipe that ran up thru the roof so all you had to do was fill the system til water shot out the roof and you were done. No expansion tank and actually no relief valve either if the pipe is large enough. I would however use a temp/press relief in any application. Ken in Oregon


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