Hot Water Storage Tank Design for Forced Air and Domestic Hotwater

[Sain6815]

Hey all,

I'm designing a heating system fueled by my woodstove coil that will utilize a Wooden, Insulated, EDPM Lined 200 gallon Tank. The tank will be non pressurized. The primary purpose will be to circulate heat through my heat exchanger in my air handler's plenum.

I really want to have a coil inside of the storage tank that will preheat my domestic hot water before it enters my electric hot water heater. I want to go with this option so I wont need a circulator pump and can rely on city water pressure.

My question is this, should i buy a coil for this? If so where? (can't find any info on the web for this.... even called a supply house today)

OR - should i make my own copper coil from tubing. Thanks in advance!

Hopefully this is clear , let me know if you have any questions.

 

[maple1]

Hey all,

I'm designing a heating system fueled by my woodstove coil that will utilize a Wooden, Insulated, EDPM Lined 200 gallon Tank. The tank will be non pressurized. The primary purpose will be to circulate heat through my heat exchanger in my air handler's plenum.

I really want to have a coil inside of the storage tank that will preheat my domestic hot water before it enters my electric hot water heater. I want to go with this option so I wont need a circulator pump and can rely on city water pressure.

My question is this, should i buy a coil for this? If so where? (can't find any info on the web for this.... even called a supply house today)

OR - should i make my own copper coil from tubing. Thanks in advance!

Hopefully this is clear , let me know if you have any questions.

What do you have for a stove? A coil on a wood stove won't make enough heat to make much of a dent in your space heating demand.

 

[Sain6815]

What do you have for a stove? A coil on a wood stove won't make enough heat to make much of a dent in your space heating demand.

Will utlize a coil in firebox

 

[Sain6815]

From the research I've done. I've read of people heating anywhere from 40 to 500 gallons of water to 175-185 degrees by utilizing a coil in the firebox. So thats the research/idea I'm working off of.

 

[maple1]

You should really run some numbers to try to see if it would be worthwhile. Most plenum HXs need a steady supply of 160+ water at flow rates in the 5-10gpm range. I don't think a coil in a wood stove can come close to keeping up with that - at most, they might keep their DHW tank hot with constant recirculation. Problem number 1 would be that the stove coil would dirty up in short order with the cold incoming water condensing creosote - so HX efficiency of that coil will be going slowly downhill as soon as it sees first fire.

Would be interesting to hear more feedback though, from those who have been there done that, but I don't think I have read of any success stories.

But on your specific coil question, I think I would just get a coil of copper & use that. I preheat my DHW, using two coils (200') of 3/4" pex, stuffed inside my tank enclosure right against my LP tanks - most of it right on top of them. There is some pressure drop at the taps on the hot side vs. cold side, but certainly liveable with. PEX doesn't really conduct heat real good for HX purposes, but it made doing it easy and does most of my DHW heating in heating season with intermittent usage.

 

[Sain6815]

Thanks for the caution. I feel a bit in uncharted territory maybe that is a sign of foolishness or courage.... or both.

Aside from the creosote buildup on the coil, are you meaning the the discharge of the heat exchanger will be more rapid than the coil can make up?

I've only seen these type systems used in radiant floor heat or radiators not home heating via central air. That's the uncharted territory I speak of. I do live in SC where did heating demand is lower.

If anyone has tried this I'd love to hear about the experience.

 

[maple1]

Aside from the creosote buildup on the coil, are you implying the the discharge of the heat exchanger will be more rapid than the coil can make up?

Yes. There are many unknowns though, such as your heat demand, and stove size. But the HX ability of a typical plenum coil is much greater than the HX ability of a typical stove coil. If you can get the straight up facts from someone who uses a stove coil in a stove such as yours, on what their actual gpm rate & temp rise at that steady flow is - you could come up with a half an idea on prospects. A commonly accepted dT value for a heating coil or emitting system loop is 20°.

Also radiant infloor heat usually only needs hot supply in the area of around 100-120° (tops).

 

[Sain6815]

Thanks. That's very good insight. I have an older buck stove. It's the large 28000 with large wood capacity.

Also, this was my initial inspiration. Jamie from Myth busters heats his home with loose copper coils in his fireplace.

 

[peakbagger]

You might want to PM member Tom in Maine regarding using EPDM in hot water storage tanks. He has forgotten more than most folks ever knew about storage tanks and gained some very expensive experience on why EPDM is a real bad idea.

He can supply you a custom made liner that will hold up to hot water if you dont want to buy one of his stock ones. http://www.americansolartechnics.com/

 

[S.Whiplash]

Thanks for the caution. I feel a bit in uncharted territory maybe that is a sign of foolishness or courage.... or both.

Aside from the creosote buildup on the coil, are you meaning the the discharge of the heat exchanger will be more rapid than the coil can make up?

I've only seen these type systems used in radiant floor heat or radiators not home heating via central air. That's the uncharted territory I speak of. I do live in SC where did heating demand is lower.

If anyone has tried this I'd love to hear about the experience.

Generally a woodstove coil can't produce enough usable btu's for in-floor heating, never mind a water to air hx in a furnace with a much higher demand. Other concern is house insurance, if you have any you can kiss it goodbye once you put a hot water coil inside your woodstove.

 

[Fred61]

Don't waste your money. You will need more plumbing components than you think and the output will never pay back. If your 200 gallons of storage starts out at 80 degrees you will be lucky to get it to 100 degrees in 24 hours

If you think you want to continue with the project, google "hilcoil". I just ask that you keep us informed on the progress (also with pics) and results.

 

[electrathon]

Not sure how many here have read my write up on my stove coil, I have a large write up posted about a year ago.

I heat most of my domestic hot water in the winter months with my stove. I have a preheat tank going into a insta-hot gas fired water heater. It works well.

Sain is on track with what he is trying to do, he is still in the initial design stages. I am sure as his ideas come together, ideas will evolve. I feel he will not gain as much heat as he is trying to get, but will gain some. Some is still heat gained, the only real question is will the gain be worth the work and cost compared to paying for gas or electricity.

 

[Fred61]

ain is on track with what he is trying to do, he is still in the initial design stages. I am sure as his ideas come together, ideas will evolve. I feel he will not gain as much heat as he is trying to get, but will gain some. Some is still heat gained, the only real question is will the gain be worth the work and cost compared to paying for gas or electricity.

That's kind of a mixed message.

 

[peakbagger]

I do have friend that has been heating a ranch with a combination of an old fisher mamabear and a stove coil for about 20 years. The stove is in the basement but he added a large addition at one point so heat didn't get out to the addition. He has a "coil" made out of sections of slant fin that pretty well covers the top of the stove. He had patio blocks on top of the slant fin. When the water temp goes over a setpoint a circulator turns on and sends the hot water around the house radiators if there is a call for heat, otherwise it goes to a fan forced unit heater in the basement of the addition. It did cut his wood usage somewhat compared to no coil but the big advantage is the entire house has fairly even heat. He doesn't have a storage tank.

 

[electrathon]

That's kind of a mixed message.

I meant to say that he is heading in a direction. It is a good direction, but the exact path is still unfolding. The question is if the end gain is worth the work, but there is going to be a gain.

 

[Sain6815]

Well a decision has been reached. I've heard from everyone I've talked to that heat output won't be worth the effort and money. Just going to keep wood stove as is and purchase a new ventless log set for the living room to replace the old inefficient terrible one. Will have heat source on both levels of the home then.

We were very happy with the gas logs in our last house which was a single level ranch 1200 sq ft.

Our current home is larger and two stories so this method will hopefully be more cost effective than running straight electric. Most importantly, this route makes my wife much happier!