Using a Flue for Water Heating

[ruddycrazy]

Hiya Guy's,

           I was thinking of making up a new firebox just for heating water for hydronic heating pipes under my house and the wife suggested just putting some coiled copper around the flue of our exsisting slow combustion heater. Now i've never heard of this being done before so this is why I'm asking the question. Would it be detritmental to the operation of the fire or would it work. The idea is put up an insulated tank and install a 12 volt pump to circulate the water. Also I'll hook up the outlet to my instant gas hot water heater which has a temp valve in place and by using preheated water it wouldn't use as much gas. So would this idea work or is it a bad idea to use the flue for heating?

Cheers Bryan

[Titantornado]

You just got to be careful not to take too much heat from the flue.  Too much would effect draft, cause creosote and/or condensation build-ups.  How much is too much?  I'm not sure, but I doubt copper tubing wound around a flue pipe would rob too much heat.

[Tom in NH]

My great uncle used to do something similar. He preheated the water with a coil around the stove pipe before sending it down into the firebox of his wood fired kitchen range. The water pipe emerged and went into a storage tank that sat behind the stove. It did not affect the operation of his wood stove. Sometimes you have to be careful not to cool the smoke too much or the creosote will condense inside the chimney and cause all kinds of problems with blockages and risk of chimney fires. But this was not a problem for my uncle. --tom

[whatsnext]

If you have a real slow combustion stove taking heat out of the flue could just stop the whole cumbustion process by stopping the draw. Even if you don't stop things you may get creosote to build up in the chilled area of the flue leading to a possible flue fire. Why not just put a tank on top of the stove?

John...

[stevesteve]

Hi Bryan,

I am going to try to build a small scale burner for wood/waste (paper etc.) and have designed a heating system as you describe. I was slightly concerned about corrosion between the cast iron flue and the copper pipe so my plan was to embed the copper in a layer of fire cement.

So far I have bought the copper tube, fire cement, flue and am looking out for a firebox (not intending to purchase any more so just keeping my scavenging head on!).

How were you thinking of keeping the copper in contact with your flue?

Were you going to rely on convective flow or were you thinking of using a pump?

[Shadow]

Heres a link to some good information on a system similiar to what you want to do.

      http://www.michigan-horse.org/preheater/

                                             Make sure it has a safety valve somewhere in the setup for when water gets too hot!   Good Luck    

[altosack]

Hi Bryan,

It is much better to put the coil (iron pipe, not copper) in the firebox, not around the flue. The heat transfer by radiation rather than convection on a relatively small area will be much better, and it will not have the detrimental effects of cooling a portion of the flue as others have mentioned.

If you're making a new firebox, and have the welding skills, even better would be to enclose the firebox in a water jacket; particularly if it's just for heating water, enclose all surfaces except the door to get as much transfer to the water as possible, and little transfer out to the room. There's an excellent article in Home Power many years ago (Issue 36, page 18) that explains exactly how to do what you want. The pdf will cost $5 from their web site, but I think it's exactly what you're looking for.

Dave

[ruddycrazy]

Hiya Guy's,

           Thanks for the replies on my water heater idea, last night I measured the temp of the flue which I should mention is about 5 metres long and straight, it's 6 " diameter and the temp at the base was 170 degrees C and about 2 metres up the temp was 140 degrees C. As I have a safety guard around the flue I could make a coil from 3/4" copper and have it retained by the guard. I'm thinking of just installing a small water pump to circulate the water to the storage tank which will be about 2 metres above the circuit. As we only use the fire for about 7 months a year the system wouldn't be use 24/7 all year round and as soon as I get my F&P windmill hooked up to the house system any dumpload will go straight into a heating element in the storage tank.

Cheers Bryan

[ruddycrazy]

Hiya Shadow,

            Thanks for that link mate that looks like a good way to do it but my wood heater cost about $2,500 and has a outer shell all round so you don't burn yourself if you touch the sides. There's no way I'm going to tamper with the fire itself and if the creosote becomes abit of a problem it would just mean cleaning the flue on a regular basis which is just a case of getting on the roof and a 10 minute job to clean it. Anyway I'm still trying to get my wife to agree to let me build another fire outside where I'm buiding a stone laundry and retaining wall and the fire would be right next to the wood pile.

Cheers Bryan

[Mink]

Hi Bryan,

I'm surprised you haven't seen the commercially made flue heaters. They keep them in stock at my local wood heater shop (Ballarat, Vic.) and only cost about $200 or maybe a little more. They are simply a short SS flue section with a SS channel wrapped around it. A good friend of mine has one installed and never uses his slow combustion stove any more as it meets most of his water heating needs - he has a couple of panels as well. However, you should try to see one in operation as his makes a real racket. When it's going it sounds like a really loud kettle on a hot stove about to boil. The plumber at work has installed a couple and said it's important not to put it too close to the fire - he suggested one section up. I could probably get you his No. if you wanted to ask him - email me if you want.

Michael