DIY wet back for wood stove

[mattyg]

Has anyone made one / retrofitted one to a wood heater? If so, please fill me in!

I have a BIG wood heater (arrow 2400) in my house which is good for heating about 35 squares. My house is about 20 squares and to get the heater up to operating temp can be too hot to sit in front of so I'd like to transfer a bit of that heat to the HWS.

The hot water service is already a low pressure type in the ceiling space so that part is easy.
I would have thought there would be a heap of diy projects on the net for this but I haven't been able to find any. As far as I can see you just need a constantly rising pipe ladder or coil through the back of the heater and then a rising hot water line to the tank to ensure thermo siphon effect. Is there any more in it than that?? Water volume in heat? Pipe diameter? Pipe material? Pipe wall thickness?

Any help appreciated. Cheers Matt

[B.S.F.]

We used to have an Everhot 204 slow combustion stove with water jacket. There is a hot water coil listed on the Everhot site. I'm sorry but I don't remember the size of the copper tubes to the tank. They were 3/4" or 1".
Everhot (http://www.woodstoves.com.au/html/everhot_parts.html)

[digger]

Does this help with things?

http://www.fastonline.org/CD3WD_40/JF/JF_OTHER/SMALL/How%20to%20build%20a%20safe,%20effective%20wood-fired%20hot%20water%20heater....pdf


this seems to make it very clear and offer assistance...
http://www.pivotstove.com.au/kb/how-to-set-up-your-wood-stove-with-hot-water-and-radiators

We have two galv water pipes (capped) hanging out of the wall directly behind the Wood Combustion Heater so that this can be done here also, as we already use solar heating for the main water heating. (boosted by electricity).

This is of interest to me, as if I can easily convert the already fitted WCH then it would definitely suit. I believe there are also systems to help transfer heat about the house (helpful as we have 10' ceilings here in an old solid farm house).

Watching with interest!

[JDNSW]

Not directly useful, but I have a factory wetback in my kitchen stove. This heats both hot water and, via a circulating pump, the whole house through pipes in the slab. This wetback comprised a steel (might be cast iron, not sure) box surrounding the fire on all four sides except for the opening for the fire door, with a 1.5" pipe across the top of the fire.

The wetback has connections top and bottom at the back. These are connected to 1" copper pipe risers via shutoff valves, but note that these valves must never be shut while the fire is lit, to avoid explosion. The piping, as pointed out, must rise without any high points, and if just going to a hot water system (in my case directly above the stove) going in top and bottom, with the hot (upper) pipe from the fire going to the bottom of the hot water system.

In my case the water circulating is not directly the hot water system - a special system, it had a coil of 1" pipe to transfer the heat. The rising pipes have tees for the house heating, and a small circulating pump, controlled by a thermal switch that does not allow it to operate until the water returning from the HWS has reached the set hot water temperature. There is also a switch that bypasses the on/off switch if the temperature of water coming from the stove approaches boiling.

John

[123rover50]

I have been thinking of doing this as well.
We have a Solar heater and tank on the roof that has pressure relief valves already.
I was thinking of winding a coil of half inch copper tube around the inner flue.
We have about 10 ft between the fire and where it goes through the roof lining. Then T into the Solar pipes with the cold running down the outside of the outer flue into the bottom of the coil. The top of the coil running to the top solar pipe.
Our firebox does not have room for a tank so thought this might work.

Keith

[JDNSW]

I have been thinking of doing this as well.
We have a Solar heater and tank on the roof that has pressure relief valves already.
I was thinking of winding a coil of half inch copper tube around the inner flue.
We have about 10 ft between the fire and where it goes through the roof lining. Then T into the Solar pipes with the cold running down the outside of the outer flue into the bottom of the coil. The top of the coil running to the top solar pipe.
Our firebox does not have room for a tank so thought this might work.

Keith

I think it will probably work, but to ensure that thermosyphon action happens you need to ensure there are no high points. All piping should be well insulated. (Not that would round the flue, of course!)

John

[Judo]

How hot do heaters and flus get? Would copper ever get too soft? While water is running through I'm sure it would be fine, but would you allow for any case where the copper is empty? Just curious...

[JDNSW]

The actual firebox may rarely get hot enough to melt copper, and certainly hot enough to weaken it (but note that copper being a good thermal conductor, provided wall thickness is adequate, heat will rapidly be carried away from the heat source) - obviously not with water in it, but the outside of the inner flue will not get hot enough to worry.

One aspect to allow for is thermal expansion of copper - substantially greater than that of iron or steel.

John

[mattyg]

Thanks everyone for your input. That is an excellent article Digger, thanks. I'm under control with the piping from heater to tank. Just the design of the actual heat exchanger in the firebox that I'm still iffy about.

I was going to go with 3/4" heavy wall, hard drawn copper brazed into a ladder type heat exchanger across the back wall of the firebox. This should keep it away from the flue and I can put a plate of steel between it and the front portion of the firebox to prevent damage when loading wood in.
The heat exchanger pipe will leave the rear of the firebox via brass connectors.

I mightn't get around to it for a little while but I will post up build details when I do

[Rurover]

Matt,

Another simpler option is to install a section of flue that has hot water connections. Like this...SolarOz Accessories - Solaroz Flue Jacket - SolarOz | Solar Hot Water Systems Australia | DIY Solar Hot Water Installation (http://solaroz.com.au/shop/solaroz-accessories/solaroz-flue-jacket/prod_172.html)
or this... http://www.axeman-fireflue.com/

Not as attractive as a hidden wetback, but still does the job and is easy to install.

Alan

[123rover50]

Thanks.
Good idea.
On second thoughts I may purchase one of those instead of winding my own coil.
Time seems more precious as I get older.
Keith

[dero]

I have seen all the above suggestions in practice & they all work , as long as you have thermal siphoning you will get hot water .
If you plan on putting a plate in to protect the coil [ good idea ] may I suggest mesh to allow direct heating .
Don't forget that every mm of space you use means less wood space ie shorter logs . Pay good heed to the warning about the valves & potential for explosion , personally I would not put any valves in the system .

[dromader driver]

One of the relo's has an outdoor water boiler to run the shower in the bush hut. We suspect that due to the pipe work being too small it tends to become a steam generator with associated rumblings and spitting. No valves in the circuit just a large coil in the top of a 20 gal drum with fire below. Tank is about 2metres head above and adjacent.

Takes about 30mins to heat 120 litres to boiling. Shepherds crook for over pressure.

Be careful !

[Tank]

Before you go modifying water heater connections to your stove it would pay to check with your Home insurance co. and Council and have/get proper approvals/permission.
Your Household insurance could be Null and Void if your house burns down or the Hot Water Service blows up, esp. if there is non-approved or faulty workmanship done by an unlicensed installer.
Domestic Wood Heater/Slow Combustion Stoves owners have found out the hard way after their house has burnt down that they have no insurance because of lack of Approval by council or homemade modifications.
You need to have council approval for the Stove/DWH and has been installed (and maintained) by an approved/licensed installer, or kiss your expensive home insurance bye, Regards Frank.





Has anyone made one / retrofitted one to a wood heater? If so, please fill me in!

I have a BIG wood heater (arrow 2400) in my house which is good for heating about 35 squares. My house is about 20 squares and to get the heater up to operating temp can be too hot to sit in front of so I'd like to transfer a bit of that heat to the HWS.

The hot water service is already a low pressure type in the ceiling space so that part is easy.
I would have thought there would be a heap of diy projects on the net for this but I haven't been able to find any. As far as I can see you just need a constantly rising pipe ladder or coil through the back of the heater and then a rising hot water line to the tank to ensure thermo siphon effect. Is there any more in it than that?? Water volume in heat? Pipe diameter? Pipe material? Pipe wall thickness?

Any help appreciated. Cheers Matt