Energy Matters Forums

[gpigeon]

I am seeking member's experiences with wood fired heaters fitted with water coil or box.
I have in the past had a pot belly stove fitted with a 1" copper pipe coil around the bottom of the firebox. It worked well but used a lot of wood and the coil required constant attention because of creosote / tar build up. I think this could be a problem with any of these stoves because the tar condenses on the cooler water jacket.
I now have a more modern "kent" style wood fire. A few years ago I cut a 200 m square out of the back of it and welded a SS316 water box into the hole. This works "sort of" ok but does not heat the water to a very high temp.
Anyone out there got any advice about stove models, etc? Any home built units operating well?
Regards.
BW

[bradley.jarvis]

We have a Stanley Traditional combustion stove with a stainless steel wetback. The water box is basically at the back of the firebox and takes the place of the back firebrick. I think that you want to have the water jacket where the heat is, so the bottom is no good because you get less heat, this also seems to minimise the amount of creosote buildup because of the extra heat. Make sure you are not running your fire too cold, you should not be getting black smoke out the flue, you should have a good red hot bed of coals in the firebox. Get a fireplace that is rated to the size of your house too, don't go bigger because it will just use more wood to run at the right heat. There are also the bakers ovens which are a wood heater and cooker in one unit, so you get more use out of your wood.

I have also seen and have heard positive comments on the flue water jackets. they are basically coil of copper wrapped around the flue and fitted on the first big of chimney coming out of the heater. They are not as efficient as the wetback but easy to retrofit a standard heater.

We are looking at getting a Thermolux combustion stove in the future, which will do hydronic heating/hot water and cooking. The good thing with these(and similar stoves) are they are fully insulated so running in summer is not too bad but because they are insulated they heat water really well because the heat is focused onto the water jacket and ovens. You then heat the house via water radiators, a much more efficient way of heating multiple rooms than with a standard wood heater which basically radiates out from where it is.

Thanks Brad

[GeoffHammond]

I have also seen and have heard positive comments on the flue water jackets. they are basically coil of copper wrapped around the flue and fitted on the first big of chimney coming out of the heater. They are not as efficient as the wetback but easy to retrofit a standard heater.

We've got one of these on our Ned Kelly wood box (made by Daylesford Engineering, I believe) and a 300 and sumthing litre storage tank otherwise heated by our solar dingus. It does recover some heat that would otherwise be discharged into the Great Outdoors - making it *more* efficient, I reckon, but I don't think it heats as much water as a true wet-back might.

Last year, I only lit the fire of an evening, so they were quite hot to get things up to the temperature we want in the house and showers the next morning were fine. This year, I am experimenting with running the woodbox on low pretty much all the time. Showers in the morning are not quite as hot.

[Nifty]

Not a specific answer for you, but we installed a Rayburn wood stove to give us cooking and heating. As it happens, we removed the boiler which was the back of the firebox (and had to replace it with a big firebrick). It's cast iron and very heavy - a replacement costs about $1100. Apparently this system works very well - the design is basically unchanged from more than 100 years ago. We'd like to install it again one day. But a purpose-made boiler is probably a better option than copper coils, I'm thinking, except for the cost and the fact that it is likely to be part of an existing stove.

[andarm]

I replaced the first piece of 900mm flue with a double skinned stainless piece. the two cylinders (one inside the other) is full of water and the flue gases run through the middle of the inner cylinder. There is a one inch spigot top and bottom to let the water thermosiphon up into our SHWS. Great in the winter when there is limited sun and the fire is going anyway - no extra wood burned. The only thing is your HWS must be vented to atmosphere i.e. low pressure as you can't connect an uncontolled heat source to a pressure system.

[Tracker]

.
Can only comment that "Gordon" has been into these Wet-Backs for ages.

I thought a Wet-Back was a Mexican Border-Crosser..
.
.

[470rigby]

A few years ago I cut a 200 m square out of the back of it and welded a SS316 water box into the hole. This works "sort of" ok but does not heat the water to a very high temp.

Stainless Steel has very low Thermal Conductivity and is a poor choice in heat exchangers...maybe part of the cause of the indifferent performance? BTW... are you a Welder? Welding SS to Mild Steel is quite a feat!

[gpigeon]

I replaced the first piece of 900mm flue with a double skinned stainless piece. the two cylinders (one inside the other) is full of water and the flue gases run through the middle of the inner cylinder.

This sounds like a good idea. What gap (space) did you leave between the inner and outer skin?
Do you get excessive creosote buildup on the cooler jacketed area?
How thick was the ss jacket?
I made mine from 5 mm SS316 plate to withstand the 25 psi (175 kPa) pressure. Because your jacket is circular in section it should be possible to drop to 3mm thk.

Yes, I'm aware of the potential problems with pressure build up in a closed system. My system is supplied from an open tank on a hill, so excessive pressure, if any, just pushes the water back up the hill.

On the controversial question of wood heating being much less efficient than electric/coal/gas heating......according to my literature, the energy content of wood is about 25% that of coal. However, once you burn coal/gas you can never "grow" any more! unless you want to hang around for a couple of million years.
With timber, I can go and plant another tree and in 20 yrs time I have more fuel. During that tree's growth it will have removed from the enviroment many tons of CO2. So, in my lifetime I will have harvested and planted around 4 lots of trees, leaving my debt due to the enviroment as zero.How will all the electricity & gas consumers pay their debt?
If I leave the tree there to die and decay as nature ordained, the "nett" contribution to the enviroment will be zero. Thus, better to harvest it and burn it when it is young!
Is this all a bit simplistic or what????
Regards.
BW

[470rigby]

If I leave the tree there to die and decay as nature ordained, the "nett" contribution to the enviroment will be zero. Thus, better to harvest it and burn it when it is young!
BW

The Wood Heater Industry claims that burning wood does not release any more carbon dioxide than the eventual biodegradation of the wood if it was not burned. Wood burning is thus claimed to be "carbon neutral" - the CO2 released to the atmosphere by combustion is recycled continuously into new plant growth as part of the carbon cycle, while the energy released during combustion is simply a form of stored solar energy.

But, that applies to trees that are already grown. What is not clear is whether planting and growing trees specifically for wood burning is carbon neutral.

Google might provide the answer but my goddammed PC won’t let me get that far before it crashes!

Doe anyone else have the answer?

[470rigby]

I am seeking member's experiences with wood fired heaters fitted with water coil or box.

Fitting a Wetback reduces Firebox dimensions and will restrict the size and amount of wood that can be burnt; thus reducing energy output and burn times. There are also some issues related to higher emission levels from the flue. Presumably, this is due to lower combustion temperatures. Australian Wood Heater manufacturers seem oblivious to this, but NZ manufacturer Metro actually do not approve some of their models to be fitted with Wetbacks if on small sized blocks.

Quote “All ECO Metro’s “with wetbacks” can be installed within New Zealand on any property that is two hectares or more in size. Most ECO models can be installed with a wetback in clean air zones, some restrictions apply to some models/regions.”

For the link see
http://www.metrofires.co.nz/generic/spe ... tions.html