Wood-burning stove for long logs?

[Tom at Heartwood]

I would like to be able to burn long logs, say 1m, in a stove that has a glass front and a back-boiler that could provide domestic hot water.

 

I like seeing a fire in the house, rather than having a boiler and accumulator tank hidden away in an out-house, but I also would like efficency of handling and combustion to heat an old farm house.

 

Can anyone suggest makes or models or sources of information on such things please? I think that I have seen French stoves years ago that are something like this but can't seem to track any down on-line.

 

Any suggestions are welcomed.

 

Cheers, Tom.

[Monikieman]

i Tom,

 

not sure about a metre long, but have a look at

 

Wood Burning and Multi-fuel Stoves from Dowling Stoves.

 

as Steve Dowling fabricates them.

 

Dare say they will have lovers and haters but he might be able to customise something for you.

 

Cheers

 

Brian

[growforest]

I've got a Dowling and it's great. Adds a nicely industrial feel to the living space, and is a very efficient burner. There's nothing to go wrong, really. Steve will build you whatever size you want.

[Alycidon]

How big is the room you want to put it in?, a stove that big will have a massive heat output. In excess of 25kw. Al boiler stoves have two heat outputs, heat output to room and heat output to water. Make sure that the stoves heat output to room does not exceed the volume of said room divided by 12 for poor insulation, 14 for average and 16 for good insulation.

 

So a monster room of say 10m x 6m x 2.5m = 150 cu m, divide say 14 for average insulation = 10.71kw heat to room required to get the inside temp to 22 deg C if its 1 deg C outside. A stove of that size will in that room deliver a comfortable temperature and show good flames in the firebox. very few boiler stoves deliver that kind of heat to room, the Brosely eVolution 26 delivers 10kw to the room and 16kw to the water, that is a massive stove

 

ahttp://www.broseleyfires.com/Wood-Burning-Stoves/eVolution-26-Boiler_Wood-Burning-Stove.htmlnd

 

 

That is the biggest boiler stove I am aware of.

 

Hope that helps.

 

A

[djbobbins]

No boiler option (AFAIK) but the big DRU stoves will burn logs up to 60cm and handily have a side door as well as the front door for direct loading of long logs.

 

I can understand the preference for seeing a fire in the room, however have you considered that with a stove and backboiler you won't get any RHI, whereas if you had a stove for "top-up" heat and focal point, plus a chip or big log boiler outside, you should be able to qualify for RHI payments on the output from the boiler (and maybe even a grant towards the install, at the moment).

[Alycidon]

No boiler option (AFAIK) but the big DRU stoves will burn logs up to 60cm and handily have a side door as well as the front door for direct loading of long logs.

 

I can understand the preference for seeing a fire in the room, however have you considered that with a stove and backboiler you won't get any RHI, whereas if you had a stove for "top-up" heat and focal point, plus a chip or big log boiler outside, you should be able to qualify for RHI payments on the output from the boiler (and maybe even a grant towards the install, at the moment).

 

 

 

Very sensible comments, side doors on big stoves are an excellent idea and there is NO RHI or reduced rate VAT on boiler stoves, only boilers. It is viewed by the tax authorities (when I last checked a couple of years ago) that a boiler stove has a glass to view the flames and is visually pleasing to the eye, often used as a centerpiece for a room. A boiler is a steel or cast iron box that has no glass, no visual appeal and is solely designed to heat water. It would be tucked away out of sight somewhere.

 

A

[oldoaktree]

Aarrow Stratford TF90 Multi-Fuel / Wood-Burning Boiler Stove

 

Have a look at this

[Ashes_Firewood]

Imagine if everyone had stoves capable of burning 1m long logs... would save so much time!

[face cord]

Doesn't increasing the log size make the fire/stove less efficient though?

[openspaceman]

Doesn't increasing the log size make the fire/stove less efficient though?

 

 

Efficient isn't a very good word to describe conversion of the chemical energy into sensible heat. You can describe how much of the heat released gets into the room and how much is lost up the chimney and you can work out how complete the combustion has been, then you might work out a figure for the overall conversion of energy to useable heat in the room. There is no reason why big logs should be any different but there are problems.

 

Natural gas is a good fuel, you can have a naked flame from a pipe and provide enough oxygen from the air around the pipe to completely oxidise the gas in a yellow flame but as the flame gets bigger the surface area of the flame is inadequate for diffusion of oxygen into the gas and sooty carbon is given off. So you arrange to premix the gas and air before the flame in the right proportions, the result is a blue clean flame.

 

Wood is a bit more difficult, first you have to heat up the surface to drive off moisture, then raise it to pyrolysis temperature, about 270C, to expose some fresh char and ignite it, once the char is burning hot enough it then ignites the pyrolysis offgas if the air is available, this will normally be a diffusion flame in a small fire. At the same time this burning surface is heating up the interior of the wood until it too dries, pyrolyses and ignites.

 

If the wood is in small pieces, like chips, you can continually feed them in with the correct amount of oxygen in air to continuously burn the lot, in practice you will supply double the actual amount of air required to keep things clean. The surface area for reaction is large in relation to the mass of fuel and the conditions are constant. At home you probably do this by throwing on a new split log now and again and the air:fuel ratio constantly varies. With good fuel excess air is the main reason for poor energy transfer into the room as it inevitably means more heat goes up the chimney.

 

Large logs have high mass, poor surface area to mass ratio and probably higher moisture content. The effect of this is adding a large log quenches the combustion temperature while the log heats up and then once it strats burning there is need for more air for the combustion to complete. With big dry logs or logs that have been left to smoulder (char burning but pyrolysis offgas and moisture being driven off unburnt as smoke) they gradually heat up in the box anf go through the drying then offgassing cycle until the firebox is full of hot offgas. Open the door at this stage and you have a potentially lethal deflagration as the stovve contens are all above the autoignition temperature of the woodgas just waiting for oxygen to react with.

 

The whole log burners I dealt with got around the problem by being configured as down draught or cross draught gasifiers. The lot was loaded as a batch and the cool primary air cooled the logs so they just burned at the gasification front, all the offgass then burned in a secondary area. Even so it was always necessary to have a large thermal load to even out the peak from the process.