Any idea what percentage of rated output a woodburner will average?

[Big J]

Just out of curiosity, I was wondering was percentage of the stated output a woodburner will average in continuous operation. Our stove is a Champion Stove Company Dominator (named after the classic motorbike) and is somewhere in the region of 20kw. Don't know exactly as I had it made to order. It is huge though, at 700mm deep, 425mm wide and 600mm high. You can get about a third of a barrow of firewood into it in one go.

 

The reason I ask is that we have a modern oil fired central heating system too. It's rated to 28kw but will not heat the house up anywhere near as quickly as the stove.

 

So I'm wondering, if the stove is in the region of 20kw, what will it actually be producing? For reference, we burn about 0.8 cubic metres a week, and the heating doesn't ever come on unless the average day and night temperature is below 0. The house is 127 square metres, and the living room is usually 20-24c, the rest of the house 17-19 with a small oil filled radiator in my daughter's room just in case.

 

Just curious as I was trying to work out how much oil we'd use if we didn't have the stove and could not come up with a figure as the average oil consumption varies so massively depending on where you are in the country and what your preferred house temperature. Given that our house is over 200 years old, I'd think we'd be using over 4000 litres a year (on the basis that we use about 1200 litres just for hot water and those occasional times the heating comes on or if we're on holiday).

[Stubby]

I understand were you are coming from BigJ . My Burley is rated at a nominal 5kw but punts out nearer 8kw once up to temp . If you slide the air control wide open is probably more than 8kw but no need to over drive it as it heats my small house fine .

[difflock]

Well seeing as a single 2250 litre fill heats our 3750 sq ft house for over 12 months, when agumented by a nominal 8kw wood burning Morso, "quite a lot", would be the only answer.

The oil heats underfloor and 2 rads in the winter and DHW in the summer.

but back when the children were at home and we were heating the upstairs as well, I guesstimated an average output of 5kw by 24 hours = 120kwHrs = a 30kw boiler running for 4 solid hours.

mth

[openspaceman]

Just out of curiosity, I was wondering was percentage of the stated output a woodburner will average in continuous operation. Our stove is a Champion Stove Company Dominator (named after the classic motorbike) and is somewhere in the region of 20kw. Don't know exactly as I had it made to order. It is huge though, at 700mm deep, 425mm wide and 600mm high. You can get about a third of a barrow of firewood into it in one go.

 

That could be as much as 44kg of softwood at 20% mc which will release 176kWh, so how long would that take and how much heat stays in the building.

 

You can treat the stove as a black box with wood and air as the inputs and flue gases as the losses, so you need to know the temperature of the flue at the point it is of no further benefit to the house. If it has an insulated chimney then for practical purposes this is at the flue exit into the chimney but with a masonry chimney heat may still be absorbed and slowly released through the chimney breast into the dwelling area.

 

Ideally you need to know the CO2:CO ratio as an indicator of completeness of combustion and the oxygen remaining in the exhaust to work out the excess air

 

So I'm wondering, if the stove is in the region of 20kw, what will it actually be producing? For reference, we burn about 0.8 cubic metres a week, and the heating doesn't ever come on unless the average day and night temperature is below 0. The house is 127 square metres, and the living room is usually 20-24c, the rest of the house 17-19 with a small oil filled radiator in my daughter's room just in case.

 

That's too many variables but your 0.8m^3 of solid wood will be around 320 kg of oven dry softwood or 400kg at 20%, about 1600kWh, say you burn that at 75% efficiency which gives an average of 7kW continuous. Assuming 85% efficient oil burner that should be delivered by 140 litres of oil.

 

The thing about most stoves is that the heat exchange area is limited to the sides of the box, it's easy to over drive them by making the box hotter but this leads to higher losses as the flue gases are dumped at higher temperatures, a boiler is more steady as it cannot get above 100C and if it has a accumulator and thermostat it can absorb more heat without significantly increasing the flue temperature.

[Woodworks]

20 kWh wood burner and 1200 litres of oil

 

Fit some insulation man. Our 8 kWh wood burner will heat our whole house with ease and keep thinking of getting a smaller one as we cant open it up without overheating the place. 200 year old barn with 120 m2 of flooring and some of it double hight.

[Big J]

20 kWh wood burner and 1200 litres of oil

 

Fit some insulation man. Our 8 kWh wood burner will heat our whole house with ease and keep thinking of getting a smaller one as we cant open it up without overheating the place. 200 year old barn with 120 m2 of flooring and some of it double hight.

 

Your climate is distinctly different in Devon. Winter is much longer here, a fair bit colder and we burn through the summer months too. We've friends in Uffculme and my brother's in Exeter. It's like a different country down there!

 

That being said, you should try keeping an old house warm in Aviemore - that's another level again.

 

We do like a warm house, but it's fairly well insulated (usual loft insulation) and warm for it's age. Our previous cottage was far harder to heat.

 

Thanks for all of the info Openspaceman - very interesting. We burn a mixture of probably 75% softwood, 25% hardwood. The burn is always kept in the efficient zone of operation courtesy of a flue thermometer.

 

140 litres a week would seem about right, which would be about 600 litres a month for 6 months (Oct-March inclusive) plus about 300 litres a month for the rest of the year. 5400 litres a year - ouch!

[SteveA]

If you want to burn about an 1/8th of the amount of wood (for the same amount of heat you're currently getting) you'd do well to look at installing a batch box rocket mass heater. Much less pollution too.

cheers, steve

[Big J]

If you want to burn about an 1/8th of the amount of wood (for the same amount of heat you're currently getting) you'd do well to look at installing a batch box rocket mass heater. Much less pollution too.

cheers, steve

 

Not something I've come across before. Practical in a domestic setting?

 

I'm not that bothered about how much we use. We burn about 15 tonnes a year, which out of a turnover of over 1000 tonnes a year through the yard is inconsequential. I was just curious to see what sort of heat output we were getting.

 

Sat in the living room at present, doors open to the rest of the house. Thermometer at far side of the room to the stove (and 2ft above the ground) is reading 22.5c. It's 3.3c outside with a stiff breeze and intermittent sleet and hail. Flue thermometer is reading exactly half way through the safe zone of operation (with vents nearly fully closed) and we'll have burned two barrows in two days on account of it being the weekend and us being here all day.

[better dayz]

Don't you grudge the time spent processing transporting stacking ?

[Big J]

Don't you grudge the time spent processing transporting stacking ?

 

Takes very little time. I don't have to split the firewood small at all (45-50cm long, and about 4 times the cross section of an average log) so three or four cube an hour through the processor. 45 minutes for my wife and I to stack a 4 cube load at home in lines outside (initial drying) and then 6 months later when it goes into the store (which holds 21 cube but is still too small!!) it took three of us 90 minutes to chuck 20 cube into the store. Having a big stove that takes big logs really cuts down on handling.

 

Not sure what else we could do to increase the thermal efficiency of the house. The loft is insulated, most of the windows are double glazed and there are no noticeable draft. Additionally, a friend of ours who took a pHd in Thermography (thermal imaging of buildings) did a quick survey of our house and said for it's age, it's quite well insulated.

 

It's just the incredibly rubbish climate here!