How to calculate cost of running a woodburner vs gas central heating

Ally_E.

Hello, what's the best way to calculate the cost of heating a room with a wood burner vs gas central heating? The wood burning stove is 4.8kw with 74% efficiency. Any advise is much appreciated.

Benny2020

Wood is free so the cost is nothing.

QrizB

If wood is free, its cheaper than gas

If you don't get wood for free, how much do you pay?

victor2

We have a 5KW log burner (put in for cosmetic purposes more than practical!) and buy the logs for it. If I run it for a typical winter evening, I'll get through about £3 of logs. The living room is toasty warm, the open plan downstairs is comfortable, the central heating gets switched off by the main thermostat and the bedrooms get cold, even with the doors left open. It saves around £1 off the gas bill by not having the central heating running.
So, no saving, just the pleasure of sitting near a burning log fire.
I avoid getting free firewood because timber has to be of a low moisture level to burn well and cleanly. We have our chimney swept annually, but missed one in the pandemic and it was fine, so will just have it done bi-annually now. The sweep said if you burn damp wood or  timber that has not been allowed to dry out thoroughly, you'll get much more soot and need sweeping more often.

Petriix

victor2 said:

We have a 5KW log burner (put in for cosmetic purposes more than practical!) and buy the logs for it. If I run it for a typical winter evening, I'll get through about £3 of logs. The living room is toasty warm, the open plan downstairs is comfortable, the central heating gets switched off by the main thermostat and the bedrooms get cold, even with the doors left open. It saves around £1 off the gas bill by not having the central heating running.
So, no saving, just the pleasure of sitting near a burning log fire.
I avoid getting free firewood because timber has to be of a low moisture level to burn well and cleanly. We have our chimney swept annually, but missed one in the pandemic and it was fine, so will just have it done bi-annually now. The sweep said if you burn damp wood or  timber that has not been allowed to dry out thoroughly, you'll get much more soot and need sweeping more often.

The trick with burning free wood is to leave it to dry for 18 months before you burn it. 

victor2

Petriix said:

victor2 said:

We have a 5KW log burner (put in for cosmetic purposes more than practical!) and buy the logs for it. If I run it for a typical winter evening, I'll get through about £3 of logs. The living room is toasty warm, the open plan downstairs is comfortable, the central heating gets switched off by the main thermostat and the bedrooms get cold, even with the doors left open. It saves around £1 off the gas bill by not having the central heating running.
So, no saving, just the pleasure of sitting near a burning log fire.
I avoid getting free firewood because timber has to be of a low moisture level to burn well and cleanly. We have our chimney swept annually, but missed one in the pandemic and it was fine, so will just have it done bi-annually now. The sweep said if you burn damp wood or  timber that has not been allowed to dry out thoroughly, you'll get much more soot and need sweeping more often.

The trick with burning free wood is to leave it to dry for 18 months before you burn it. 

The wood we buy has had at least 2 years to dry out. Haven't got the space or patience to store wood like that at home!

Ally_E.

We used a mix of free wood (yeay!), kilned dried logs in 40kg bags and briquettes from B&Q. How do I calculate how much cost of wood we're using up without counting the actual amounts of wood/briquettes? Is there some way to calculate based on kW of the stove?

Trying to see if buying more wood (a tonne at a time) and using the woodburner more next winter would make financial sense. Working from home, so I could move my office to the living room and enjoy the stove majority of the day and have the bedrooms TRVs on low to prevent damp. 

On a colder day we use around 3 logs and 1 briquette to heat the room from 16 to 20 degrees, on a warmer day that's 2 logs and 1 briquette. If I move to the living room to work from, the starting temp would be 14 or less and heating to around 18.

If the amount of saving is minimal, it might not be worth the effort tending to the fire all the time and moving the office to living room, which would remove that work-life separation.

Ally_E.

victor2 said:

Petriix said:

victor2 said:

We have a 5KW log burner (put in for cosmetic purposes more than practical!) and buy the logs for it. If I run it for a typical winter evening, I'll get through about £3 of logs. The living room is toasty warm, the open plan downstairs is comfortable, the central heating gets switched off by the main thermostat and the bedrooms get cold, even with the doors left open. It saves around £1 off the gas bill by not having the central heating running.
So, no saving, just the pleasure of sitting near a burning log fire.
I avoid getting free firewood because timber has to be of a low moisture level to burn well and cleanly. We have our chimney swept annually, but missed one in the pandemic and it was fine, so will just have it done bi-annually now. The sweep said if you burn damp wood or  timber that has not been allowed to dry out thoroughly, you'll get much more soot and need sweeping more often.

The trick with burning free wood is to leave it to dry for 18 months before you burn it. 

The wood we buy has had at least 2 years to dry out. Haven't got the space or patience to store wood like that at home!

apparently wood like willow dries out much quicker, but if you don't have the space - you don't have the space whatever the type of wood it is

2 years is for wood like oak

QrizB

Ally_E. said:

We used a mix of free wood (yeay!), kilned dried logs in 40kg bags and briquettes from B&Q. How do I calculate how much cost of wood we're using up without counting the actual amounts of wood/briquettes? Is there some way to calculate based on kW of the stove?

Seasoned (dry) wood contains around 4kWh/kg (briquettes will be similar). A 40kg bag will contain 160kWh.

160kWh of gas on the current 7.5p/kWh variable tariff costs around £12.

If you're paying more than £12 for a 40kg bag of wood, it's definitely more expensive than running the GCH.

Exactly how much cheaper wood needs to be for it to work out cheaper than gas will depend on the efficiency of your fire vs. the efficiency of your boiler. The boiler, if relatively modern, will be 85-90% efficient and you say your stove is 74% efficient.

If your boiler is 85% efficient, a kWh of useful heat will require (1 / 0.85) kWh of gas and will cost 8.8p.

Your stove is 74% efficient and so a kWh of useful heat will need (1 / 0.74) kWh of wood. For it to be cheaper, the price of wood needs to be  74 / 85ths of the 7.5p/kWh price of gas, ie. 6.53p. That's 26p/kg or £10.45 a bag.

Ally_E.

Thanks, that's really helpful. The boiler can run up to 88% efficiency when I turn the flow temp down. 

Ally_E.

Found this really useful fuel comparison table in one of the old threads https://nottenergy.com/resources/energy-cost-comparison/3/ Cost of 1kW/h of burning wood with 85% stove efficiency is 7.09p and central heating with 88% efficiency is 8.3p.

For me that would mean around 8.34p for the woodburner and 8.3p for GCH, so the same thing. Not worth the effort of moving the office to living room.