In theory wood is a renewable, but as we saw in the last month’s post it’s not a green fuel.
How to burn less firewood and reduce emissions
A wood burner is most efficient and produces least pollution when it burns dry wood with a hot flame.
The more complete the combustion of any fuel, the cleaner it burns.
How combustion works
Three things are needed for combustion:
- Ignition (a match or spark)
- Comburent (the oxygen in air)
- Combustible (a fuel such as wood)
Without ignition you have no fire.
The more comburent a fire has access to, the hotter it will burn.
When you light a fire there will be little to no draught (airflow). That’s why fires smoke a lot when you light them. There is a lot of fuel and very little airflow. If you blow on the fire (adding more comburent air) it burns better and smokes less.
Once the fire is going, if you add lots of thick wood to a fire and it has very little comburent (air intakes closed) you stifle the flame. It will burn slower and dirtier (richer) producing less heat and more emissions.
The more smoke you see, the more pollution you’re producing.
The less wood you use and more comburent (air intakes open) the hotter, faster and cleaner (leaner) the fire will burn.
Incomplete combustion (smouldering) produces a lot of pollution and toxic gases including carbon monoxide.
Indicators of complete/incomplete combustion
- Flame colour
- Smoke
- Clean/dirty chimney
- Flame colour
You can tell how hot/clean a flame is burning by its colour.
- blue, very hot, complete combustion burns off most gases
- yellow/orange, hot, some unburnt emissions
- red/smouldering embers, incomplete combustion produces carbon monoxide and other emissions
- Smoke
When a wood (or any fuel) burns hot and clean the emissions are almost invisible. If you look at the smoke coming out of the chimney, there will be very little smoke when dry wood is burning hot. You might be able to see fast-rising shimmering heat with very little smoke. When you start a fire or red embers are smouldering there will be a lot of slow-moving grey/black smoke gathering in the flue. Smoke will rise slowly out of the chimney. This is a sign of incomplete combustion.
- Clean/dirty chimney
If you’ve been burning wood for a while, look at the chimney. If it’s black on the outside and creosote is forming inside, this is a sign of incomplete combustion. Incomplete combustion is caused by burning wet wood or slow-burning/smouldering embers. Creosote can catch fire, so inspect and clean the chimney at least once a year.
How to prep firewood
- Prune trees, cut dead ones down in winter.
- Plant new trees early spring.
- Cut wood into a size that fits in the burner.
- Split anything thicker than an arm.
Stack wood so air circulates through the wood pile. Protect the pile from rain. Leave to dry for at least spring-summer-autumn in hotter climes. Dry for longer and/or indoors in colder climes.
Dry hardwood, chopped into halves/quarters burns well. Whole, big pieces of trunk do not, they smoulder producing a lot of smoke, CO and CO2. Big pieces don’t burn well at first, then produce too much heat, then smoulder.
Starting a hot fire
You don’t need to use firelighters. Most chemical firelighters are made of kerosene, paraffin and resin. They are toxic to dogs and children if swallowed, can cause eye and skin irritation. Dry pine cones are a great way to start a fire. If there are no pine trees in the area you can use natural fire starters.
Use two biggish pieces of wood either side of the pine cones to let air flow through as the fire starts. Place twigs, finger-thin kindling on the cones, then a slightly bigger piece or two on the kindling.
- Open all air intakes
- Open ash box a little
- Light pine cones
- Close glass door
The fire should start burning hot immediately. There should be very little smoke.
A blowtorch starts the fire even hotter and faster. 1 small propane gas canister lasts the whole winter.
You can also build and light a fire from the top down. A top down fire works well in smaller stoves.
In an open fireplace a tent-shapped build of wood placed vertically in a corner should burn hotter and faster. Less air flows through a horizontal stack so the fire is more likely to smoulder.
Air intakes and flue dampers
Use the air intakes to control how much comburent air the fire gets. If you close the intakes completely this will stifle the fire, making it smoke. You can do this for just a short time, if the fire is too hot.
The ash box and primary air intakes in the grate/ash box supplying air under the wood can be closed after the fire is burning well. The secondary air intakes in the door or going to the combustion chamber should be left at least half open while the fire is burning. It’s better to add less wood than reduce airflow.
In an open fireplace you can use bellows or a blowpipe once the fire is lit. These add a lot of comburent air so the flame burns hotter and cleaner.
If the fire is too hot, start with less wood next time. This reduces the amount of wood you’re burning and the fire burns hotter and cleaner. Add wood gradually rather than building a big fire.
The flue damper should be kept open when the fire is burning. This helps the fire burn hot, reduces emissions and the chance of backdraft. Adjust the secondary air in the stove door, and/or primary air in the ash box under the grate to increase/reduce the amount of air.
Only close the damper if the fire is completely out. This can stop warm air going up the flue and cold air coming down it. If the fireplace has a fire glass door you can leave the damper open.
Use the fan/s on the stove to blow ambient air around it and into the room.
Free firewood
Plan ahead for free firewood. If this winter someone needs a hand pruning their trees you’ll have dry firewood next winter.
Using less firewood
Using less firewood (or none at all) is obviously the best way to reduce emissions.
Before getting a heat pump and double glazing I was spending about €300/year on firewood. The wood burner was my only source of heating.
A heat pump + double glazing meant 60% savings on my heating bill. Double glazing stopped the house losing a lot of heat at night. The heat pump reduced the amount of firewood I’m using by 70%.
In next month’s post we’ll take a look at efficient catalytic stoves that produce more heat and lower emissions, and how to turn an old insert into a more efficient wood burner.
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Very interesting post, learned how to burn my wood more efficiently, why cutting it smaller is better and that we do good by drying our firewood. Now I just need help pruning! Any volunteers want to be paid in firewood? ☺️
Thanks Lilia.
Pruning anytime 😉
Thanks for reading. Feel free to comment.