Modern and efficient wood burning stoves are a relatively new trend. However, the act of burning wood for heat dates back hundreds of thousands of years. Most of us have lived in or spent time at a home with an open fire where the attitude is often if it burns it goes on. Chucking all manner of junk onto my great-grandparents open fire forms some of my earliest memories. Today, we tackle another one from the FAQ: what wood to use in a wood burning stove?
Wood or Multi-Fuel Stoves
Multi-fuel stoves are a topic for another day. It goes without saying that you can burn wood in a multi-fuel stove. However, you cannot burn alternative fuels in a wood only stove. For now, let us concentrate on wood as the source of fuel for our stove.
Best Wood Type for Burning
I’m often asked, “what wood should I use in my stove?” The quick answer is, “you’re asking the wrong question”. Like any fuel, the heat available per unit of wood is measured in calories. The calorific value of freshly felled Ash, for example, is almost double that of Norway Spruce. This gap is generally present when comparing any hardwood species to softwood. This can lead to the false conclusion that hardwood is better than softwood for burning in a stove.
The moisture content of the wood you put in your stove is of far greater relevance. As this Forestry Commission Technical Supplement for Fuel Suppliers document details, a tonne of oven dried timber has similar calorific value across all species. If anything, the softwood species nudge ahead with a slightly greater output. So, what is the right question to ask?
Ideal Moisture Content for a Wood Burning Stove
By far the most important consideration is the moisture content of the wood you put in your log burner. Most manufacturers agree that less than 20% moisture is the golden number to shoot for. Timber with a moisture content of 25% and above will have a lower calorific value and will produce less heat. In addition, the excess water will leave more deposits in the stove and flue system, reducing their lifespan.
The best way to test the moisture content of your wood is with an inexpensive moisture meter. Simply insert the probes into the wood and press the test button. Remember to check several pieces of wood from different parts of the same pile or batch.
Using properly seasoned wood will, regardless of species, ensure you get lots of heat out of your stove. In so doing, you will prolong the life of all of the components of your burner and chimney system and reduce the build-up of creosote and other nasty deposits.
How to Season Wood for Burning
While the calorific value of a tonne of kiln dried wood is roughly similar, the energy in green unseasoned wood varies greatly by species. This is because the moisture content of freshly cut timber ranges from about 30-65%. Using the previous example, Ash has a wet moisture content of 32% versus 65% for Norway Spruce. In general, hardwoods have a lower starting moisture content and are denser than softwoods. This means hardwood requires less seasoning and tonne-for-tonne takes up less space.
Kiln drying speeds up the seasoning process. You will need to purchase kiln dried wood for the first few heating seasons if you don’t already have a dry supply in storage. A cheaper long-term solution is to acquire or purchase green wood and season it yourself.
The process of seasoning wood for fuel is straightforward. Ideally, you should cut and split your wood prior to seasoning. Keep wood off the ground on a pallet or in crates, shelter it from the rain, and use an open fronted design to allow air movement. In simple terms, the idea is to keep the rain off and let the wind in. The amount of time required to dry the wood will vary by species, the drying environment, and the starting moisture content. The latter will depend on when the tree was cut down. For example, a tree felled in winter will have a lower sap content and therefore less moisture.
As a rule, expect it to take 1-2 years to season wood to the required 20% or less. Use a moisture meter to monitor progress and storing wood indoors for a short period prior to use can reduce the moisture content further.
Don’t put Junk Wood in Your Stove
The term wood burning stove is used more widely than log burner. However, the latter term more accurately describes the fuel you should use. Only put fully seasoned, cut and split logs into your stove. Regardless of species, you cannot go far wrong if you abide by this one rule.
You can, of course, burn processed timber, off cuts and scrap wood. However, timber for construction or other purposes are often chemically treated. Do not burn painted, treated, or laminated woods in your stove. Think of it like this, your stove is on the Paleo Diet and has no stomach for chemicals, paints, nails, glues, or other processed rubbish. Randomly acquired cuts of wood don’t come with an ingredients list so it can be difficult to determine if it’s been treated or not. As the saying goes, junk in equals junk out; while it’s tempting to burn processed wood, the damage to your equipment will cost you in the long run.
Many of those asking what wood to use in a wood burning stove expect a complicated answer or an ordered list of tree species. In terms of cost, storage space and time to season, the tree species is a relevant consideration. However, it is far more important to ensure that the wood you put in your stove is untreated with a moisture content of <20%. Stick to this rule and you can expect plenty of heat and longer component lifespan.