Saturday, 14 August 2010

Using a moisture meter to check firewood

People tell you that wood needs to be seasoned before it is burned as fuel, but what does this actually mean?

When a tree is felled in winter, the moisture content of the wood will be over 50%, and even higher if it is felled in summer. By splitting the logs and/or cutting them to short lengths, you can accelerate the rate at which the wood will dry out. You can check how dry it is using a moisture meter. The picture below shows the spikes on the end of my moisture meter - you just jab these into the end of a log and it gives you a reading. Better still, split a log and test what had been the inside of it, to see the moisture content at the centre.

The reason the wood needs to have dried out is that when you put a log into a fire or stove, before it can burn all of the moisture must be driven off, and this uses some of the heat from the fire - so, the wetter the log, the less heat you get out. But there's another problem too - wet wood doesn't burn as cleanly, which is bad from the point of view of pollution, and also can deposit tar in the chimney, potentially creating a fire risk.

So how dry should your wood be before you burn it? This varies from one stove to another, but the moisture content should be 30% at most, and ideally should be 20%. How long it takes to get this dry depends on how the wood is stored. Ideally it should be split as soon as possible, then stored off the ground in a place where air can circulate through it and the rain is kept off it. Drying to 30% should usually happen within a year, and 20% in two years, although in ideal conditions these times can be reduced.

Here's an example of how important it is to split logs. These two logs were both felled about two years ago, and have been freshly sawn from the centre of 2m lengths. The first one was split straight after felling, and the moisture content is just over 20%:

The second one has only just been split, and the moisture content is still over 40% (the meter only reads between 3 and 40%):

The good news is that a short split log like this dries out very quickly in the summer - I checked it again a week later and it had already dropped to about 30%, having been left in the sun.



Giles said...

Interesting. I was always told that cutting a log into shorter lengths was better than splitting it, as the moisture is lost via the xylem and phloem tubes rather than through the longitudinal fibres exposed when you split it.

I suppose either splitting or chopping into smaller logs is better than nothing!

Mike Pepler said...

I think you are right, but as you say, splitting is better than doing nothing, and works fairly well. The big thing for us is that by leaving the logs in long split lengths we can easily store them and handle them, and more importantly cut them to the right length when you know who's going to be burning them. It's meant we can easily store them for 2 years, which has proved to be plenty of time for them to dry to 20%.


Anonymous said...

Proper testing: 1.) split a piece of your firewood 2). place the tester in the newly exposed SIDE of the wood, NOT the end!

Mike Pepler said...

Yep, splitting and testing the side is a good way to check the centre. As you'll see in the post, one of the 'ends' of a log I tested had only just been cut, so would give a similar result to splitting and testing.