How to build a DIY timber drying kiln

[Big J]

It's about time that I started a thread as a guide for people out there wanting to build a kiln on a budget.

 

I have run about 8-9 kiln loads of timber through mine now, making small and large changes on the way. The system that I have now I am very happy with and would whole heartedly recommend.

 

Shopping list for building your kiln:

 

* 18-20ft insulated lorry back (or insulated shipping container, though they are considerably more expensive). Best place to get one is a commercial vehicle breakers, or a company that runs a large commercial hire fleet including refrigerated lorries. You really need at least two - one as a kiln and one as storage for when the kiln opens. When the kiln first opens, it will fill both the kiln and the storage box, as you want to be able to access the timber for sales. Aim to pay about £500 for each box, though expect to pay more and don't take anything on that isn't in good condition, as they degrade reasonably quickly as kilns.

 

* Heat recovery unit - I use a Vent Axia unit that extracts roughly 200 cubic metres of air an hour. This is about right for a kiln of this size. I got mine off ebay almost new for £290 delivered. You will need 100mm ducting to go with it, which is very cheap.

 

* Heaters - you need to have 2-3kw of heating in the kiln. I use oil filled radiators and they are fine and cheap. A more expensive but more convenient option are tube heaters, as they can be mounted just under the roof and are therefor out of the way. You need a plug in thermostat (£20 or less) to control the heaters too.

 

* Large fan - I use a carpet dryer, which I regard as being the best fan for the job. It is around 850w, moves over 3000 cubic metres of air an hour in quite a well directed manner. It's also very compact.

 

* Cheap weather station - you need to buy one with indoor/outdoor temperature and humidity. It lives in your storage box, and looking at it you will know exactly what the kiln is doing by the temp/relative humidity.

 

 

That is it for the shopping list. Now for setup:

 

* Make sure that the box is very well supported from underneath. There could be 8 tonnes of timber going into it green, and it doesn't matter how strong the box is, it will still deform to some degree. Bear in mind that any deviation from straight in the floor is mirrored in the timber, resulting in boards that are not flat.

 

* Mount the heat recover unit just under the roof inside close to the far end. Two pipes go outside and you want those to be as short as possible (to avoid condensation forming in the outflow pipe as the air is heavy with moisture and has been cooled by the air coming in). Pipe the air inflow outlet to the other end of the box and have the air outflow inlet right at the heat recovery unit. This will be much clearer when I get the photos up. Basically, you want the dry air piped to one end, and the wet air collected at the other. Your heat recovery unit comes with a condensate pipe - pipe that outside.

 

* Mount the heaters just under the roof if using tube heaters, or sit the heaters adjacent to the fan if using oil filled radiators. Set your thermostat to about 32 celcius if drying sawn timber, or 38 if drying firewood (you can cook firewood as hot as you like, but most electrical gear like the fan and heat recovery unit aren't guaranteed past 40 celcius).

 

* Place the carpet drying fan in the middle of the box on the left side with the outlet facing up. This will collect the newly heated air (if using oil filled radiators) or will direct the air towards the heaters (if using tube heaters). I have a small plywood splitter directly above the fan directing the air to all corners of the box.

 

* If drying sawn timber, screw bearers (I used 38x38mm) to the floor at 18 inch intervals. Ensure that they are accurate as they are the bearers you will use to stack your stack. To aid accurate stacking as the stack gets higher, I have lines penned onto the wall to indicate exactly where the bearer on the floor is. You need to have perfectly vertical sticker lines for straight timber. It's something I am very anal about.

 

* Put your sensor for your weather station on the opposite side of the box to the fan, halfway up the wall.

 

That's set up - now for usage guidelines:

 

* Timber stacking. As previously mentioned, accurate stickering is crucial. Don't cut corners on this. When stacking, stack from floor to 1ft from the ceiling. Leave an unobstructed 1ft gap on the right side (far side from the fan) and 18 inches on the near side, where your electrical control equipment is. You would be surprised how often you might want access. 1ft is too narrow for most whilst 18 inches is OK. This will give you a 5ft wide stack if using a lorry back. Make sure when stacking that the sides of the stack are as straight as possible, with no stickers protruding past the edge of the stack and that all stickers and timber are supported. It's much better to have unsupported stickers in the middle of the stack than unsupported ends of stickers. You would be amazed how easily timber deforms if given the opportunity. Try to fill the kiln end to end as well, leaving no gaps at the end. My box is 18ft - I have two 9ft stacks for simplicity.

 

* Turn everything on and close the doors. With the thermostat set at 32 celcius, the temperature will vary between 30 and 34 depending on external conditions. More important than temperature is the relative humidity (RH). The RH will sit at around 83% for the first couple of weeks of the cycle and then start to drop steadily at 1-2 percentage points per day. Once the RH reaches 20-25%, at least 95% of your timber will be ready. Obviously, mixing species and thicknesses complicates things, but it's almost always accurate. My last kiln opened yesterday and was showing 23% RH prior to opening. The driest timber was the elm at 7% and the wettest some stubborn 2 1/4 inch oak at 18%. Oak is notoriously hard to dry though. It was a kiln of predominantly 1 and 1.5 inch timber though, hence the higher reading on the thicker Oak.

 

* Timber storage - just make sure that your timber storage box is either heated or dehumidified. To figure out what moisture content your timber will settle at, type in equilibrium moisture content calculator into google, and type in your temperature and RH. You want to control the environment so that your EMC is less than 12%.

 

* Firewood - for firewood drying, don't screw down the stickers so you have a flat floor. Best suggestion is forklift a couple of metre cube crates into the entrance. Then use a pallet truck to wheel them to the back and repeat. I'm going to be testing this tomorrow for the first time so I'm sorry that I cannot say any more! Use a higher temperature (38c) and you won't need to go as low as 25% on the RH. 50% would probably suffice to get it to 20-25%.

 

Hope that helps more people to get kiln drying - it's really simple and well worth doing. Timber is quite hard to shift unless it's ready to use, and air drying can only do so much.

 

I might actually do a short video to explain the kiln tomorrow.

[Big J]

Forgot to mention timescales:

 

* Air dried sawn hardwood should take about 4-6 weeks, green 7-9 weeks. I'd always advise air drying even if you can only manage 3-6 months - the difference in quality is huge. It also saves electricity - the kiln costs about £8 a day to run.

 

* Firewood - not sure yet as I'm only just doing this now, but less than two weeks I'd imagine. Firewood will give up it's moisture far quicker due to proportionally higher surface area and higher temperature afforded by not giving a monkeys about defects!

[Chris Gagen]

Very detailed, liking a lot!

 

do you think it would be possible to harness the heat of a charcoal kiln in some manner? also I have a hay drying fan (tractor mounted) that reckons it produces a wind speed in excess of 80mph! any scope to factor these into a firewood/kindling wood drying apparatus?

[Big J]

Very detailed, liking a lot!

 

do you think it would be possible to harness the heat of a charcoal kiln in some manner? also I have a hay drying fan (tractor mounted) that reckons it produces a wind speed in excess of 80mph! any scope to factor these into a firewood/kindling wood drying apparatus?

 

You can have an inconsistent heat source if you are producing firewood but not for sawn timber. The fluctuations in temperature (and therefor humidity) with cause too much defect in the timber.

 

But for firewood, I'd say go for it!

[s.varty]

i,m confused when it comes to kiln drying, my question is always "why?". If you can get timber down to say 10% moisture,but store the logs in your log store wont the logs absorb the moisture in the air and you end up with logs no dryer than if they had been air dried, i realise there must be a reason why you kiln dry them, but i cant figure out why.

[Big J]

i,m confused when it comes to kiln drying, my question is always "why?". If you can get timber down to say 10% moisture,but store the logs in your log store wont the logs absorb the moisture in the air and you end up with logs no dryer than if they had been air dried, i realise there must be a reason why you kiln dry them, but i cant figure out why.

 

Yep, unless stored in a humidity controlled environment, they will take on moisture.

 

The above guide is meant primarily for sawn timber. The firewood aspect is just a bonus really. The advantage of the kiln is that I think that I can get near enough green timber down to 25% in two weeks. Takes a year to do that air drying.

[s.varty]

Yep, unless stored in a humidity controlled environment, they will take on moisture.

 

The above guide is meant primarily for sawn timber. The firewood aspect is just a bonus really. The advantage of the kiln is that I think that I can get near enough green timber down to 25% in two weeks. Takes a year to do that air drying.

 

Thanks for that, didnt realise you could reduce the moisture content that quick.

[bobh]

Really useful - thanks for taking the time

That gives me a bit to think about. If wanting a smaller setup would it be possible to insulate a shed (obviously with the floor beefed up to allow for the weight) was thinking something like kingspan with a ply lining.

[teepeeat]

Big J - does the £8 a day include heating as well??

[L W Woodlands]

Really useful - thanks for taking the time

That gives me a bit to think about. If wanting a smaller setup would it be possible to insulate a shed (obviously with the floor beefed up to allow for the weight) was thinking something like kingspan with a ply lining.

 

Hi, have you looked up solar kilns? saw one recently basically a wooden shed with large sloping south facing window, has some vents, fans and screens to protect timber from direct sun. apparently produces very good results.