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Firewood kilns

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Can someone give me a idea of cost of instalation typical drying times. How much your putting through yours how much it costs to run ie is a log one better than a woodchip one and rhi payment received. Looking at one just wanted some feedback really. Oh and who is best to get to install one.



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We're a bit different to most because we dry a large amount slowly in a heated shed as well as some chip but hopefully this is still of some use to you.


It takes around 3 weeks to dry 90tons, we dry 12-1300 tons a year and that takes 150 tons of chip. The chip will usually need to be a certain size - you unfortunately won't be able to use just any chip.


Woodchip boilers are more efficient but you'll need to buy or hire a chipper and they cost more to buy.


Unless you can justify the cost of a chp boiler rhi has dropped to a point where I can't see it covering drying.

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First I would say be very careful if you go through a "renewable heating" company. My experience of 3 have been shocking and I've heard plenty of horror stories too!

We have 2 boilers but I will give you the figures for the 130kw eco Angus.

£18k complete to build (bought all the parts myself and got a plumber to fit it)

Dries 30 cubic metres in 80 hours down to 20% inside the log.

Uses 1/2 a cube a day in the summer and 3/4 of a cube in the winter.

We get about £12k a year on average back from the RHI.

If you can afford it go for a chip boiler, they will be a lot more than our setup but in all honesty log boilers are a pain. Burn time is only 3/4 hours Max so if you go for a log boiler don't expect to run it overnight.

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Dries 30 cubic metres in 80 hours down to 20% inside the log.


Sounds like yours runs at a fairly high temperature in the kiln to get it down to 20% in 3 days and a bit.


In the hardwood lumber industry the kiln just finished off the drying after the boards had been in stick for a season, not sure about imported softwood, but the same can apply with logs. They don't necessarily have to be dried from 45-50% down to 20% in the kiln.


BTW is that 20% measured straight after the kiln is opened or after the log has "normalised"?


I found my test pine log which was oven dried still hadn't gained much moisture and was about 10% from sitting in the shed, I had expected it to get up to about 17% by now.


I shall have to devise a rewetting experiment to see if the higher temperature treatment resists re adsorption of moisture from the air.

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Here is a quick guide for you


Two methods of drying logs


1. Forced Air drier

An insulated container or building with air heat exchangers at one end an some open vents at the other. A boiler heats water, which is converted to hot air via the heat exchangers and blown through the container or building to dry the logs.


The effectiveness of this and the time it will take to dry logs will depend on how the logs are stored ( metal mesh stillages are best to ensure airflow), the temperature of the air and the amount of air passing through the kiln. The hotter the temperature the better so long as there is a reasonable airflow. All things being equal , in the summer when the outside air temperature is 20 degrees your drier will be able to achieve 20 degree higher temperatures inside the drier. The difference in drying time between 40 degrees and 60 degrees is huge.


2. A Traditional Kiln

An insulated building or container with radiators in and powered vents that open once the air reaches a certain relative humidity level. At this point fans blow the wet air out and switch off, close the vents and the air heats up again with the process being repeated until the logs are dry. This is generally a more efficient method of drying logs - set up is tricker though.


A Constant Heat and Insulate, Insulate , Insulate

I cannot stress the importance of this enough. It takes a large amount of energy to heat the logs up to the point where the moisture begins to escape into the air. If you are only running your kiln 12 hours a day you are repeatedly having to do this and drying times reflect this. Keeping the temperature constant in the kiln rapidly increases drying time. Insulation is key, it costs next to nothing to insulate your container or building and makes a massive difference to the effectiveness of your kiln


Boiler Choice

Essentially two principle biomass choices that can power wither type of drying method.


a) Woodchip

Expensive to install, hardly any hassle to fuel, provides a constant heat 24 hours a day.so that temperature is min


b) Batch fed (Logs)

Cheap to install, requires regular refueling, about 30% less effecient, a nightmare to fuel over night to keep the temperature in the kiln constant. Best to turn the heat exchnagers right down to minimum so that temperature is maintained with as little draw on the boiler as possible.



Unless you are going to install a boiler of greater than 200kwh then forget any sort of real payback on your investment, even then returns are not what they were


Drying Times

These vary massively on the setup ( boiler type, boiler sizee, type of kiln, size of kiln, how the logs are stored, outside air temperature). My 10ft insulated forced air dryer, with 100kwh of heat exchangers on full blast ( wood chip boiler) will dry 10m3 from green to 20% in just two days in the height of summer with a temperature of 68 degrees in the container. In the winter when its freezing outside in may take twice this long. Initially I get the temperature up as high as possible and have the fans on a low speed, the speed of the fans is then increased as the logs dry through the cycle.


Hope this helps

Edited by Duffryn

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I thought this additional high level info might help. In calculating out a return on investment


3,200kwh in a tonne of 20% m/c wood. It doesn’t matter whether this is softwood or hardwood there is roughly the same energy per tonne. Softwood will take up more space and need more refueling than hardwood


Woodchip boiler

90% efficient so you will generate 2,880 kwh an hour, multiply this by the relevant RHI rate say 5.2p per kwh (current rate for boilers over 200kw in size and below 500kw) and you will generate £151 per tonne of fuel


Batch fed boiler

60% efficient so you will generate £100 per tonne of fuel


Fuel Cost

If you are paying £40 per tonne for green biomass wood, then you are looking at closer to £60 cost per tonne once it has dried. Then you have to cut, chip and load it.


My personal opinion

Unless you can use free or nearly free fuel or even better get paid to take the fuel as waste then the return on investment is no longer there with RHI at current rates

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Has anyone built a kiln that's heated via ground source heat pumps?


Not done it but wouldn't advise it other than low temperature lumber drying.


The thing is that heat pumps have a Coefficient of Performance which varies inversely with the difference in temperature you are pumping through.


So assuming your heat source can stand the heat removal ( and the one I remember couldn't as the pipes under the lawn froze the soil and could be seen as a network of ridges ) then it's good for underfloor heating as you only have to pump up to about 25C and the COP will be better than 3, that is 1kW(electrical) in will deliver 3+kW(thermal) out.


However as has been discussed log drying benefits from higher temperatures than 25C and the COP would drop if you could find a pump capable of running at 60C out.

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