Boosting a Solar Kiln with Propane Heat

      Thoughts on modifying a solar kiln with supplemental propane heat to kill bugs and set pitch. April 30, 2009

I am in a northern climate with current temps at around freezing to +10c highs. Winter temps much lower (-20C or lower). I am just completing a greenhouse style solar kiln with clear polycarbonate roof about 18'x10' (single layer). Unfortunately I was not able to angle the kiln to face due south so I only get sun from about 11am onward in its south-west orientation.

I have a large amount of eastern white pine that is air dried but has old house borer infestation. I also believe I have an infestation of PPB in some air dried maple (about four years) that has been stored flat stacked in another location.

As such I need to run the kiln to sterilize all this stock in the cold months. I tried adding a 14000btu propane "sunflower" type heater and it wouldn't raise the temp more than 10C. Added a 30,000btu construction heater (propane) and it seems to be able to keep up with heat loss but have not tried to hit 160F yet but want to get that high to set sap and kill bugs.

Questions: would stapling up a layer of 6mil poly vapor barrier 3" below the polycarbonate roofing substantially decrease my heat loss but still allow solar input? Am I better off to forget about solar in the winter and try to insulate the solar collector roof with say, 1" foil faced foamboard that could be removed in summer? Keep in mind I am not drying from green here although I would like to dry from about 12% down to 6-8% in winter but bug eradication is the priority.

Lastly, the kiln gets up to 30C or more for about four hours on a sunny day at present with no vents open. Will the solar design will still work with my SW orientation (just slower drying). Should I be considering scrapping the solar, insulating and just using conventional heat?

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
A second layer would be very helpful. Insulating the roof when you raise the heat is even better. Solar is somewhat questionable, but should work.

From the original questioner:
Is a propane construction heater a reasonable choice for supplemental heat? I believe they will pump out a lot of moisture, would this be detrimental to already seasoned wood if I were to place it back in for six hours to heat treat it?

From Gene Wengert, forum technical advisor:
Propane heaters, if they are safe around wood in this situation, should be fine.

From contributor L:
Don't forget that propane heaters need a lot of oxygen and if your kiln is air tight, it will use up the oxygen in minutes. You need enough fresh air to keep it going and that takes care of the moisture from combustion. Don't try sticking the heater outside and having it blow in a door or opening because they usually not raise the temp enough and you will waste a great deal of energy.

From the original questioner:
I had thought of trying to duct it in from outside so you saved me the trouble there. I was going to try making up a duct from flexible metal dryer vent and run it up into the rear of the propane heater as combustion air inlet. I would still need to crack open a top vent to allow air exchange or no air would flow in the vent to feed the burner. I could then regulate the flow of combustion air by closing the kiln vents so it didn't let too much air in. I am concerned about too much heat as there's no thermostat on the salamander type propane heater.

I have added a 220v 4800w construction heater on a thermostat so with luck I can use propane to jack the heat up to temp and maybe, just maybe, the electric heater might be able to maintain the temp. Could also keep the propane on but throttled back if the electric needed just a little help.

Those 4800w heaters are close to 19,000btu I believe, my propane is 35,000 max. I put polycarbonate roofing on (supposed to be good to 250F) but still a bit concerned about damaging it. I've added 6mil poly to the underside of the rafters and may add 1" foamboard in channels attached to the underside of the rafters that can be removed easily for normal solar operation and added back for HT or setting sap. The extra R7 of the foam board should help a lot when supplemental heat is on. Does this all sound reasonable or is there a better way?

From contributor U:
Keep in mind that the primary byproduct of burning propane is H2O, something that may be working against your drying efforts.

From the original questioner:
It was a concern of mine too but in tests the heat from the burner has the RH dropping from about 40-50%. The propane burner will only be used to set the sap in pine and to heat treat all lumber so any charge will see a max of 24 hours with propane. Even if the RH were driven way up, the exposure will be so short it shouldn't matter from what I have learned. I did have to add 2" Styrofoam insulation to the inside of the rafters to get efficient heating. Now I'm waiting for a method to monitor the temp and I will try my first charge.

From contributor U:
If your heater has a way of re-ignition, they do make thermostats that will turn the power on and off. Depends on what type of heater it is. If you can regulate the output of your heater you may be able to turn it down to give you what youre looking for, with the heat loss factored in. Manually adjust it for the 24 hours

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