I built an electric kiln in my wood storage shed. I use it to KD wood that I first AD to below 20%. I can also bring the load up to 140F after it's dry should I need to kill any bugs, though I have rarely needed to do so. All that's needed is a fan or blower to recirculate the air and an oil filled room radiator if the box is insulated well enough. The photo shows the kiln. It's 4 feet high x 4 ft deep and 10 feet long. It holds about 400 bf of stickered wood with enough space at the front and back for the 600 cfm blower to circulate air through the stack. The door and floor is just 2" thick foam. The walls and ceiling have 3-1/2" fiberglass with a vapor barrier over it, then 2" of foam insulation over that.
You can buy these oil filled radiator heaters at Home Depot, etc. for about $50. You want the old type with mechanical controls, not a digitally controlled one because it won't survive at 140F. These heaters can produce about 1500 W and that is more than enough for the kiln size I have even in Winter when the temp. is 0F. It costs about $50/month to operate in Winter, so the cost/bf is really low. The advantage of these heaters is 1) they are cheap, and 2) they are safe. Depending upon the model you pick you may have to bypass a hi temp. cutout thermostat in order to get to 140F.
I started with a dehumidifier in this kiln during the drying process, but soon learned it's not necessary as long as I start with wood below 20%. Instead, I use a simple inlet and outlet vent to allow high humidity air out and drier fresh air into the kiln. After I get down to 7% MC I close the vents and increase the temp to 140F if I need to sterilize the wood. I use digital controllers on the outside of the kiln to control the radiator and another for monitoring the RH inside. I formerly used it to control the dehumidifier until I found it unnecessary.
I've been using the kiln now for 2 years. I've only sterilized loads twice with it, but have had at least 8 drying runs through it w/o issue.
The total cost was less than $500. I wanted to build it on the outside of my shed but this was the cheapest and fastest option.
John

John
That oil filled radiator can get up to 150 degrees? It takes a lot of heat to get 400 board feet up to 133 in the center of all the stock. How long does your sterilization cycle take?

I don't know if it can get to 150F but it gets up to 140F w/o a problem, as long as there's no hi temp. cutout. 400 bf of oak weighs about 1400 lbs. The specific heat of oak is about 0.57 BTU/lb-F so to increase the temp from 110F (where I finish the KD cycle) to 140F takes about 24,000 BTU = about 7 KW. The heater can output 1.5 KW. Worst case it takes about 1 KW to compensate for heat loss from the kiln, which leaves 0.5 KW to go into raising the temperature of the wood. So, it takes at least 14 hours to get to wood up to 140F. Of course, it takes longer because the thermal conductivity of wood is not like metal. I could figure it but my guess is 50% more time is plenty, so 14 hours becomes 21 hours. I leave the wood at 140F for a minimum of 24 hours after the kiln reaches 140F, which gives some additional cushion. If I were really worried about bugs, as in I was seeing live ones or visible frass, I would put a temperature probe into the middle of a couple of boards to verify I have 140F for at least 3 hours.
There are other, more industrial, heater options that won't get hot enough to cause a fire. I use the oil filled radiator because it's cheap and it works for my setup. The key to making it possible to use such a low wattage heater is a well insulated chamber.
I hope that helps.

John

Thanks Rich and John for you input on this. John you mentioned pictures of your kiln? Did you post them?

Well, I had posted a photo, but it's never a sure thing on this site, so let's try again. The one shot shows the kiln ready to close up. You can see the dehumidifier but like I said earlier, I no longer use it. The oil filled radiator is to the right behind one of the other door panels. You also can see the blower on the far left of the opening. It blows air down the left side and then around the back so it can flow through the stack.
Another photo shows the RH and temperature controllers. Inkbird brand from Amazon. The RH controller is just there now so I know what the RH is inside so I can decide how to adjust the vents. The oil filled radiator is plugged into the temperature controller and controlled from the outside by it.
I follow EBAC's drying schedule for the species and thickness of wood I have in the kiln, as far as possible, but usually only heat to 110F max, although the kiln has no trouble going up to 130 or 140F.
I can report that those PVC stickers don't like going up to 140F unless you want noodles! I'm back to using wood stickers in the kiln, but the PCV ones are great for air drying.
Hope this helps.

John

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It's going to take an incredibly long time to get the center of a 12" wide board up to 133 degrees when the heat source is only 7 degrees higher.