Guide to building a timber frame home

Steam Box Design

Our steam box was originally built for bending large boat timbers, such as the keel and planking. The original box was constructed from 20 ft long 2"x12" Fir boards, and had been sitting outside for several years. Although the original design allowed for a baffle to be inserted, effectively shortening the box to 10 ft, the end of the box had some serious rot, so it was cut down to a 10 ft length. The box is tilted down towards the steam inlet, allowing some of the condensed steam to run back into the boiler. The boiler here is a 5 gallon safety can. The burner is a 250,000 BTU propane burner, originally from a large space heater.

A crucial modification to our steam box to allow steaming of multiple boards for precise times was the addition of two sets of 3/4" dowells. Originally, we tried steaming 3 or 4 boards at a time, spaced by stickers, but we over-steamed the lot, turning it into firewood. The dowells allow us to open the door and quickly remove or insert boards as they reach their target steaming time of 1 hour per inch of thickness. You can also see a small dam (heavily reinforced with white caulk) which stops the condensed steam from running back passed the steam inlet, and dripping out the bottom of the door. We're stilling fine tuning the process, in terms of how the number of boards in the box affects the steam circulation.

We've always run the propane burner full-on, which means a steady stream of propane at 10 psi. Our propane tank holds 100 lbs of propane, which is enough for more than 30 hours of operation. During extended steaming operations, the system loses water. Since a cold start (bringing 5 gallons of water up to a boil and building a nice head of steam) requires over a half hour, any wood in the box would be ruined by shutting down and refilling. Our current solution is to boil a couple gallons of water in kettles on the kitchen stove, to momentarily shut of the flame, disconnect the safety can from the radiator hose, and add the boiling water. A 1/4" hole is drilled through the top of the box near the far end, releasing a trickle of steam during operation.

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