Start from the oven-dry weight, not the green weight
Moisture content in wood is water weight divided by oven-dry weight, which is why a figure over 100 percent is ordinary rather than impossible. Twelve thousand board feet is 1,000 cubic feet; at 45 lb per cubic foot that is 45,000 lb on the sticks. At 65 percent the oven-dry wood inside it weighs 45,000 / 1.65 = 27,273 lb and the water weighs 17,727 lb. Divide by green weight instead and you get 29,250 lb of water, an error of 65 percent in the direction that makes the fuel bill look far worse than it is.
Coming out at 7 percent leaves 1,909 lb of water still in the wood, so 15,818 lb — about 1,895 gallons — has to be evaporated. That is the number everything else hangs off.
Three bills, not one
At 1,050 BTU per pound the evaporation alone is 16.6 million BTU. Warming 27,273 lb of dry wood through 110 degrees at 0.32 adds 0.96 million, and warming the water it is carrying — about 9,818 lb averaged across the run — adds another 1.08 million. Useful heat, 18.6 million BTU. Add 35 percent for vents and shell and 22 percent lost between the burner and the chamber, and you buy 32.2 million BTU, or 322 therms, which at $1.20 is $387 for the charge and $32 per thousand board feet.
On those numbers evaporation is 51 percent of the fuel, the two warming terms together are 6 percent, and losses are the remaining 42 percent. Change the moisture drop to 12 percent down to 7 and the picture inverts completely: the water term collapses and the fixed cost of heating the mass and feeding the leaks becomes most of the bill. The page names the largest share for whatever you put in.
The last point is the expensive one
Going one point further down — 6 percent instead of 7 — is another 273 lb of water and about $5.65 in fuel on this charge. That looks trivial next to $387, and in fuel it is. What it is not trivial in is time: the last points come off slowly, and the chamber is occupied for the whole of it. The real cost of overdrying is a scheduling cost, and it belongs on the throughput page rather than this one.
What this cannot see
Nothing here models a schedule. It assumes the charge is warmed once and held, which is not what a dry kiln does — it climbs, it holds, it conditions, it cools, and each stage has its own loss. It also assumes latent heat is a single number when it is a curve that steepens as bound water comes out of the cell wall. Read the output as the shape of the bill and the size of each part of it, not as a meter reading.
Questions people ask
Why is the vent and shell loss a percentage on top rather than an efficiency?
Because it is a different kind of loss and mixing them hides which is which. Vent and shell loss is heat that got into the chamber and then left it, and it scales with how long the run is and how leaky the building is. Burner efficiency is heat you paid for that never reached the chamber at all. Keeping them on separate lines lets you see whether the money is going out of the vents or up the flue, and they respond to completely different fixes.
What latent heat figure should I put in?
Whatever your own reference gives for the temperature and pressure you dry at. This page will not name one. Be aware that the single-figure model is weakest at the dry end: bound water held in the cell wall below fibre saturation takes appreciably more energy per pound than free water does, so a run that finishes at 6 or 7 percent is under-costed by a figure taken from free water alone.
Does this work for a dehumidification kiln?
Not as written. A dehumidification kiln recovers the latent heat by condensing the water vapour and putting the heat back into the chamber, so the evaporation term is not a straight fuel cost the way it is in a vented kiln — it is a compressor duty with its own coefficient of performance. Running this page for a dehumidification chamber gives an upper bound, not a bill.
The number this gives is nothing like what my kiln actually costs. Which is wrong?
Probably the vent and shell figure, and possibly the density. Those two carry most of the uncertainty. The way to settle it is to weigh a charge in and out — the weight loss is the water, directly measured, no meter needed — and read your own fuel meter across the same run. That gives you a real BTU per pound of water for your building, and once you have it you can work backward through this page to find what your vent and shell loss really is.
Should I add the fan power?
This page does not include it and on a long run it is not negligible. Kiln fans run continuously for days and most of that electrical energy ends up as heat in the chamber, which is not entirely wasted. If you want it in, meter the fans, convert kWh to BTU at 3,412 BTU per kWh, and either add it as fuel or count it against the heat demand — but not both.