The conversion. Episode fifty-three of the Gas Service Animated series.

Move a furnace from natural gas to propane — or back — and you're changing the two numbers that define its fire: how rich the gas is, and how hard it's pushed. Get the conversion right and the appliance never knows the difference. Get it wrong and you've built a soot machine or a carbon-monoxide factory. There is exactly one right way: the manufacturer's kit.

The physics first. Propane carries about two and a half times the Btu per cubic foot, and it arrives at ten to eleven inches of water column instead of three and a half. Richer fuel, pushed harder — so to land the same firing rate, the metering hole must shrink. That's the whole conversion in one sentence: same Btu, much smaller orifice.

The kit is the law. Burner orifices sized for the model. A conversion spring or regulator insert for the gas valve. Often a pilot orifice. And the label that re-rates the appliance for its new fuel. Drilling orifices, stretching springs, or ‘close-enough’ parts-bin conversions are how legal appliances become evidence. If there's no listed kit for the model, the model doesn't convert.

See what the mistakes look like. Natural-gas orifices left in on propane: two and a half times rich, pushed three times as hard — a roaring overfire that soots the exchanger and pours out carbon monoxide. Propane orifices run on natural gas: a starved, lazy fire that won't heat the house. The appliance always tells you; episode sixty-two taught you its language.

The procedure, in order. Gas off; swap every burner orifice to the kit spec. Convert the valve — spring or insert — per the instructions. Swap the pilot orifice if the kit includes one. Fuel on, leak-check every joint you touched. Then set the manifold pressure to the nameplate for the new fuel — ten to eleven inches propane, three and a half natural — with a manometer, not a memory.

Then prove the burn. Adjust primary air until the flame reads right — episode sixty-two — and finish with the analyzer: air-free carbon monoxide inside the appliance's limit, stable numbers at steady state — episodes forty-one and forty-two. A conversion isn't done when it lights; it's done when the numbers say the fire is the fire the nameplate promised.

Last, the paperwork on the appliance: apply the kit's conversion label at the rating plate, note the kit number, date it. The next tech — maybe you, in five years — must be able to read what fuel this appliance now expects. An unlabeled conversion is a trap set for a stranger.

One interaction to remember: altitude. High-elevation installs derate and sometimes re-orifice on their own schedule — the code's equivalent-orifice tables and the manufacturer's altitude instructions stack with the fuel conversion. Two tables, one appliance: reconcile both before you size the hole.

Conversion in one breath. Richer gas, harder push — smaller orifice, by the kit and only the kit. Swap, convert the valve, leak-check, set the manifold by manometer, prove it with the flame and the analyzer, and label it. Same appliance, honest new fuel. Next: LP safety and the boundary.