How to Convert a Furnace From Natural Gas to Propane

Why this matters

Propane carries about two and a half times the heat of natural gas per cubic foot, so a furnace burning propane through natural gas orifices over-fires badly: sooted heat exchanger, flame rollout, carbon monoxide. The reverse mistake, propane orifices left at a natural gas manifold setting, under-fires the furnace by roughly 40 percent and can condense in a vent and heat exchanger that were never built for it. Both mistakes come from treating the conversion as an orifice swap. It is three changes that only work together (orifices, regulator, manifold setting) followed by a proof that the result burns clean, and every part of it comes from the furnace maker's kit for that model. A universal orifice set, a drilled orifice, or a natural gas valve "turned all the way up" is not a conversion.

Who owns which part

Your work is inside the furnace, from the appliance shutoff valve forward. The tank, its first-stage regulator, the second-stage regulator at the house, and the gas piping to the furnace belong to the propane supplier or a licensed gas fitter, as your state and locality assign that work, and many places require a permit or a supplier inspection before a converted appliance goes into service. Confirm who owns what before the day of the job. If the inlet pressure you measure in step 7 is wrong, that is a call to the supplier, not a reason to touch their regulator.

Start state and isolation

Safety first. If you smell gas, stop: everyone leaves, no switches or phones inside, and the supplier or utility is called from outside. Propane is heavier than air and pools in basements, crawlspaces and the bottom of the furnace cabinet, so a leak may not reach nose height. Wear a personal CO monitor whenever the furnace is firing.

Isolate both energies before you open the burner compartment. Close the appliance gas shutoff and confirm it holds. Turn off the furnace service switch, lock it and tag it, and prove the furnace dead at the board with a tester proven on a known live source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2). A service switch on a stud near a furnace has been mistaken for something else more than once.

What you need: the maker's conversion kit for this model and this altitude, the kit instructions, the rating plate's propane values (inlet range, manifold pressure, input), a manometer that resolves 0.1 in w.c., leak solution, and the combustion analyzer.

Check the kit against the furnace before you open anything. The kit part number must be the one the installation instructions list for this model, and the kit's orifice chart must cover this input and this elevation. An orifice chart that stops below your altitude means you need the high-altitude version, and you find that out now rather than with the manifold off.

The conversion, with one job's readings

The job: a single-stage 80 percent furnace, plate input 60,000 BTU/h, three in-shot burners, installed new in a house that moved from natural gas to a propane tank. Plate propane values: inlet 11 minimum, 13 maximum in w.c.; manifold 10.0 in w.c. Elevation is inside the kit's standard chart.

1. Remove the manifold and swap the orifices. Disconnect the manifold from the valve outlet, take it out, and back out each natural gas orifice. Install each propane orifice from the kit, snug by hand then to the kit's direction, with sealant only where the kit calls for it; compound that reaches the bore restricts the orifice and starves that burner. Why every one: a single missed orifice is one burner over-firing at two and a half times the heat per cubic foot. On the job: three natural gas orifices out, three kit orifices in, and the tech read the drill-size stamp on each one against the kit chart before threading it. All three matched the chart's entry for 60,000 BTU/h at this elevation.

2. Convert the regulator. Depending on the valve, the kit swaps the regulator spring and cap, adds a conversion insert, or replaces the valve; two-stage valves usually convert both stages. Follow the kit's sequence exactly and keep the removed natural gas parts in the bag the kit came in, taped inside the cabinet, so a reconversion does not start with a hunt. Why: the natural gas spring cannot hold a regulated outlet near 10 in w.c., and the adjusting screw will run out of travel trying. On the job: the helper doing this step set the new spring on the bench and was called away. That gets caught in step 8, which is why step 8 exists.

3. Change anything else the kit names. Some kits include a different pressure switch, a pilot orifice on older standing-pilot designs, or a burner baffle. If the kit does not name a part, do not change it; a pressure switch swapped on a hunch defeats a safety the maker certified. On the job: this kit named nothing else.

4. Reinstall the manifold and burner assembly. Seat the manifold, align each orifice centered on its burner throat, reconnect to the valve outlet. A crooked orifice fires off-center and can impinge the flame on the heat exchanger wall.

Restore and prove

5. Restore gas and leak test the joints. Open the appliance shutoff and bubble-test the valve inlet connection right away, since it is under pressure now. The manifold and orifice joints only see gas while the valve is open, so they are tested in step 6 with the burners firing.

6. Restore power and fire it. Remove your lock and tag, confirm everyone's hands are out of the cabinet, close the service switch, and call for heat with the burner door handled as the maker directs. Stand to the side of the burner opening for the first light, not in front of it, because a missed or crossed orifice announces itself as rollout at ignition. With burners lit, bubble-test every orifice-to-manifold joint and the manifold-to-valve union. If ignition rolls out or the flames lift and lick outside the burner face: kill the call at the thermostat, shut the gas, re-isolate, and go back to step 1.

7. Read inlet static and under load. Manometer on the inlet tap, all other propane appliances off, then again with the furnace firing and the water heater and range running. Both must fall inside the plate's propane range. On the job: 11.6 in w.c. static, 11.1 under load. Inside 11 to 13, so the supplier's regulators are doing their job.

8. Set the manifold. Move to the outlet tap and set the manifold to the plate's propane value by the method in How to Set Gas Manifold Pressure on a Furnace. On the job: the first reading was 3.6 in w.c., and the adjusting screw had turned clockwise to the end of its travel at 4.1. A propane manifold that cannot get past about 4 in w.c. is a natural gas spring, which is exactly what the helper had left on the bench. At 3.6 in w.c. against a 10.0 target, flow through the propane orifices would be about the square root of 3.6 / 10.0, which is the square root of 0.36, or 0.60: the furnace was firing near 36,000 BTU/h on a 60,000 plate. Gas off, power locked out and proven dead again, spring fitted, steps 5 and 6 repeated. Second reading 9.6, adjusted to 10.0, cap on, re-read 10.0.

9. Check combustion and temperature rise. There is usually no meter to clock on a propane house, so input rests on the orifice stamp plus the manifold reading, and the analyzer is the only direct evidence the fire is right. Read CO and O2 in the flue against the maker's limits (See related: Gas Furnace Combustion Analysis and Tuning), then temperature rise against the plate range (See related: How to Measure Temperature Rise on a Gas Furnace). On the job: combustion inside the maker's limits on the first read, and rise inside the plate range after the blower had run steady.

10. Restore the house. Return the water heater and range to how you found them and confirm both light. Take the manometer off, reinstall both tap plugs, and bubble-test each with the furnace firing.

What changes the job

Two-stage and modulating furnaces. Both stages get set to their own propane values, and modulating furnaces often need the board told the fuel type through a setting or a dip switch in addition to the hardware. A board left on natural gas logic on a modulating furnace mis-fires across its range even with correct orifices.

High altitude. The kit chart's altitude rows govern, and some makers require a separate high-altitude orifice kit on top of the propane kit. Above the chart's top row, call the maker before you install anything.

Going the other way. Propane to natural gas is the same sequence with the natural gas kit, and the natural gas manifold commonly sits near 3.5 in w.c. (See related: Gas Pressure Set-Up Numbers Field Card). The parts you taped inside the cabinet are where it starts.

The label and the record

The kit's conversion label goes on the furnace next to the rating plate, filled in completely: the fuel the furnace now burns, the input, the date, the company, and the person who did it. It is the first thing the next tech reads, and a furnace with propane orifices and no label is one somebody will set up as natural gas. Some kits also include a sticker for the gas valve itself.

The ticket carries the kit part number, the orifice stamp read on each burner, inlet static and under load, manifold as found and as left, the combustion readings, temperature rise, every joint and tap bubble-tested, and where the natural gas parts are stored. On this job it also carries the 3.6 in w.c. first reading and the missed spring, because a shop that records its catches is a shop that learns which step gets skipped.

References

• Furnace installation instructions and the maker's conversion kit instructions for the model: parts, orifice chart, altitude rows, manifold values
• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, Article 120
• 29 CFR 1910.333 Selection and use of work practices
• See related: How to Set Gas Manifold Pressure on a Furnace; Gas Pressure Set-Up Numbers Field Card
• See related: Gas Furnace Combustion Analysis and Tuning; How to Measure Temperature Rise on a Gas Furnace