Propane to Natural Gas Conversion

Purpose

Converting an appliance from propane to natural gas is three parts changes and one piping decision, and shops routinely do the first and skip the last. The orifices change, the appliance regulator changes, the manifold pressure gets reset. Then the pipe has to be re-checked, because natural gas carries roughly a third of the energy per cubic foot that propane does, so the same input in Btu per hour needs roughly two and a half times the volume through it.

The whole exercise is legitimate only where the manufacturer lists a conversion kit for that model. A drilled orifice has no listed input, and the appliance that comes out of that job over-fires quietly for a season, cracks a heat exchanger, and starts making carbon monoxide in a house nobody will connect to a plumber's visit from the spring before.

Scope

Covers converting existing appliances from propane to natural gas at a residential or small commercial property that has just been connected to a main: houses, small retail, small restaurants, offices, multifamily.

It does not cover the sizing recalculation, which the gas line sizing for an added appliance SOP owns and which step 2 hands off to, or the meter set and turn-on, which the gas meter set coordination SOP owns. It does not cover removing the propane tank, regulators or buried line, which belongs to the propane supplier and, for customer-side piping, to the gas line abandonment and capping SOP.

Roles and the handoff

Procedure

1. Confirm a listed conversion kit exists for this exact model and serial before anything comes apart. Acceptance: the manufacturer's kit number identified against model and serial, the kit in hand with its instructions, and the jurisdiction's rule on who may perform a conversion recorded. Wrong: a kit for the same product family but a different serial range. Stop: no listed kit means the appliance is replaced, not converted; drilling or swapping orifices from another appliance is not an option at any price. Hazard: an orifice with no listed input is an uncontrolled firing rate, and over-firing a sealed chamber is how a heat exchanger cracks and starts putting products into the supply air.

2. Re-run the piping capacity for the new fuel before you remove a single part. The same input needs far more volume on natural gas, and the table column changes too, because sizing tables are gravity specific: natural gas sits near 0.60 and propane near 1.50. Acceptance: every section re-checked at the natural gas gravity and the adopted allowable pressure drop, under the gas line sizing for an added appliance SOP, with a pass or fail on each. Wrong: converting appliances on piping sized for propane, the commonest way a conversion produces a house full of lazy burners. Stop: any failed section means the conversions wait until the piping is corrected. Hazard: none, arithmetic done before tools come out, which is why it belongs here rather than after.

3. Isolate the appliance and prove it dead before opening the burner compartment. Close the appliance shutoff, crack the joint on the appliance side, and check with a listed leak-detection solution and a combustible gas indicator against an outdoor background. Acceptance: no flow, no bubble growth, no indicator response above background. Wrong: a valve that turns without sealing, giving a soft continuous hiss. Stop: a valve that will not hold means isolation moves upstream before the burner box opens. Hazard: this releases propane, which is heavier than air and collects at floor level in exactly the space you are about to kneel in, so no ignition source is operated and the low reading is taken first.

4. Change every orifice the kit lists, counting them out and counting them in. Acceptance: orifices removed equals burner count, orifices installed equals the count the kit lists, and the marking stamped on each new one matches the kit's table for this model and for natural gas. Wrong: a six burner range with five converted, which lights fine and runs one burner at propane orifice size on natural gas, so that burner is starved and the customer thinks it is a bad burner. Stop: a count that does not reconcile means every orifice comes back out and gets read again. Hazard: reaching into a burner box puts your hands on sheet metal edges and an igniter, so the appliance stays isolated from step 3 and power to it stays off until step 7.

5. Convert the appliance regulator exactly as the kit directs. Depending on the appliance this is a spring change, a reversible plug or cap, or a whole regulator. Acceptance: the kit's step performed and the visible indicator, usually a marking or the cap orientation, reading the new fuel. Wrong: natural gas orifices with a regulator still set for propane, running the burner far above the manifold pressure the new orifice was sized for. Stop: a regulator with no listed conversion sends the appliance back to step 1's decision. Hazard: confirm by feel that the cap is fully seated, because a partly engaged conversion plug reads converted and behaves like neither fuel.

6. Fill in and fit the conversion label next to the rating plate. NFPA 54, the National Fuel Gas Code, in the edition your authority having jurisdiction has adopted, requires a converted appliance to carry a label identifying the fuel it now burns. Acceptance: the kit's label completed with the new fuel, the date, the kit number and who performed the conversion, fixed beside the rating plate where the next tech reads it before touching anything. Wrong: a converted appliance whose plate still says propane, so the next service call diagnoses against the wrong manifold pressure and the wrong orifice table. Stop: a kit with no label means you make a durable one carrying the same information rather than skipping it. Hazard: none, a label, but it is what stops the next person setting this appliance up for a fuel it no longer burns.

7. Reintroduce gas, leak check, set manifold pressure, then clock the input at the meter. With no ignition source operated and the indicator in hand, open the shutoff a quarter turn, let it come up, open fully, and leak check every joint. Read manifold pressure at the regulator outlet tap with the burner at full rate and set it to the plate value; natural gas appliances commonly sit near 3.5 in wc and propane near 10 in wc, and the plate governs, not the typical. Then, with only this appliance firing, time the meter's test dial: cubic feet per hour is 3600 times the dial size divided by seconds per revolution, and input is that times the operator's heating value. Acceptance: manifold pressure at the plate value and the clocked input within the tolerance the kit states, both recorded, with any altitude derate the instructions require applied first. Wrong: a blue flame accepted as proof, which it is across a wide range of inputs. Stop: an input outside tolerance once manifold pressure is correct means an orifice is wrong, and the fix is at step 4, never at the regulator screw. Hazard: this puts fuel back in with the burner lit and a panel off, so the tap plug goes back in and is leak checked before the appliance runs unattended, and a personal carbon monoxide monitor stays at your breathing zone; the OSHA limit is 50 ppm as an 8 hour time weighted average at 29 CFR 1910.1000 Table Z-1.

8. Prove combustion and venting on this appliance and on every other one in the building. Acceptance: smooth ignition with no rollout, no spillage at a draft hood or relief opening a few minutes in with the building in its worst case, meaning exhaust fans on and doors closed, and every other gas appliance verified on the new fuel with its loaded inlet pressure inside its plate range. Wrong: commissioning only what you converted, when every appliance in the building just changed fuel supply. Stop: spillage or a rising monitor shuts and tags that appliance and scopes the venting before anything else. Hazard: several appliances fire at once here, so the monitor stays on you and the worst-case depressurisation test runs with the customer out of the mechanical space.

The record this produces

One conversion record per appliance, plus one property record, filed against the address:

• Make, model, serial and plate input for each appliance, and the kit number matched to it
• Appliances with no listed kit, and what was done instead
• The re-sizing result: each section's demand and capacity at the natural gas gravity, pass or fail
• Orifice count removed and installed, and the marking read on the new ones
• Regulator conversion method, the visible indicator confirmed, and the label wording and date
• Manifold pressure set, plate value, and the tap it was read at
• Clocked input: dial size, seconds per revolution, cubic feet per hour, heating value, resulting input, and the plate input it was compared with
• Spillage and carbon monoxide results for every gas appliance in the building

Worked pass: a house connected to a main after twenty years on propane

Three gas appliances: an 80,000 Btu per hour furnace, a range, and a tank water heater. Operator's published heating value 1,030 Btu per cubic foot.

• Step 1: FAILED at the water heater. The furnace and range both have kits listed for their model and serial. The water heater's manufacturer lists no conversion for that serial range. Stop rule taken: it was quoted for replacement rather than re-orificed, and shut down for the day it stayed.
• Step 2: re-sizing run under the sizing SOP at natural gas gravity. Every section cleared, because the propane-era piping had been run generously; the record notes that this is luck rather than a rule.
• Step 3: furnace and range each isolated and proved. No bubble growth, indicator 0 against an outdoor background of 0, low readings taken first.
• Step 4: furnace, one orifice out and one in, marking matching the kit table. Range, four surface burners plus one oven burner: five out and five in, all markings checked.
• Steps 5 and 6: both regulators converted by reversing the kit's plug, cap markings now reading natural, and labels completed with fuel, date, kit number and tech beside each rating plate.
• Step 7, first reading on the furnace: manifold pressure read 3.7 in wc against a plate value of 3.5. Clocked on the meter's 2 cubic foot test dial with nothing else firing: 2 cubic feet in 89 seconds, so 3600 times 2 divided by 89 is 80.9 cubic feet per hour, and 80.9 times 1,030 is 83,300 Btu per hour against a plate input of 80,000. That is 3,300 over, or 4.1 percent high.
• Step 7, after setting manifold pressure to 3.5 in wc: 2 cubic feet in 92 seconds, so 3600 times 2 divided by 92 is 78.3 cubic feet per hour, and 78.3 times 1,030 is 80,600 Btu per hour, 600 over plate, or 0.8 percent high, inside the kit's stated tolerance.
• Step 8: furnace ignition smooth, no rollout, no spillage at the flue with the dryer and bath fans running and doors closed. Range flames stable at all five burners. Monitor at background throughout. The water heater was not commissioned because it was not converted.

The clocked input is what makes this pass worth reading. At 3.7 in wc the furnace fired 4.1 percent over its plate with a flame that looked exactly like the one at 3.5 in wc, and nothing on the appliance would have told anyone. A shop that sets manifold pressure and stops has verified the pressure, not the input, and the input is what the heat exchanger was designed around.

References

• The appliance manufacturer's conversion kit, its instructions and its rating plate, which set the kit's applicability by serial, the orifice markings, the regulator conversion, the manifold pressure and the input tolerance step 7 measures against
• NFPA 54, the National Fuel Gas Code, in the edition your authority having jurisdiction has adopted, for the requirement that a converted appliance carry a conversion label and for the gravity-specific sizing tables step 2 uses
• The gas operator, for the published heating value at your location used in the step 7 clock
• 29 CFR 1910.1000 Table Z-1 for the carbon monoxide limit governing the monitor at steps 7 and 8
• See related: the gas line sizing for an added appliance SOP, the gas meter set coordination SOP, the gas line abandonment and capping SOP