Propane Conversion on a Gas Clothes Dryer
A cottage owner buys a used 20,000 Btu/h gas clothes dryer rated for natural gas and asks for a propane conversion. The conversion kit includes new burner orifices and a regulator spring. The propane supply is a 500-gal tank with a two-stage regulator; the technician measures 11 in. w.c. at the dryer inlet. After installing the orifices and spring, the dryer lights but the flame is long, yellow, and lifts off the ports; the burner tube shows soot after 10 minutes. The manifold pressure gauge reads 6.5 in. w.c. The dryer sits in a 3 ft by 3 ft by 8 ft closet with a solid door and one 8 in. by 8 in. metal grille (75% free area). The flexible connector is 6 ft long. The homeowner says the dryer worked fine on natural gas at the old house. The technician has a manometer, CO analyzer, and combustion-air calculator.
1. What is the most likely reason for the yellow flame and soot, and how would you verify it?
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Propane appliances typically require about 10 in. w.c. manifold pressure unless the nameplate says otherwise. A reading of 6.5 in. w.c. shows the regulator is not set correctly or the wrong spring/regulator was installed, but the yellow, lifting flame and soot also suggest the burner may be overfired or has the wrong orifice-if a natural-gas orifice was left in, propane would pass too much gas. Verify the orifice stamp against the conversion kit, confirm the correct spring and regulator, check supply pressure under load, and adjust manifold pressure to the nameplate value. Then set the primary air shutter for a stable blue flame with slight yellow tips. Do not simply open the air shutter to hide soot; correct the gas input and air first, then measure CO.
2. Calculate the combustion-air requirement for the closet and decide whether the existing grille is adequate.
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Closet volume is 3 × 3 × 8 = 72 ft³. For 20,000 Btu/h, the unconfined-space threshold is 20 × 50 = 1,000 ft³, so the closet is confined. Two permanent openings are required, each at 1 in² free area per 1,000 Btu/h, but each opening has a 100 in² minimum. The existing grille is 8 × 8 = 64 in²; at 75% free area it provides only 48 in², and it is only one low opening. It is inadequate. The door must be provided with two properly sized openings, or the appliance must receive direct outdoor combustion air. A louvered door alone does not fix the problem unless it provides the required free area in the required high/low arrangement.
3. What would you do before leaving the job, and what appliance and vent checks are required?
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Correct the manifold pressure, orifice, and air adjustment, then verify the flame is stable and the CO level in the flue and ambient space is acceptable. Check the vent connector and dryer exhaust separately; clothes-dryer exhaust must not be connected to the gas vent. Confirm the flexible connector is not concealed, kinked, or longer than allowed, and that a shutoff valve and sediment trap are installed. Leak-test all connections, check clearances to combustibles, and confirm combustion air is permanent. Install or verify a CO alarm, instruct the owner on the propane conversion, and document the orifice size, manifold pressure, CO readings, and air-opening calculations.