Combustion & gas: Furnace tips

Short, practical technician tips on combustion & gas for furnace. Check each one against the manual for your exact model before acting.

Burner firing with inducer off ruins the pressure switch

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Overheated diaphragm; don't jumper the switch.

Check gas pressure before adjusting air

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High manifold pressure mimics "not enough air", low pressure mimics "too much air" on the analyzer.

CO means air is wrong either way

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Too little OR too much combustion air makes CO; use CO2/O2 to tell which.

Flame colour read

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Blue inner cone with light-blue outer = good; yellow tips = short on primary air (CO risk); orange flecks = dust burning, not an air problem; noisy, lifting flame = too much primary air.

Flame rod = ionization electrode = flame electrode

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Same part, several names.

Flame rod cleaning

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Oxide film on the rod causes burners to light then drop out; clean with sand cloth/steel wool, check ceramic for cracks, don't crack it on reinstall, rod is hot.

LP regulators are usually external

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NG valves have integral regulator; LP often has a separate one.

Modulating furnaces need the matching communicating stat

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With a plain stat they run as single-stage.

NG→LP conversion = change spuds

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LP is heavier, flows less through the same orifice; swap orifices and set 10.5 in. WC.

Rollout switch is manual reset — never just reset it

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Tripped rollout = cracked HX, rusted burners, poor draft, wrong flue height, blocked passages; find the cause.

Starved combustion in tight houses

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A negative-pressure house starves the burner and lowers efficiency; consider a makeup air unit or two-pipe direct vent.

Trial for ignition settings are mfr-specific

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On selectable-TFI modules, check the manual before changing.

Yellow flame = IAQ clue

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Yellow instead of blue burner/pilot flames on gas appliances is on the residential IAQ pollutant checklist.