Combustion & gas: General tips

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

Cast iron pipe is prohibited for gas

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; black iron or CSST typical. Label exposed gas pipe "Gas" every 5 ft (not required for steel or in the appliance room).

Hybrid gas/electric chillers

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Let owners switch to the cheaper energy.

Leave the first two threads bare

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When doping gas pipe so compound doesn't plug orifices.

Rubber mallet tap on a stuck steam trap/vent/valve

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Can free it temporarily to confirm which component is at fault.

Two gas shutoffs on RTUs

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One outside the cabinet for emergency; gas pipe ≥ 3.5" above roof.

Valve names don't matter, function does (Pro Tip)

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Hot-gas bypass, liquid injection, head pressure control are functions; the hardware may be an AXV-style, TXV-style or EEV-style body. Identify what it does in the circuit.

Water valve problems by symptom

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Too little flow: leaking/chattering/sediment-bound/misadjusted valve, clogged screen, leaking bellows. Too much: supply pressure too high (add regulator), valve leaking or opened too far; water running during off cycle = leaking valve, flush debris from seat. Constant flow hides air in condenser and ruins hot-gas defrost.