Airflow & ducts: Furnace tips

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

Annual furnace inspection checklist

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Louvers clear; burners for rust/dirt/water marks; flue pipe joints; burned wires; blower door seals return; return duct sealed to furnace; run test; analyze flue gas. Tell owner to shut down for new noises, odours, excess moisture, burned/rusty parts, or flu-like symptoms that clear when away (CO).

Blower service

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Clean blower yearly; a few drops SAE 30 once or twice a year if oilable; summer cooling usually uses the high blower tap; replace glazed/cracked belts.

Door interlock switch

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Blower door must be fully seated or the interlock kills power; "dead furnace" is often just the door.

Filter arrow points toward the furnace

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(direction of return airflow) — teach the customer.

G doesn't run the blower in heat mode

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On most furnaces — a temp or time-delay switch does, so cold duct air isn't blown.

Heat exchanger 3-point check (GAMA)

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(1) watch flame before/after blower starts: any change = HX/gasket breach; (2) visual with mirror & light; (3) tracer-gas/chemical test at supply registers. Cracks may only open when hot. HX wall thinned ≥50 % = replace. Combustion gas in supply air can also come from mechanical-room depressurization, not just a crack.

LP pilot failure pools gas in low spots

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LP is heavier than air; smell = gas present; ventilate basements/crawlspaces. LP systems must be 100 % shutoff.

PSC speed taps

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More winding in the circuit = slower; fewer turns = faster. Change speed by moving the lead to a different tap.

Record baseline at install

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Fuel type, model, filter size, manifold pressure, stack temp; leave a copy with owner and on file.

Stack effect

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Heating: warm air rises, upper floors slightly positive, lower floors negative pulling cold air in. Cooling: reverse. Combustion appliances pull house negative → add dedicated fresh-air duct/fan.

V-belt width must match pulleys

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On belt-drive blowers check belt rides fully in both grooves; wrong width = slip and wear.