Condensing gas furnace with PVC vent pipes and metal ductwork in a utility room

Generic photo: Wtshymanski · CC BY-SA 4.0

Furnace runs a long time but the house never reaches set point

When the burners light and warm air comes out, the furnace is usually working; the heat is being lost or not delivered. Check thermostat settings, filter and registers, then duct leaks, insulation and sizing. Only after that look at weak burner output or ignition problems, which are gas-side and go to a licensed gas technician.

Cross-checked ~45 min easyBrands: All

Before you start

If you smell gas or a CO alarm sounds: get everyone out and call the gas utility or 911 from outside
Do not adjust the gas valve, manifold pressure or burners; that is licensed gas-tech work
Kill power at the furnace switch before opening the blower compartment

Diagnostic steps

Unlock all 6 steps with expected values

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