Furnace runs but produces insufficient heat; home never reaches set temperature.

Safety first: Turn off power and gas before opening burner compartment.

Steps:

• Verify that the thermostat is set to HEAT and the temperature is above room temperature. Replace thermostat batteries if applicable.
• Replace the furnace air filter with a clean, properly sized filter.
• Check that all supply registers are open and that return grilles are not blocked by furniture or rugs.
• Inspect burners for dirt or rust buildup and clean with a soft brush and vacuum.
• If the problem persists, a professional should verify gas input rate (manifold pressure and orifice size) and temperature rise across the furnace per CTC specifications.