Furnace runs but produces insufficient heat; supply air feels lukewarm and house never reaches set temperature.

Safety: Gas pressure adjustments must be done by a licensed technician. Homeowners should focus on airflow and duct issues.

Steps:

• Check filter and registers: Ensure good airflow by using a clean filter and opening all registers.
• Inspect ducts: Look for disconnected or crushed ducts, especially in unconditioned spaces.
• Observe burner flame: Through the sight glass, flames should be mostly blue and steady. Yellow, lazy flames may indicate dirty burners.
• Thermostat calibration: Compare thermostat reading with a separate thermometer to ensure it is accurate.

When to call a technician: If airflow and ducts are fine but heat is still weak, a technician should check gas input, temperature rise, and burner condition.