Furnace runs but does not keep the house warm; short cycles on and off frequently without reaching set temperature.

Safety first: Turn off power before removing panels.

• Replace filter: Install a new filter of the correct size and MERV rating. Avoid overly restrictive filters unless ductwork is designed for them.
• Open registers: Ensure most supply and return registers are fully open and not blocked.
• Check thermostat placement: If the thermostat is near a supply register, sunlight, or other heat source, it may shut the furnace off early. Consider relocating the thermostat with the help of a professional.
• Monitor run time: After changes, observe furnace run times. Persistent short cycling may indicate an oversized furnace or duct issues that require professional evaluation.