Symptoms
- Furnace runs long cycles but house stays cooler than the setting
- Some rooms feel fine, others stay cold
- Heating bills higher than usual for similar weather
Likely causes
- CommonDirty filter restricting airflowLess air moving over the heat exchanger means less heat delivered to the house.
- CommonClosed or blocked vents and ductsReduced airflow to rooms means they get less of the heated air being produced.
- PossibleThermostat set too conservatively or miscalibratedA thermostat reading warmer than actual room temperature will undershoot the true setting.
- PossibleUndersized furnace for the home's size or insulation levelA furnace can be too small for the square footage, especially after an addition.
- PossibleAging furnace losing efficiencyOlder units lose heating capacity as components wear, especially past 15–20 years.
What to do
- Replace the air filter and check that all supply and return vents are open and unobstructed.
- Check attic or basement ductwork for obvious disconnected or crushed sections if accessible.
- Compare thermostat reading to a separate room thermometer to check for a big mismatch.
- Check attic and wall insulation levels if the home loses heat quickly; poor insulation can outpace any furnace.
- Note the furnace's age; units over 15–20 years old naturally lose some capacity and efficiency.
- If everything checks out and heating is still inadequate, have a technician assess whether the furnace is properly sized for the house.
Tools and parts
- Room thermometer
Standard furnace air filterSearch: “furnace air filter size replacement”
When to call a pro
Call an HVAC technician for a load calculation if the furnace consistently can't keep the house warm; it may be undersized or losing capacity with age.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.