Symptoms
- System runs long or constantly
- Temperature stalls a few degrees short of the setting
- Some rooms colder or hotter than others
Likely causes
- CommonDirty air filter restricting airflowA clogged filter chokes airflow and reduces heating or cooling capacity.
- CommonClosed or blocked vents and registersFurniture or closed vents reduce the air actually reaching rooms.
- PossibleUndersized or aging HVAC systemAn older or undersized unit may not have enough capacity for extreme weather.
- PossibleThermostat set too aggressively for outdoor conditionsOn very hot or cold days, systems can only get so far below or above outdoor extremes.
- PossibleRefrigerant or heating component issueLow refrigerant or a failing heat exchanger reduces real output even though the system runs.
What to do
- Replace the air filter if it's dirty or overdue.
- Walk the house and open any closed vents, especially in unused rooms.
- Check that furniture, rugs, or curtains aren't blocking vents or the return air grille.
- Note the outdoor temperature and adjust expectations; most systems struggle to close more than 15–20 degrees of difference.
- Check that the outdoor unit isn't blocked by plants, snow, or debris.
- If airflow and filters are fine but the gap persists for days, the system may be undersized or losing refrigerant/heat output.
Tools and parts
- Replacement filter
- Flashlight
HVAC air filterSearch: “HVAC air filter correct size”
When to call a pro
Call an HVAC technician if filters and vents are fine and the system still can't reach the set temperature, especially if it runs nonstop.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.