Heater Not Reaching Set Temperature
Quick Answer
If your heater runs but can't reach the set temperature, check your air filter first - a dirty filter can reduce heating capacity by 15% or more. Also check for drafts, blocked vents, and ensure the thermostat isn't near heat sources or cold drafts.
Safe Checks You Can Do
- 1Replace air filter - A clogged filter significantly reduces heating output.
- 2Open all vents - Ensure all supply vents are fully open and unobstructed.
- 3Seal drafts - Check windows and doors for cold air leaks. Use weatherstripping as needed.
- 4Check thermostat placement - Move if near drafts, sunlight, or heat sources that affect readings.
Likely Causes
- Dirty air filter
Reduces airflow and heating capacity. The most common and easiest fix.
- Undersized furnace
If the furnace is too small for your home, it may never reach set temp in cold weather.
- Poor insulation
Heat loss through walls, attic, or windows can exceed what the furnace can produce.
- Duct leaks
Leaky ductwork in unconditioned spaces loses heated air before it reaches rooms.
- Aging furnace
Older furnaces lose efficiency. A 20-year-old unit may be 30% less efficient than when new.
When to Call a Pro
- Problem persists after filter replacement and vent check
- Furnace is running constantly but house is still cold
- You've noticed a gradual decline in heating performance
- Furnace is more than 15 years old
- Energy bills have increased significantly
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