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Here's What to Check If Your Furnace Leaves Your Home Cold


Is your house cold? When there’s a problem with a furnace, it doesn’t always mean that the equipment fails to turn on. Sometimes a heater just doesn’t do the job that well. There may be cold spots or cold rooms in the house, lukewarm air coming out of vents, or various other issues that leave you curious as to what could be going on with the furnace.

Hopefully, a few simple furnace repairs or a little tweaking here and there will solve the problem. But not all issues are mechanical, but some are. Since most homes in our area have heat pumps installed, we’ll cover a few common problems associated with this equipment and how to check for possible causes before calling a heating repair contractor.

When The Furnace Blows Lukewarm Air

Lukewarm air is not hot enough to heat your home sufficiently. Some possible problems with the furnace can strain the equipment because it constantly tries to heat the home to the temperature on the thermostat setting but can’t. This is more likely a mechanical issue that only a heating technician can troubleshoot. However, there are some things you can check first.

Possible Causes:

  • Dirty air filter causing blockage
  • Safety feature on the furnace
  • Leaky duct
  • Tripped breaker

What To Check:

  • Check the filter’s location and condition
  • Check for a tripped breaker
  • Inspect the ductwork for a leak

Some HVAC filters are installed wrong or are the wrong size and then get sucked up into the return air handler. This blocks the furnace. Check the position and that it’s not turned backward. Also, make sure to change the filter if it’s dirty, this is another type of blockage. If the breaker has tripped, turn it back on to see if it trips again. If so, the fan motor may be bad. Heat could be escaping from a duct, so check the entire system for a breach.

When The Air Isn’t Warm Enough From Just One Vent

If comfortable warm air is coming from most of the vents in your house and just one or two vents seem to be flowing with cooler air, it’s likely a breach in the duct system that leads to that room. Joints can come loose and rodents or squirrels can chew right through the metal duct material. There could also be a blockage of some sort.

What To Check:

You will need to carefully inspect the duct system, whether in the attic or crawl space, for holes or connections that aren’t sealed. The entire duct system should be completely sealed and wrapped with insulation or the warm air will escape. If there’s a problem, it will need to be fixed – sometimes by means of a professional. If there’s not, the duct system in the area needs to be inspected inside for a blockage.

Air Is Too Hot, Too Cold, Or Temperature Fluctuates

If your home is never heated to the degree that you set on the thermostat, it may need to be calibrated. Thermostats can lose calibration if they get dirty or accidentally bumped. Older mercury thermostats can easily get off balance.

Possible Causes:

  • Thermostat may be malfunctioning
  • Wire to thermostat may be loose
  • Thermostat temperature calibration may be off
  • Thermostat is just too old
  • Thermostat is dirty
  • Engine performance is weak

How To Check Calibration:

  • Ask a heating technician to check the thermostat and calibrate if necessary
  • Tape a glass tube thermometer with a paper towel padding (behind it) a few inches away from the thermostat to see if temperatures match

The fix will depend on what’s actually wrong with the thermostat. If it’s the furnace motor, you’ll need to call a heating contractor. Hopefully, it’s just dirty, needs to be calibrated, or there’s a loose wire connection. Otherwise, you may need to replace the thermostat.