Furnace Not Turning On? Emergency Troubleshooting Before Calling HVAC
It is freezing and your furnace won't start. Here are the 6 things to check before paying for an emergency HVAC service call.
Check 1: Thermostat Settings
Start with the thermostat. Make sure it is set to Heat mode (not Cool or Off), the temperature is set above the current room temperature, the fan is set to Auto (not Off), and the batteries are not dead if it is a battery-powered thermostat. Try setting the temperature 5 degrees above room temperature and wait 3 to 5 minutes for the furnace to respond. If your thermostat screen is blank, replace the batteries or check that it has power.
Check 2: Air Filter
A severely clogged air filter restricts airflow and can cause the furnace to overheat and shut down via the high-limit safety switch. Pull out the filter and look through it. If you cannot see light through it, it is clogged. Replace it with a new filter and try again. Filters should be replaced every 1 to 3 months depending on household factors like pets and allergies. This is the simplest fix and the most commonly overlooked.
Check 3: Circuit Breaker and Power Switch
Check your electrical panel for a tripped breaker labeled Furnace, HVAC, or Heater. Reset it if tripped. Also check for a power switch on or near the furnace — it looks like a light switch and is often accidentally turned off. Some furnaces also have a separate breaker or fuse at the unit itself. Make sure all power sources are on.
Check 4: Gas Supply
For gas furnaces, verify that the gas supply valve is open. The valve is on the gas pipe leading to the furnace and has a handle that should be parallel to the pipe (open) not perpendicular (closed). If you smell gas, do NOT flip any switches. Leave the house immediately and call your gas company emergency line from outside. Never try to diagnose a gas leak yourself.
Check 5: Pilot Light or Igniter
Older furnaces have a standing pilot light that can blow out. Look through the small window on the furnace — you should see a small blue flame. If it is out, follow the relighting instructions on the label attached to your furnace. Most modern furnaces use electronic ignition instead of a pilot light. If you hear clicking sounds followed by no ignition, the igniter may have failed. Igniters cost 20 to 40 dollars.
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