Heater Not Turning On? 9 Things to Check Before You Call
The first cold night of the year in San Antonio is when a lot of heaters will not start. Many of those calls turn out to be a setting, a switch or a filter. Here are the safe checks to make first, and the signs that you need a technician.
1. Check the thermostat first
Most "heater won't turn on" calls start at the thermostat. After months of cooling, it is easy to miss a setting.
- Mode: it must say HEAT, not COOL, OFF or AUTO set too low.
- Setpoint: set it at least 3 degrees above the room temperature so it calls for heat.
- Batteries: a blank or dim screen often means dead batteries. Replace them, even if the screen still shows something.
- Schedule or hold: a smart or programmable thermostat may be following an old summer schedule.
If the screen is blank with fresh batteries, or the settings look right and nothing happens, see how to reset your thermostat.
2. Check the breaker and the furnace power switch
Your heater needs electricity even if it burns gas. Look for a tripped breaker labeled furnace, air handler or HVAC, and reset it once. Then find the power switch at the indoor unit. It looks like a light switch on the wall or ceiling near the furnace, often in the attic or a closet, and it is easy to flip off by mistake.
If the breaker trips again, leave it off and call. A breaker that keeps tripping points to an electrical problem.
3. Make sure the furnace door is closed tight
Most furnaces have a safety switch behind the front panel. If the panel is loose or was put back crooked after a filter change, the furnace will not run at all. Push the panel firmly into place and wait a few minutes.
4. Change the air filter
A clogged filter chokes the airflow. The furnace can overheat, trip its high-limit safety switch and shut itself off, or start and stop over and over. Put in a clean filter of the right size, with the arrow pointing toward the unit. See how your air filter affects your heater.
5. Check the gas supply
If you have a gas furnace, make sure the gas valve on the line to the furnace is open (the handle runs in the same direction as the pipe). If your gas stove or water heater also will not light, the problem may be with the gas service, not the furnace.
If you smell rotten eggs or gas, leave the house, do not flip switches, and call your gas company from outside.
6. Listen for the ignition
Most furnaces in San Antonio homes use electronic ignition, not a standing pilot light. When the thermostat calls for heat, you should hear the inducer fan start, then a click or glow, then the burners light. If the burners light and shut off after a few seconds, a dirty flame sensor is a common cause. Read what a flame sensor does. Burners, gas valves and ignition parts should be serviced by a licensed technician.
7. Check the condensate drain on a high-efficiency furnace
High-efficiency furnaces (often with white plastic vent pipes) make water when they run. If the drain line or trap clogs, a float switch can shut the furnace off to prevent a leak. Look for water in the pan or around the unit. If you see standing water, call for service.
8. If you have a heat pump
Many San Antonio homes heat with a heat pump. In cold, damp weather the outdoor unit goes into defrost for a few minutes, the fan stops and you may see steam. That is normal. If the outdoor unit is covered in thick ice, will not run at all, or the thermostat shows AUX or EM heat all the time, read EM heat vs. heat and see heat pump repair.
9. Read the status light
Most modern furnaces have a small light on the control board, visible through a window in the lower panel. It blinks a code, and the chart that explains the code is usually printed on the inside of the furnace door. Write the code down before you turn the power off. It helps the technician find the problem faster.
When to stop and call
- You smell gas: leave the house and call your gas company from outside.
- A carbon monoxide alarm sounds: get everyone out and call 911.
- The breaker trips again after one reset.
- The burners light and shut off over and over, or the furnace keeps locking out.
- You hear banging, squealing or grinding, or smell burning plastic or wiring. See heater smells like burning.
- It runs but blows cold air. See heater blowing cold air.
Cowboy's answers 24/7 with our own dispatch on Blanco Road. On every heating call we check gas safety before we call it fixed. See heating repair in San Antonio, furnace repair, or call (210) 495-7771.
Frequently asked questions
Why is my heater not turning on?
The most common causes are simple: the thermostat is not set to HEAT or has dead batteries, a breaker tripped, the furnace power switch is off, the door panel is loose, or a dirty filter caused a safety shutdown. Gas supply, ignition and control board problems need a technician.
Why won't my furnace turn on when the thermostat is set to heat?
Check the breaker, the power switch near the furnace, the door panel and the filter. If all of those are fine, read the blinking status light on the furnace and call for service.
Can a dirty air filter stop a furnace from turning on?
Yes. A clogged filter can make the furnace overheat and trip its high-limit safety switch, which shuts the burners off. Replace the filter and give the system a few minutes to reset.
How do I reset my furnace?
Turn the power switch at the furnace off, wait about 30 seconds, and turn it back on. If the furnace locks out again, do not keep resetting it. Write down the status light code and call a technician.
Should I relight the pilot light myself?
Most furnaces in San Antonio homes use electronic ignition and have no standing pilot. If yours does have a pilot, follow the instructions on the furnace label. If you smell gas, do not try to light anything; leave and call your gas company.
Related service: heating repair in San Antonio · furnace repair · heat pump repair. Cowboy's AC answers 24/7 at (210) 495-7771.
