Solved! How to Fix a Furnace That’s Not Turning On

Before you call an expensive technician, consider these simple solutions for a furnace that won't heat up.

furnace not turning on repair

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Q: We recently moved into our home, and now that it’s getting cold, we’re concerned because the furnace isn’t turning on. We know we need to get it working before the pipes freeze but have no idea what the problem is. Can we troubleshoot the issue of a furnace not turning on before calling a technician?

A: Apart from causing the discomfort of a cold house, a malfunctioning furnace could lead to costly damage to your home—including, as you mention, pipes freezing. Cold temperatures or the resulting condensation can also potentially damage windows, and structural damage can also occur if a house is left unheated for several months of cold temperatures.

Getting your furnace going during cold weather is a high priority that should be tackled immediately, but it needn’t necessarily mean calling a maintenance specialist. There might be straightforward solutions to furnace problems that you can undertake, from setting the thermostat to reigniting the pilot light on a gas furnace. So read on for some troubleshooting tactics to try right now.

Make sure the thermostat is on.

The most obvious solution for a furnace not turning on is often the one that is most frequently overlooked: Some folks neglect to check the thermostat—the part of your heating system that assesses the room’s temperature and allows you to set and adjust the desired temperature. A thermostat is typically installed on one of the main walls in your home, around the same height as a light switch.

If the device isn’t getting power from your home’s electric wiring system, it can’t tell the furnace when to turn on. So if you have a programmable thermostat, check to ensure the display is active, the furnace is on, and the unit is set to heat. Manual thermostats can accidentally get knocked to an off position from dusting or other household cleaning chores. Turn the heat up to a temperature that typically causes the furnace to switch on; this can vary from thermostat to thermostat. If it doesn’t turn on after a minute or so, increase the temperature significantly as you continue troubleshooting.

RELATED: The Best Programmable Thermostats for the Home

Ensure the gas is turned on.

Any gas furnace will have a valve or gas cock located within 6 feet of the unit. Furnace valve design varies, but they often include a little disk with a small handle or lever on it. Other gas valves can look like little boxes, while others still can resemble a lever about the size of your index finger (often brightly colored).

furnace not turning on check gas valve

Bob Vila

This valve may have been closed by accident or by someone working on the furnace who forgot to turn it back on. While valves can vary depending on make or manufacturer, a gas valve will typically be in the open position—which means there is gas flowing through the pipe—if the handle on the valve is parallel with the pipe. If the handle is perpendicular to the pipe, the valve is likely closed. To open it, turn the little handle so that it’s parallel to the pipe.

If the furnace still doesn’t kick on once the valve is open, check another gas appliance in the house, such as a stove or fireplace, to see if it’s still working. If the other gas appliances don’t turn on, you may not have gas coming into your house. This can be the result of crews working somewhere on the gas line, or there might be a gas leak, a break, or even a blockage in the line. A gas leak can be a dangerous situation, so if you suspect this to be the case, call 9-1-1 immediately and follow their instructions.

To assess the situation, go outside and check both the street-side and the internal house-side valves to see if they are on. If they are not on, turning the valve to the open or on position (as described above), should get gas flowing into your home once again. If your furnace and other appliances don’t start working, contact your utility company to find out if they’re working in your area or if there have been any service interruptions. If that’s not the case, check the status of your account. Whether due to a technical glitch or human oversight, a missed or incomplete utility payment can result in a service interruption.

RELATED: How to Locate Your Home’s Gas Shut-Off Valve

Check the furnace’s power switch position.

Sometimes the reason for a furnace not turning on is because it’s simply turned off. Locate the power switch on the furnace itself, which is likely on a nearby wall, or even on a ceiling or floor joist near the unit. It will often look like a regular light switch, but it may or may not be labeled. Typically, but not always, the on position is up, though this will depend on the installation. It’s easy for someone unfamiliar with the house to accidentally turn the furnace off, mistaking it for a light switch. Flip this switch back to the on position and wait for the unit to start up. Some furnaces can take several minutes before they resume functioning.

furnace not turning on check power switch

Bob Vila

Replace an air filter.

A clogged air filter will stop a furnace from heating properly. Restricted airflow can cause the heat in the heat exchanger to continually increase. Every furnace has limit switches that activate when temperatures exceed a designated point, shutting down the furnace. Furnace air filters should be replaced every 1 to 3 months, regardless of any heating or airflow issues. An air filter can become clogged with dust, hair, or other household debris, making your furnace less efficient.

To find the filter, locate the spot where air flows into the furnace when it is working. Slide the old filter out manually and examine the side for a model number and size, which indicate the correct replacement to buy. When replacing the filter, check the frame for an arrow mark, which indicates the direction the air is flowing. Position the arrow so that it faces the furnace side when sliding the new filter into place.

furnace not turning on check filter

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RELATED: Change Filters Frequently to Optimize Furnace Efficiency

Check the ignition sensor.

If you’re confident in your DIY skills, you may wish to clean the ignition sensor (aka flame sensor) on your furnace. The ignition sensor is a safety device that detects whether the furnace is burning gas. The sensor uses the increasing temperature in the furnace to identify flames. If the sensor cannot detect a flame, it will shut down the furnace to avoid unburned gas from escaping. Residue buildup can prevent the flame sensor from operating correctly. Cleaning it can allow the furnace to turn on and function properly. If this troubleshooting tactic feels beyond the scope of your DIY skills, it may be time to call a pro. Consider the basic how-to instructions below:

  • Before attempting to clean your sensor, shut off your furnace. A furnace doesn’t have an On/Off switch, so to turn it off, find the gas valve that controls the gas going into your furnace, and turn the little handle on the valve to the closed or off position (perpendicular to the pipe).
  • Then, look for a porcelain base with a small metallic rod in front of the pilot flame inside the furnace. Use a screwdriver with the appropriate head to loosen and remove the screw beside the sensor and then slide the dirty sensor out from its current position.
  • Gently clean the metal rod with a wad of steel wool or a clean, straight-edged knife to remove any buildup. Rub very carefully and patiently; you don’t want to detach the rod from the base.
  • Once clean, slide the sensor back into place, replace the screw keeping the sensor in place, and switch it back on.

If after trying these solutions you still have a furnace not turning on, it might be time to consider calling in a furnace repair professional. Check online directories to find repair technicians in your neighborhood. Your local home renovation shop can also be a great resource for finding furnace repair technicians.