Why Does the Thermostat Say the Heat’s On But the Furnace Isn’t Running?

Your thermostat is set to “heat” mode but the furnace refuses to turn on. This is quite frustrating, not to mention uncomfortable. Even more disconcerting is not knowing the reason. Here, we’ll detail why the thermostat says the heat is on but the furnace isn’t running, plus what you may be able to do about it.

The Thermostat Isn’t Wired Correctly

If heating issues start immediately after installing a new thermostat, it may have been wired wrong. For example, if the W and Y wires are reversed, the cooling and heating functions will be as well. Look at how the wires are configured and determine if all are in the right terminals (both at the thermostat and the furnace). Correct the issue if you can, or call a heating professional or electrician if not comfortable doing so.

Broken or Corroded Wires

A thermostat will malfunction if one of its wires breaks. Worn insulation, frayed conductors, and corrosion can cause a wire to fail. Wear and tear happen over time but can lead to dangerous situations if a conductor is exposed. Solid copper wires are durable but can break if nicked by a stapler or flexed too much by settling wall studs. Damaged wires should be replaced immediately by a professional.

The Thermostat Is Set Too High/Too Low

Your furnace may be operating normally, but the thermostat is set lower than the temperature it can produce. This can occur during extreme cold. The unit can be strained in unusual weather conditions. Reset the thermostat to a more practical temperature or check the furnace to determine if it needs to be fixed.

The Thermostat Needs to Be Calibrated

If the thermostat is inaccurate, try to recalibrate it. To do so, place a thermometer a couple of inches away from the unit, not near a hole or anything else that can disrupt the temperature reading. Wait a few minutes and compare the readings. They should not differ by more than one degree. If you have an older thermostat with a mercury dial, try leveling it and then testing its accuracy again.

Its Sensor Has Malfunctioned

A faulty sensor won’t know there’s a need for heat. Therefore, it won’t tell the thermostat to turn on. It can’t sense the temperature or compare it to what it should be. Or, perhaps the sensor is getting the wrong reading. 

Your Home May Not Have Enough Insulation

The heat is on, but so much is leaking through cracks in walls and around doors and windows you don’t feel it. Poor insulating materials can be problematic as well. This can be fixed by adding insulation or sealing leak points with caulking, foam, or other materials. Old single-pane windows should be replaced with energy-efficient ones to keep in the heat.

The Thermostat Needs New Batteries

If the batteries are dead, the thermostat may say the heat is on, but the furnace isn’t running. Look for a low battery warning light; some units beep if the battery is low. Replacing the battery should fix the problem. To avoid issues, replace the battery seasonally.

The Thermostat Needs to Be Cleaned

Dirt can cause a thermostat to malfunction. Since it is used (and touched) frequently during hot or chilly times, cleaning is important. It removes dirt from the inside and keeps the unit free of germs. 

To clean a thermostat, remove the cover (by unscrewing it or snapping it off, depending on the model). Use a small, dry, soft brush to dust the inside. If there’s debris between electronic components, use a piece of paper to slide and loosen it for safe removal. Then dust off the cover and reinstall it.

Other Reasons the Thermostat Says the Heat Is On But the Furnace Isn’t Running

The issue may not be coming from the thermostat itself. If this is the case, other possible causes include

• Power outages
• Tripped circuit breaker
• The furnace power switch is off
• No gas supply
• Dirty/frosted heat pump coils
• Dirty pilot ignitor
• Dirty furnace filter

Contact Hays Cooling, Heating & Plumbing

If the thermostat says the heat is on but the furnace isn’t running, our qualified specialists can help. We’ve been providing high-quality heating repair in Phoenix, AZ, since 2001. A heating issue can impact your comfort and safety. Whether the problem originates from your furnace, heat pump, or thermostat, Hays can diagnose and resolve it. Schedule service on the web or call 602-362-7917 and we’ll send a technician right away.