Picture of the front of a typical apartment size furnace.

Furnace Not Kicking On When Temp Drops

You might notice your furnace not kicking on when the temp drops in your home. I.e. The thermostat says that the heat is on yet the heat still does not work. Or the t-stat never calls for heat when this happens. In fact, a broken thermostat, incorrect wiring, tripped circuit breakers, blown fuses, problem with the gas supply, or a broken HVAC system could cause this. Here, we detail some of these causes, and then offer troubleshooting routines and advice to solve them. First, we look at what might go wrong in the furnace itself. Then we talk about thermostat issues that can also cause the the furnace not to kick on.

Furnace Not Kicking On When Temp Drops: Furnace Issues 

1. Clogged Furnace Filters and Fan Blades


Dirty air filters can constrict airflow though the furnace’s heat exchanger, and cut the heat output to near zero. Clogged filters reduce how much the furnace heats. How so?  Dirty filters and fan blades move air less well. Plus, they make more noise besides. Finally, they could restrict the airflow so much that protective circuits shut down the system so it no longer kicks on.


    1. Replace furnace filters monthly.
    2. Plus, hire a pro home heating technician yearly, to check your furnace. He should clean out dust buildup inside.
    3. Also, have him remove blockages in the air hander, inspect the control units for damage, clean out air ducts, check heat registers around your home for clogs, and so on.

2. Dirty or Frosted Heat Pump Coils 


Dirty heat pump coils, again, slow airflow through the condenser outside, and the evaporator unit inside. Thus, dirt can interfere with the heating system’s efficiency. Dirty coils lowers the BTUs per hour that a heat pump can pump for example. That could mean that the system cannot keep your quarters as warm as your thermostat setting calls for. And once again, the system might shut down completely to prevent damage due to overheating.


    1. These coils have closely spaced metal fins that often become logged with dirt. So have a pro clean these with forced air or steam in extreme cases.
    2. Or, try vacuuming the coils with a household hose and brush sweeper. Be careful not to bend the fins though, as they’re delicate. Bending them too much can forever restrict airflow and thus, reduce how well your heating system (heat pump) heats your house.
    3. Replace any blown thermal fuses and the like.

3. Loss of Electricity or Gas Supply to Furnace


Though the furnace turns on, you may notice that the heating elements such as burners, electric coils, or heat pump compressor does not come on as well.  This can happen due to a loss of electric power to said elements due to tripped circuit breakers, failed heating elements, broken relays, or a faulty control board.  Or in the case of gas furnaces, it could be a loss of gas supply or faulty ignitor (the device that lights the gas coming out of the burner when the t-stat calls for heat, and again, faulty wiring or broken control board.


    1. Check circuit breakers. Make sure all breakers that feed the furnace and any outdoor components are ON.  For gas heating, assure that your gas supply is okay by checking that your other gas appliances are still working and that no one has mistakenly closed the gas valve that feeds the furnace.  Also, are you up to date on your gas bill? If breakers keep tripping, then seek help from a qualified electrician or HVAC technician.
    2. Check connections in furnace. Most furnaces have at least a couple cables that plug into other cables, to control boards, or relays.  So make sure that these are all tight and properly seated, because one loose connection can prevent heat from activating.
    3. Make sure gas burners are clean.  It’s a useful maintenance practice to clean the burner jets yearly, as some being clogged can cause the protective circuits to prevent the gas valve that feeds the burner from opening.

Furnace Not Kicking On When Temp Drops: Thermostat Problems

1. Thermostat Wrongly Wired

Picture of needle nose pliers being used to straighten the ends of thermostat wires before connecting them to the new wall plate terminals. Furnace Not Kicking On When Temp Drops.
Use needle nose pliers to straighten thermostat wires before connecting to new wall plate terminals.


If the furnace stopped kicking on when you installed a new thermostat, you may have made mistakes in how you wired it. Perhaps you’ve reversed the W and Y wires. This would run the cooling stage though the thermostat is actually calling for heat. Other symptoms may appear too. But these depend on which wires and how many of them you have connected to the wrong t-stat terminals.


Check for correct wiring at both ends of the thermostat cable. At the thermostat, attach all wires to the right terminals in the wall plate. Then, do the same at the furnace end. Get help from a professional heating system repairman if you feel insecure matching these wires up with the right terminals.

2. Broken Thermostat Wiring 

Picture of the Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected.
Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected.


The solid copper wires in the thermostat cable can break if flexed too much. Or, perhaps the installer stapled the cable with excessive pressure or stapler misalignment, and unknowingly nicked the cable.

This issue normally shows up during new thermostat testing. But sometimes, it might not surface for years. Plus, settling of wall studs can flex these wires that installers often fasten to them. So, after decades, this flexing can wear wire coverings, causing faulty furnace heating operation.


Replace the cable between the thermostat and furnace. Avoid tightly stapling the wire, and don’t fasten it more than you must for a secure installation. Lay it loosely.

3. Thermostat Temp Set Too Low

Picture of an installed and operating RTH7600D Programmable Thermostat by Honeywell.
Honeywell Programmable Thermostat RTH7600D


You can set thermostats way lower than current room temperature, and this would cause the furnace not kicking on when the indoor temp drops. E.g. You might set the heat temperature to 65 degrees, when the room is already 70 degrees. In this case, the furnace would not kick on, even if the room temperature were to fall another few degrees.


    1. Raise t-stat temp setting.  Try setting the thermostat to a temperature above the current room temp.


As should be clear, the Honeywell thermostat not heating up problem in homes has many causes. Plus, most of them lie beyond the thermostat. A poorly insulated home, a furnace that’s too small, leaky windows and doors also contribute. All of these make the HVAC system work too hard to heat your home. In many homes there’s little coolness to spare due to poor HVAC sizing. Sadly, the fix for this often costs a lot. Why? Because it means replacing windows and doors, or adding more insulation. Or you may need to upgrade your furnace or add more supplemental heaters to your home.

But we hope that now, you know what might cause the Honeywell thermostat not heating up your home issue. Knowing the causes often helps in choosing the right fixes. Even though the best fix may cost the most, at least now, you have the data in front of you. Good luck, and choose the fixes wisely.

Related Posts to Furnace Not Kicking On When Temp Drops

    1. Honeywell Thermostat Not Reaching Set Temperature Troubleshooting
    2. How to Calibrate Honeywell Thermostat Temperature, RTH9580WF

References for Furnace Not Kicking On When Temp Drops

    1. Honeywell Thermostat Troubleshooting – A Six Step Guide, from All Things HVAC

Revision History

    • 2023-02-16: First published.