Picture of the Honeywell RTH9580WF thermostat, showing that the Heat is ON.

Thermostat Turns On Fan But Not Heat

You might notice that you have no heat even though the fan in your furnace is blowing when the thermostat calls for heat. This can happen for many reasons. Incorrect wiring, tripped circuit breakers, blown fuses, loss of gas supply, burned out heating elements, or a broken HVAC system could cause this. Here, we describe some of these causes, and then offer troubleshooting procedures and repair hints to resolve them.

Warning !

Some of the procedures below are highly dangerous if you don’t know what you’re doing.  So if you’re not comfortable working with high voltages or natural gas, then please consult the appropriate professionals to solve your heating problem for you.

Why Thermostat Turns On Fan But Not Heat 

Note that the thermostat itself is rarely (if ever) the cause of this particular symptom.  So we won’t be checking it out very much here.

1. Dirty Filters


The dust filter in furnaces often accumulates so much dust and debris that little air can pass through it. This in turn can trigger protection circuits, that shut off the heat source because there’s not enough air circulation to prevent component overheating. Thus, you might see the blower run, but only get cold air when you should be feeling warm air.


    1. Clean or replace filter. For units with removable filters that you slide in and out, buy a new one, available at most home improvement centers.  If you have an electric air cleaner then remove and hose down all its pieces, including the fins and coarse dirt traps.  Then let them dry thoroughly before switching on the cleaner again.
Picture of a Siemens indoor load center circuit breaker box, front view, with door open.
A Siemens indoor load center circuit breaker box, front view, with door open.

2. 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.

3. Dirty or Frosted Heat Pump Coils 


Dirty heat pump coils, 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. In extreme cases, this feels like there’s not any heat at all being produced.


    1. Clean out evaporator and condenser coils. 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. 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.

4. Extremely Cold Temperatures Outside Make it Seem Like There’s Not Heat When Thermostat Turns On the Fan


It’s common for furnaces to fall behind when the weather outside is very chilly.  They might run constantly yet still fail to keep up. Now there may be nothing wrong with the t-stat per se, or the furnace either for that matter. The issue might instead be that your central heating system is too small to overcome the entering cold from outside. Again, a very cold day can make it feel inside like there’s not heat.


    1. Close all windows and drapes, especially during the night.
    2. Check multiple heating stage operation.  If you have a two- or three-stage heat, check that all stages are working as they should.
    3. Replace windows. If your windows are old or are single-pane, try upgrading to at least double-pane. We like triple-pane.  Why?  Because these offer greater insulating. Thus, they keep the heat inside better.

5. One Stage Not Working in Multistage Systems


You may be getting SOME heating, but not enough to hold room temperature at the desired value.


    1. Check for proper compressor running. If you see the thermostat turn on yet not the heat, check that your compressors are working.
    2. Check changeover valve in so equipped systems.  For heat pump systems, a faulty or badly wired changeover valve may also cause weak heating. In this case, your HVAC system may deliver chilled air though the thermostat is really calling for heat.


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 Thermostat Turns On Fan But Not Heat

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

References for Thermostat Turns On Fan But Not Heat

    1. Furnace Repair: 19 Common Furnace Problems (And How to Fix Them) from CentralHtg.com

Revision History

    • 2023-02-16: First published.