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

Thermostat Says Heat On But Furnace Not Running

You might see your thermostat that says that the heat is on, and yet you furnace is not running. This can happen for many reasons. A broken thermostat, incorrect wiring, tripped circuit breakers, dirty filters, blown fuses, or a broken HVAC system could cause this. Here, we describe some of these causes, and then offer repair hints to correct them. First, we look at what might go wrong in the thermostat itself. Then we talk about the power and furnace issues that can also cause the heating not to function.





Thermostat Says Heat On but Furnace Not Running Causes

Thermostat Badly Wired

Picture of needle nose pliers being used to straighten the ends of thermostat wires before connecting them to the new wall plate terminals.
Use needle nose pliers to straighten thermostat wires before connecting to new wall plate terminals.

Problem

If the heat problem happened when you installed a new thermostat, you may have erred 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 heating. Other symptoms may arise too. These depend on which wires and how many of them you have connected to the wrong thermostat terminals.

Fixes

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 HVAC repairman if you feel insecure matching these wires up with the right terminals.





Broken Thermostat Wiring 

Picture of a thermostat wall plate, mounted, with wires connected.
Thermostat wall plate, mounted, with wires connected.

Problem

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

This issue normally shows up during new thermostat testing. But sometimes, it might not appear 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, and cause faulty furnace operation.

Solutions

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.





Thermostat Temperature Set Too High or Too Low

Picture of an installed and operating programmable thermostat.
A typical programmable thermostat.

Problem

You can set thermostats way lower than the heating system can deliver. During a cold time outside for example, you might set the heat temperature to 74 degrees. But your furnace moves only enough BTUs to heat to 72 degrees. On particularly cold days, it may feel like there’s no heat output, even though the furnace is operating normally. In this case, the furnace is indeed running.  But it’s not producing enough heat to overcome the extreme cold.

Fixes

    1. Try setting the thermostat to a realistic temperature for your furnace. A reasonable temperature is one that your home heaters can reach based on current weather conditions. If that setting does not feel comfortable, then your furnace may be at fault, or simply too small for the job.
    2. So, check your furnace for proper working as discussed elsewhere in this piece, and fix as needed.

If you do these things but still want higher temps in the home, then try the following.

    1. Add more heaters around the house, such as electric baseboard heaters or portable space heaters.
    2. Or, upgrade the central heating unit to a larger size.  Discussed more in the next section.
    3. Add more insulation around your home.
    4. Install better sealing windows and doors can also raise the high temperature in your home that your furnace can reach.
    5. Upgrade windows and doors to models with higher R factors.




Thermostat Says Heat On but Furnace Not Running: Furnace Issues 

Clogged Furnace Filters and Fan Blades

Problem

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.

Fixes

    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.

Wrongly Sized or Worn Furnace System can Cause Less than Desired Heat

Problem

Sometimes, home builders cut corners to cut costs when figuring furnace size. They want the cheapest furnace they can get by with.  Sadly though, these cheapo models are often too small to regulate the heating temperature well. So, on very frosty days, you might see the problem show up.

Also, over time with heat pumps especially, the compressors lose efficiency. Why? Because either their valves start leaking. Or the refrigerant leaks out through small holes in the piping.  As more refrigerant escapes, the less efficient the heating system becomes. In this case, the unit produces little or no heat even though it’s running.

Fixes

    1. Have an HVAC tech check refrigerant pressure and fill with enough refrigerant to bring this reading into line with what the HVAC system specs say it should be.   This value differs depending on the model of heating system you have.
    2. The tech should also read the head and tail pressures while the compressor runs. Find the right values for all of these specs on a sticker somewhere inside the furnace.  Then read your actual values.  Now if the ratio between head and tail pressure is too low, it is likely time to replace the compressor.
    3. Also, the changeover valve might be cheap enough to replace. So too would the compressor. This repair may be the right thing to do as long as the system is less than fifteen to twenty years old. But if it is older, then replace the whole system.




Dirty or Frosted Heat Pump Coils can Trigger the Thermostat Says Heat On but Furnace Not Running Issue

Problem

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. Clogged coils may cause overheating and trigger a furnace shutdown as well, even though the thermostat is calling for more heat.

Fixes

    1. These coils have closely spaced metal fins that often become plugged 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 can be quite delicate. Bending them too much can forever restrict airflow and thus, reduce how well your heating system (heat pump) heats your house.

Extremely Cold Temps Outside Can Cause the Furnace Not Running Problem

Problem

It’s common for furnaces to fail to keep up 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 thermostat per se. And, there might be nothing the matter with the furnace either. The issue might 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 the furnace is not running at all.

Fixes

    1. Close all windows and drapes, especially during the night.
    2. If you have a multi stage heating system, check that all heating stages are working as they should.
    3. 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.

One Stage Not Working in Multistage Systems

Problem

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

Fixes

    1. If you see notice your thermostat not heating up the home, check that your compressors are working.
    2. Check all the heating stages in the furnace (burners, elements, heat exchangers), as some have more than one. For multi stage heating, stage one should come on first. As soon as room temperature falls below the set point by half a degree or so, you should hear it start up. If it does not, find out why. Then, the second stage should kick in if the room temp hits two or three degrees less than set temperature. Again, if it does not, this may be what’s causing your thermostat to fail to start the furnace running.
    3. 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 asking for heat.

Too Little Insulation or Too Much Outside Air Getting In.

Problem

If your house lacks enough insulation, this worsens the furnace not heating issue. Indeed, much warmth leaves through cracks around windows, walls, and doors. Thus, the furnace works harder to bring the house up to the set temperature. Indeed, there may be many BTUs leaking in in this way. So many BTUs, that the central heating system cannot backfill. The furnace cannot thus, maintain the set temperature. So, your thermostat never reaches that set heating temperature.

Fixes

    1. Find the most leaky areas of your home. Do that by walking around inside the house, feeling for cold spots. Look for places where chilly air leaks in. Then, seal these spots with the calking, hardening foams, and other materials.
    2. Be sure that the louvers on any exhaust fans to the outside that you have, close completely when the fan is off.  These include bathroom or kitchen vent fans.
    3. Replace faulty windows and doors. Again, single pane windows are huge energy hogs. We suggest replacing these with at least double pane models.
    4. Also, remember the basement. We’ve found that replacing our old single pane basement windows with glass block units helped a lot. See our   How to Install Glass Block Windows   post for details on that project.
    5. These new windows took a big load off of our HVAC unit. Indeed, the set temperature grew much easier to maintain over a wide range of heating values. Basement glass block windows, in our case indeed lessened our Honeywell thermostat not heating up problem.




Conclusion

As should be clear, the problem of thermostat says heat on but furnace not running in homes has many causes. Plus, most of them lie beyond the thermostat. A poorly insulated home, faulty wiring, 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, the furnace itself, or adding more insulation. Or you may need to add more supplemental heaters to your home.

But we hope that now, you know what might trigger this 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.

Related Posts to Thermostat Says Heat On but Furnace Not Running

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    3. Thermostat Waiting for Equipment Message
    4. How to Set Thermostat Temperature Offset
    5. Programmable Thermostat Pros and Cons

References for Thermostat Says Heat On but Furnace Not Running

    1. Why Thermostats Don’t Start the Heat

Revision History

    • 2023-01-13: First published.
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