Picture of a Honeywell thermostat not reaching set temperature, with the set temperature at 83 degrees while the current temperature Is 77 degrees.

Honeywell Thermostat Not Reaching Set Temperature Troubleshooting




A Honeywell thermostat not reaching set temperature can occur for many reasons.   For example, poor thermostat placement, to a broken heating / cooling system.  Here, we discuss some of the reasons why this happens.  We then offer troubleshooting tips and fixes to solve these issues.  First, we look at what problems happen in the thermostat itself.  Then we go over the furnace / HVAC issues that can also cause too little heating and cooling.



Possible Reasons for the Honeywell Thermostat not Reaching Set Temperature

Thermostat Frozen

Problem

Now and then with smart thermostats, a snap of static electricity near the thermostat, can freeze the touch screen.  Also, a power surge in the home’s electrical system can do the same.  Then too, the data shown onscreen no longer updates.  Further, the heating or cooling may stay on or off. No matter how far from set temperature the room temperature is, the HVAC system may run and run and run.   Or, it may not come on at all.

Solutions

When Honeywell thermostat not reaching set temperature happens, reboot the thermostat.  Turn off power to the thermostat for ten to twenty seconds. Then turn it back on.  This is easy with many smart models because they can detach from furnace power. Simply unsnap them from their wall plates.  Then, wait a few seconds,  and finally re-attach them to reboot.  Or, if your thermostat does not come off of the wall easily, cycle it at the breaker box.  You can cycle main power to the whole furnace system by switching off and on the breakers that feed it. Doing this power cycles the thermostat as well, and thus, reboots it.

Honeywell thermostat not reaching set temperature. Picture of the Honeywell RTH9580WF wireless thermostat, displaying its -Home- screen. We set the temperature set to 69 degrees, and placed the thermostat into temporary hold mode until 10:00 PM.
Honeywell thermostat not reaching set temperature. The RTH9580WF wireless thermostat, displaying its -Home- screen. We set the temperature to 69 degrees, and placed this thermostat into temporary hold mode until 10:00 PM.

Thermostat Lost Power

Problem

The screen on smart thermostats looks blank or dark.  No readings or status messages show up.  Further, the furnace and / or condenser does not run.  Plus, you hear no humming at the furnace.  There may be no indication of power loss on today’s non programmable models, except that their backlights aren’t working.  But the effect is still the same.  That is, that the HVAC system does not function at all, no matter what the thermostat setting.

Fixes

Honeywell thermostats get power from the furnace / HVAC system usually. So, check that all parts of that system have power.  Often there’s one circuit breaker that feeds the furnace itself, and another that delivers power to the outside unit.  So check that none of these has tripped off.

Check that the heat works.  The heating and cooling subsystems often get power from separate circuit breakers. So, the thermostat could get its power from either one, although usually not both.  A power loss in either system could cause Honeywell thermostat not reaching set temperature.  By checking that the heat works, you’ve ruled out this cause of thermostat power loss.

Also, check that the cooling works as you did with the heat above.  Your thermostat may get power from the cooling subsystem (Rc wire).  A downed cooling system may therefore, also take the thermostat offline.  If so, check for and if necessary, restore power to the cooling system to restore the thermostat to working order.




Thermostat Not Calibrated Correctly Might Cause Honeywell Thermostat Not Reaching Set Temperature

Problem

The current temperature reading on the thermostat may not show actual room temperature.  Perhaps the thermostat reads low for example. It may read 72 degrees. But the actual room temperature is 76 degrees. You have problems in this case.  Indeed, you may see the set temperature never hit, as the room is much warmer than what the thermostat says. Plus, your furnace may have hit capacity.  So, it’s not able to raise the room temperature higher.

The reverse could happen with the cooling system.  In that case, the thermostat reads high.  That is, it reads 76 degrees when the actual room temperature is 72 degrees. Here again, the cooling system may be pumping out all the cool air it can.  So, it cannot lower room temperature further.  Thus, the thermostat never reaches its set temperature.

Fixes

Calibrate your thermostat on those models that allow this.  To do that, buy an accurate tube-style thermometer. Then, place it near the thermostat.  Next, wait for a half hour for the readings to stabilize.  Then, note the difference in reading between the thermometer and the thermostat reading.  In the 72 degrees Vs. 76 degrees example above, the difference is four degrees.  If the thermostat reads four degrees warmer than the thermometer, then adjust thermostat calibration four degrees in the negative.  If the thermostat reads four degrees cooler, then adjust the reading four degrees in the positive.




Thermostat Incorrectly Wired

Problem

If the problem occurred when you installed a new thermostat, you may have wrongly wired the thermostat.  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 arise too.  This depends on which wires and how many of them connect 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 uncomfortable ringing out these wires.

Honeywell thermostat not reaching set temperature: Picture of the screen of the Honeywell RTH8580WF thermostat, in System mode.
Honeywell thermostat not reaching set temperature: Screen display of the Honeywell RTH8580WF thermostat, in System mode.

Thermostat Not Level May Cause Honeywell Thermostat Not Reaching Set Temperature

Problem

Poor leveling on an older thermostat can affect how well it responds to room temperature changes.  Those models relied on a bimetal spring to sense temperature.  They attached a mercury switch to the end of that spring.  Then, as room temperature rose and fell, the spring changed size.  This tipped the mercury switch on or off to bring room temperature near the thermostat set temperature.

Proper leveling was essential for this tipping to happen correctly.  Indeed, if tilted too far off of level, the furnace might run all the time, or never at all.  The mercury inside does not make or break the contacts  as it should.

Fixes

Check that your thermostat is level, and level it if not.  If it’s not too far out of level, try loosening the screws that hold it against the wall. Then, tilt it toward level.  Finally, while holding the thermostat in that position, tighten the screws back up again.

But if the thermostat is too far from level, you may have to drill new mounting holes.  In that case, remove the screws and wires if necessary. Then reposition the thermostat over the new holes.  Then, drive the screws into those new holes.  We suggest putting wall anchors into these holes for a stronger hold.

If you still have a mercury thermostat, it may be time to replace it with a current model.  Today’s thermostats have more accurate sensors and more features.  Plus, these newer ones work well, no matter how far off of level you install them.



Honeywell Thermostat Not Showing Correct Temperature

Problem

Your living quarters may reach the set temperature.  But, the thermostat does not show that temperature.  This can happen you place the thermostat on an outside wall. Too much outside temperature affects the thermostat reading inside.  Or, the thermostat  might be on a wall shared with the furnace / utility room.  In this case, the furnace heat can also affect the thermostat negatively.  Or, the sun is shining on it, making it read too high.  For these situations, someone installed your thermostat in a poor location.

Fixes

Always place thermostats on an inside wall.  This wall should be far from heating / cooling sources.  So, move your thermostat to a better spot.  If you must have it on an outside wall, be sure to well insulate that wall.




Broken Thermostat Wiring can Cause Honeywell Thermostat Not Reaching Set Temperature

Problem

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

This problem would normally show up during testing of a new thermostat.  But it often does not surface for years.  Plus, settling of wall studs can flex these wires that installers often fasten to them.  After decades, this flexing can erode wire coverings, causing faulty HVAC operation.

Fixes

Replace the cable between the thermostat and HVAC unit.  Avoid tightly stapling the wire, and don’t fasten it more than necessary.  Lay it loosely.

Thermostat Temperature Offset Too Large

Problem

Let’s say that you set the thermostat to 75 degrees. Plus, the furnace can heat the house to a max of 78 when running full tilt.  Further, let’s say that you set your temperature offset to -5 degrees.  In that case, your thermostat may try to heat your house to 80 degrees.  Not the displayed set temperature of 75 degrees.  So, an extreme temperature offset can look like a Honeywell thermostat not reaching set temperature issue.  But it isn’t here.

Fixes

Adjust the offset temperature.  This helps assure that your set temperature does not exceed your HVAC capacity.



Thermostat Temperature Set Too High or Too Low

Problem

You can set thermostats way higher or lower than the HVAC system can deliver.  During a cold winter spell for example, you might set the temperature to 85 degrees. But your furnace can deliver only enough BTUs to heat to 78 degrees.  Or, during a summer heatwave, you might set the temperature to 72 degrees.  Yet your cooling system may be too small to cool your living space down to 72. Perhaps it can only hit 75 degrees.

Fixes

Try setting the thermostat to a realistic temperature.  A realistic temperature is one that your HVAC system can deliver.  If that setting is not comfortable, then your HVAC may be to blame.  So, check it for proper function as discussed elsewhere in this post. You could also add more heating / cooling units around the house.  Or, upgrade the central HVAC unit to a larger size.




Furnace / HVAC Issues can Cause Honeywell Thermostat Not Reaching Set Temperature

Clogged Furnace Filters and Fan Blades

Problem

Dirty air filters can restrict airflow though the HVAC unit and throughout the home. Clogged filters reduce how much the system can heat and cool.  How so?  Dirty fan blades move air less well.  Plus, they put out more noise, and also, again, reduce your furnace’s heating and cooling ability.

Fixes

Replace furnace filters monthly. Plus, hire a pro HVAC technician yearly, to check your system.  He should clean out dust buildup inside.  Also, have him remove blockages in the air hander, inspect the control units for damage, and so on.

Dirty Heat Pump Coils can Trigger Honeywell Thermostat Not Reaching Set Temperature

Problem

Dirty heat pump coils, again, slow airflow through the condenser outside, and the evaporator unit inside.  Thus, dirt can interfere with the system’s  efficiency.  Dirty coils lowers the BTUs per hour that the heat pump can pump. That could mean that the system cannot keep your quarters as warm / cool as your thermostat setting calls for.

Fixes

These coils have closely spaced metal fins.    Have a professional clean these with steam.  Or, try vacuuming them with a household sweeper that has a  hose and brush.  Do not bend the fins though, as they’re delicate. Bending them too much can forever restrict airflow and thus, reduce your system’s heating / cooling abilities.



Furnace Ran Out of Fuel

Problem

If the thermostat calls for heat, yet you’re getting none, you might be out of fuel.

Fixes

So, check your oil and propane gas tanks.  Are they empty?  Or, perhaps the gas company cut your service.  In that case, pay your bill and get it back on, ASAP.

Have tanks filled.  Check that your gas is on.  Try another gas appliance in your home, such as a water heater or stove.  If it’s working, then you have gas.

Evaporator Coils Frosted Up

Problem

On humid days, when an HVAC runs most, frost buildup can occur.  This happens either in the furnace on warm days, or in the outside unit on cold days.  So, why should you care?  Because frost lowers airflow through the coil fins. Thus, this ice interferes with the necessary heat exchanging that allows proper heating and cooling.  No matter where the frosting happens, the effect is the same; reduced heating and cooling function.  Frost-up thus, can cause the Honeywell thermostat not reaching set temperature problem.  Frosting makes your HVAC unit work harder to provide the same amount of heating / cooling.  This means higher energy bills for less comfort.

Fixes

Turn your HVAC system off for a half hour to an hour.  This allows the frost to melt away, restoring proper operation to the system.



Extreme Temps Outside Often Cause Honeywell Thermostat Not Reaching Set Temperature

Problem

It’s common for HVAC’s to fail to keep up when the weather outside is extreme.  They might run constantly but still not 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 HVAC system is too small to overcome the entering heat or cold.

Fixes

Close all windows and drapes.

Switch heat pump systems over to emergency heat on those very cold winter days.  This turns on the electric heaters in the air handler. These often produce hotter air output than when just the heat pump runs by itself.

But doing this consumes more electricity.  So, it might rise your heating bills.  But you will likely feel greater comfort too.

If you have a multi stage cooling system, check that both stages are working properly.

If your windows are old or are single-pane, try upgrading to at least double-pane. We prefer triple-pane, as these offer greater insulating.

One Stage Not Working in Multistage Systems

Problem

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

Fixes

If your system fails to reach the set temperature when heating, check the following.  All stages in the heating system should work well.  Stage one comes on first. This happens when room temperature falls below the set point by half a degree or so.  If it does not, figure out why.  Then, the second heating stage, if present, should fire up when room temp falls a few degrees below set temperature.  Again, if it does not, this may be why you are seeing your Honeywell thermostat not reaching set temperature.




If you see the Honeywell thermostat not reaching set temperature when cooling, check that your compressors are working.  Check all the stages in the cooling system, as some have more than one.  For multi stage cooling, stage one should come on first. As soon as room temperature rises above the set point by half a degree or so, you should hear it.  If it does not, find out why.  Then, the second stage should kick in when the room temp hits two or three degrees above set temperature.  Again, if it does not, this may be what’s causing your Honeywell thermostat not reaching set temperature issue.

For heat pump systems, a faulty or badly wired change over valve may also be to blame. In this case, your HVAC system may deliver air conditioning when the thermostat is really calling for heat.

Too Little Insulation or Too Much Outside Air Getting In

Problem

If your house lacks enough insulation, this worsens the Honeywell thermostat not reaching set temperature issue.  Indeed, much heat or coolness leaves through cracks around windows, walls, and doors.  Thus, the HVAC works harder to bring the house to the set temperature.  Indeed, there may be many BTUs leaking out in this way.  So many, that the HVAC system cannot backfill.  It cannot thus, maintain the set temperature. So, your thermostat never reaches that set temperature.

Fixes

Find the most leaky areas of your home.  Do that by walking around inside the house, feeling for hot or cold spots.  Look for places where cold air comes in.  Then, seal with the right materials.  Do the same on hot days, looking for places where excess warm, humid air comes in.  Be sure that the louvers on any exhaust fans you have, close completely when the fan is off.

Replace faulty windows and doors.  Again, single pane windows are huge energy hogs.  We suggest replacing these with at least double pane models.  Also, don’t forget about the basement.  We’ve found that replacing our old single pane basement windows with glass block units helped a lot.  These new windows took a big load off of our furnace.   Indeed, the set temperature grew much easier to maintain over a wide range of set values. Basement glass block windows, in our case indeed lessened our Honeywell thermostat not reaching set temperature problem.

Honeywell thermostat not reaching set temperature: Picture of the Honeywell RTH9580WF smart thermostat, front view after setup.
Honeywell thermostat not reaching set temperature: The RTH9580WF smart thermostat, front view after setup.

Improperly Sized or Worn Out HVAC / Furnace System is Another Reason for Honeywell Thermostat Not Reaching Set Temperature

Problem

Sometimes, home builders cut corners to save money when figuring HVAC unit size.  They want a cheaper unit, one that might be too small thus, to regulate the temperature well.  So, on very hot or cold days, you might see the Honeywell thermostat not reaching set temperature problem show up.

Also, over time with heat pumps especially, their compressors lose efficiency.  Why does this happen?  Well, either because their valves are starting to leak. Or it occurs due to refrigerant leaking out through small holes in the piping.

Fixes

Have an HVAC tech check refrigerant pressure in your system. That person should also read the head and tail pressures while the compressor runs.  Find the right values for all of these on a sticker somewhere inside the HVAC unit.  Have him add refrigerant if these read below optimal values.

But if the head / tail pressure difference is too low, then you have major problems.  This means that the change over valve is leaky, or compressor valves are inefficient.  The changeover valve might be cheap enough to replace. So too would the compressor.  This repair may be worth it as long as the system is less than fifteen to twenty years old.  If it is older than that though, then you should replace the whole system.




Conclusion

As should be clear, the Honeywell thermostat not reaching set temperature problem has many causes.  Plus, most of them lie beyond the thermostat itself.  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 heard.  In many homes there’s little heat or coolness to spare due to poor sizing of these units.  Sadly, the fix for this often costs a lot.  Why?  Because it involves replacing windows and doors, or adding more insulation. Or you may need to upgrade your furnace or add more heating and air conditioning units.

But we hope that now, you know what might cause the Honeywell thermostat not reaching set temperature 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 information in front of you.  Good luck, and choose the fixes wisely.

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References

Revision History

  • 2018-04-01: Revised the post content.
  • 2017-03-13: Originally published.