Picture of the Honeywell RTH9580WF Smart Thermostat, front view after initial setup.

Honeywell Thermostat Not Cooling Down, How to Fix




A Honeywell thermostat not cooling Down can happen for lots of reasons. For example, poor thermostat placement, to a broken cooling system. Here, we discuss some of these reasons. We then offer troubleshooting methods and fixes to solve these cooling issues. First, we examine what problems happen in the thermostat itself. Then we talk about the furnace / HVAC issues that can also cause not enough cooling.



Possible Reasons for the Honeywell Thermostat not Cooling Down the House

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 cooling system may stay off. No matter how far from set temperature the room temperature is, the HVAC system may not come on at all.

Solutions

When Honeywell thermostat not cooling down 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.

Thermostat Lost Power

Problem

The screen on smart thermostats looks blank or dark. No readings or status messages appear. Further, the condenser does not run. Plus, you hear no humming at the HVAC unit. There may be no indication of power loss on today’s non programmable thermostat models, except that their backlights don’t light up. But the effect is still the same. That is, that the HVAC system cools not 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 out.

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 cooling down the house. By checking that the heat works, you’ve ruled out this cause of thermostat power loss.  In this way, a faulty heating system can make the cooling system malfunction as well.

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 down. 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 Cooling Down the House

Problem

The current temperature reading on the thermostat may not show actual room temperature. Think about your cooling system. 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 cools down your home as much as you’d like.

Fixes

Calibrate your thermostat on those models that support this. To do that, buy an accurate 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 cooling problem happened when you installed a new thermostat, you may have wrongly wired it. Perhaps you’ve reversed the W and Y wires. This would run the heating stage though the thermostat is actually calling for air conditioning. 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 matching these wires up with the right terminals.

Honeywell thermostat not cooling down: Picture of the screen of the Honeywell RTH8580WF thermostat, in System mode.
Honeywell thermostat not cooling down: Screen display of the Honeywell RTH8580WF thermostat, in System mode.

Thermostat Not Level May Cause Honeywell Thermostat Not Cooling Down your House

Problem

Poor leveling on older thermostats 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 needed for this tipping to occur as designed. Indeed, if tilted too far off of level, the cooling system might never cool at all. The mercury inside does not make or break the contacts as it should.

Fixes

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

But if the thermostat is too far from level, drill new mounting holes. To do that, remove the screws and wires if they get in the way. Then reposition the thermostat over the new holes. Finally, 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.  They’ll cool the house in any position.



Honeywell Thermostat Not Showing Correct Temperature

Problem

Your living quarters may cool down to the temperature you desire. 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 HVAC / utility room. In this case, cooled air from the HVAC can also affect the thermostat. 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 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 Cooling Down Your Home

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 cable.

This problem normally shows up during new thermostat testing. But sometimes, it does 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 HVAC cooling 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 HVAC system can cool the house to a low of 72 when running full tilt. Further, let’s say that you set your temperature offset to +5 degrees. In that case, your thermostat thinks that the room is five degrees warmer than it actually is.  So, it may try to cool your house to 70 degrees, not 75 degrees. The room temperature here, never reaches the set temperature (75) because it cannot reach the actual temperature of 70 degrees.  So, an extreme temperature offset can look like a Honeywell thermostat not cooling down issue. But it isn’t here.

Fixes

Adjust the offset temperature. This helps assure that your set temperature does not exceed your HVAC capacity.  Avoid setting the temperature offset to more than a couple or three degrees.



Thermostat Temperature Set Too High or Too Low

Problem

You can set thermostats way lower than the HVAC system can deliver. During a hot spell for example, you might set the cool temperature to 74 degrees. But your air conditioning unit moves only enough BTUs to cool to 78 degrees when it’s so hot outside.

Fixes

Try setting the thermostat to a realistic temperature. A realistic temperature is one that your HVAC system can deliver based on current weather conditions. 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 cooling units around the house. Or, upgrade the central HVAC unit to a larger size.




HVAC Issues can Cause Honeywell Thermostat Not Cooling Down your House

Clogged HVAC Filters and Fan Blades

Problem

Dirty air filters can restrict airflow though the HVAC unit and around the home. Clogged filters reduce how much the system cools. How so? Dirty filters and fan blades move air less well. Plus, they make more noise, and also, again, reduce your HVAC’s cooling ability.

Fixes

Replace air conditioner 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 cooling air hander, inspect the control units for damage, and so on.

Dirty Heat Pump Coils can Trigger Honeywell Thermostat Not Cooling Down your House

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 a heat pump can pump. That could mean that the system cannot keep your quarters as cool as your thermostat setting calls for.

Fixes

These coils have closely spaced metal fins. Have a pro 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 cooling airflow and thus, reduce how well your HVAC system cools your house.



 

Evaporator Coils Frosted Up

Problem

On humid days, when an air conditioning unit runs most, frost buildup can occur. This happens either in the inside unit. So, why care? Because frost lowers airflow through these coils. Thus, this ice interferes with the necessary heat exchanging that allows good cooling. No matter where the frosting happens, the effect is the same; reduced cooling. Frost-up thus, can cause the Honeywell thermostat not cooling down problem. Frosting makes your HVAC unit work harder to provide the same amount of 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 Cooling Down your House

Problem

It’s common for HVAC’s to fail to keep up when the weather outside is very warm. They might run constantly but 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 air conditioner either. The issue might be that your HVAC system is too small to overcome the entering heat from outside.

Fixes

Close all windows and drapes, especially during the day.

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.  They keep the coolness inside better thus.

One Stage Not Working in Multistage Systems

Problem

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

Fixes

If you see the Honeywell thermostat not cooling down, 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 start up. 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 cooling down your house.



For heat pump systems, a faulty or badly wired changeover valve may also be the culprit. In this case, your HVAC system may deliver heat when the thermostat is really calling for cooling.

Too Little Insulation or Too Much Outside Air Getting In

Problem

If your house lacks enough insulation, this worsens the Honeywell thermostat not cooling down issue. Indeed, much 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 in 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 cool temperature.

Fixes

Find the most leaky areas of your home. Do that by walking around inside the house, feeling for hot spots. Look for places where warm air comes in. Then, seal with the calking, hardening foams, and other materials. 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 HVAC unit. Indeed, the set temperature grew much easier to maintain over a wide range of cooling values. Basement glass block windows, in our case indeed lessened our Honeywell thermostat not cooling down problem.

Honeywell thermostat not cooling down: Picture of the Honeywell RTH9580WF smart thermostat, front view after setup.
Honeywell thermostat not cooling down: The RTH9580WF smart thermostat, front view after setup.

Improperly Sized or Worn Out HVAC / Furnace System is Another Reason for Honeywell Thermostat Not Cooling Down your Home

Problem

Sometimes, home builders cut corners to save money when figuring HVAC unit size. They want the cheapest unit they can get by with, one that might be too small thus, to control the cooling temperature well. So, on very hot days, you might see the Honeywell thermostat not cooling down problem show up.

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

Fixes

Have an HVAC tech check refrigerant pressure. She 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 her add refrigerant if these read below recommended values.

But if the head / tail pressure difference is too small, 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 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.




Conclusion

As should be clear, the Honeywell thermostat not cooling down problem 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 cool 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 air conditioning unit or add more of these devices.

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

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References

Revision History

  • 2018-04-02: Originally published.