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 from improper placement of the thermostat to a malfunctioning heating / cooling system.  Here, we discuss the various reasons why this might occur, and offer troubleshooting tips and solutions to address these causes.  First, we look at what problems are likely to occur within and around the thermostat itself.  Then later, we summarize the sorts of furnace / HVAC issues that can also produce too little heating and cooling and prevent the thermostat set temperature from being achieved.

Possible Issues with the Thermostat

Thermostat Frozen

Problem

Occasionally, particularly in smart thermostats, a snap of static electricity in the winter time near the thermostat, or a power surge in the home’s electrical distribution system, can trigger a lockup in these little computer thermostats.  Their buttons and touchscreens become unresponsive to touches and presses, and the data displayed onscreen is no longer updated.

Solutions

When this happens, try rebooting the thermostat.  Remove power from the thermostat for ten to twenty seconds, and then reconnect.  Many smart models can be momentarily detached from furnace power by simply unsnapping them from their wall plates, and then re-attaching them after a short time.  Or, cycle main power to the entire furnace system, which power cycles the thermostat as well.

 

Thermostat Lost Power

Problem

The screen on smart thermostats may appear completely blank or dark, the furnace and / or condenser is not running, and you hear no humming at the furnace.  There may not be any indication on today’s non programmable models, except that their backlights aren’t working.

Solutions

Since, in many installations, Honeywell thermostats receive their operating power from the furnace / HVAC system, check that all components of the HVAC system have power.  Often there’s one circuit breaker that feeds the furnace itself, and another that delivers power to the outside condenser unit.  Be sure that none of these has tripped off.

Verify that the heat works.  The heating and cooling subsystems are often powered by separate circuit breakers, and the thermostat could be powered by either one of these, although not generally both.  A low of power in either one could result in the failure of the set temperature being reached.  By verifying that the heat works, you’ve eliminated this possible cause of thermostat loss of power.

Verify that the cooling works.  Ditto.  If your thermostat is powered from the cooling subsystem (Rc wire), then this system must be properly energized in order for the thermostat to operate.

 

Thermostat Not Calibrated Correctly

Problem

The current temperature reading on the thermostat may not be the actual room temperature.  If the thermostat is reading low (reads 72 degrees for example, when the actual room temperature is 76 degrees), then you may observe the set temperature not being reached, as the room is much warmer than what the thermostat indicates, and your furnace may have reached capacity and be unable to raise the room temperature any higher.  The same is true for cooling.  In that case, if the thermostat reads high (reads 76 degrees when the actual room temperature is 72 degrees), then the cooling system may, again, be pumping out all the cool air it can muster, and be unable to lower the actual room temperature any further.

Solutions

Calibrate your Honeywell thermostat on those models that allow this.  Buy an accurate tube-style thermometer, place it near the thermostat, wait for a half hour for the readings to stabilize, and then note the difference in reading between the thermometer and the thermostat readout.  In the 72 degrees Vs. 76 degrees example above, the difference is 4 degrees.  If the thermostat reads four degrees warmer than the thermometer, then the

 

Thermostat Incorrectly Wired

Problem

If the problem occurred when you installed a new thermostat, you may have incorrectly wired the thermostat.  Perhaps you’ve reversed the W and Y wires, which would cause the cooling stage to activate when the thermostat is actually calling for heat, or vice versa.

Solutions

Verify that the wiring at both ends of the thermostat cable is proper.  At the thermostat, check that the various colored wires are attached to the expected terminals.  Do the same at the furnace end as well.  Get help from a professional HVAC repairman if you’re uncomfortable ringing out these wires.

 

Thermostat Not Level

Problem

Properly leveling a thermostat can affect how accurately it measures and responds to room temperatures.  Particularly in the older style models that relied on a bimetal spring and mercury switch to sense and respond to temperature changes, proper leveling during installation is essential to correct thermostat operation.  If tilted too far up or down, the mercury inside the switch will not make or break the contacts inside properly, or it may not even work at all.

Solutions

Verify that your thermostat is level, and level it if it isn’t.  If it’s not too far out of level, you might be able to loosen the screws that hold it against the wall, tilt it toward level, and then tighten the screws again.  But if the thermostat is too far off from level, drilling one or more new holes may be necessary.  In that case, once you’ve made the new hole(s), remove the screws and wires if necessary, then reposition the thermostat over the new holes and drive the screws into the new holes.  We recommend wall anchors be placed into these holes for a more secure hold.

Of course, if you still have a mercury style thermostat, it may be time to replace it with a more accurate, fuller featured model; one that is not so sensitive to how level it is.

 

Honeywell Thermostat Not Showing Correct Temperature

Problem

Your living quarters may have actually reached the desired temperature.  However, the thermostat is not displaying the correct current temperature.  This can happen when thermostat is mounted on an exterior wall, and is therefore, too much influenced by the temperatures outside.  Or, it might be on a wall that is shared with the furnace / utility room.  Or, the sun is shining on it.  For these situations, your thermostat may have been improperly located.

Solutions

Thermostats should always be placed on an interior wall that does not contain or is close to the heating / cooling sources that they control.  So, consider moving your thermostat to a better location.  If you must have it on an exterior wall, that wall should be VERY WELL insulated.

 

Faulty Thermostat Wiring

Problem

While rare, the solid copper individual wires within the thermostat cable can break; particularly if flexed too often.  Or, perhaps the installed inadvertently 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.  However, it often does not surface for years after the installation.  Home settling and expansion and contraction of wall studs that these wires are often fastened to over the years, can eventually erode their insulation causing a short, or break them, causing some of the heating and cooling stages not to function.

Solutions

Replace the cable between the thermostat and HVAC unit.

 

Thermostat Temperature Offset Too Large

Problem

If the thermostat is set to 75 degrees for example, and the furnace can heat the house to a max of 78 when running full tilt, and your temperature offset is set to -5 degrees, then your thermostat may be calling for enough to heat your house to 80 degrees, not the displayed set temperature of 75 degrees.

Solutions

Adjust the offset temperature to assure that none of your typical set temperature preferences exceed the capacity of your heating / cooling system.

 

Thermostat Temperature Set Too High or Too Low

Problem

Thermostats can typically be set to temperatures way higher or lower, than the connected HVAC system is capability of delivering.  During a cold winter spell for example, you might be able to set the temperature to 85 degrees. But your furnace can deliver only enough BTUs to heat your living space to 78 degrees.  Or, during a summertime heatwave, you might set the temperature to 72 degrees.  Yet your cooling system may not be large enough to actually cool your living space down to 72, and might only be able to maintain a temperature of 75 degrees.

Solutions

Try setting the thermostat to a realistic temperature; one that your central heating / cooling system is capable of delivering.  If that temperature range is not comfortable, then check your heating / cooling system for proper operation as discussed elsewhere in this post, add supplemental heating or cooling units, or upgrade the furnace to a larger model.

 

Furnace / HVAC Possible Issues

Clogged Furnace Filters and Fan Blades

Problem

Dirty air filters can restrict airflow though the HVAC unit and throughout the home, which reduces the system’s heating and cooling capacity.  Dirty fan blades move air less efficiently, are noisy, and again, reduce your furnace’s ability to heat and cool your home.

Solutions

Replace furnace filters monthly, and hire a professional HVAC technician annually, to check your system for dust buildup inside, blockages in the air handling system, and so on.

 

Dirty Heat Pump Coils Outside and Inside

Problem

Dirty heat pump coils, again, restrict airflow through the condenser outside, and the evaporator unit inside.  Thus, dirt can interfere with the system’s heat exchanging efficiency.  This effectively reduces the BTUs per hour that the heat pump can pump, and that could mean that the system might not be able to keep your living environment as warm or as cool as your thermostat is calling for.

Solutions

These coil assemblies contain closely spaced metal fins which are best cleaned by a professional, with steam.  However, you can try vacuuming them with a household vacuum cleaner and hose with a brush.  Just be careful not to bend the fins; they’re rather delicate. Bending them too much can permanently restrict airflow and thus, permanently reduce your system’s heating / cooling abilities.

 

Furnace Ran Out of Fuel

Problem

If the thermostat reports that it is indeed calling for heat, yet you’re getting very little or no heat, check oil and propane gas tanks for adequate fuel supply.  Or, perhaps the gas company cut your service for some reason.

Solutions

Have tanks filled.  Verify that your gas is turned on, by checking another gas appliance in your home, such as a water heater or stove.

 

Evaporator Coils Frosted Up

Problem

Particularly on humid days when an HVAC system tends to run the most, frost buildup can occur either within the furnace on warm days (inside coils function as evaporator), or within the outside unit (where the compressor is) on cold days.  Frost constrains airflow through the coil fins and thus, interferes with the heat exchanging that must go on with these coils in order for the unit to heat and cool the inside of the house fully.  Regardless of where the frosting occurs, either in the inside or outside evaporator units, the effect is the same; drastically reduced heating and cooling function.  Frost-up can cause the thermostat not to reach the desired set temperature.

Solutions

Turn your HVAC system off for a half hour to an hour, to allow the frost to melt away.

 

Extreme Temperatures Outside

Problem

It’s common for furnaces and air conditioners to fail to keep up when the weather outside is voraciously hot or unusually cold, even when running constantly.  There may be nothing wrong with the thermostat per se, and there might be nothing the matter with the furnace.  The issue could just be that your HVAC system doesn’t produce enough heating or cooling to completely offset the heat or cold coming in from the outside.

Solutions

Close all windows and drapes.

Switch heat pump systems over to emergency heat on those very cold winter days.  This fully activates the electric heaters within the air handler, which often produces hotter air output than when just the heat pump is doing the heating.  This will use more electricity, and therefore, result in higher heating bills.  But you will likely experience greater comfort.

If you have a multistage cooling system, make sure that both stages are operating to maximize the cooling effect.

If your windows are old or of the single-pane variety, consider upgrading the most drafty ones to at least double-pane, and preferably triple-pane.

 

One Stage Not Working in Multistage Systems

Problem

You may be experiencing SOME heating or cooling, but not enough to maintain room temperature at the desired value.

Solutions

If your system is not reaching the correct temperature when heating, check that all the stages in the heating system are functioning fully.  Stage one typically should come on first, as soon as room temperature falls below the set point by half a degree or so.  If it does not, investigate why.  Then, the second heating stage (if your unit has one) should come on when the room temperature is two or three degrees less than the set temperature.  Again, if it does not, this may be why you are unable to achieve the set thermostat temperature.

If your system is not reaching the correct temperature when cooling, check that all the stages in the cooling system are functioning fully.  Stage one typically should come on first, as soon as room temperature rises above the set point by half a degree or so.  If it does not, investigate why.  Then, the second cooling stage (if your unit has one) should come on when the room temperature reaches two or three degrees higher than the set temperature.  Again, if it does not, this may be why you are unable to achieve the set thermostat temperature.

For heat pump systems, a malfunctioning or improperly wired changeover valve may also be to blame. In this case, your HVAC system may be delivering air conditioning when the thermostat is actually calling for heat.

 

Inadequate Insulation or Too Much Air Getting In from Outside

Problem

If your house doesn’t have enough insulation in its walls, doors, and windows, much of the heat or coolness that your HVAC system produces escapes through the cracks.  This means that it must work excessively hard in order to get the house to the desired set temperature.  Indeed, there may be so many BTUs leaking out of your house that the climate control system cannot backfill with enough to maintain the set temperature.

Solutions

Identify the most leaky areas of your home.  Walk around inside the house, feeling for hot or cold spots.  Look for places where cold air is entering, and seal with appropriate materials.  Do the same on hot days, looking for places where excess warm, humid air is coming in.  Be sure that the louvers on any exhaust fans you may have, close completely when the fan is not operating.

Replace faulty windows and doors.  Again, single pane windows are huge energy hogs.  We recommend replacing any of these with at least double pane models, and our preference is triple pane.  Also, don’t forget about the basement.  We’ve found that replacing our old single pane basement windows with glass block units really took a lot of load off of our furnace, and set temperature became much easier to maintain over a wide range of values.

 

Improperly Sized or Worn Out HVAC / Furnace System

Problem

Sometimes, home builders cut huge corners to save money when calculating the size of HVAC unit that a particular home needs for adequate heating and cooling.  They opt to install a unit that might not be big enough therefore, to regulate well the temperature.  So, particularly on very hot or very cold days, you may notice your furnace just doesn’t keep your place comfortably warm or cool.

Also, over time with heat pumps especially, their compressors lose efficiency either due to their valves beginning to leak, or due to the refrigerant leaking out through microscopic holes in the piping.

Solutions

Have an HVAC tech check the refrigerant pressure in your system, as well as the head and tail pressures while the compressor is operating.  The correct values for all of these are often found on a sticker somewhere inside the unit.  Have him add refrigerant if these are found to be low.

But if the head / tail pressure difference is too low, then this means that the changeover valve is leaky, or compressor valves are inefficient.  The changeover valve might be cheap enough to replace, and so too would the compressor as long as the system is not move than fifteen to twenty years old.  If it is, then your best bet is to replace the whole system with a current model.

 

References

Revision History

  • 2017-03-13: Originally published.