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

Thermostat Reads Higher than Actual Temperature, How to Fix

When a thermostat reads higher than actual temperature, your home can get too cold in the winter.  It can also become  too warm in summer.  Now At least several factors affect the thermostat temperature reading besides actual room temperature.  These include aging components drifting out of tolerance in older thermostats.  Or, an improperly positioned thermostat might read incorrectly. Also, broken electronics inside the thermostat or lopsided thermostat installation (not level), cause faulty readings.  So here, we discuss some of these problems and offer suggestions for fixing them, to address this annoying and potentially costly situation.

Why Thermostat Reads Higher than Actual Temperature and How to Fix

We offer these causes and fixes in  order of severity.  That is, the most involved or costly problems appear near the bottom of this list.  The simpler adjustments and cost-free tweaks appear near the top.

First, Verify that your Thermostat is Indeed Reading Incorrectly

To start, obtain an accurate thermometer.  Then, place it as close to the thermostat as practical.  This assures that it’s reading the same air temperature that the thermostat sees.  We use an inexpensive refrigerator thermometer for this purpose, as shown next.

However, for the absolute BEST results, get a thermometer that reads in the 70-degree range midway up its scale.  Also, be sure it’s big enough for you to easily read.  You want to be far enough away from it so that your warm breath does not impact the temperature readings.

Picture of a Honeywell smart thermostat alongside a reference thermometer. This makes it easy to compare the temperature your thermostat reads with the reading from a known-good thermometer nearby.
A Honeywell smart thermostat alongside a reference thermometer. This makes it easy to compare the temperature your thermostat reads with the reading from a known-good thermometer nearby. Thermostat Reads Higher than Actual Temperature.

Now to the causes and fixes of incorrect thermostat temperature readings.

1. Assure that Thermostat Vents are Not Blocked by Dust, Lint, Curtains, Furniture

Picture of the Honeywell RTH9580WF Internet Thermostat, being snapped onto wall plate.
Honeywell RTH9580WF Internet Thermostat, being snapped onto wall plate. Thermostat Reads Higher than Actual Temperature.

This is of big concern in modern day thermostat models, that generate some heat from the microprocessors, memory, and other computer components inside.  If the vents are blocked by dust, furniture, Etc., the thermostat can grow extra warm inside, where the temperature sensor is found.  Thus, this vent blocking can cause higher-than-normal temperature readings.

The solution is to keep your vents open.  Vacuum the thermostat once per season, or more often if your thermostat is in a dusty, “linty” location.  Or, remove the cover and gently blow out the inside with canned air.  Or, if you don’t have canned air, use a soft, fine bristled brush and GENTLY brush out the dirt.  Note however, that some thermostats have no vents on the sides. For these, take them off the wall, and dust out in back of the unit.

2. Thermostat May Need Calibrated

Picture of the Honeywell RTH9580WF thermostat, displaying its -Indoor Display Offsets- screen with a minus 1 degree temperature offset highlighted.
Honeywell RTH9580WF thermostat, displaying its -Indoor Display Offsets- screen with a minus 1 degree temperature offset highlighted. Thermostat Reads Higher than Actual Temperature.

See our post  How to Calibrate Honeywell Thermostat Temperature RTH9580WF  for details.  This explains how to assure that your thermostat is adjusted correctly for reading the right room temperature, for a typical digital smart thermostat.  A temperature offset that is too far in the negative can make the thermostat read a few degrees higher than the actual temperature in the room.

Follow the instructions in your thermostat’s manual to adjust its display offset temperature reading.

3. Thermostat Could Need Leveling

Picture of the Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected.
Honeywell Smart Thermostat RTH9580WF wall plate, mounted, with wires connected. Thermostat Reads Higher than Actual Temperature.

While leveling is not as important with today’s digital thermostats, we suggest  that you ensure that your thermostat is indeed level, especially if yours is reading room temperature incorrectly.  Not only does leveling the thermostat improve its cosmetic appearance.  It also maximizes airflow through the thermostat, helping to ensure accurate temp readings.

If its manual says to level it, then by all means, make sure the thermostat is level (horizontal and plum).  Sometimes you can get away with simply loosening its mounting screws enough to re position the unit. But if that doesn’t give you enough play to properly set it level, then don’t be afraid to take the unit off the wall and make new, more level screw holes for it.

4. Thermostat Temperature Sensor is Broken

All thermostats have some form of sensing device that detects the temperature near to the thermostat.  Now in today’s t-stats, a temperature sensitive resister such as a thermistor or varistor, can start to offer different resistance values for a given temperature.  This confuses the computer inside the thermostat. This misleads it into reading a higher than actual temp.

Older thermostats also have a sensor.  But this is usually a mechanical bi-metal spring attached to a reading gauge needle.  Some of these may offer a calibration adjustment.  But others, especially if they become bent due to tampering, are best just replaced.  Many of these mercury thermostats have two sensors.  One reads the temperature for viewing.  The other tips the mercury switch on and off as the room heats and cools. In these models, a faulty spring in the visual thermometer portion of the unit can cause exaggerating readings.

In any case though, if you have one of these mechanical thermostats, we highly recommend that you buy a newer, all-electronic model.

5. Thermostat is in Drafty, Extra Warm or Cool Location

Keep all heat or cold-producing appliances well away from your thermostat.  That includes microwave ovens, stoves, sunlamps, most fans, hair dryers, radios, — ANYTHING that produces heat!

Furthermore, do not allow sunlight or lamplight to shine directly on your thermostat, as these can make it read higher than actual room temperature. Keep candles and auxiliary heaters away!

Also, do not install a thermostat on any exterior walls or near windows, doors, as the outside climate will impact the accuracy of thermostat temperature readings.  Keep it away from heated pipes and ducts as well.

Hire an HVAC technician to relocate your thermostat to an interior wall that does not get any direct heating or cooling from the furnace / AC system that it controls.  Mount it approximately five feet above the floor

6. Faulty Wiring can Cause Excess Heat in and Around the Thermostat

Picture of the Honeywell Smart Thermostat RTH9580WF wall plate, mounted, but with wires not yet connected.
Honeywell Smart Thermostat RTH9580WF, wall plate mounted, but with wires not yet connected. Thermostat Reads Higher than Actual Temperature.

In rare cases, incorrectly wired thermostat installations have been known to cause higher than expected temperature readings.  This can occur if, for some reason, the wires inside the thermostat housing become warm due to excess current; often caused by a short circuit.

Check that the thermostat is cool to slightly warm to the touch.  It should not be very hot.  If it is, then call a qualified HVAC technician immediately to check the wiring between the HVAC unit and thermostat, for correct hook up.  Excess warmth in or around the thermostat could indicate a fire hazard, which obviously, you should take care as soon as possible.

7. Thermostat May Read Correctly But Furnace Running Too Much, or AC Running Too Little

It could be that though your thermostat appears to read too high a temperature, that it’s actually reading correctly, and your room actually IS warmer than you think.

For this issue, check that your heating is not running excessively.  If so, this could mean that your thermostat is broken in a different way than simply reading the wrong temperature.  Or, if in summer, perhaps your air conditioning is not running enough to cool your place down to the set temperature on the thermostat.  In either case, call in your trusty HVAC repairman to check out the system to see what might be the matter.

Conclusion

If a thermostat reads higher than actual temperature, don’t take it lightly.  Fix it right away.  Why?  Because this issue can seriously impact how comfortable you feel in your home.  How?  It can make you feel extra cold all year round.  And, especially in the summertime, your air conditioning may run excessively, raising your energy bills.  In winter, it can cause your furnace not to run enough, causing excessive coldness in your living quarters.  So do it up right, and make sure all is well with your thermostat.  Check that it’s reading the right temperature at least once per year, keep it clean and level, and free of any blockages around its vents.

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References for Thermostat Reads Higher than Actual Temperature

Revision History

  • 2019-04-15: Shortened post URL and tweaked key phrase targeting.
  • 2019-02-05: First published.