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

Thermostat Reads Higher than Actual Temperature

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 we discuss some of these problems, and offer suggestions for fixing them.

Why Thermostat Reads Higher than Actual Temperature: 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, Check 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 Free of Obstructions

Picture of attaching a typical thermostat to its wall plate.
Attaching a typical thermostat to its wall plate.

 

 

This is of big concern in modern day thermostat models. Why? Because these can generate some heat in their computer components.  Now the vents might have obstructions like dust, furniture, Etc. If so, then the thermostat can grow extra warm, Thus, it displays 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  for details.  This explains how to adjust your thermostat to correctly display room temperature.  A temperature offset that is too far negative can make the thermostat read higher than the actual temperature.





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

3. Thermostat Reads Higher than Actual Temperature Because it 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, still make sure that your thermostat is indeed level.  Not only does leveling improve  appearance.  It also promotes airflow through the thermostat. And this helps  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 not, then take the unit off the wall and make more level screw holes for it.

4. Thermostat Temperature Sensor is Defective

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 failing. This misleads the thermostat 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 it is best to replace these, as they aren’t as accurate as today’s digital models.  Many of these older thermostats have two sensors.  One reads the temperature for viewing.  The other tips a 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.

Finally, if you have a mechanical thermostat, you should consider an all-electronic model upgrade for best accuracy.

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 let sunlight or lamplight shine on it. These can make it read higher than actual room temperature. Keep candles and auxiliary heaters away as well !

Also, do not install a thermostat on any exterior walls or near windows or doors. Why not?  Because the outside climate can impact the accuracy of readings. Furthermore, keep it away from heated pipes and ducts as well.

Hire an HVAC technician to relocate your thermostat. Have them move it to an interior wall. That wall should not get any direct heating or cooling from the furnace.  Finally, mount it at least five feet above the floor

6. Faulty Wiring can Cause Excess Heat in and Around the Thermostat and Make It Read Higher than Actual Temperature

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, incorrect thermostat wiring causes higher than expected temperature readings.  This can occur if, for some reason, the wires inside the thermostat become warm from excess current.

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.  Excess warmth in or around the thermostat could indicate a fire hazard.

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

It could be that though your thermostat reads too high, that it’s actually reading correctly. So 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 defective.  Or, if in summer, perhaps your air conditioning is not running enough to cool your place adequately.  In either case, call in your trusty HVAC repairman. He can then 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. Furthermore, 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

    1. Why Does my Thermostat Read the Wrong Temperature?   From Hyde AC