In this short podcast episode, Bryan talks about why your thermostat is reading the wall, not the actual air temperature.
A thermostat is an instrument designed to read temperature (and sometimes humidity); it displays those readings based on sensors and makes decisions. Most of these sensors are thermistors, which are resistors that change resistance with changes in temperature. Thermistors have low mass and respond to radiant, convective, and conductive heat.
Air coming through the wall (convective heat), the temperature of the things around the thermostat (radiant heat), AND the wall temperature (conductive heat) can influence your thermostat's reading. We may see a service call where the thermostat reads above the air temperature and runs the air conditioning unit even when the air temperature is already below the set point. Our first move as technicians should be to ask customers when they noticed the problem; the timing can give us some clues about what is going on. Looking behind the thermostat is one of the first things we should do, as air can move through the hole where wires come through, especially if there is air leakage elsewhere. If there is a temperature difference and an airflow path, that could be the source of the issue.
Use a reliable temperature probe near the thermostat, off the wall and away from the thermostat casing and electronics, to get a reliable temperature measurement to compare to the thermostat. If the probe and thermostat disagree after several minutes, we should investigate deeper for a warm wall. Infrared thermometers and thermal imagers can help you identify warm surfaces. Keep in mind that thermostats may have sensors in different locations, offsets, and programmed corrections as well.
If you find an open wire penetration, use non-flammable insulation and/or an appropriate putty to close it. Be careful not to block the sensing vents, too. Let everything settle and repeat the comparison. Some other solutions include changing the location of the thermostat or remote sensor.
This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, Copeland, and Santa Fe Dehumidifiers.
Resources
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