Quick answer
A thermostat reading the wrong temperature is usually in the wrong spot, not broken. Sun, a supply register below it, an exterior wall or cold air leaking from the cable hole behind the base all skew the sensor. Compare it with a separate thermometer beside it for 15 minutes: within 0.5 to 1 °C is normal. A bigger steady gap needs an offset, a remote sensor or a new location.
When a thermostat is reading the wrong temperature, the sensor is almost always reporting honestly on the air right around it. The trouble is that the air around it is not the air you are sitting in. Before blaming the thermostat, tape a simple thermometer to the wall beside it, leave both alone for 15 minutes with the furnace off, and compare. Agreement to 0.5 to 1 °C is as good as household thermostats get.
Location errors: sun, registers, outside walls and the hole behind the base
Location causes more wrong readings than failed sensors do. Each of these fools a thermostat in a predictable direction:
- Direct sun. A thermostat that catches an hour of afternoon sun reads several degrees high, shuts the heat off early and leaves the rest of the house cool. The error comes and goes with the time of day.
- Above or beside a supply register. Warm air washes over it the moment the furnace starts, the thermostat is satisfied in minutes and the cycle ends before the rooms catch up.
- On an exterior wall. The wall surface runs colder than the room in January, so the reading is low and the house overheats.
- The cable hole behind the base. The thermostat wire comes through a hole drilled into the stud cavity, and that cavity connects to the basement or the attic. Cold air, or warm air off a duct in the same cavity, leaks out of the hole straight onto the sensor. Take the thermostat off its base and plug the hole with putty or a pinch of insulation; it is the cheapest fix on this page.
- Near a lamp, a television, the kitchen doorway or the top of the stairs, where local heat or a stairwell draft does the same thing.
Sensor drift, dust and self-heating on smart thermostats
When the location is fine and the reading is still off, look at the thermostat itself. Electronic sensors drift slowly with age, and a mat of dust inside the housing insulates the sensor so it lags behind the room. Pull the cover and blow the dust out gently.
Smart thermostats add an error of their own: the screen, processor and Wi-Fi radio make heat inside a small case. The software subtracts an estimate of that heat, and the estimate can be off after a restart, an update or a stretch with the display lit, which shows up as a reading 1 to 2 °C high. If the gap is steady, use the calibration offset. Many Honeywell and Ecobee models have one in the installer or threshold settings, where you enter a correction so the display matches your reference thermometer.
Old mercury thermostats: level and the heat anticipator
A round mercury thermostat reads wrong for two reasons no digital model has. First, it must be level: the mercury bulb rides on a coiled bimetal strip, and a body that has been bumped off level shifts the switching point by the same error every cycle. Second, it has a heat anticipator, a small resistor with a sliding pointer that warms the coil during a cycle so the furnace stops slightly early. It is meant to be set to the current drawn by the furnace's gas valve circuit.
Set too low, the anticipator makes cycles short and the house runs cool; set too high, each cycle overshoots. The setting is often wrong after a furnace replacement, because the new furnace draws a different current than the one the thermostat was matched to. If you replace a mercury thermostat, keep it out of the garbage: mercury goes to a household hazardous waste depot.
How far off is normal, and when it needs fixing
- Within 0.5 to 1 °C of a separate thermometer held beside it: normal. Two consumer sensors rarely agree closer than that.
- A steady 1 to 3 °C gap that never changes: use the offset setting or seal the cable hole, then recheck.
- A gap that changes with the time of day or with furnace cycles: a location problem. An offset cannot fix an error that moves.
- A reading that jumps by several degrees, or a number that does not move while the furnace runs: a failed sensor. Replace the thermostat.
- The thermostat matches the thermometer and other rooms are far off: the thermostat is fine and the issue is air balance. See why one room is colder than the rest.
Relocate the thermostat or add a remote sensor?
Relocate when the thermostat sits in sun, over a register or on an outside wall. The right spot is an interior wall about 1.5 m above the floor, in a room you use, away from supply air, windows and exterior doors. Thermostat relocation means fishing a new cable from the furnace, so the cost depends on how finished the walls and basement ceiling are, and it is quoted in writing before the work.
Add a remote sensor when the thermostat's spot is acceptable but is not where you live: a hallway thermostat in a house where the family sits in a back family room, or a bedroom that matters at night. Ecobee, Nest and Honeywell Home each sell wireless room sensors for their compatible models. Depending on the brand, the thermostat averages several rooms or follows one chosen sensor on a schedule, and our Ecobee and Nest comparison covers how each handles it.
If the reading is right and the furnace still does not respond, the question becomes whether the thermostat or the furnace is at fault. ZK Mechanical installs, moves and sets up thermostats across Burlington, Oakville, Hamilton and Mississauga. See smart thermostat installation or request a written quote.