The thermostat insists the room is at target and you are sitting there in a jumper. Or the heating runs long past comfortable because the thermostat thinks the room is colder than it is. In almost every case the sensor is working correctly and is being told a misleading story by where it has been mounted.
The short answer
Check what the thermostat is near. Direct sunlight, a radiator, a draught from a door, an external wall, a lamp or a television all bias the reading. The correct position is an internal wall, about 1.5 metres up, in a room you actually use, away from heat sources and draughts. If it cannot be moved, most smart thermostats have a calibration offset you can set instead.
What biases the reading
- Direct sunlight. The easiest to spot: the reading climbs on sunny afternoons and the heating stops while the room is still cold.
- An external wall. Colder than the room air, so the thermostat reads low and overheats the house.
- Near a radiator. Reads high as soon as the heating starts, so it shuts off before the room is warm. This produces short cycling.
- In a draught. Near a front door, a letterbox or a poorly sealed window, so it reads low every time someone comes in.
- Above a heat source. A television, a lamp, a router or a cable box underneath adds a couple of degrees.
- In a hallway. Hallways are colder than living rooms and rarely occupied, which makes them a poor proxy for comfort.
- Its own electronics. A thermostat with a bright display and a wireless radio generates a small amount of heat itself, which is why calibration offsets exist.
Measure the room independently
Put a simple thermometer on a table in the middle of the room, at sitting height, away from anything warm. Leave it half an hour. Compare with what the thermostat reports. Now you know the size and direction of the error.
Look at what surrounds the thermostat
Sun through a window at any time of day, a radiator below, a door nearby, anything electrical underneath. Photograph the wall if it helps you think about it.
Set a calibration offset
Most smart thermostats allow a correction of a degree or two, usually in the app under advanced or installation settings. If your independent thermometer consistently reads two degrees lower, apply that offset. This is the practical fix when relocating is not possible.
Move it if you reasonably can
The ideal position is an internal wall, roughly 1.5 metres from the floor, in a room you spend time in, with clear air around it. Many smart thermostats are wireless and battery powered, which makes relocating straightforward.
Add a remote sensor
Many systems support additional sensors in other rooms, and can average them or prioritise a chosen room at certain times. This is the best answer for a house where the thermostat is stuck in a hallway.
Check for short cycling
If the heating turns on and off every few minutes, the thermostat is near a heat source or the differential is set too tight. Widen the differential or move the thermostat. Short cycling is inefficient and hard on the boiler.
Balance the radiators
If one room is always cold regardless of the thermostat, the problem is distribution rather than measurement. Adjust the lockshield valves so the far radiators get their share of flow.
Check the batteries
Low batteries cause erratic readings and intermittent communication before they cause an outright failure. Replace them annually rather than waiting.
Be careful about what you are actually measuring. Air temperature and comfort are not the same thing. A room at 20 degrees with cold walls and a draught feels considerably colder than a room at 20 degrees that is well insulated. If the thermostat is accurate and the room still feels cold, the issue is the room rather than the reading.
Remote sensors are usually the real answer
Most houses were wired with the thermostat in the hallway, because that is where the cable run was convenient. A hallway is colder than a living room, is full of draughts from the front door, and is somewhere nobody sits. Using it to decide the temperature of the whole house is a compromise that predates anything smart.
Remote sensors fix this properly. Put one in the living room and one in the main bedroom, and set the system to follow the living room in the evening and the bedroom at night. That is the feature that makes a smart thermostat genuinely better than a dial, and many people never set it up.
If the thermostat reading is wildly wrong, ten degrees or more, do not calibrate around it. That suggests a failing sensor or a wiring fault rather than placement, and a large calibration offset masking a hardware problem will cause trouble later.
Where should a thermostat be placed?
On an internal wall, about 1.5 metres up, in a room you use, away from radiators, draughts, direct sunlight and electronics. Hallways are the traditional position and the worst one.
Why does my heating stop before the room is warm?
The thermostat is reading higher than the room, usually because it is near a radiator, in sunlight, or above a heat-producing device. Check the surroundings and apply a calibration offset if you cannot move it.
What is short cycling?
The heating switching on and off every few minutes. It happens when the thermostat is too close to a heat source, or the temperature differential is set too narrowly. It is inefficient and hard on the boiler.
Are remote sensors worth adding?
For most houses, yes, and they are the single most useful smart thermostat feature. They let the system heat based on where you actually are rather than where the wiring happened to be convenient.
How much calibration offset is reasonable?
A degree or two is normal and expected, largely to account for the thermostat’s own heat. An error of several degrees means placement or a fault, and calibrating around it hides the real problem.
The bottom line
Measure the room with an independent thermometer so you know the size and direction of the error, then look at what surrounds the thermostat. Apply a small calibration offset if it cannot be moved, and add remote sensors if your system supports them, because heating the room you are sitting in rather than the hallway is what a smart thermostat is actually for.
