Where You Put A Thermostat Decides What It Reads

A thermostat measures temperature at exactly one spot and runs the entire heating system from that reading. If the spot is unrepresentative, the house is heated to the wrong temperature no matter what number you set.

One sensor stands in for every room

The control loop is simple. The thermostat compares its reading against the setpoint, calls for heat until the reading passes it, and stops.

Nothing in that loop knows about the rest of the house. The bedroom could be considerably colder and the system would still shut off, because the hallway sensor is satisfied.

Every complaint about a house that is too hot upstairs and too cold downstairs starts here, with a single reading standing in for conditions it cannot observe.

The classic bad locations

Direct sun on the wall raises the sensor well above room temperature, so the system shuts off early and the house stays cold on the brightest days.

An exterior wall, a spot near a drafty door, or an unheated stairwell does the opposite, reading low and running the system past the point where the occupied rooms are comfortable.

Kitchens are unreliable for the same reason, since cooking dumps heat into the room for an hour at a time and the thermostat treats it as evidence the house is warm.

The device heats itself

A connected thermostat with a display and a radio dissipates a small amount of power continuously, and that heat lands on the same board as the sensor.

Manufacturers compensate in software, estimating self-heating and subtracting it, but the compensation assumes typical airflow behind and around the unit.

Mounting it inside a recessed niche or behind a curtain changes the airflow, and the correction that was calibrated for open wall mounting starts reading high.

Cycling behavior is part of the reading

A thermostat has to allow the temperature to drift a little before acting, or the system would switch on and off constantly and wear itself out.

That deadband means the room genuinely swings above and below the number you set. A wide band gives long, efficient cycles and noticeable temperature swings.

Systems that anticipate, starting earlier and stopping before the setpoint is reached, hold the room closer to the number while running the equipment less often.

Remote sensors change what the loop controls

Adding sensors in occupied rooms lets the control loop average several readings or follow whichever room is currently in use, rather than the hallway where the wiring happened to run.

This does not fix airflow imbalance between floors, since one heating zone still delivers one amount of heat everywhere it goes.

What it does fix is the mismatch between where comfort matters and where the measurement happens, which is the cheaper half of the problem.