House temperature swings between heating or cooling cycles
A small swing is built in. Thermostats let the room drift before starting the system and run a little past the setpoint before stopping, often about 1°F each way. Larger swings usually come from a slow cycle rate or wide differential, a thermostat in a poor spot, or a schedule or sensor change moving the target.
Is some swing normal?
Yes. A thermostat that switched the system on and off at the exact setpoint would start it constantly, so every model allows a little drift. Honeywell says its thermostats are designed to control temperature to plus or minus 1°F. Google gives a worked example for Nest: with a 70°F target and a 1°F maintenance band in cooling, the thermostat cools to 69°F, stops, and starts again when the room reaches 71°F.
That means a 2°F spread from the low point to the high point is ordinary on a Nest with that band. If your swing is about that size, the thermostat is behaving as its maker describes.
Is the swing bigger than the thermostat's own band?
The display can help here. Watch the current temperature on the thermostat through a full cycle: note the reading when the system starts and when it stops.
If the thermostat's own reading moves about a degree or two, the control is doing its job and the bigger swing you feel is happening somewhere else in the house. Skip to the location and sensor questions below.
If the thermostat's reading itself moves several degrees between starts, look at the settings that control how far it lets the room drift.
Is the cycle rate or differential set wide?
These two settings decide how far the room drifts before the next run. On Honeywell Home T4 Pro and T6 Pro Smart thermostats it is the cycles per hour setting, which Honeywell rates at a 50% load. On ecobee it is the temperature differential, set in the thresholds on the thermostat. On Google Nest the maintenance band is adjusted automatically.
Sensi spells out the tradeoff on its cycle rate page: a slower cycle rate allows a wider temperature swing, and a faster one gives tighter control with shorter cycles. The spec sheet for the Sensi smart thermostat 1F87U-42WF shows how much difference the choice makes. For cooling, the rated differential is 0.9°F at Fast and 1.7°F at Slow.
| Brand and model | Setting that widens or narrows the swing | Who usually changes it |
|---|---|---|
| Honeywell Home T4 Pro, T6 Pro Smart | Cycle rate (cycles per hour), cooling and heating set separately | Installer |
| Emerson Sensi (1F87U-42WF) | "Cycle Rate": Fast, Medium or Slow | Owner, in the Sensi app |
| ecobee | "Heat Differential Temperature" and "Cool Differential Temperature" | Owner or installer, on the thermostat |
| Google Nest | Maintenance band | The thermostat, automatically |
Narrowing the swing has a price: more starts per hour. How the two styles of setting compare is covered in differential vs cycles per hour.
Is the thermostat in a spot that warms or cools faster than the house?
A thermostat in direct sun, in a draft or near a vent sees its own small climate. It may decide the house is warm enough while the far rooms are still cold, or keep cooling a hallway that is already comfortable while the living room swings.
Sensi says a poor thermostat location, or equipment sized wrong for the home, is behind nearly every case of a digital thermostat reading inaccurately. Google's list of bad spots for a Nest Temperature Sensor names vents, direct sunlight and drafts, and recommends chest height. If the reading on the screen does not match the room, start with thermostat shows the wrong room temperature.
Is the target itself changing?
A swing that follows the clock is often a schedule, not a control problem. A lower heating setpoint overnight or a higher cooling setpoint during the day will look like a swing if you only glance at the room. Eco and Away modes do the same. If the setpoint on the screen changes without you touching it, see thermostat is not following its schedule.
Remote sensors can also move the target in practice. A thermostat that averages several rooms, or follows the rooms where it detects people, will hold that average steady while individual rooms drift. Room sensors vs the thermostat sensor explains how that works by brand.
What should you check first?
Does the setpoint on the screen change during the day?
- Yes
A schedule, Eco or Away is changing the target. Review the schedule.
- No
Does the thermostat reading itself move more than about 2°F between starts?
- Yes
The cycle rate or differential may be set wide. Check the setting for your brand.
- No
Is the thermostat near a vent, sunny window or outside door?
- Yes
If the room never gets to the setpoint at all, rather than swinging around it, see why the room never matches the set temperature.
Frequently asked questions
- How much should the temperature swing between cycles?
- About a degree either side of the setpoint on the thermostats we checked. Honeywell says its thermostats are designed to control temperature to plus or minus 1°F, and Google's Nest example lets a 70°F room move between 69°F and 71°F. A room you measure elsewhere can differ from the thermostat's spot.
- Why does it feel colder than the thermostat says right before the heat comes on?
- Because the thermostat waits for the room to drift before it starts the heat. At the bottom of that drift the room is at its coolest point of the cycle. A slower cycle rate or wider differential makes that low point lower.
- Can I make the temperature steadier?
- On some models. A faster Sensi "Cycle Rate" gives tighter control, a higher Honeywell Home cycle rate does the same through the installer, and ecobee's differential thresholds can be narrowed on the thermostat. The cost is more frequent starts. Google Nest manages its band itself.
- Why does the swing change from room to room?
- The thermostat only controls to the temperature at its sensor, or at the sensors it is averaging. Rooms with sun, outside walls or long duct runs warm and cool at their own pace, so they can swing more than the room the thermostat is in.