Why doesn’t the Nest Learning Thermostat start my system immediately when the temperature is above or below the target temperature?
Sometimes you may set your thermostat to 70°F/21ºC, but it's 69°F/20.5ºC inside and the heat isn't turning on. That's because all thermostats, including the Nest Learning Thermostat, maintain a range of about 1ºF/.5ºC around your target temperature. This is called a maintenance band - it prevents excessive wear and tear to your equipment.
The Nest Thermostat won’t turn the heat or AC on every time the temperature in the room changes slightly - that would result in your system turning on and off in a matter of seconds. It would be inefficient, noisy, and uncomfortable.
So the Nest Thermostat has a maintenance band of 1ºF (0.6ºC) above and below the target temperature for cooling and 0.7ºF (0.4ºC) above and below for heating. That means the Nest Thermostat will switch your system on when the room temperature is around 1ºF away from your target temperature, then it’ll run your system until the temperature is slightly beyond your target temp.
Sometimes it may seem like the Nest Thermostat's maintenance band is larger than 1ºF, but it’s most likely that your system takes a few minutes to start heating or cooling after your Nest Thermostat tells it to. In that time the temperature can drift beyond the maintenance band. Your Nest Thermostat may also limit your system’s cycle time to protect your equipment, which can mean that it takes longer to reach the temperature you want.
For example, let’s say you set your Nest Thermostat to heat to 70ºF. Here’s what will happen:
- When the room cools to about 69ºF, your Nest Thermostat will turn on the furnace.
- The heater will start heating the room. It might take a few minutes for the pipes or ducts or radiators to warm up and start warming everything else in the room.
- The Nest Thermostat will keep running the heat until the room temperature reaches about 71ºF. Then it will turn off the heat.
- Depending on your equipment, the fan might continue to run for a while to get the warm air into the room, or the radiators might stay warm for a while and continue to warm the air in the room.
- In any case, the room temperature is intended to peak around 71ºF. Unlike other thermostats, the Nest Thermostat learns how your room temperature behaves after heating is complete and adjusts its behavior for your house and equipment so that it heats the right amount.
- If its cold outside, the room temperature will eventually start to fall again, go past 70º to 69º, and the cycle will repeat.
In virtually every home, the difference between the temperature in one room and the next, or between the temperature at head height and at the floor is over 2ºF. So the Nest Thermostat balances between heating your home to a comfortable temperature and cycling your system too often.
Some heating and cooling systems may continue to heat or cool for a while after they’ve been turned off. If that makes the system heat or cool beyond the maintenance band, your Nest Thermostat will adjust how long the system runs to correct for it.