Why the room never matches the set temperature
Often the thermostat is doing what it was told. It lets the room drift a little around the setpoint, it may cap how high or low you can set it, and Eco, Away or Auto mode can change the target. If it calls for heat or cooling and the room still falls short, the cause is beyond the thermostat.
Is the thermostat set to the right mode and setpoint?
Start with the basics, because they catch more people than faults do. Honeywell's T4 Pro and T6 Pro manuals give the check for heat in one line: press the Mode button to set the system to Heat, and "Make sure the desired temperature is set higher than the inside temperature." For cooling it is the reverse: Cool, with the setpoint below the room temperature.
Then confirm the reading is right. If the thermostat thinks the room is already at the setpoint, it will stop, even if your own thermometer disagrees. Sensi says that when a digital thermostat does not read accurately, the cause is nearly always a poor location or heating and cooling equipment that is the wrong size for the home. If the number on the screen is the problem, see thermostat shows the wrong room temperature.
Does the thermostat let the room drift on purpose?
Yes. Thermostats generally do not hold the room at exactly the setpoint. They wait until the room has drifted a little before it starts the system, then runs a little past the setpoint before it stops.
Google calls this the maintenance band on Nest thermostats: "The maintenance band allows the temperature in your home to change by a few degrees before it turns your system on or off." Google's example: with a target of 70°F and a 1°F band, a Nest in cooling mode cools the room to 69°F, then starts cooling again when it reaches 71°F. On the Nest Learning Thermostat (4th gen), the band appears in the Google Home app under Settings, Equipment, Advanced configuration, as "Heat temperature difference" and "Cool temperature difference".
ecobee widens the gap while you are out. Its thresholds page says that during Away activities the thermostat adds an extra 1°F to your set temperature when deciding to call for heating or cooling.
So if the room is a degree or so off and the thermostat is idle, it is probably inside its band. The temperature differential page explains the setting on each brand, and cycles per hour covers the Honeywell control that affects how tightly the temperature is held.
Is a setpoint limit stopping you?
Many thermostats let an installer or owner cap how warm or cool you can set them. If the limit is below the temperature you want, the room will never get there because the thermostat will not ask for it.
| Thermostat | Heating limit(°F) | Cooling limit(°F) | Where it is set |
|---|---|---|---|
| Honeywell Home T4 Pro, T6 Pro | Maximum Heat Setpoint (ISU 431): 40 to 90, default 90 | Minimum Cool Setpoint (ISU 430): 50 to 99, default 50 | Installer setup |
| Honeywell Home T9 | Minimum and maximum heat: 40 to 90 | Minimum and maximum cool: 50 to 99 | Not stated |
| ecobee | Heating Range, default 45 to 79 | Cooling Range, default 65 to 92 | Main Menu > General > Preferences |
| Emerson Sensi | Temperature Limits: Not stated | Temperature Limits: Not stated | Sensi app: settings gear > System settings > Temperature Limits |
Each brand shows the limit differently. On the T4 Pro, Honeywell says that if you are in the correct mode but cannot set the heat anywhere from 40°F to 90°F, "you may have range-stops restricting your setting. Check with the installer." On the T9, if the minimum and maximum heat setpoints are set to the same value, the heat setpoint cannot be adjusted from the home screen. Sensi shows "Limit" on the thermostat when someone tries to go past a Temperature Limit, and it moves schedule setpoints so they stay inside the limits. Sensi limits can only be changed in the app, not at the thermostat.
Is Auto mode keeping heat and cool apart?
In Auto mode the thermostat holds a heating setpoint and a cooling setpoint at the same time, and it keeps them a minimum distance apart. Each Honeywell family handles that gap its own way:
- T5 and T6 Pro Wi-Fi. Resideo requires a 1.5 degree difference between the heating and cooling setpoints. Its support page adds: "You may notice that your home naturally warms or cools up to 1.5 degrees past your setpoint… This is normal."
- T10 and T10+ Pro. When installer option ISU 3015 is set to 0, Honeywell says it enforces a 1.5°F differential behind the scenes.
- T4 Pro and T6 Pro. ISU 303, Auto Changeover Differential, ranges from 0 to 5°F with a default of 0°F. Honeywell notes that this differential is not the deadband, and that an algorithm fixes the deadband at 0°F.
- FocusPRO 6000. Heat and cool settings must be at least 3 degrees apart. The guide says that separation is fixed and cannot be changed.
See Auto mode for how changeover works.
Is Eco, Away or a schedule changing the target?
The temperature you remember setting may not be the one the thermostat is using now.
On the Nest Thermostat E and Nest Learning Thermostat, Eco replaces your setpoint with the Eco temperatures. Google allows them in a range of 40°F to 70°F for heating and 76°F to 90°F for cooling. "When you manually switch to Eco, your thermostat will ignore all scheduled temperatures until you manually switch it back to heating or cooling." If a Nest is showing Eco, compare its Eco temperatures with what you expect; Nest Eco temperatures explains them.
On ecobee, Away activities add the extra 1°F mentioned above. A schedule period with a lower heating or higher cooling setpoint can also be in effect without you noticing. If the target keeps changing behind your back, see thermostat is not following its schedule.
After a long setback, some thermostats start early so the room is ready on time. If the house is still catching up in the morning, recovery mode explains what the thermostat is planning.
What should you check now?
Is the mode Heat or Cool, with the setpoint on the right side of the room temperature?
- Yes
Does the screen show Eco, Away or a hold, or does the setpoint differ from what you chose?
- Yes
- No
Is the thermostat actively calling for heat or cooling right now?
- Yes
The thermostat is asking for heat or cooling and the room still falls short. That points past the thermostat. Check the signs at the bottom of this page.
- No
- Yes
- No
Set the mode and move the setpoint past the room temperature. If it will not go far enough, a setpoint limit is in place.
Frequently asked questions
- Why does my thermostat stop at 90°F when I try to turn up the heat?
- On Honeywell Home T4 Pro and T6 Pro thermostats, the Maximum Heat Setpoint is 90°F by default, and the heat cannot be set above it. An installer may also have set a lower limit. Honeywell says to check with the installer if you cannot use the full 40°F to 90°F range.
- Why does my Sensi thermostat say "Limit"?
- Someone has set Temperature Limits in the Sensi app, and you tried to adjust past one. Sensi says the limits cannot be changed at the thermostat. Open the app, then the settings gear, System settings and Temperature Limits.
- Is it normal for the room to go a degree past the setpoint?
- Yes, within limits. Google describes a maintenance band on Nest thermostats that lets the temperature move a few degrees before the system turns on or off, and Resideo says homes with its T5 and T6 Pro Wi-Fi thermostats may drift up to 1.5 degrees past the setpoint.
- Can Eco mode stop my Nest from reaching my temperature?
- Yes. In Eco, a Nest holds the Eco temperatures instead of your setpoint. Google gives the allowed Eco range as 40°F to 70°F for heating and 76°F to 90°F for cooling. If you switched to Eco by hand, it ignores the schedule until you switch back.