ecobee AUX heat max outdoor temperature setting
It is a ceiling for backup heat. ecobee says "Aux Heat Max Outdoor Temperature" prevents auxiliary heat (AUX) from running above a specific outdoor temperature, so on milder days the heat pump has to do the heating alone. It is a threshold setting you can only change on the thermostat itself, under Installation Settings, not in the ecobee app.
What does "Aux Heat Max Outdoor Temperature" do?
It sets an outdoor temperature above which the ecobee will not run auxiliary heat (AUX). ecobee's own definition is one line: the setting prevents auxiliary heat from running above a specific outdoor temperature.
Think of it as a ceiling. When it is colder outside than the number you set, the thermostat is allowed to add backup heat whenever its other rules say the heat pump needs help. When it is warmer than that number, the heat pump has to manage alone, even if the room is slow to warm up. Backup heat costs more to run than the heat pump. ecobee does not give a figure, but Google, writing about its own thermostats, puts AUX at about 2 to 5 times the cost of the heat pump.
The setting only appears when the ecobee is set up for a heat pump with backup heat. ecobee notes that the thresholds you see depend on your equipment configuration.
Where is it on the thermostat?
Thresholds live only on the thermostat. ecobee says they cannot be reached in the app.
- Most ecobee models: Main Menu, then General, then Settings, then Installation Settings, then Thresholds.
- Smart Thermostat Lite and Essential: Main Menu, then Settings, then Installation Settings, then Thresholds.
ecobee puts a warning on this menu: if you are unfamiliar with the capabilities of your HVAC equipment, contact ecobee Support before changing anything. These are installer settings, and the value on your thermostat may have been chosen to suit your particular heat pump.
Why might you not see a number to change?
ecobee thermostats can decide staging on their own. ecobee says the "Configure Staging" option is set to "Automatically" by default, and that in this mode the thermostat uses its own algorithms to decide heating and cooling calls, staging and fan runtime.
For a heat pump with AUX, automatic staging offers a simpler choice called "Aux Savings Optimization". ecobee says it decides when AUX comes on based on the gap between the setpoint and the current temperature, and lets you favor savings, comfort, or a balance of both. If you want to set individual thresholds yourself, ecobee says to switch "Configure Staging" to "Manually".
Which other ecobee settings work alongside it?
The outdoor ceiling is one of several AUX rules. ecobee lists these thresholds for heat pumps with backup heat:
| Setting | What ecobee says it does | Default |
|---|---|---|
| "Aux Heat Max Outdoor Temperature" | Prevents auxiliary heat from running above a specific outdoor temperature | Not stated |
| "Compressor Min Outdoor Temperature" | Sets the lowest outdoor temperature at which the compressor operates | 35°F |
| "Compressor to Aux Temperature Delta" | Sets the temperature difference before auxiliary heat is engaged | Not stated |
| "Compressor to Aux Runtime" | Sets how long the compressor must run before switching to auxiliary heat | Not stated |
| "Aux Heat Min on Time" | Sets the minimum time auxiliary heat runs during a call | Not stated |
| "Aux Reverse Staging" | Turns auxiliary heat off near the end of a heating cycle so the heat pump finishes the job | Not stated |
ecobee adds that you can set only one of the two Compressor to Aux rules: either by temperature gap or by runtime, not both.
The two outdoor settings work as a pair. "Compressor Min Outdoor Temperature" is the floor for the heat pump; "Aux Heat Max Outdoor Temperature" is the ceiling for backup heat. ecobee does not describe in these articles what happens if the two values are set so that they leave a gap, so leave that combination to your installer.
How does it compare with other brands?
Every brand has an equivalent outdoor limit for backup heat, under a different name:
| Brand and model | Setting | Factory value and range |
|---|---|---|
| ecobee | "Aux Heat Max Outdoor Temperature" | Not stated |
| Google Nest | AUX lockout temperature, set for you by Heat Pump Balance | Google uses 40°F as its example |
| Honeywell Home T6 Pro Programmable | "Outdoor Lockout Backup Heat" (installer option 356) | Off from the factory, or 5 to 65°F |
| Emerson Sensi | "Aux Lockout" | Minus 4 to 80°F |
On a Nest, the comparable choice is a preference rather than a number; see Nest Heat Pump Balance.
When does this setting matter to you?
Mostly when backup heat seems to run on days that are not very cold. If you see AUX for long stretches in mild weather, the outdoor ceiling may be set high, and why a thermostat shows AUX covers the other common causes, such as a big morning setback. A setting that is too low can show up the other way, as a house that struggles to warm up on cold mornings.
ecobee also has a system mode called "Aux" that is a different thing altogether: its heating FAQ says that mode uses only the furnace. If your ecobee is in that mode by accident, see emergency heat on the thermostat. More ecobee pages are listed on the ecobee hub.
Frequently asked questions
- Can I change the AUX outdoor limit in the ecobee app?
- No. ecobee says threshold adjustments can only be made on the thermostat itself and are not available in the ecobee app. You will find them under Installation Settings, then Thresholds.
- What is the default for this setting?
- ecobee does not publish a default or a range for "Aux Heat Max Outdoor Temperature" in the support articles we checked. The value on your thermostat is either the factory value or whatever your installer chose.
- Is this the same as the compressor minimum outdoor temperature?
- No, it is the mirror image. "Compressor Min Outdoor Temperature" sets the lowest outdoor temperature at which the heat pump compressor runs, and ecobee gives 35°F as its default. The AUX setting sets the highest outdoor temperature at which backup heat may run.
- Should I lower it to save money?
- A lower ceiling means backup heat is allowed on fewer days, which usually means less use of the expensive heat. The risk is a house that warms slowly on cool days. ecobee recommends contacting its support before changing thresholds if you are unfamiliar with your equipment.