Carrier Comfort TC-NHP01-A Owner's Manual page 22

WHAT IF . . .
The battery icon is displayed
The battery icon appears on the display only when the battery starts to lose power.
Replace it when one- third of the icon is black. If your thermostat is battery
powered and you wait until the icon is just a silhouette with no black bars, you will
slowly lose thermostat functions until it doesn't work at all.
"Aux heat on" is displayed (heat pump systems only)
Aux heat on is auxiliary heat. It applies only to the heat pump version. If it
is on, you will see aux heat on. When it is displayed, it means the system
has determined that it's too cold outside for your heat pump to heat the
house without help, so it's getting help from the electric heaters. This hap-
pens automatically.
"Em heat" is displayed (heat pump systems only)
Em heat is the display term for emergency heat and applies only to the heat pump
version. You have to turn on emergency heat manually – it doesn't happen
automatically – so if you see Em heat on the display, it means somebody has
turned it on.
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