Short Answer
Complete Explanation
Emergency heat, commonly shown as “Em. Heat” on thermostats and HVAC control panels, is a backup heating mode used by heat‑pump systems. When the outdoor temperature drops below a level at which the heat pump can efficiently extract heat (typically around 30–40 °F or 0–4 °C), the system switches to an auxiliary heating source, such as electric resistance coils or a gas furnace, to maintain indoor comfort.
- Definition:
Emergency heat is a supplemental heating function activated when the primary heat‑pump cycle cannot meet the thermostat’s demand. - How it works:
The thermostat detects a prolonged low‑temperature condition or a failure in the heat‑pump’s outdoor coil and signals the control board to energize the auxiliary heater. - Typical activation temperature:
Manufacturers set the switch‑over point between 30 °F and 40 °F (0 °C–4 °C), though the exact threshold varies by model. - Energy consumption:
Auxiliary heat sources are usually less efficient than the heat pump, consuming more electricity or fuel to produce the same amount of heat. - Indicator purpose:
The “Em. Heat” light alerts occupants that the system is using a higher‑cost heating method, prompting possible adjustments to temperature settings or to call for service if the mode stays on for an extended period.
Common Misconceptions
Emergency heat is always a sign of system failure.
It is a normal, intentional backup that engages under very cold conditions; it only indicates a problem if it remains on when outdoor temperatures are above the set threshold.
The “Em. Heat” light can be turned off manually to save energy.
Disabling the indicator does not stop the auxiliary heater; the thermostat must be set to a mode that disables emergency heat, which is generally not recommended during extreme cold.
FAQ
When should I be concerned if the “Em. Heat” light stays on?
If the indicator remains lit while outdoor temperatures are above the manufacturer’s switch‑over point, it may signal a malfunction in the heat‑pump’s outdoor coil, a refrigerant issue, or a control problem. Contact an HVAC technician for diagnosis.
Can I manually switch off emergency heat to reduce my electric bill?
Most thermostats allow you to disable the emergency heat mode, but doing so in very cold weather can leave the home under‑heated and may damage the heat‑pump compressor. It is generally safer to let the system manage the mode automatically.
Does using emergency heat increase my heating costs significantly?
Yes. Because auxiliary heating sources are less efficient, running them can raise energy consumption by 30‑100 % compared with normal heat‑pump operation, depending on the type of auxiliary heater and the duration of use.
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