What Is EM Heat on a Thermostat and When to Use It
Learn what EM heat means on a thermostat, how it works with heat pumps, when to use it, and practical tips to manage energy use and comfort.
EM heat, short for emergency heat, is a mode on heat pump thermostats that activates auxiliary heating when the outdoor temperature makes the heat pump inadequate.
What EM heat is and how it works
EM heat, short for emergency heat, is a mode on heat pump thermostats that activates auxiliary heating when the outdoor temperature makes the heat pump unable to meet the home’s heating demand. In practice, the system may switch to electric resistance heaters inside the house or engage a separate auxiliary stage to deliver warmth. This backup heating path ensures comfort during very cold snaps, even if the heat pump alone cannot keep up. Understanding what is happening physically helps homeowners manage energy use and avoid confusion when the thermostat display shows EM heat. The concept is straightforward: when the outdoor unit cannot pull enough heat from outside, the system relies on supplemental heating. This is a safe, intentional design to protect comfort and prevent rapid temperature drops in your living spaces.
Distinguishing EM heat from normal heat operation
In normal heat pump operation, the outdoor unit extracts heat from the outside air and uses the refrigerant cycle to warm indoor spaces efficiently. When EM heat is engaged, the thermostat signals auxiliary heating elements, often electric resistance coils, to contribute warmth. This path is less energy‑efficient than the standard heat pump cycle, and you may notice warmer air that comes with a higher energy draw and a louder system sound. Display indicators on many thermostats label this mode as EM heat or Aux heat, or show a separate symbol. The key idea is that EM heat provides a reliable fallback rather than an energy optimization feature, so understanding when it runs helps you balance comfort and cost.
When EM heat kicks in
EM heat is typically activated when outdoor temperatures fall to levels where the heat pump cannot comfortably reach the set indoor temperature. Thermostats use sensor data and demand, sometimes coupled with defrost cycles, to determine if the heat pump alone can satisfy the heating load. In many systems, EM heat comes on automatically during extreme cold or when there is a sudden spike in indoor temperature demand. It may also be triggered after several failed attempts by the heat pump to deliver warmth, or when a thermostat is set to a tight temperature target. Knowing these triggers helps you anticipate energy use and avoid unnecessary runs.
How to use EM heat wisely
Use EM heat only when necessary to maintain comfort during cold days. For daily operation, keep the thermostat in Auto or Heat mode so the heat pump can prioritize the efficient cooling cycle and minimize auxiliary heating. Pre-warming rooms before occupants arrive, sealing drafts, and improving insulation reduce reliance on EM heat. If your climate requires frequent EM heat, consider upgrading to a more efficient heat pump or adjusting the thermostat programming to reduce extended auxiliary heating. Smart thermostats can optimize recovery times and occupancy patterns to lower overall energy usage while keeping comfort intact.
Signs EM heat is active or needed
Common signs that EM heat is active include a noticeable difference in heating efficiency, a louder running sound, and air that feels warmer or less consistent than when the heat pump is doing the work. The thermostat display may show EM heat or AUX heat. If you regularly notice these indicators, evaluate whether indoor temperatures are acceptable and whether the outside conditions justify continued reliance on auxiliary heating. Quick checks include ensuring the heat pump is not in a mode that bypasses efficient operation and verifying that the home’s envelope is well sealed to minimize heat loss.
Troubleshooting EM heat issues
If EM heat seems to run more often than expected, verify that the thermostat is configured for a heat pump system rather than a conventional furnace. Confirm that the system is set to Auto or Heat with a proper fan setting and that the thermostat’s sensors aren’t obstructed or exposed to sunlight or drafts. Inspect air filters, confirm adequate refrigerant charge with a professional, and ensure the defrost cycle is operating correctly. If EM heat persists unexpectedly, contact a qualified HVAC technician to assess system performance and confirm the thermostat programming aligns with the heat pump model.
Energy considerations and costs with EM heat
EM heat can substantially increase energy use because electric resistance heating is less efficient than a running heat pump in milder conditions. While it provides dependable warmth during very cold weather, expect higher bills if the system spends long periods in EM heat. To manage costs, rely on the heat pump whenever possible, improve home insulation, and use programmable schedules to avoid heating when spaces are unoccupied. Smart thermostats can help by ramping up heat gradually and recovering to comfort temperatures only when needed.
Compatibility and thermostat requirements
Not every thermostat will expose a separate EM heat control; many modern heat pump thermostats label the secondary heat as AUX or Emergency Heat. Ensure your thermostat is compatible with a heat pump system and supports auxiliary or emergency heat. If you replace a thermostat, verify that the new unit can properly manage the heat pump’s stages, defrost cycles, and any backup heaters. Reading the installation manual and confirming with the manufacturer or a licensed technician helps prevent misconfigurations that trigger unnecessary EM heat.
AUTHORITY SOURCES
To deepen understanding and verify best practices, consult reputable sources on heat pumps and thermostat operation. These references explain how heat pumps work, what EM heat does, and how to optimize performance across different climates.
Questions & Answers
What exactly is EM heat on a thermostat?
EM heat, or emergency heat, is a backup mode on heat pump thermostats that engages auxiliary heating when the heat pump cannot meet the heating demand. It keeps indoor temperatures stable, but it uses more energy. This mode is intended for extreme conditions or temporary losses of efficiency.
EM heat is the backup heat on a heat pump. It kicks in when the main system can’t keep up, so your home stays warm, but it uses more energy.
How do I know if my thermostat is using EM heat?
Many thermostats display EM heat or AUX heat when the backup heating is active. Look for indicators like EM heat or AUX on the screen, or a separate light or symbol. If in doubt, check your thermostat’s manual for how it shows auxiliary heating.
Check the thermostat display for EM heat or AUX indicators. If you see the symbol, auxiliary heating is active.
Is EM heat necessary in cold climates?
EM heat is often necessary when outdoor temperatures are very low and the heat pump cannot meet the indoor heating demand. It helps keep rooms comfortable, but it should be used judiciously to avoid excessive energy use.
In very cold climates, EM heat is often needed to stay warm, but try to limit its use to avoid higher energy costs.
Will EM heat damage my heat pump?
EM heat itself doesn’t damage the heat pump; it is a designed fallback. Prolonged reliance on EM heat increases energy use and can stress the system if the underlying issues aren’t addressed. Regular maintenance helps prevent ongoing problems.
EM heat won’t damage the unit by itself, but constant use can stress components and raise costs. Regular service helps.
Can I disable EM heat on my thermostat?
Some thermostats allow you to disable EM heat, but this is not recommended in cold climates where the heat pump may require backup heat. If you’re unsure, consult the thermostat manual or a licensed technician before modifying the setting.
You can often disable it, but in cold climates that’s not advised. If unsure, check the manual or ask a pro.
Does EM heat cost more to run than regular heat?
Yes, EM heat generally costs more to run than the regular heat pump mode because it uses electric resistance heating. The difference is most noticeable during very cold days when EM heat runs longer.
EM heat usually costs more because the backup heat uses electric resistance. It’s best to minimize its use when possible.
What to Remember
- Use EM heat only when needed to maintain comfort in cold weather
- EM heat uses auxiliary heating, typically electric resistance, which increases energy use
- Keep your home well insulated to minimize reliance on EM heat
- Set the thermostat to Auto or Heat to let the heat pump operate efficiently when possible
- Regularly review thermostat settings and consider professional checks for persistent EM heat usage