What Is EM Heat on a Thermostat?

EM heat, also known as emergency heat or auxiliary heat, is a heating system function found on heat pumps that utilizes electric resistance heaters to provide heating when the outdoor air temperature is too cold for the heat pump alone to work efficiently. EM heat provides supplementary heating when needed to ensure comfort is maintained inside the home.

Understanding what EM heat is, why it exists, and how it works on a thermostat can help homeowners use this function effectively and efficiently.

How Does a Heat Pump Work?

Before diving into what exactly EM heat is, it’s helpful to understand how a heat pump works. A heat pump is an HVAC system that provides both heating and air conditioning in one unit.

Heat pumps move heat rather than generate it directly. In the winter, they extract heat from the outside air and pump it indoors. In the summer, they do the reverse and take heat from inside the home and discharge it outdoors.

A heat pump system consists of an outdoor unit and an indoor air handler connected by refrigerant lines. The main components include:

  • Outdoor Coils – Absorb or release heat from/to the outdoor air
  • Compressor – Circulates and compresses the refrigerant
  • Reversing Valve – Reverses the direction of refrigerant flow for heating or cooling
  • Expansion Device – Allows the refrigerant to expand and lower in temperature
  • Indoor Coils – Absorb or release heat from/to the indoor air
  • Blower – Circulates air across the indoor coils

Under moderate outdoor temperatures, this heat pump cycle works well for both heating and cooling. However, in very cold weather, around 35°F or lower, a heat pump starts to lose efficiency and capacity.

This is where EM or auxiliary heat comes in.

What is EM Heat?

EM stands for “emergency” or sometimes “auxiliary.” EM heat refers to electric resistance heaters built into a heat pump that provide supplemental heating when it’s extremely cold outside.

These heaters consist of coils or strips of wire that heat up when electrical current passes through them. They provide 100% efficient heat, in contrast to the heat pump’s coils which move existing heat rather than create it directly.

EM heating engages to boost heating capacity when:

  • Outdoor Temperatures Are Very Low – Below 35°F is usually when EM kicks in. The colder it is, the less heat a heat pump can extract from outdoor air. EM heat provides extra heating ability.
  • Defrost Cycle – Heat pumps will periodically go into a defrost mode to melt frost off the outdoor coils. EM heat helps maintain indoor temperatures meanwhile.
  • Insufficient Heat Pump Output – If the heat pump alone can’t keep up with heating demand, EM will engage to boost the heat output.

So in essence, EM functions as a backup or supplemental heating source for extra cold days or high heat requirements in a home.

How Does the EM Heat Function Work on a Thermostat?

Thermostats with EM heat will have settings to control when and how this backup heating engages.

There are two main ways EM heat can be activated:

1. Automatic Emergency Heat

In “Auto” EM mode, the heat pump controls when emergency heat turns on automatically. This is the default setting in most cases.

  • The heat pump monitors conditions and decides when supplementary electric heat is needed.
  • EM will kick in at low outdoor temperatures, during defrost cycles, or if the heat pump capacity is insufficient to maintain the set temperature.
  • This ensures comfort is maintained while allowing the heat pump to work as efficiently as possible.

2. Emergency Heat Only

The homeowner can manually switch to “EM Heat Only” mode on the thermostat. This option runs only the emergency heat and disables the heat pump.

  • EM Only may be selected if the heat pump is malfunctioning and immediate heat is needed.
  • It can also be used to provide maximum heating output, though only temporarily since it is expensive to run.
  • A manual switch to EM Only should prompt a service call to address any underlying issue with the heat pump.

So in summary, EM functions automatically in Auto mode to supplement the heat pump, while EM Only manually activates only the electric resistance heaters.

Why Does a Heat Pump Need EM Heat?

EM heat exists because heat pumps become less efficient at extracting heat from frigid outdoor air. Supplemental electric heating is needed in cold climates to make up the difference.

There are a few reasons why heat pump performance drops off when temperatures plunge:

Lower Air Density – Cold air is less dense and has fewer heat-containing molecules for the heat pump to extract thermal energy from.

Frost Buildup – Below 35°F, moisture in outdoor air can freeze into frost on the heat pump coils. This frost insulates the coils and blocks airflow.

Refrigerant Pressure – Refrigerant pressure and temperature drop in cold weather. This makes it harder for the heat pump to release enough heat indoors.

Output Capacity – The heating capacity and performance of a heat pump system declines steadily as the air temperature falls.

EM heat provides a boost to account for this inevitable cold weather drop-off in heating capability. It ensures the indoor temperature can be maintained at the desired level even when it’s frigid outside.

Signs Your EM Heat May Be Running

Some indications your emergency or auxiliary heat has activated include:

  • Higher Energy Bills – EM heat uses expensive electric resistance heating. Energy bills will likely spike in months it runs frequently.
  • Warmer Air Temperatures – Noticeably hotter air coming from the vents may indicate EM heat has kicked in to boost heating output.
  • EM Indicator on Thermostat – Many programmable thermostats have an “EM Heat” or similar indicator that shows when it’s running.
  • Heating System Stops Blowing Hot Air – EM only heats when called for, so you may notice it cycling on and off more than the heat pump’s constant airflow.
  • Outdoor Temperatures Below 35°F – This cold outdoor threshold is generally when EM starts automatically supplementing the heat pump.

Paying attention to these indicators can help you understand when and how often your emergency heat runs. This allows you to use it wisely.

EM Heat Energy Use and Costs

A key disadvantage of EM heat is that it’s extremely energy intensive and expensive to operate. Emergency heaters consume 3-5 times more power than a heat pump alone.

Why is EM heat so energy hungry? A few reasons:

  • Electric Resistance Heating – EM uses resistive coil heaters which convert 100% of their electrical input directly into heat. This makes them energy guzzlers.
  • No Heat Extraction – Heat pumps move existing heat. EM creates heat from scratch, which is inherently less efficient.
  • Supplementary Role – EM is designed for temporary supplemental use only for the coldest periods. Running it regularly wastes energy.

For homes in cold northern climates that rely on EM heating often, it can add hundreds of dollars per month to energy bills in the winter. In milder climates, this cost may be reduced, but still significant.

Using EM heat judiciously and sparingly is important to control energy costs. Higher efficiency heat pump systems can also reduce dependence on auxiliary heating.

Best Practices for Using EM Heat

To maximize comfort while minimizing energy use and costs, here are some tips for effectively operating EM heat:

  • Set the thermostat to “Auto” EM control and leave it alone to run automatically as needed. Don’t override it regularly.
  • Have your heat pump inspected before winter to ensure it’s running optimally and won’t require excessive EM heating.
  • Upgrade to a larger capacity, hyper-efficient heat pump if your current system needs EM for long periods each winter.
  • Improve home insulation and seal air leaks to reduce heating load demands on the heat pump.
  • Make sure the thermostat is located on an interior wall away from drafts or cold spots that could falsely trigger more EM heating.
  • Lower the thermostat a couple degrees on extremely cold nights to reduce how often EM heat engages.
  • Clean or replace air filters monthly to allow proper airflow and efficiency.
  • Only use the “EM Heat Only” setting temporarily if the heat pump fails. Call your HVAC contractor for repairs.

Taking these steps will help ensure your EM heat provides an effective efficiency boost when you really need it, while minimizing energy waste and costs.

Heat Pump Thermostat Settings for EM Heat

There are a few key settings related to EM heat that can be adjusted on heat pump thermostats:

EM Heat Auto/On – Auto allows EM to run automatically as needed. On forces it to stay activated manually.

EM Heat Lockout Temperature – Prevents EM from running above a set outdoor temperature threshold (e.g. 40°F).

Defrost Cycle Settings – Controls EM activation and indoor fan operation during defrost mode.

Staggered Start – Delays auxiliary heat startup by a set time (e.g. 30 minutes) to let heat pump ramp up first.

Balance Point – Prevents EM usage above a certain outdoor temperature where heat pump output balances heating needs.

Consult your thermostat manual to configure these settings for optimal efficiency, comfort and costs.

Heat Pump Thermostat Wiring for EM Heat

Proper wiring allows connected equipment like EM heat to communicate with the thermostat:

  • W2 Wire – Activates first stage heat and compressor for heat pump.
  • W3 Wire – Activates EM/auxiliary heat specifically.
  • O Wire – Reversing valve for changing heat pump from cooling to heating.
  • Y Wire – Compressor operation.
  • G Wire – Fan control.
  • C Wire – Provides continuous power to thermostat.

A qualified technician should handle any thermostat wiring for EM heat to ensure it functions properly.

Alternatives to EM Heat

Because of the high energy demands of electric resistance EM heating, alternatives may be worth considering:

More Efficient Heat Pumps – Newer cold climate models work at lower temps with less EM needed.

Variable Speed Systems – Slow ramp-up reduces EM cycling and saves energy.

Gas Furnace – Integrated with a heat pump provides efficient combustion heating as an alternative to EM.

High-Efficiency Electric Furnace – Saves 20-40% on heating costs compared to EM resistance heaters.

Wood Stoves or Pellet Stoves – Provides supplemental heat source without the high electrical costs.

Solar Heating – Renewable daytime heat source to reduce heat pump load.

For major home renovations or new HVAC system installation, weighing the pros and cons of these alternatives can lead to lower long-term energy costs compared to heavy reliance on EM heating.

Signs of EM Heat Malfunction

Some issues to watch out for if your EM heat is not functioning properly:

  • No heat despite cold outdoor temperatures
  • Heater staying on non-stop with no cycling
  • Burning smell from heater activation
  • High temperature limit tripping
  • Heater blowing cold air
  • Unusually high energy bills
  • EM indicator light not coming on
  • Thermostat EM control issues

Troubleshooting problems with EM heat may require the expertise of an HVAC technician. Do not attempt to service EM heaters on your own due to risk of electrical shock.

EM Heat Maintenance Tips

Proper maintenance is crucial to ensure an EM heating system remains in good working order:

  • Inspect auxiliary heaters each fall before using for winter. Check for dust/debris accumulation, signs of water intrusion or animal infestation, rust, and any damaged components.
  • Have a professional service technician do a thorough inspection and tune-up of the full heat pump system annually.
  • Vacuum around the emergency heater vents regularly to remove dust and dander.
  • Check that heater fins are straight and in good condition. Carefully straighten any bent fins with a plastic spatula to maintain airflow.
  • Confirm electrical connections are intact with no frayed wires or loose connections.
  • Clean or replace thermostat air filter each month during winter for optimal airflow.
  • Verify exhaust and combustion air vents are properly sealed and ventilated if combined with a natural gas furnace.
  • Test emergency heat function on milder fall days to confirm proper operation before freezing temps hit.

Following diligent maintenance procedures will keep your EM heating primed and ready when you need that critical boost in frigid weather.

FAQs About EM Heat on a Thermostat

Some frequently asked questions about EM heat function and operation:

What temperature does EM heat turn on?

The most common outdoor temperature when EM activates automatically is 35° F and below. This threshold can vary based on thermostat programming and the heat pump’s specifications.

Does EM heat use more electricity?

Yes, EM heat runs on high-energy electric resistance heating, which uses 3-5 times more electricity than the heat pump compressor alone. This leads to substantially higher energy bills when used extensively.

Why does my house get hotter when EM heat turns on?

The greater heating capacity of EM electric coils leads to hotter airflow temperatures. This sudden boost may feel excessive at first but helps offset the heat pump’s performance decline in cold conditions.

Should I leave my thermostat on EM heat all the time?

No, you should avoid running EM heat manually or continuously. This will waste significant energy and costs. Leave the thermostat in Auto EM mode so it only operates temporarily as needed.

Why does my heat pump run on defrost while EM heat is on?

A periodic defrost cycle melts frost buildup on the heat pump’s outdoor coils. EM activates temporarily to maintain heating comfort indoors until defrost is complete.

Can I replace my EM heaters with a more efficient system?

Yes, upgrading to integrated multi-stage heat pumps, gas furnaces, or variable capacity systems can provide more comfortable and affordable alternatives to EM resistance heating.


Understanding what is EM heat on your thermostat, why it exists, and how it operates allows you to utilize this function effectively when your heat pump needs a cold weather boost. Used properly and sparingly, it kicks in to prevent your home from dropping below the desired temperature on frigid days. Keeping up with maintenance, adjusting settings wisely, and exploring upgrades can help balance comfort and costs when using EM heat. With these insights, you can stay cozy warm all winter long without breaking the bank on energy bills!