Your thermostat has a setting labeled EM Heat, EMH, or sometimes EMER. Most homeowners have no idea what it does, when to use it, or why their heating bill spikes when they switch to it during a cold snap.
EM Heat is one of the most misunderstood thermostat settings in any home with a heat pump. Used correctly, it keeps your family warm when your primary heating system fails. Used incorrectly, it can double your electric bill in a single month.
This guide breaks down exactly what EM Heat is, how it differs from auxiliary heat, when you should manually switch to it, and what to do if your thermostat is stuck in emergency mode. By the end, you will know when EM Heat is protecting your home and when it is quietly draining your wallet.
What EM Heat Means on Your Thermostat
EM Heat stands for emergency heat. It is a backup heating mode found exclusively on thermostats connected to heat pump systems. When you activate EM Heat, your thermostat shuts down the outdoor heat pump completely and warms your home using only a secondary heating source.
That secondary source is built into your indoor air handler or furnace and is usually one of the following:
- Electric resistance heat strips (the most common backup in our region)
- A natural gas furnace (in dual-fuel hybrid systems)
- A propane or oil furnace
If your thermostat does not have an EM Heat option, you most likely have a standard gas furnace or all-electric system rather than a heat pump. The setting only exists when your home has both a primary heat pump and a backup heating element.
EM Heat vs. Aux Heat: What Is the Difference?
This is the single biggest source of confusion homeowners have about heat pump thermostats, and getting it wrong can cost you hundreds of dollars per winter. Both settings activate your backup heating source, but they work very differently.
Auxiliary Heat (Aux Heat)
Aux Heat is automatic. Your thermostat turns it on by itself when it detects that the heat pump alone cannot keep up with the temperature setting. When Aux Heat activates, both your heat pump and your backup heating source run simultaneously, working together to restore indoor temperature faster.
You will typically see the Aux Heat trigger when:
- Outdoor temperatures drop into the low 30s or below, and the heat pump cannot extract enough heat from the outside air
- You raise the thermostat setting by more than two or three degrees at once
- The heat pump enters a defrost cycle to clear ice off the outdoor coil
Aux Heat is normal, expected, and built into the design of modern heat pump systems to operate during cold weather.
Emergency Heat (EM Heat)
EM Heat is manual. You have to physically switch your thermostat to it. When EM Heat is on, the outdoor heat pump shuts off entirely, and only the backup heating source runs. There is no teamwork happening, just a single, less efficient system trying to warm the entire house on its own.
The simplest way to remember the difference: Aux Heat is your system asking for help. EM Heat is you taking over and telling the heat pump to stop.
When You Should Manually Switch to EM Heat
EM Heat is intended for genuine emergencies, not for general cold weather. There are really only three situations where flipping that switch makes sense.
1. Your Heat Pump Has Failed
If your outdoor unit has stopped working entirely, whether from a mechanical failure, electrical issue, or physical damage, EM Heat lets you keep your home warm while you wait for a service technician. This is the textbook use case.
2. Your Outdoor Unit Is Encased in Ice
During an ice storm or freezing rain event, your heat pump can become covered in ice that the defrost cycle cannot keep up with. If ice builds up heavily on the outdoor unit, the fan blades can be damaged when the system tries to operate. Switching to EM Heat shuts down the outdoor unit entirely, protecting it from further damage until the ice melts or a technician can clear it.
3. Your Heat Pump Is Running Constantly but Not Keeping Up
This one requires judgment. If outdoor temperatures are unusually low and your heat pump has been running nonstop without reaching your thermostat setting, EM Heat can provide stronger immediate warmth. But before flipping the switch, it is worth asking whether the issue is actually extreme cold or whether something is wrong with the system. Frequent reliance on EM Heat during normal winter weather is almost always a sign of an underlying problem.
When You Should Not Use EM Heat
Most situations where homeowners reach for EM Heat are the ones they should leave alone. Here are the common mistakes we see in North Georgia homes.
Do not use EM Heat just because it is cold outside. Modern heat pumps are 2 to 3 times more efficient than electric resistance heating, even at temperatures near freezing. If your system is working correctly, the regular Heat setting will handle cold weather just fine, with Aux Heat kicking in automatically when needed.
Do not use EM Heat to warm the house faster. It feels intuitive that emergency heat would be stronger, but in reality, running on backup alone is less efficient and more expensive. Your home will reach temperature faster with the heat pump and Aux Heat working together.
Do not leave EM Heat on for days at a time. If you have switched to EM Heat because your heat pump is broken, that is a temporary measure. Every day you run on emergency heat alone is a day of dramatically higher utility costs and additional strain on the backup system, which was not designed to operate continuously.
Why EM Heat Costs So Much More
The reason EM Heat is so expensive comes down to physics. A heat pump does not generate heat; it moves heat from outside air into your home. Even when it is 30 degrees outside, there is still heat energy in the air that the heat pump can extract and concentrate indoors. This makes heat pumps remarkably efficient.
Electric resistance heat strips, on the other hand, generate heat by forcing electricity through metal coils, the same principle used in a hair dryer or space heater. Every unit of electricity becomes one unit of heat. There is no multiplication, no efficiency gain.
The result is that running on EM Heat alone can use two to three times more electricity than running on the heat pump for the same amount of warmth. Homeowners who accidentally leave EM Heat on for an entire winter month often see their electric bill double.
What to Do If Your EM Heat Indicator Light Is Stuck On
Most heat pump thermostats display a small indicator (often a red light or a label on the screen) when emergency heat is active. If you see that indicator on, but you never switched the system to EM Heat, something is wrong. There are a few possibilities.
The system activated EM Heat automatically due to a heat pump fault. Some thermostats are programmed to switch to emergency heat when they cannot communicate with the outdoor unit. This usually means the heat pump has stopped responding and needs professional diagnosis.
The thermostat is bumped or programmed incorrectly. Check that the mode is set to Heat, not EM Heat. It is surprisingly easy to switch the setting accidentally when adjusting the temperature.
The thermostat itself is malfunctioning. A failing thermostat may incorrectly display the EM Heat indicator or be unable to switch out of emergency mode.
If the indicator remains on after you confirm the mode is set to Heat, schedule a service call. Continuing to operate in emergency mode will significantly increase your bill while a technician diagnoses the underlying issue.
How to Switch Your Thermostat to EM Heat (the Right Way)
If you have determined that your situation actually calls for emergency heat, the process is straightforward on most thermostats:
- Locate the system mode setting on your thermostat (Heat / Cool / Off / Auto / EM Heat or EMER)
- Switch the mode from Heat to EM Heat
- Confirm the indicator light or display changes to show that emergency mode is active
- Set your desired temperature, but keep in mind the system will be slower to recover
- Schedule a service call as soon as possible if your heat pump is the reason you switched
Once your heat pump is repaired or the situation that prompted EM Heat has resolved, switch back to the regular Heat setting immediately. The system will not switch back automatically.
Frequently Asked Questions About EM Heat
Should I use Heat or EM Heat in cold weather?
Use the regular Heat setting in almost all cases, even during cold winter days. Modern heat pumps are designed to handle freezing temperatures, and your thermostat will automatically activate Aux Heat as backup support when needed. Only switch to EM Heat if your heat pump has failed or is at risk of damage from ice buildup.
How long can I run emergency heat?
Technically, your system can run on EM Heat for as long as needed without mechanical damage. The real limit is your electric bill. Backup heat strips use significantly more electricity than your heat pump, so every additional day on EM Heat adds noticeable cost. Treat it as a short-term solution while repairs are underway.
Will emergency heat raise my electric bill?
Yes, often dramatically. Electric resistance heat is one of the least efficient ways to warm a home, and running it as your only heat source can double or even triple your daily heating costs. Homeowners who leave EM Heat on for an entire month often see bills far higher than those in a typical winter month.
Can I use emergency heat as my regular heat?
You should not. Backup heat strips are not designed for continuous, daily operation as a primary heating source. Beyond the cost, prolonged use accelerates wear on components engineered to operate only as a temporary supplement. If you find yourself relying on EM Heat regularly, your heat pump likely needs service.
Does EM Heat mean my heat pump is broken?
Not necessarily, but if EM Heat activates without you switching it on, that is often the signal of a heat pump problem. Homeowners who manually choose EM Heat are usually doing so on purpose. Homeowners who notice the EM Heat indicator on without changing the setting themselves should have the system inspected.
Get Heat Pump and Furnace Help in North Georgia
If your heat pump is forcing you to rely on EM Heat, or if your indicator light is stuck on and you cannot figure out why, our team can help. MR. HVAC has been serving Cherokee County and the surrounding North Georgia communities for over 25 years, and we know how heat pump systems behave in our specific climate.
Whether you need a fast diagnosis of a heat pump that quit during a cold snap, a thorough heat pump repair visit, or a furnace maintenance tune-up to ensure your backup heating source is ready when you need it, we are a phone call away. Routine furnace repair and seasonal checkups can also catch the kind of issues that send heat pump systems into emergency mode in the first place. For homeowners running aging equipment, a complete system replacement can be the more cost-effective long-term solution.
Call MR. HVAC or schedule online. We proudly serve Canton, Woodstock, Roswell, Alpharetta, and the surrounding North Georgia communities.
