The EMER setting on your thermostat is one of the most misunderstood buttons in your house. Most homeowners think it means “heat harder” or assume it is a backup mode that just kicks on automatically. The truth is more specific. So what does emer mean on a thermostat? It stands for Emergency Heat, and it bypasses your heat pump entirely to run on the electric resistance or gas backup heat source directly. After diagnosing thousands of HVAC issues, I can tell you exactly when this mode is useful and when it is burning money for no reason.
Heat Pumps Explained Briefly
The EMER setting only exists on thermostats controlling heat pump systems. A heat pump is essentially an air conditioner that runs in reverse during winter — extracting heat from outdoor air and pumping it inside. The system also has a backup heat source (called auxiliary heat or aux heat) for very cold days when the heat pump cannot keep up.
Aux heat is usually electric resistance heat strips inside the air handler, though some hybrid systems use a gas furnace as backup. In normal operation, the heat pump does most of the work and the aux only kicks on when the temperature differential becomes too large.
What EMER Actually Does
When you switch to EMER, the thermostat tells the system to stop using the heat pump compressor entirely and run only on the backup heat source. The outdoor unit (the fan and compressor sitting outside) shuts off. Heat comes purely from the electric strips or gas backup inside the air handler.
This is fundamentally different from AUX heat, which runs the heat pump and backup together. EMER is for when the heat pump itself has failed and you need to keep warm until a technician arrives.
When to Use Emergency Heat
- Heat pump outdoor unit is frozen over: Ice has built up on the coils and the unit cannot defrost itself. EMER prevents further damage while the unit thaws.
- Heat pump is making unusual noises: Grinding, screeching, or banging suggests mechanical failure. Switch to EMER to stop the compressor before damage spreads.
- Outdoor fan stopped working: Heat pump compressors can overheat without proper airflow. EMER protects the equipment.
- You smell electrical burning at the outdoor unit: Immediate shutoff via EMER is correct, followed by a service call.
- Service technician advises it: If a tech tells you to use EMER until they arrive, do exactly that.
When Not to Use Emergency Heat
EMER is not a “make it warmer faster” button. Running emergency heat costs significantly more than normal heat pump operation. Electric resistance heat — the most common backup — is roughly 2 to 3 times more expensive per BTU than a properly operating heat pump.
Skip EMER if your house just feels cold and the heat pump is running normally. Lower the thermostat at night and raise it slowly in the morning instead — the heat pump can keep up if you give it time. Skip EMER during normal cold snaps when the aux heat is already cycling on automatically as needed.
EMER Versus AUX — The Critical Difference
If the thermostat shows AUX (auxiliary heat) on its own without you setting it, that is normal. The system has automatically engaged the backup heat to supplement the heat pump because the demand is higher than the heat pump alone can meet. Both heat sources are running together.
If you manually switch to EMER, only the backup runs. The heat pump is fully off. EMER is a user-initiated emergency setting; AUX is an automatic supplement that requires no action from you.
The Cost of Running EMER
Quick math on a typical home. A 3-ton heat pump in heating mode operating at 35°F outdoor temperature has a Coefficient of Performance (COP) of roughly 3.0 — meaning it produces 3 units of heat for every 1 unit of electricity consumed. Electric resistance heat has a COP of exactly 1.0. Running EMER instead of heat pump cuts your efficiency to one-third.
For a 2,000 square foot home in 35°F weather, that difference adds roughly $8 to $14 per day to your electric bill. Run EMER for a week instead of fixing the actual problem and you have spent more than a service call would have cost.
Common Reasons People Accidentally Turn It On
Smart thermostats like Nest, Ecobee, and Honeywell T6 Pro have EMER buttons in their menus. Users tap through screens, see Emergency Heat, and switch it on thinking it will make the system run harder. The system runs hotter for a minute (because the strips heat up faster than the heat pump) and they assume the button “worked.”
The bills the following month tell the real story. A homeowner I worked with left EMER on for three months without realizing — their electric bill jumped from $180 to $560 per month before they noticed.
How to Set It Correctly
For normal winter operation, your thermostat should be on HEAT mode, not EMER. The system controls both the heat pump and aux heat automatically based on demand. Set the temperature once and let it work.
If you have a programmable schedule, avoid large setbacks (more than 4 to 6 degrees) on heat pump systems. Big setbacks trigger aux heat to recover quickly, which wastes the efficiency advantage of the heat pump. Smaller, more frequent adjustments work better.
When to Call a Technician
If you find yourself reaching for EMER more than once or twice a winter, the system needs service. Common underlying issues include low refrigerant, dirty outdoor coils, faulty defrost controls, or undersized equipment for the home. None of these resolve themselves over time.
A typical HVAC service call runs $90 to $150 for diagnostic plus parts and labor. Refrigerant top-up is $250 to $500. Defrost control board replacement is $300 to $600. Each is far cheaper than a season of running emergency heat unnecessarily.
The Short Answer
So what does emer mean on a thermostat? Emergency heat — a manual override that turns off your heat pump and runs only on backup heat. Use it only when the heat pump itself has failed or is unsafe to run. Otherwise leave the thermostat on regular HEAT mode and let the system manage itself. Knowing the difference between EMER and AUX is one of those small homeowner literacy items that can save hundreds of dollars per winter.
