EM Heat is the emergency heat mode on a heat pump thermostat: it shuts the outdoor unit off completely and runs the backup heat source alone, which in most North Georgia homes means the electric resistance strips inside the air handler. The setting exists so you can keep the house warm when the heat pump has failed or is iced over, not for ordinary cold weather, because resistance strips use two to three times the electricity of a working heat pump for the same warmth. The thermostat will not switch back on its own, so a system left in EM Heat runs on strips until someone moves it back to Heat.
WHAT EMH OR EMER ON A THERMOSTAT MEANS
Thermostat makers abbreviate the same mode three ways. Most digital thermostats spell it out as EM Heat, some models shorten it to EMH on a crowded display, and older slide-switch thermostats and a few manufacturer-branded stats label the position EMER. All three mean emergency heat, and all three do the same thing when selected: the Y and O signals to the outdoor unit stop, the W terminal energizes the backup heat, and the indoor blower runs the strips as the only source of warmth. A thermostat with none of these settings is wired to a gas furnace or a straight electric furnace, since the mode only exists when there is a heat pump to bypass.
The backup source behind EM Heat is usually a bank of electric heat strips rated between 5 and 20 kilowatts, sized at install to carry the house on its own. Some homes pair a heat pump with a gas furnace instead, and on those systems EM Heat runs the furnace alone.
EM HEAT VS. AUX HEAT
Aux Heat and EM Heat use the same strips but are controlled differently, and that difference is where most of the confusion and most of the wasted electricity comes from. Aux Heat is automatic. The thermostat energizes the strips on its own when the heat pump is falling behind the setpoint by about two degrees, during a defrost cycle when the outdoor coil has iced, or after a large setpoint jump, and the heat pump keeps running the whole time. The strips drop out again as soon as the room catches up. Seeing Aux Heat on the display during a 28-degree morning is normal operation.
EM Heat is manual. The homeowner selects it, the outdoor unit stops, and the strips carry the entire load for as long as the mode stays selected. Aux Heat is the system adding help; EM Heat is the homeowner taking the heat pump out of service. The two labels are not interchangeable, and a thermostat parked on EM Heat all winter produces the biggest heating bills we see on service calls.
WHEN TO USE EMERGENCY HEAT
Three situations call for EM Heat, and each one is temporary.
- The heat pump has failed. The outdoor fan is not turning, the compressor is silent, or the breaker trips every time the unit tries to start. EM Heat keeps the house warm until heat pump repair can be scheduled.
- The outdoor unit is encased in ice after freezing rain and the defrost cycle cannot clear it. Running a heat pump with its fan blades frozen into a block of ice breaks blades and burns out the fan motor, so shutting it down protects the equipment until the ice melts.
- The heat pump is running nonstop on an unusually cold night and the house is still losing ground. This one takes judgment. A healthy heat pump with working Aux Heat holds a North Georgia house through the teens, so a system that cannot keep up at 25 degrees usually has a refrigerant charge, defrost, or strip problem that EM Heat is masking rather than solving.
The answer to whether to run Heat or EM Heat in cold weather is Heat, in nearly every case. Modern heat pumps pull usable heat out of outdoor air well below freezing, and the thermostat brings the strips in automatically on the few nights they are needed.
WHEN NOT TO USE EM HEAT
Do not select EM Heat because it is cold outside. The Heat setting with automatic Aux Heat already covers cold snaps, and it does so with the heat pump providing most of the warmth. Do not select it to warm the house faster, either. The strips alone are not stronger than the heat pump and strips running together, so recovery from a setback is slower on EM Heat, not quicker. And do not leave it on for days once the emergency has passed. Heat strips are built as a supplement, and running them as the sole source for weeks puts hours on the sequencers and limit switches that were never meant to cycle that often.
EM Heat will not damage the system if it runs for a long time. The limit is the electric bill. Every day on strips alone costs more than the same day on the heat pump, and a month on EM Heat shows up as a utility bill well above a normal winter month. Treat it as a bridge to a repair, not as a regular heating mode, and if the house seems to need it every winter, the heat pump itself needs service.
WHY EM HEAT COSTS SO MUCH MORE TO RUN
A heat pump does not make heat. It moves heat from outdoor air into the house, and even at 30 degrees there is enough heat energy in the air for the refrigerant to absorb it outside and release it inside. A well-sized heat pump in metro Atlanta delivers roughly two to three units of heat for every unit of electricity it draws across a typical winter. Electric resistance strips work like a hair dryer: current runs through a coil, the coil glows, and one unit of electricity becomes exactly one unit of heat. On EM Heat the house gets all of its warmth from that one-to-one source, which is why the meter spins two to three times faster for the same indoor temperature.
EM HEAT INDICATOR ON WHEN YOU DID NOT SELECT IT
Most heat pump thermostats show a red light or an on-screen label when emergency heat is active. When that indicator is on and nobody chose EM Heat, one of three things has happened. Some thermostats drop into emergency heat on their own when they lose communication with the outdoor unit, which usually means the heat pump has stopped responding and the heat pump needs diagnosis. The mode may also have been bumped while someone was adjusting the temperature, since EM Heat sits one click past Heat on many menus, so the first check is confirming the system mode reads Heat. A thermostat that will not leave emergency mode after the setting is corrected has a failed relay or control board of its own.
An EM Heat light that comes on by itself is not proof the heat pump is broken, but it is the most common way homeowners find out that it is. If the indicator stays on after the mode reads Heat, schedule service rather than waiting out the bill.
HOW TO SWITCH TO EM HEAT AND BACK
On most thermostats the system mode menu cycles through Heat, Cool, Off, Auto, and EM Heat or EMER. Select EM Heat, confirm the indicator or label shows emergency mode is active, and set the temperature a degree or two lower than usual, since the strips recover slowly. Schedule service the same day if a failed heat pump is the reason. Once the heat pump is repaired or the ice has cleared, move the mode back to Heat by hand. No common residential thermostat exits emergency heat on its own, and the mode will stay selected through the spring if nobody changes it.
GET THE HEAT PUMP BACK ONLINE
MR. HVAC has repaired heat pumps and their backup heat strips across Canton, Woodstock, Roswell, Alpharetta, and the rest of Cherokee County and North Georgia for more than 25 years. If EM Heat is the only thing keeping the house warm, schedule heat pump service online or call (770) 213-4111, and if the outdoor unit is past the point of repair, financing and current Georgia rebates apply to a new heat pump installation.
