Emergency Heat

What emergency heat actually does

A heat pump normally heats a house by moving heat from outdoor air to indoor air, which is efficient even in cold weather. It also has a backup heat source for the coldest conditions — usually electric resistance strips in the air handler, sometimes a gas furnace in a dual-fuel system.

Emergency heat is a mode that turns the heat pump off completely and heats the house using only that backup. When you set the thermostat to EM HEAT, the outdoor unit stops running entirely, and the electric strips or furnace carry the whole load by themselves.

The name is literal. It is an emergency setting — a way to keep the house warm when the heat pump cannot run at all, while you wait for a repair. It is not a colder-weather mode or a way to heat the house faster, even though it is frequently mistaken for both.

When you should actually use it

There is really one situation that calls for emergency heat: the heat pump itself has failed or been damaged and cannot run safely.

That covers a compressor that has quit, an outdoor unit that has been crushed by a fallen limb or encased in ice and cannot defrost, a refrigerant leak that has left the system unable to produce heat, or any condition where the outdoor unit needs to stay off until a technician can look at it. In those cases, switching to emergency heat keeps the house warm on the backup source without asking the damaged heat pump to run.

If you have been told by a technician to run emergency heat until they can come out, that is a legitimate use. If you switched it on yourself simply because it got cold, that is almost certainly a mistake — the system was already handling the cold automatically, and you have now turned off the efficient part of it.

EM heat versus regular heat

This is the distinction that causes the most confusion and the most wasted money, so it is worth being precise.

Regular heat — the plain HEAT setting — runs the heat pump as the primary source and lets the system bring on backup heat automatically only when it is needed. That automatic backup is called auxiliary heat. On a cold morning, a system in normal HEAT mode uses the heat pump for most of the work and the strips only to fill the gap. This is how the system is designed to run all winter.

Emergency heat — the EM HEAT setting — shuts the heat pump off entirely and runs on backup alone, all the time, regardless of how mild it is outside. Nothing about the weather triggers it; you chose it, and it stays chosen until you change it back.

So the practical rule is simple. In normal winter weather, leave the thermostat on HEAT and let the system manage its own backup. Only move it to EM HEAT when the heat pump is actually broken. The two settings look almost identical on the thermostat and cost very different amounts to run.

Why leaving it on is expensive

Electric resistance heat — the most common backup — is one of the least efficient ways to heat a home. A heat pump running normally delivers several units of heat for each unit of electricity it consumes, because it is moving existing heat rather than generating it. Electric strips deliver roughly one unit of heat per unit of electricity, because they generate heat directly, like a giant toaster.

When you run emergency heat, you are heating the entire house with only that inefficient backup and leaving the efficient heat pump switched off. In mild weather especially, that is a dramatic difference in operating cost for exactly the same comfort.

The trap is that the house still feels fine. Emergency heat does keep you warm, so nothing prompts you to change it back — until the electric bill arrives. Every winter, some of the higher heating bills people cannot explain trace back to a thermostat that got left on EM HEAT and never got switched off.

What to do instead

If it is simply cold outside and the house is not quite keeping up, the right setting is normal HEAT, not emergency heat. The system will bring on its backup automatically as needed, which is the efficient way to get the same result. If you find yourself reaching for emergency heat because the house feels cold in ordinary winter weather, that is a sign the heat pump may not be working correctly — and the fix is a service call, not the EM HEAT button, because running on backup only hides the underlying problem while it costs you money.

A quick self-check: look at whether the thermostat is already set to EM HEAT without your realizing it, since a system stuck in that mode will run up costs and produce no obvious symptom other than the bill. Then look at the outdoor unit — if it is iced over or blocked, that is worth addressing, though a unit that repeatedly ices needs a technician.

Beyond that, why a heat pump is underperforming — charge, defrost, the reversing valve, airflow — is diagnosed with instruments by a qualified technician. Emergency heat is a way to stay warm while that happens, not a solution in itself.