What EM Heat Means on Your Thermostat (and Whether You Should Turn It On)

It usually happens on the first truly cold morning of the year. You walk past the thermostat, notice it says EM HEAT or AUX, and have no memory of ever pressing anything. Or the house is chilly, you scroll through the modes looking for more heat, and there it is: Emergency Heat.

Sounds serious.

Here is what it actually is, when your heat pump uses it on its own, and the one situation where you should turn it on yourself.

What emergency heat is

Nearly every heat pump system has a backup. On most systems around here that backup is a set of electric resistance strips inside the air handler, basically a very large version of the coils in a toaster. On a dual fuel system, the backup is a gas furnace. Either way, it is a second heat source that lives alongside the heat pump.

When you switch the thermostat to EM HEAT, you are turning the heat pump off completely and running the house on that backup alone. That is the whole definition. Emergency heat is not extra heat. It is different heat, and on an all-electric system it is by far the most expensive kind.

EM heat versus AUX heat

These get mixed up constantly, and the difference matters.

AUX heat (auxiliary, sometimes shown as "Aux" or a little flame icon) comes on automatically. The heat pump is still running, and the backup strips kick in for a while to help it catch up. You do not control this, and seeing it flicker on now and then in winter is normal.

EM heat (emergency) is manual. You choose it. The heat pump shuts off entirely and the backup carries the whole load until you switch it back.

So if your thermostat is showing AUX and you never touched it, the system is doing what it was designed to do. If it is showing EM HEAT and you never touched it, someone in the house probably did, or a power blip reset the thermostat to a default. Either way, it is worth switching back to regular HEAT and watching what happens.

When a heat pump switches to auxiliary heat on its own

A heat pump moves heat from outside air into the house. As it gets colder outside, there is less heat to move and the system slows down. The thermostat is watching two things: how far the room is from the set temperature, and how fast it is closing the gap. When the heat pump falls a couple of degrees behind and is not catching up, the thermostat calls for AUX.

The other trigger is defrost. In damp cold, which is most of our winter, frost builds up on the outdoor coil. The system periodically runs a short defrost cycle, and during those few minutes the strips come on so the vents do not blow cold air. If you see steam rising off the outdoor unit on a January morning, that is defrost, and it is normal.

At what temperature does a heat pump stop working?

Later than most people think. Older units started losing ground somewhere in the mid 30s. Modern cold-climate heat pumps, including most of what we install, hold their rated output well into the teens and keep producing usable heat below that.

Now look at our weather. A typical Puget Sound winter night sits in the 30s and 40s. We get a handful of mornings in the 20s and, every few years, a stretch in the teens. That means a properly sized modern heat pump should carry a Snohomish County home on its own almost every day of the year, with AUX helping out only on the coldest mornings. If your backup heat is running most of the winter, something is off, and we get to that below.

Should you turn EM heat on?

Only if the heat pump is actually not working. That is the emergency the setting is named for.

Signs of that:

• The outdoor unit is not running at all, or is making a noise it has never made before. (If the outdoor unit is silent, check its breaker first. We cover what a tripping heat pump breaker usually means in our guide to why breakers keep tripping.)
• The outdoor unit is iced into a solid block, not just frosted, and is not clearing itself.
• The vents are blowing cold air in HEAT mode for more than a few minutes, and it is not defrost.
• A technician told you to switch to EM until the repair is done.

In those cases, yes, flip it to EM HEAT so the house stays warm, then call us for heat pump repair. In every other case, leave it on regular HEAT. Turning on EM heat because the house feels a little slow to warm up does not make it warm up faster. It just switches you to the expensive heater.

Why the January power bill jumps

More often than not this is the backup strips running far more than they should, and there are a few common reasons.

The biggest one is the thermostat schedule. With a furnace, dropping the temperature 8 or 10 degrees at night and cranking it back up in the morning saves money. With a heat pump, that same setback makes the system fall way behind at 6am, which triggers AUX for an hour. Keep setbacks small, 2 to 3 degrees, or skip them entirely.

Beyond that: a clogged filter or dirty outdoor coil that keeps the heat pump from keeping up, low refrigerant, a stuck reversing valve, or a system that was undersized for the house in the first place. That last one is why we do a real load calculation on every heat pump installation and back it with a sizing guarantee. If what we recommended cannot do what we promised, we replace it.

A note on dual fuel, since it changes this whole conversation

If you have gas service, there is a version of this where the backup is not electric strips at all. A dual fuel system pairs a heat pump with a gas furnace, and the thermostat hands off between them automatically based on outdoor temperature. The heat pump does the work on the mild days, which here is most of them, and the furnace takes over on the cold mornings when it is the cheaper option.

For a lot of Puget Sound homes with an existing furnace in decent shape, this is the best of both, and it makes the EM heat question mostly go away. Worth asking about if you are due for a replacement.

What to do this month

If your thermostat is showing EM HEAT right now and the heat pump is fine, switch it back to HEAT. That is the whole fix for a lot of people.

If you are not sure whether the heat pump is fine, a fall tune-up answers that before the cold gets here. We have a $19.99 annual plan for gas furnaces and a $29.99 twice-yearly plan for heat pumps, and plan members get a lifetime warranty on any system we install. Call (425) 496-7300 and we will take a look, explain what the thermostat is telling you, and leave you with a heating setup you do not have to think about all winter.