What Is Emergency Heat on My Thermostat (and When Should I Use It)?

If your home is heated by a heat pump, there’s a good chance your thermostat has a setting most homeowners are afraid to touch: emergency heat, often labeled “Em Heat.” Every fall, as folks across the Oklahoma City metro flip their systems over to heating for the first time, we field the same question — what does this button do, and am I supposed to be using it?

The short answer: for the vast majority of the winter, you should leave it alone. But it’s worth understanding what it’s for, because using it at the wrong time quietly runs up your electric bill, and knowing when it is the right call can keep your family warm on the night your heat pump quits.

First, how a heat pump heats your home

Emergency heat only exists on heat pump systems, so it helps to know what makes a heat pump different from a gas furnace. Instead of burning fuel, a heat pump moves heat. In summer it pulls heat out of your house; in winter it runs in reverse, pulling warmth out of the outdoor air — yes, even cold air — and bringing it inside. It’s a remarkably efficient way to heat, which is why heat pumps have become common on Oklahoma homes.

But there’s a catch. The colder it gets outside, the less heat there is in the air for the pump to grab. On a mild 45° morning a heat pump heats a home easily. On a hard-freeze night in the teens, it can struggle to keep up on its own. That’s where backup heat comes in — and that’s the source of most of the confusion.

Auxiliary heat vs. emergency heat: the key difference

Nearly every heat pump has a backup (or supplemental) heat source built in. On an all-electric system that’s usually a set of electric resistance heat strips inside the air handler. On a “dual fuel” system it’s a gas furnace. That backup gets used in two very different ways, and telling them apart is the whole ballgame:

• Auxiliary heat (Aux) turns on automatically. When the thermostat sees that the heat pump alone can’t keep up — on a cold morning, or when you’ve jumped the setting up several degrees at once — it quietly brings the backup in to help while the heat pump keeps running. This is the system doing exactly what it was designed to do. You don’t control it; the thermostat does.

• Emergency heat (Em Heat) is something you turn on manually. When you select it, the thermostat shuts the heat pump off entirely and heats your home using only the backup source. There’s nothing automatic about it — it’s an override that takes the efficient part of your system out of the equation.

So aux heat means “heat pump plus a little help.” Emergency heat means “heat pump off, backup only.” That distinction is why one is normal and the other is a special-occasion setting.

When you should actually use emergency heat

Emergency heat is named literally. It’s for when the heat pump itself has a problem and can’t run safely or effectively. The clearest example: the outdoor unit stops working. Maybe a winter storm iced it over solid, a limb fell on it, it’s making an awful noise, or it simply won’t come on. Switching to Em Heat bypasses the failed outdoor unit and keeps warm air coming from your backup source until a technician can get out to you.

A few situations where Em Heat is the right move:

• The outdoor unit has clearly failed — it won’t run, it’s damaged, or it’s encased in ice.
• You hear grinding or loud mechanical noises from the outdoor unit and want to stop it from running until it’s inspected.
• A technician has advised you to run on emergency heat temporarily until a scheduled repair.

In every one of these cases, emergency heat is a bridge to get you through until the heat pump is fixed — not a destination.

When you should NOT use it

Here’s where good intentions cost money. Because a heat pump can look like it’s “not doing much” on a cold day — the air from the vents is warm but not furnace-blast hot — some homeowners assume it’s broken and flip to emergency heat to force stronger heat. That’s almost always a mistake.

Running on Em Heat means you’re heating your entire home with the backup source alone. Electric heat strips are the most expensive way to heat a house, and running them around the clock instead of the efficient heat pump can send your electric bill climbing noticeably. Skip emergency heat when:

• It’s just a cold day and the heat pump is working — your normal “Heat” setting already calls for aux heat automatically when it’s needed.
• You want the house to warm up faster. It won’t meaningfully; you’ll just pay more.
• The heat pump is running fine and you simply don’t understand why the air feels cooler than a furnace. (Cooler-but-steady vent air is normal for a heat pump — here’s more on how heat pumps compare to gas furnaces in Oklahoma.)

If you ever do switch to emergency heat, remember to switch it back to “Heat” once the problem’s resolved. It’s easy to forget, and a thermostat left on Em Heat for weeks is a common reason we hear “my bill was huge last month.”

A note for Oklahoma’s fall weather

Early fall is exactly when this question comes up, because it’s the first time all year many homeowners are running heat. Oklahoma’s autumn is famous for wild temperature swings — 80° one afternoon, a frost warning two days later — so your heat pump may kick on for the first time and behave in ways you haven’t seen since last winter. Seeing the “aux heat” indicator on a chilly morning is normal and nothing to override. Seeing the heat pump struggle on genuinely mild days, or seeing aux heat run nonstop, is worth a look from a professional before the real cold arrives.

Fall is also the ideal time for a heating tune-up. A technician can confirm your heat pump reverses into heating mode correctly, check that the backup heat and the automatic aux staging are working, and make sure defrost cycles are functioning — all the things you’d rather discover in October than during the first hard freeze.

The bottom line

Emergency heat is a safety-net setting, not an everyday one. Leave your thermostat on the regular “Heat” mode and let it bring in auxiliary heat on its own when the weather demands it; reserve “Em Heat” for the times your heat pump has genuinely failed and you need warmth until it’s repaired. If your heat pump is short on heat when it shouldn’t be, aux heat is running constantly, or your outdoor unit won’t come on this fall, the fix is a diagnosis — not a bigger electric bill.

Triple Play Home Services handles heat pump repairs, tune-ups, and installations across Edmond, Oklahoma City, and the surrounding metro, with flat-rate pricing you approve up front before any work begins. Call (405) 500-5333 to get your system checked and ready for winter.