Auxiliary vs Emergency Heat: What Your Thermostat Is Telling You

TL;DR: Aux heat (auxiliary heat) is a normal backup that kicks in automatically when it's too cold outside for your heat pump alone, usually below 35-40°F. Emergency heat is a manual switch you flip only when the heat pump itself has failed, and it can run your electric bill up by $150-$300 a month if left on by mistake. If "Aux" or "Em Heat" stays lit for more than a day or two straight, something's wrong with the outdoor unit.

_Last reviewed: July 2026 · 7 min read_

You glance at the thermostat, see "Aux Heat" or "Emergency Heat" glowing, and wonder if you should be worried or if it's just doing its job. The short answer: it depends on which word you're seeing and how long it's been lit. Here's what each one actually means and when it's telling you to call someone.

Okoniq Property Hub helps landlords log HVAC service dates and thermostat quirks per unit, so a spike in aux-heat runtime doesn't get missed across a whole portfolio.

What does "Aux Heat" mean on a heat pump thermostat?

Aux heat means your heat pump has switched on a secondary heat source, usually electric resistance coils, because the outdoor unit alone can't pull enough heat from the cold air. Heat pumps work by moving heat from outside air into your house, and that process gets less efficient as the temperature drops. Most systems start leaning on aux heat somewhere between 35°F and 40°F outside, though the exact threshold depends on your equipment and how the installer set the balance point.

This is normal, automatic, and temporary by design. The thermostat is supposed to blend in aux heat during cold snaps or right after you bump the setpoint up a few degrees, since the heat pump alone can't catch up fast either. If you check the thermostat on a 20°F morning and see "Aux" lit, that's the system doing exactly what it's built to do.

The problem shows up when aux heat runs far more than it should for the outdoor temperature. That usually points to a heat pump that's losing capacity, similar to how a furnace shows warning signs before it fails outright, as covered in 8 signs your furnace is failing. Low refrigerant, a failing compressor, or a dirty outdoor coil can all force the system to lean on backup heat even when it's only 45°F outside.

What triggers Emergency Heat, and how is it different from Aux?

Emergency heat is a manual override you switch on yourself when the heat pump's outdoor unit has stopped working entirely, whether from a mechanical failure, an ice buildup it can't defrost, or an electrical fault. Unlike aux heat, which the thermostat controls automatically in the background, emergency heat is a setting you have to flip on purpose, usually a switch labeled "Em Heat" on the thermostat.

When emergency heat is on, the outdoor unit is fully bypassed. All your heating comes from the electric resistance coils (or in some systems, a gas furnace acting as backup). That's useful for a night or two while you wait on a repair, but it's not meant to run for weeks. Resistance heat is roughly 3 times more expensive to run than a working heat pump for the same amount of warmth, since it's converting electricity to heat directly instead of moving existing heat around.

If you find the "Em Heat" switch flipped on and you didn't do it, or it's been on for more than a few days without you calling for service, that's the thermostat telling you the compressor or outdoor fan hasn't run in a while. Worth checking the outdoor unit for ice buildup, which can also affect nearby baseboard heater zones if the system shares a backup loop.

Why does Aux or Emergency heat cost so much more to run?

Both rely on electric resistance heat, which costs roughly $0.10 to $0.15 more per kilowatt-hour of usable heat than a heat pump running in its efficient range, translating to $150-$300 a month in added cost for an average 2,000 sq ft home if it runs continuously through a cold month. A heat pump moves 2-3 units of heat for every 1 unit of electricity it uses. Resistance coils give you exactly 1 unit of heat for 1 unit of electricity, no multiplier.

| | Aux Heat (normal) | Emergency Heat (manual) | |---|---|---| | Triggered by | Thermostat automatically, cold weather | You, by hand | | Typical duration | Minutes to hours per cycle | Should be days at most, until repaired | | Cost vs heat pump | 2-3x higher while active | 2-3x higher, for the entire time it's on | | What it means | System is working as designed | Outdoor unit isn't running at all |

If your December bill jumped $200 over November and nothing else changed, check the thermostat history (most smart thermostats log aux-heat runtime hours) before assuming it's just a cold month. A properly sealed attic reduces how often aux heat needs to kick in at all; see attic insulation upgrade payback for what that fix typically saves.

When should you actually use Emergency Heat vs call for repair?

Use emergency heat only as a short bridge while you're waiting for a technician, not as a long-term fix. Flip it on if the outdoor unit is obviously frozen solid, making grinding noises, or not spinning at all, and you need heat that same night. Once you've switched it on, call for service within a day or two. Leaving it on for a week because "the house feels warm anyway" just means paying resistance-heat prices for a heat pump that's sitting broken outside.

Before flipping the switch, check two things yourself: the outdoor unit's breaker (a tripped breaker looks identical to a dead compressor from inside the house), and visible ice coating the coils, which sometimes clears with a 30-minute defrost cycle rather than a full repair call. If neither explains it, emergency heat is the right call until a tech can look at the reversing valve, compressor, or refrigerant charge.

FAQ

Is it bad if aux heat runs every day in winter?

Not necessarily below freezing, since most heat pumps are designed to lean on aux heat regularly once outdoor temps drop under 35-40°F. It becomes a concern if aux heat is running at 50°F or warmer, which usually points to a heat pump losing capacity.

Can I just leave the thermostat on Emergency Heat all winter to save on repairs?

You can, but it will cost far more than the repair itself. A failed heat pump repair typically runs $300-$800, while running resistance heat as your primary source all winter can add $600-$1,200 or more to electric bills over a season.

Does Emergency Heat mean my furnace is broken?

No, Emergency Heat specifically refers to bypassing a heat pump's outdoor compressor unit. If your home has a standard gas or oil furnace with no heat pump, you won't see this setting at all.

Why does my thermostat say "Aux Heat" the moment I raise the temperature a few degrees?

Heat pumps recover temperature slowly, so most thermostats automatically add aux heat for a short burst whenever you raise the setpoint more than 2-3 degrees at once. This clears on its own within 30-60 minutes and isn't a sign of a problem.

Will a smart thermostat tell me if aux heat runtime is unusually high?

Many models, including Ecobee and Nest, log daily or monthly aux-heat runtime hours you can check in the app, which is the fastest way to catch a heat pump problem before the bill arrives.

This is educational information, not HVAC repair advice. Call a licensed HVAC technician if your heat pump's outdoor unit isn't running, or if aux/emergency heat runtime seems unusually high for the outside temperature.