The short answer
Emergency heat on a Trane heat pump in Northern Virginia is a switch you flip only when the heat pump itself has quit. EM HEAT shuts the outdoor unit off and heats the house with the backup heat alone: electric heat strips on most systems, or the gas furnace on a dual fuel system. It keeps you warm, but strips cost a lot more to run. AUX HEAT is different. It comes on by itself, and seeing it on a cold morning is normal. The warning is the backup heat doing all the work for hours when nobody asked it to.
Emergency heat vs auxiliary heat
From Trane’s own page on auxiliary heat:
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| AUX HEAT | EM HEAT | |
|---|---|---|
| Who turns it on | The thermostat, by itself | You, by switching the mode |
| When | Under about 40 degrees out, during defrost, or when you raise the setpoint more than 3 or 4 degrees | Only when the heat pump has stopped working |
| What runs | The heat pump plus backup heat | Backup heat alone; the heat pump is off |
| What to do | Nothing, as long as it shuts back off | Call for service right away |
Trane’s XL824 installer guide says Emergency Heat runs the indoor heat source only, and the mode only shows up when the outdoor unit is a heat pump. On most heat pump homes around here, that indoor heat is electric strips in the air handler.
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When to switch to EM HEAT
- The outdoor unit has stopped: no fan, no hum, and the house is getting colder.
- Ice has covered the outdoor coil and it hasn’t cleared in an hour, which means defrost isn’t working (heat pump covered in ice).
- The breaker for the outdoor unit tripped. Trane’s page on a heat pump that won’t turn on says not to reset it: something in the system tripped it. Switch to EM HEAT and call.
Then call. EM HEAT is a bridge to the repair, not a way to heat the house for weeks.
When the backup heat is a warning
- AUX HEAT that comes on and stays on when it’s over 40 out. Trane calls that a sign of a problem with the heat pump or the thermostat. Low refrigerant, a defrost fault or a failing part are the usual reasons (aux vs emergency heat).
- Only the backup heat runs. Check the mode first: Emergency Heat turns the heat pump off. If the mode is right and the outdoor unit sits idle, it needs service, unless a lockout setting is holding it off.
- The power bill jumped on a mild stretch. Strip heat doing the heat pump’s job shows up on the bill before you feel it in the house.
The lockout settings
An XL824 with an outdoor sensor can stop the heat pump or the backup heat at set outdoor temperatures. Both settings ship disabled. Once enabled, the compressor lockout can be set from 5 to 70 degrees, 30 by default, and stops heat pump heating when it’s colder than that. The aux heat lockout runs from 32 to 70, 45 by default, and stops the backup heat when it’s warmer than that. The two have to sit at least 10 degrees apart. Set wrong, they can leave you on strips all winter or with no backup on the coldest night, so that’s a setting for your installer.
How cold is too cold for the heat pump itself? Trane’s page on when heat pumps stop working says today’s most efficient heat pumps hold full heating capacity down to about 32 degrees, then start to lose some, and most traditional heat pumps won’t run at all much under 0. A Northern Virginia winter rarely gets there.
What the repair usually is
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| What you notice | What it usually is | Typical repair |
|---|---|---|
| Outdoor unit hums and won’t start | The capacitor | $240 to $370 |
| Outdoor unit silent while the thermostat calls | The contactor | $290 to $425 |
| Ice that defrost doesn’t clear | The defrost board or its sensor | $450 to $1,150 |
| AUX HEAT on for hours in mild weather | Often low refrigerant from a leak | A top-up runs $190 to $740; the leak is found and priced in writing |
More in Trane heat pump not heating, heat pumps in a Virginia winter and heat pump making noise, and our heat pump repair page covers what we fix.
What a visit costs
A diagnostic visit is $99, and you get the problem and the price in writing before any work starts. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. If you want every brand’s codes and lights in one place, our error codes and warning lights guide has them.
Book it
Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
When should I use emergency heat on my Trane heat pump?
Only when the heat pump has stopped: the outdoor unit is dead, iced over and not clearing, or its breaker tripped. Switch to EM HEAT, then call for service.
What’s the difference between AUX HEAT and EM HEAT?
AUX HEAT is automatic backup that helps the heat pump when it’s cold or defrosting. EM HEAT is a mode you choose that turns the heat pump off and runs the backup heat alone.
Is it bad to run emergency heat?
It’s fine as a bridge, but electric strips cost much more to run than the heat pump does. Don’t leave it on for weeks; get the heat pump fixed.
Why is AUX HEAT on all the time?
Under about 40 degrees, during defrost, or after a big jump in the setpoint, it’s normal for it to help. If it comes on and stays on when it’s over 40, the heat pump or the thermostat needs a look.
At what temperature does a Trane heat pump switch to backup heat?
AUX HEAT helps under about 40 degrees. If an XL824 has the compressor lockout enabled, the heat pump stops at the set temperature, 30 by default, and the backup heat takes over.
