--- title: Heat Pump Aux Heat Running Too Much? NKY Winter Guide description: Heat pump stuck on aux heat in Northern Kentucky? Learn when aux heat is normal, what makes it run too often, and when to call Niceley's at 859-342-9900. --- [Niceley's Notes | Blog](https://niceleys.com/niceleys-notes-blog) # [Heat Pump Aux Heat Running Too Much? NKY Winter Guide](https://niceleys.com/niceleys-notes-blog/heat-pump-auxiliary-heat-running-too-much-kentucky) Written by [Jeff Niceley](https://niceleys.com/niceleys-notes-blog/author/jeff-niceley) | Nov 10, 2026, 12:46:26 PM Auxiliary heat is your heat pump's backup. It's normal on cold Northern Kentucky nights, during defrost, and after a big thermostat bump. It shouldn't run on mild days or stay on for long. When it does, common causes include deep setbacks, thermostat settings, a dirty filter, low refrigerant, or a failing part. Call Niceley's at 859-342-9900. ## What auxiliary heat actually is A heat pump doesn't make heat. It moves heat from the outdoor air into your house. The colder it gets outside, the less heat there is to move, and the harder the heat pump has to work. At some outdoor temperature, the heat pump can no longer keep up with what the house is losing on its own. Carrier calls this the [thermal balance point](https://www.carrier.com/us/en/residential/hvac-resources/what-is-auxiliary-heat/). Below it, the thermostat calls in a second source of heat. That's auxiliary heat, usually shown as "AUX" on the thermostat. In most homes, that backup is one of two things: - **Electric resistance heat strips** inside the indoor air handler. They work like the coils in a toaster. They heat quickly, and they cost much more to run than the heat pump. - **A gas furnace**, in what's called a dual-fuel system. The heat pump handles milder weather and the furnace takes over when it's colder. Either way, auxiliary heat is the expensive heat. The goal isn't to never use it. It's to use it only when the heat pump really needs help. ## Aux heat vs. emergency heat These two get confused all the time, and the difference matters for your electric bill. | | Auxiliary heat (AUX) | Emergency heat (EM HEAT) | | --- | --- | --- | | How it turns on | Automatically, by the thermostat | Only when you switch it on | | Heat pump status | Still running. Aux adds heat alongside it | Shut off. Backup heat does all the work | | When it's normal | Cold weather, defrost, after you raise the setpoint | Only when the heat pump has failed or is iced over and can't run | | What to do | Usually nothing, unless it runs too often | Use it as a stopgap, and call for service | [Carrier](https://www.carrier.com/us/en/residential/hvac-resources/what-is-auxiliary-heat/) and [Trane](https://www.trane.com/residential/en/resources/blog/heat-pump-auxiliary-heat/) both say emergency heat is for a heat pump that has broken down, not a way to get warmer faster. Leaving the thermostat on EM HEAT all winter runs your most expensive heat all the time, and it hides whatever is wrong with the heat pump. ## When aux heat is normal in Northern Kentucky Our winters spend many hours right where heat pumps start needing help. At the Cincinnati/Northern Kentucky airport, the normal January average low is about 23°F and the normal high is about 40°F, according to the [National Weather Service](https://forecast.weather.gov/product.php?site=MHX&issuedby=CVG&product=CLM&format=CI&version=8&glossary=0). Last January ran colder than normal. Lows averaged 17.9°F, 26 nights were at or below freezing, and the temperature hit −3°F. [Trane](https://www.trane.com/residential/en/resources/blog/heat-pump-auxiliary-heat/) says aux heat is expected when it's below about 40°F outside, or below the trigger temperature set on your system. Many systems are set so strip heat can't come on until it's colder than that. Department of Energy training material says those lockouts are [commonly set somewhere between 25°F and 35°F](https://bsesc.energy.gov/sites/default/files/Thermostats%20Dual%20Fuel%20-%20Presentation_0.pptx). Every system is different. On a typical January night in Boone, Kenton, or Campbell County, though, seeing AUX come on is normal. Aux heat is also normal in these two situations, whatever the temperature: **During defrost.** In cold, damp weather, frost builds up on the outdoor coil. The heat pump reverses for a few minutes to melt it, and aux heat keeps the air coming out of your vents from turning cold while it does. You may see steam rising from the outdoor unit. That's normal. A unit that's frozen solid after freezing rain is a different problem. See [Ice Storm Prep: Protecting Appliances and HVAC During NKY Power Outages](https://niceleys.com/niceleys-notes-blog/ice-storm-power-outage-prep-appliances-hvac-nky). **After you turn the thermostat up.** Raise the setpoint more than 3 or 4 degrees, and many thermostats bring aux heat in to close the gap faster, according to [Trane](https://www.trane.com/residential/en/resources/blog/heat-pump-auxiliary-heat/). Once the house is near the setpoint, aux should go off. ## When aux heat is running too often Aux heat is a problem when you see any of these: - AUX is on when it's above about 40°F outside - AUX stays on for long stretches. [Lennox](https://www.lennox.com/residential/lennox-life/consumer/what-does-aux-heat-mean) treats more than about 30 minutes at a time as a sign the heat pump can't handle the load - AUX comes on almost every time the heat runs, even on moderate days - The house won't reach the setpoint, even with AUX on - The electric bill jumped, and the weather wasn't unusually cold - The outdoor unit isn't running while the heat is on, or it's running and blowing cool air inside ## Common causes, starting with the ones you can check ## 1. Deep thermostat setbacks This is the most common cause, and it's the one you can fix yourself. With a gas furnace, turning the heat down at night saves money. With a heat pump, it often doesn't. When the thermostat jumps back up in the morning, the house is several degrees short. The thermostat calls for aux heat to catch up, and you pay for that expensive heat right when you were trying to save. Department of Energy training material says [the morning recovery can cancel out the savings](https://bsesc.energy.gov/sites/default/files/2024-08/Introduction%20to%20Heat%20Pumps%20Level%202%20-%20Lecture%20Notes%20%26%20Problem%20Sets.docx) from the setback. What to do: - Pick one comfortable temperature and leave it there. Or keep setbacks small. [Carrier recommends](https://www.carrier.com/us/en/residential/hvac-resources/smart-thermostats/for-heat-pump/) no more than 2 to 3 degrees. - When you want it warmer, raise it 1 degree at a time, [as Lennox suggests](https://www.lennox.com/residential/lennox-life/consumer/what-does-aux-heat-mean). - If you have a smart thermostat, turn on its heat pump or "smart recovery" setting so it warms the house gradually. - Longer setbacks while you're away for a day or more can still save energy. Just expect a slower recovery when you get back. ## 2. A thermostat that isn't set up for your heat pump A thermostat has to know it's controlling a heat pump with backup heat. If it was installed or replaced as a standard furnace thermostat, or if its settings are off, it can call aux heat too early or keep it on too long. A standard thermostat [can leave aux heat running constantly](https://www.carrier.com/us/en/residential/hvac-resources/smart-thermostats/for-heat-pump/), Carrier says. Things to look at: - Make sure the mode is set to Heat, not Emergency Heat. Someone in the house may have switched it during a cold snap and left it there. - Check whether it's a heat pump thermostat. Most have an Aux, Em Heat, or "heat pump" setting in the menu. - On a smart thermostat, check the auxiliary heat lockout or "aux heat max outdoor temperature" setting. Many allow a lockout so aux can't run above a set outdoor temperature. If you're not sure about the settings, don't guess on the wiring or installer menu. A technician can check how the thermostat is configured for your equipment. ## 3. A dirty air filter or blocked airflow A heat pump needs airflow to move heat into your house. A packed filter, closed registers, furniture over the returns, or a crushed duct means less heat delivered. The house warms more slowly, and the thermostat brings in aux heat to make up the difference. Pull the filter and hold it up to a light. If you can't see through it, replace it with the size printed on the frame. Open any supply vents you closed off. Keep the return grilles clear. For which filter to buy, see [5 Signs Your Furnace Filter Should Be Replaced](https://niceleys.com/niceleys-notes-blog/5-signs-your-furnace-filter-should-be-replaced). The same filter slot serves your heat pump's air handler. ## 4. An outdoor unit that's buried, blocked or not defrosting The outdoor unit needs air moving freely across its coil. Snow drifts, leaves, a tarp, or shrubs pressed against it all cut its output. - Clear snow and debris from around the unit. Leave about 2 feet open on all sides if you can. - Don't cover it in winter. It has to run. - Don't chip ice off the coil. The fins and refrigerant lines can be easily damaged. - If the coil stays white or solid with ice and never clears, the defrost control, a sensor, or the outdoor fan may not be working. In that case, the heat pump is putting out very little heat, and aux is carrying the house. That's a service call. ## 5. Low refrigerant A heat pump moves heat with refrigerant. It's a sealed system. If the charge is low, there's a leak. With less refrigerant, the heat pump moves less heat, and aux heat makes up the shortfall. You might notice the air at the vents feeling lukewarm, ice forming in odd places, or the outdoor unit running constantly. Refrigerant isn't a homeowner fix, and topping it off without finding the leak doesn't last. A technician has to measure the charge, find the leak, and repair it. ## 6. A failing part When the heat pump half of the system is weak or not running, aux heat ends up doing most of the work, often without you noticing until the bill comes. Common culprits: - The compressor, or the capacitor or contactor that starts it - The outdoor fan motor - The reversing valve, which switches the system between heating and cooling - The outdoor temperature sensor or defrost control board A quick check: with the heat running and AUX showing, go outside. If the outdoor unit is silent, or the fan isn't spinning, the backup is likely heating your whole house. Set the thermostat to Emergency Heat to keep the house warm, then call. Don't let it run that way for weeks. ## 7. A heat pump that's undersized, or a house that loses heat fast Some heat pumps were simply sized too small for the house, or sized for cooling with winter as an afterthought. Leaky ductwork in an attic or crawlspace, old windows, a finished basement or an addition can also add heat loss the original system wasn't designed for. Then the balance point sits higher than it should, and aux heat runs on days a properly matched system wouldn't need it. [Trane notes](https://www.trane.com/residential/en/resources/blog/heat-pump-auxiliary-heat/) that a correctly sized heat pump may seldom need its heat strips. If your system is in good shape and aux still runs on most winter days, sizing and duct losses are worth a look. The answer might be duct sealing, insulation, or, at replacement time, a heat pump designed for colder weather or a dual-fuel setup. ## If you have a dual-fuel system In a dual-fuel home, "aux heat" means the gas furnace is running, not electric strips. The same rules apply: aux on a cold night is normal, and aux on a mild day isn't. The changeover temperature is a thermostat or control setting, and it can be set too high. If the furnace side is what isn't working, see [Furnace Won't Ignite: Diagnostic Steps for NKY Homeowners](https://niceleys.com/niceleys-notes-blog/furnace-wont-ignite-northern-kentucky).   ## What to check before you call 1. Confirm the thermostat is on Heat, not Emergency Heat. 2. Look at the schedule. Reduce any setback bigger than 2 or 3 degrees. 3. Replace a dirty filter, and open closed vents. 4. Clear snow, leaves, and covers from around the outdoor unit. 5. With AUX on, check that the outdoor unit is running and its fan is turning. 6. Note the outdoor temperature when AUX comes on, and how long it stays on. If you've done those steps and AUX still runs on mild days, stays on for long periods, or the outdoor unit isn't running, it's time to call for service. ## When to call Niceley's Call if: - Aux heat is on when it's above about 40°F outside - Aux runs more than about 30 minutes at a time on moderate days - The outdoor unit isn't running, or the fan won't turn, while the heat is on - The outdoor coil stays iced and never clears - The house won't reach the setpoint - You've been on Emergency Heat because the heat pump stopped working - Your electric bill jumped without a cold snap to explain it Call Niceley's at **859-342-9900**. We're at 11 Price Avenue in Erlanger, family-run since 1994, and our heating and cooling technicians are available 24 hours a day, 7 days a week. You'll talk to a person here, not an out-of-state call center. We service all major brands. Before any repair starts, the technician tells you what's wrong, what it takes to fix it, and the price. For more on our service, see [Heat Pump Repair, Service and Installation in NKY](https://niceleys.com/niceleys-notes-blog/heat-pump-repair-service-and-installation-in-nky). The best time to catch a weak capacitor, a refrigerant leak or a misconfigured thermostat is before January. Book a [Precision Tune-Up](https://niceleys.com/heating-cooling) for your heat pump this fall. ## Heat pump aux heat questions we hear       [View full post](https://niceleys.com/niceleys-notes-blog/heat-pump-auxiliary-heat-running-too-much-kentucky) ```json { "@context" : "http://schema.org", "@type" : "BlogPosting", "author" : { "@type" : "Person", "name" : "Jeff Niceley" }, "dateModified" : "2026-11-10T12:46:26.665Z", "datePublished" : "2026-11-10T12:46:26Z", "headline" : "Heat Pump Aux Heat Running Too Much? 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