What is Auxiliary Heat? Understanding Your Heat Pump's Backup System

Have you ever glanced at your thermostat and noticed ‘Aux Heat’ on the display and felt a mild sense of unrest? You’re not alone. It’s analogous to noticing there’s a warning light lit up on your car’s dashboard — you immediately have a set of questions. And what is auxiliary heat, anyway? And more importantly: Is it going to empty your wallet?

Let’s cut to the chase. Your heat pump has a backup heat source: You may also see something like “Aux Heat” or “Emergency Heat” pop up on your thermostat, it’s your heat pump’s backup plan. Picture it as a booster shot for your heating system, one that “enables the second stage of operation to be run when the primary heat pump has reached its maximum run time to maintain the home in a comfortable condition, such as when it is true cold outside,” in the company’s words.

It’s not always a sign you need to freak about, but knowing why it comes on, what it does, and how it affects your energy bill can be your secret weapon. Time to get into the nitty-gritty.

So, What Is Auxiliary Heat?

At its most basic sense, emergency heat is a second way to heat your home when your heat pump isn’t working. On most days, even cold ones, air-source heat pumps are spectacular at pulling heat from the outdoors and depositing it inside your house, in a way an air conditioner doing the same thing in reverse is not. It’s really efficient in mild weather. But there’s a catch: Heat pumps work less and may even fail when outdoor temperatures truly drop, usually into the 30s or lower, generally between 32ºF and 35ºF.

That’s where supplemental heat comes in. It gives an “extra boost” of warmth. What is it that usually gives this lift?

• Electric Heat Strips: The most typical auxiliary heat source found in heat pump air handlers. They’re large glowing toaster elements in your system, radiant heat created through electric resistance. They’re fast, but they’re also known to be energy hogs.
• Gas Furnace: If you have a “dual-fuel system,” your heat pump could be matched with a gas furnace. This arrangement can be the better choice in cold climates, because gas furnaces tend to be more efficient than electric heat strips in really cold conditions. Your thermostat automatically alternates the heat pump and the gas furnace to choose the most economical.

So, to put it another way, it’s your system telling you, “Okay, heat pump, I appreciate you doing what you can, but I’ve got to call in the cavalry to help warm this place the rest of the way.”

When Does Aux Heat Turn On? (The Triggers)

And now, you’re thinking, “Why does my thermostat say Aux Heat?” There are a couple major situations under which your emergency heat will turn itself on automatically:

• When It’s That Biting Cold Outside: This is the most frequent trigger. Once it gets into the 30s and below outside, especially 32 to 40 degrees, your heat pump can start having trouble finding enough heat in the outside air to warm your home. Your thermostat, taking note that the heat pump isn’t doing the job, summons the auxiliary heat for support.
• When You Request a Big Jump in Temperature: Say you’ve been out of town over the holiday break, taking the smart approach of turning down the thermostat — to, oh, say, 62 — in order to save a bit of money. And now you’ve returned and everyone wants 70 degrees, yesterday. When you crank up the thermostat a few degrees at once — say, 3-4 degrees or more — your system understands the heat pump isn’t going to reach it quickly on its own, particularly if it’s already cold outside. Boom. Aux heat kicks on to speed you into that comfort zone. It’s all about speed.
• When Defrost Mode is On: Heat pumps are great, with one exception: Cold, wet weather can result in ice on their outdoor coils. When that happens, your heat pump goes into a “defrost cycle” in an effort to melt the ice. The system in this cycle reverses the direction of the refrigerant flow to heat the outdoor unit, so it is actually cooling your home for that period of time. To avoid you getting some frigid blast of air, or shivering the fuzz off of your socks, the auxiliary heat comes on to put some warm into your house.
• Thermostat Logic: Your smart thermostat is watching at all times. If your interior temperature drops 1.5 to 3 degrees below your desired level, it will turn on the auxiliary heat to warm your home back up more quickly. For your comfort, however, it’s a self-preservation mechanism.

An indicator on your thermostat, which may be a small light or a digital readout reading “Aux Heat,” will typically indicate when it is in use.

Auxiliary Heat vs. Emergency Heat: Not the Same Beast

This is a common point of confusion, and it’s critical to get this straight. Some thermostats might even display “emergency heat” when they mean “aux heat”. But the two are fundamentally different beasts, especially for your wallet.

Here’s the deal:

The big takeaway? Auxiliary heat is often a normal part of your heat pump’s operation, especially in winter. Emergency heat is your “break glass in case of emergency” button. If you flip that switch, pick up the phone and call an HVAC pro, because something’s gone sideways.

Is Auxiliary Heat More Expensive? (Spoiler: Usually, Yes)

This is where your wallet speaks. Yes, backup heat, especially if it’s electric resistance heat strips, usually costs more to operate than the heat pump by itself. Why? Since heat strips require a whole lot more energy to turn the electricity into heat (rather than just moving existing heat) compared to your heat pump.

Here’s the thing: your heat pump is the marathon runner; it’s super efficient over long stretches. Auxiliary heat is like a sprinter, with all the speed, the strength and the rushes of energy.

If you run auxiliary heat unnecessarily, you may see an increase in your energy bill. Some estimates put your energy bill’s increase at up to 50% with aux heat engaged. But if you have a gas furnace as your backup, in a dual-fuel setup, it all becomes a bit more nuanced: you’ll need to calculate the spread between the cost of natural gas and electricity, the cost of the equipment, and how long the equipment is likely to last. In cold climates, a good gas furnace is a more economical solution than electric strips.

The objective isn’t to steer entirely clear of auxiliary heat, which is necessary under some conditions. The aim is to limit its unnecessary use.

Tips on reducing/controlling hot water heating with electric.

No one wants to be “jumping up and screaming” their power bill. So, here’s some pro advice on how to keep your auxiliary heat at bay and save some money:

Set Your Thermostat Smartly: While, as with all of these challenges, we’d love to see your number hit zero or less, keep your thermostat in the mid-to-high 60s and not pushing into the 70s. A larger setting indicates a larger “heating load,” which causes your system to work harder and prompts aux heat more regularly. The sweet spot tends to be around 68 degrees Fahrenheit when you are home. When you’re away or asleep, lowering it by 7-10 degrees can save you big, as much as 10% on heating costs every year.
Ease Up on the Thermometers: This is a really big one. Steer clear of those “set it and forget it” whopper moves. If you need to adjust the heat, jump by just two degrees. Let the heat pump have its way, and then bump the temperature up again by two degrees, if necessary. This little trick gets your heat pump up to its new set point without having to call on that energy-gobbling aux heat.
Insulate and Seal Your Home: It’s similar to wrapping your house in a warm winter coat. Sealing up drafts near windows, doors, and where plumbing or wiring penetrates the walls can cut energy losses significantly. Just replacing single-pane windows with double-pane models, or making sure your fireplace flue is shut tight when it’s not in use, can save you a lot of money. The less heat that is escaping, the less your system needs to fight and the less need there is for aux heat.
Use Passive Heating: Open those curtains or blinds and allow the sun to come streaming in and naturally heat your home. Free heat, anyone?

Consider a System Upgrade:

• Dual-Fuel System: When replacing your HVAC system, you could choose to combine a heat pump with a gas furnace. That gives you the efficiency of the heat pump on milder days and the brute force of a gas furnace when it’s really freezing. It’s often the “best of both worlds” option.
• Superior Heat Pump Technology: Modern heat pumps are significantly more efficient than the ones of the past and can operate in lower temperatures better than before. Some high-end models, such as Mitsubishi’s Hyper-Heating H2I® mini-split systems, can heat down to minus 13 degrees without any need for heat strips. Although these are often used as add-ons or for supplemental heating, they are an early indicator of a very exciting future for heat pump efficiencies.

Close Off Unused Rooms: If you have extra bedrooms or other living areas you do not use, do not be afraid to close their doors and shut vents. This redirects the heat to the sections of your home you do use, making your system more efficient as a whole.

Wrap Up: In the end, it can be something simple. Keep layering, get out a warm blanket. Don’t force your heating system to work harder than necessary to keep you warm when a jumper will do the job.

Troubleshooting Auxiliary Heat Issues & When to Call a Pro

It’s okay to see “Aux Heat” on your thermostat. But if it seems to be running 24/7, even when it’s not freezing outside (i.e., above 40°F), or EVERY time your furnace is running your home never feels warm enough, well, that’s a sign that something may not be right. And this is not just about comfort; it’s about dodging a monster electricity bill.

You can quickly do the following checks:

• Inspect Your AC Unit: Is your outdoor unit covered in snow or ice? Clear any obstructions. If it is defrosting and seems to be happening slowly (more than an hour), that could be a sign there’s a problem. If it’s not powering on at all, definitely red flag.
• Check Thermostat Settings: Maybe you set it to “volcano hot” in a fit of rage. Or perhaps it is not switching heating modes properly. Perhaps if you turn the temp down a couple degrees, the ”Aux Heat” light will go away? If it remains on when it’s not too cold, you could have an issue. “Sometimes it’s as simple as resetting a thermostat. Newer thermostats might be more inclined to turn on aux heat compared to old ones as well.
• Clean Your Air Filters: A dirty air filter can block your airflow and cause your system to work overtime, which can force it to use more energy and also turn on the aux heat when you don’t actually need it. This is an easy DIY fix!

When to Call for Backup (An HVAC Professional): If you find your auxiliary heat running constantly when it’s not super cold outside (I.e., over 40°F), if your house doesn’t get as warm as you set it to, if there are unexpected noises, or if your energy bills are suddenly up in the sky, it’s time for the pros. They can tell whether it’s a bad heat pump, low refrigerant, an issue with your heat strips…or just a bad thermometer. Scheduling regular maintenance is also a savvy step for spotting potential issues early. We will be ready for firms like PV Heating, Cooling & Plumbing or Cooper Heating and Cooling to step in.

FAQ

Q: Why does my heat pump display aux heat?

A: Your thermostat shows “aux heat” when your heat pump requires help warming your home, typically because it’s extremely cold outside, you’ve recently adjusted the temperature setting or your heat pump is in defrost mode.

Q: Is aux heat more expensive to run?

A: Yes, if your electric heat is consisting of resistance, supplemental heat strips, it will consume much more power (and cost you much more money to run) than just the heat pump.

Q: What is aux heat vs emergency heat?

A: No, they are different. Auxiliary heat comes on automatically to help your heat pump and emergency heat must be turned on and should be used only in an extreme emergency if your heat pump has failed. Emergency heat is also very costly to operate.

Q: Should I shut off aux heat?

A: No, there’s no way to manually switch off auxiliary heat; it’s something that is automatically managed by your thermostat. So if you are mainly concerned about reducing the frequency with which your heat pump has to resort to the auxiliary heating, even just saving energy over the long term, your time is better spent on limiting the amount it runs than worrying about when it does.

Q: When should the aux heat not come on?

Q: Should auxiliary heat be on all the time when it’s cold outside? If it is, then you probably have a malfunction with your heat pump, or possibly even with your thermostat, and you should have it checked out by a professional.

Q: How do I know if my heat pump needs fixing?

A: Common signs your heat pump needs repairing include running constantly, not adequately heating your home, emitting strange noises, high electric bills, not turning on at all, or the aux heat running when it’s not necessary. If you’re concerned, call an HVAC technician.

Q: What’s a dual-fuel system?

A: A dual-fuel system is one in which a heat pump and a gas furnace are used together. It leans on the heat pump for efficient performance in mild cold and falls back on the gas furnace for strong, effective heating in colder, wintry conditions.

So, it really just don’t panic about aux heat.

So if you’re seeing “aux heat” on your thermostat, don’t panic just yet. It’s probably just your system doing its job, delivering that extra oomph when your heat pump needs it most. But now you’re empowered to know when it’s standard, when it’s too pricey, and when it’s time to bring in the cavalry (your HVC pro!). Keep warm, stay sharp, and keep those utility bills in line.