How Do I Know if I Have a Heat Pump? 6 Checks and What Emergency Heat Costs

How do I know if I have a heat pump? It’s a fair question. The outdoor unit of a heat pump looks almost the same as a central air conditioner. Many renters and new owners simply don’t know which one they have. This guide gives you a quick clue checker, a decision chart, six checks you can do without tools, and a calculator that shows what Emergency Heat really costs.

What a Heat Pump Does (and Why It Looks Like an AC)

A central air conditioner only moves heat out of the house. A heat pump can also run in reverse and move heat into the house in winter. A part called the reversing valve switches the direction. So on the outside, both units have a fan, a coil and copper lines. The difference is inside, and in how the system is controlled.

That’s why you can’t trust looks alone. You need clues that come from the label, the thermostat and the way the system behaves.

How it works: the model label counts as proof. The O/B wire, the Emergency Heat setting and winter operation count as strong clues. The indoor equipment only helps, because a furnace can be the backup of a heat pump. This is a guide, not a diagnosis.

How Do I Know if I Have a Heat Pump? Six Ways to Check

1. Look up the model number (the only real proof)

Go outside and find the metal data plate on the outdoor unit. Write down the brand and the full model number, or take a photo. Then search the model on the manufacturer’s site. The spec sheet will say “heat pump” or “air conditioner.”

You can also search the free AHRI Directory, where manufacturers certify their equipment. Heat pumps are listed with a heating rating (HSPF2) as well as a cooling rating. A plain air conditioner only has a cooling rating. That HSPF2 number is also useful for the calculator further down.

Don’t try to decode the model number from memory or from a forum list. Each brand uses its own system, and it changes over the years.

2. Look at the thermostat wires

Heat pump thermostats control the reversing valve through a wire on the O, B or O/B terminal. Honeywell Home’s wiring guide describes that terminal as the “reversing valve for heat pump systems.” A system with a gas furnace and a separate AC usually has a W wire for heat and no O/B wire.

Many thermostats show the wiring in their settings menu. If yours doesn’t, you can look at the labels on the base when you change the batteries. Only look. Don’t disconnect or move wires. If you plan to replace the thermostat, switch off power to the HVAC system first, as the thermostat maker’s instructions say.

3. Look for an Emergency Heat setting

Heat pump thermostats often have a mode called Emergency Heat (“Em Heat”) and a display that sometimes shows Aux Heat. A furnace-plus-AC thermostat usually just has Heat, Cool and Off. It’s a strong clue, not proof: some thermostats show these options in their menu even when they aren’t set up for a heat pump.

Look, but don’t switch to Emergency Heat to “test” it. We’ll explain why below.

4. Watch the outdoor unit on a cold day

Turn the heat on during a cold day and go outside. If the outdoor fan runs while the house is heating, it’s working as a heat pump. An air conditioner’s outdoor unit stays off in winter.

You might also see a cloud of steam and hear a whoosh now and then. That’s the defrost cycle, and it’s normal. Don’t chip ice off the coil or pour hot water on it; you can bend the fins. If the unit stays covered in a thick block of ice, call a technician.

5. Check the indoor equipment

Find the indoor unit. It’s often in a closet, basement, attic or garage. An air handler with no burner, no flue pipe and no gas line usually pairs with a heat pump or with electric resistance heat. A gas or oil furnace or a boiler usually means a separate AC, or a dual-fuel system where the furnace backs up a heat pump. So this check helps, but it can’t decide on its own. Don’t open panels you can’t identify.

6. Check the paperwork, or ask

Look for the home inspection report, the equipment manual, the installation permit or old service invoices. The landlord, seller or HVAC company that services the system can usually tell you in one phone call. A technician can confirm it in a few minutes during a routine visit.

Aux Heat vs. Emergency Heat: The Expensive Mix-Up

Most heat pumps have a backup heat source, usually electric heat strips in the air handler, or a furnace in dual-fuel homes. The thermostat uses it in two ways:

• Aux Heat turns on by itself when the heat pump can’t keep up, for example during defrost or in very cold weather. Seeing “Aux” on the screen now and then is normal.
• Emergency Heat is a manual mode. It shuts the outdoor unit off and runs only the backup heat. Chelan PUD, a public utility in Washington state, explains that it is meant for when the heat pump is broken or iced over, and warns that it uses about double the electricity.

Building scientist Allison Bailes makes the same point in Energy Vanguard: leave the thermostat on Heat and use Emergency Heat only if the heat pump has actually failed.

Why Emergency Heat Costs So Much More

Why does it cost so much more? Heat strips turn each kilowatt-hour into one kilowatt-hour of heat. A heat pump moves heat from outside, so over a season it delivers more heat per kilowatt-hour. Its HSPF2 rating tells you how much: divide it by 3.412 to get the seasonal average heat delivered per unit of electricity. An HSPF2 of 8 means about 2.3 units of heat for each unit of electricity, on average over the season.

Calculate What Emergency Heat Costs You

Estimate only. It compares the same amount of heat delivered by the strips and by the heat pump at its seasonal average efficiency. In very cold weather a heat pump works below its average, and some aux heat is normal, so real savings from switching back can be smaller than shown.

When to Call a Professional Instead

• The checks disagree, or you can’t find a readable label.
• The outdoor unit stays frozen in a solid block of ice.
• “Aux” stays on almost all the time in mild weather.
• You’re replacing the thermostat and the wires don’t match the new one’s instructions.
• You smell burning, hear grinding, or a breaker keeps tripping. Turn the system off and call an HVAC technician.

Frequently Asked Questions