Heat Pump vs. AC: What’s Actually Different?

A heat pump is an air conditioner with a reversing valve. In summer the two are functionally identical and rated the same way. In winter the heat pump moves heat into the house instead of out of it, and whether that is cheaper than the furnace you already own depends on your local electricity-to-gas price ratio, not on any general claim about heat pump efficiency.

The short version: same machine, one extra part

Open the outdoor cabinet of a central air conditioner and that of an air-source heat pump of the same size and you find the same things: a compressor, a coil, a fan, a run capacitor, a contactor, a service valve set. Same refrigerant, same indoor coil. In cooling, the two are not similar — they are the same machine doing the same job.

The difference is a four-way reversing valve in the refrigerant circuit, the control logic and outdoor sensors that operate it, and usually a second metering device so refrigerant flows correctly in both directions. That is the whole hardware gap; everything else people believe about the two systems follows from what that valve makes possible.

What the reversing valve actually does

An air conditioner has a hot side and a cold side. Refrigerant absorbs heat from your indoor air at the indoor coil, the compressor raises its pressure and temperature, and the outdoor coil dumps that heat outside.

The reversing valve swaps which coil does which job. In heating mode the outdoor coil becomes the absorber — it pulls heat out of outdoor air even when that air feels cold, because there is usable heat in it well below freezing, and the indoor coil gives that heat off. Nothing is burned. The system is not making heat, it is moving heat that already exists, which is why it can deliver more heat energy than the electricity it consumes. That single fact is the source of every real difference between the two systems: capacity, running cost, the defrost cycle, and the backup heat needed for the days when moving heat is not enough.

Cooling: genuinely identical

Same test, same ratings

Central air conditioners and air-source heat pumps are both rated for cooling with SEER2 (seasonal) and EER2 (at a single design condition). Heat pumps carry one additional rating, HSPF2, for heating. The regional efficiency floors work the same way for both: 13.4 SEER2 in the northern states and 14.3 SEER2 in the South and Southwest, with a lower floor for larger units. If you want the metric itself explained, see our guide to what SEER2 measures.

One caveat for both types: AHRI certifies matched systems — a specific outdoor unit with a specific indoor coil, not outdoor units on their own. Any headline rating is the ceiling reached by one pairing, and the combination you are quoted may be rated lower. Ask for the AHRI certificate for the exact system on your quote.

A heat pump is not a worse air conditioner

The persistent myth is that a heat pump gives up cooling performance to gain heating. It does not. At the same SEER2 rating it cools exactly as well and costs the same to run doing it — the reversing valve’s small pressure drop is already inside the rated number. If a salesperson tells you otherwise, it is worth wondering what else they are wrong about.

Heating: where they stop being the same

HSPF2, and what it does and does not tell you

HSPF2 is the seasonal heating rating, in Btu of heat delivered per watt-hour of electricity. Divide it by 3.412 for a seasonal coefficient of performance — the multiplier on your electricity. A 7.5 HSPF2 unit runs at a seasonal COP of about 2.2: roughly 2.2 units of heat per unit of electricity, averaged across a rating season rather than promised for any given January night. DOE’s federal purchasing guidance, updated December 2024, uses three useful landmarks: baseline at 14.3 SEER2 / 7.5 HSPF2, ENERGY STAR at 15.2 SEER2 / 7.8 HSPF2, and best available at 24 SEER2 / 11 HSPF2.

Capacity falls as it gets colder — exactly when you need it most

A heat pump’s job gets harder as it gets colder and its capacity falls at the same time, because there is less heat in the outdoor air to move and the refrigerant works across a wider temperature lift. A unit that comfortably heats the house at 45°F may cover only part of the load at 25°F. That is why heat pumps are specified with a balance point — the outdoor temperature below which the heat pump alone can no longer keep up, and why something has to take over below it. Cold-climate models hold capacity far better than older ones, but the physics does not disappear. In the inland Central Valley, where design winter temperatures are mild, the balance point rarely becomes the issue it is in a cold climate.

Auxiliary and emergency heat, and the mistake that doubles a winter bill

Below the balance point, a ducted electric heat pump falls back on auxiliary heat — usually resistance strips in the air handler, running in addition to the compressor. Resistance heat has a COP of exactly 1.0: the least efficient heat available, on the most expensive energy available. In a dual-fuel system the backup is a gas furnace instead, a much better arrangement where gas is already at the house.

Ice and steam on the outdoor unit is normal

In heating mode the outdoor coil runs colder than the outdoor air, so moisture condenses on it and freezes. Every heat pump therefore runs a defrost cycle: it briefly reverses into cooling to warm the coil, melts the frost, and returns to heating. The unit may steam dramatically, the fan may stop, and a loud whoosh signals the reversing valve shifting; cool air can come from the registers for a minute or two. All of it is correct operation, and it is the commonest reason a first-winter heat pump owner calls for service they do not need.

What each one costs to install

Ranges below are contractor-reported market estimates, not figures from any published index — no authority publishes installed residential HVAC prices. Read them as a frame for a quote, not a price to expect.

The comparison people usually get wrong is the one in that table. A heat pump looks expensive against an air conditioner because it is doing two jobs. The fair comparison is against an air conditioner plus a furnace, and against that pairing the gap narrows considerably. Our heat pump cost guide breaks the install down line by line, and the AC replacement cost guide does the same for cooling-only.

Running cost is a ratio, not a price

Almost every claim about heat pump running costs is written as though electricity has one price and gas has another. Both vary enormously by state, and only the ratio between them decides which system is cheaper to run.

The inputs, named. EIA put California residential electricity at 33.25 cents per kWh in May 2026 against a U.S. average of 18.44 cents (Electric Power Monthly, Table 5.6.A), and California residential gas at $20.72 to $22.96 per thousand cubic feet in early 2026, roughly $2.00 to $2.21 per therm. Now price a million Btu of delivered heat — this arithmetic is ours from those two published prices, not an EIA result:

• 95% AFUE gas furnace at $2.00/therm: about $21 per million Btu delivered. At $2.21/therm, about $23.
• Heat pump at a seasonal COP of 3.0, California electricity: about $32 per million Btu delivered.
• The same heat pump at the U.S. average electricity price, still measured against California gas: about $18, which would put it below the furnace. That is a deliberately mixed comparison, not a national result: the only figure changed is electricity.

Same equipment, opposite answer, and the only thing that changed was the electricity price. California gas is not cheap; it is among the most expensive residential gas in the nation. California electricity is simply more expensive still.

Expressed as a rule you can apply to your own bills: a heat pump at 7.5 HSPF2 breaks even against a 95% AFUE furnace when a kilowatt-hour of electricity costs no more than about 2.3 times the same quantity of energy bought as gas — the seasonal COP divided by the furnace’s AFUE. At a more efficient seasonal COP of 3.0, the break-even ratio rises to about 3.2 to 1. California currently sits near 4.9 to 1, which is why gas heating still wins there on running cost alone.

What the efficiency upgrade is worth, at federal assumptions

DOE’s Federal Energy Management Program publishes, by climate region, the maximum price premium at which an efficient model still pays for itself. California sits in DOE’s Hot-Dry/Southwest region; at its assumptions, over a 15-year life:

DOE’s cost-effective premium ceiling for the ENERGY STAR level in that region is $728 in 2023 dollars — spend more than that on the upgrade and the energy savings do not return it.

What both of them run on now

New equipment of either type now ships with an A2L refrigerant, usually R-454B or R-32. A2Ls are classified as mildly flammable. They are not more efficient than R-410A and they cost more per pound, not less, so nobody should sell you a system on the refrigerant alone.

Owning an R-410A system is not a reason to replace it. Manufacture and import of new residential R-410A equipment ended January 1, 2025, but servicing and recharging installed equipment is legal indefinitely, and EPA’s May 2026 final rule removed the installation compliance date entirely for systems whose components were all manufactured or imported before that date. The only refrigerant-driven argument for replacement is cost: rising R-410A prices can tip an already-close call on an old system with a large leak.

Who should buy which

1. A working gas furnace, and only the AC has failed. Replace the air conditioner. Buying heating capability you will not use on running-cost grounds solves a problem you do not have.
2. No gas at the house, or electric resistance heat today. Buy the heat pump, and it is not close. Against resistance heat at COP 1.0, a heat pump at seasonal COP 2.2 or better cuts heating energy by more than half.
3. Both appliances near end of life, gas at the house. The genuine decision. Price a heat pump against a furnace-plus-AC pair, apply the ratio arithmetic above to your own bills, and check what your utility will rebate — rebates are the only incentive money left, since there is no federal residential tax credit for either system in 2026.
4. You want a heat pump for reasons other than running cost — one appliance instead of two, no combustion indoors, electrification plans. Those are legitimate; say so out loud rather than let them be dressed up as savings.
5. Gas at the house and cold winters. Ask about dual fuel: heat pump above the balance point, furnace below it.

Whichever way you go, correct sizing decides more than the badge does. Equipment is selected on rated capacity at your design conditions — an ACCA Manual J load calculation with a Manual S selection, not on the tonnage of whatever is out there now; our Manual J guide covers what to ask for. When you are ready for numbers, see what the work should cost.

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