Air-source heat pump (ASHP)

Short definition

An air-source heat pump (ASHP) extracts heat from outside air and moves it indoors using a refrigerant cycle. It heats in winter and — being reversible — cools in summer with the same equipment. ASHP is WA’s flagship electrification appliance: PSE, Seattle City Light, Tacoma Power, and Snohomish PUD all offer rebates that stack with the federal IRA 25C tax credit and HEAR Act income-qualified rebates.

What it is

An ASHP works the same way a refrigerator does, run in reverse and scaled up. The refrigerant cycle:

1. Evaporator coil (outdoor unit). Cold refrigerant absorbs heat from outside air, even at temperatures well below freezing.
2. Compressor. Pressurizes the refrigerant, raising its temperature.
3. Condenser coil (indoor unit). Hot refrigerant gives up heat to indoor air or to a hydronic loop.
4. Expansion valve. Drops pressure and temperature; cycle repeats.

A reversing valve flips the cycle direction so the same equipment cools in summer.

ASHP form factors common in WA:

• Ductless mini-split. One outdoor unit serves one to several indoor wall-mount cassettes, each with its own remote and zone. Cheapest install ($4,000–$8,000 single-zone). No ductwork required.
• Ducted central ASHP. One outdoor unit feeds a conventional ducted air handler indoors. Replaces a gas furnace or electric resistance furnace. $12,000–$28,000 installed.
• Air-to-water (hydronic) ASHP. One outdoor unit feeds a hydronic loop indoors — radiators, baseboard, or radiant floor. The replacement path for older WA hydronic homes. $15,000–$35,000 installed.

Performance is measured by coefficient of performance (CoP). ASHP CoP at moderate ambient (40–50°F) typically runs 2.5–4.5 — meaning two to four units of heat delivered per unit of electricity consumed. CoP drops as outdoor temperature falls; at 25°F, a standard ASHP may be at CoP 2.0; below 0°F, a standard unit needs cold-climate engineering to keep useful capacity.

Why it matters to a homeowner

ASHPs are the most consequential residential energy-system shift happening in WA right now. Three reasons.

Operating cost. WA’s mild winters keep ASHP CoP in a favorable range nearly all heating season. Even at WA electric rates, a CoP of 3 typically beats gas-furnace operating cost on heating, and the same equipment provides cooling — a win on Seattle’s increasingly hot summers.

Rebates and incentives. As of early 2026, the rebate stack on a typical WA ASHP install can include:

• PSE, SCL, Tacoma Power, or SnoPUD rebate (varies by service territory; commonly $1,000–$3,000+).
• Federal IRA 25C tax credit: 30% up to $2,000.
• HEAR Act (income-qualified): up to $8,000.
• PSE specifically has a dedicated rebate path for “electric resistance to heat pump” conversions, useful for homes with electric baseboard or electric furnaces.

Specifics rotate; verify the current rebates for your utility before quoting. The rebate stack regularly takes $5,000–$10,000+ off a $20,000 install for income-qualified WA homeowners.

Cooling. Most WA homes built before 2000 don’t have central AC. An ASHP replacement of an old gas furnace adds cooling for free — same equipment heats and cools.

The catch is install quality. An undersized unit runs auxiliary resistance heat constantly during cold snaps, killing the efficiency advantage. An oversized unit short-cycles. A botched refrigerant charge degrades CoP year-round. Ask whether the contractor does a Manual J load calculation, verify their EPA Section 608 refrigerant certification, and check that the indoor and outdoor unit pairing is on the manufacturer’s matched-system list.

When you’ll encounter this term

• An old gas furnace or electric resistance system at end of life.
• A PSE, SCL, Tacoma Power, or SnoPUD rebate flyer.
• An IRA 25C tax credit form referencing “heat pump” or “ENERGY STAR-certified.”
• A real estate listing for a newer or recently retrofitted WA home.

Common variants and disambiguation

• ASHP vs. HPWH. Same physics, different application. ASHP heats your house. Heat-pump water heater (HPWH) heats your domestic hot water tank.
• Ducted central ASHP vs. ductless mini-split. Ducted uses existing or new ductwork. Ductless mini-split is wall-mount cassettes per zone, no ducts.
• Air-to-air vs. air-to-water. Air-to-air heats indoor air directly. Air-to-water heats a hydronic loop. Air-to-water is the path for hydronic-heated WA homes considering electrification.
• Standard ASHP vs. cold-climate ASHP (CC-ASHP). Standard ASHP loses capacity quickly below ~25°F. CC-ASHP maintains capacity to -15°F or below — needed inland and east of the Cascades.

Common failure modes

• Defrost cycle frequent. Outdoor coil ices over and the unit defrosts repeatedly. Indicates refrigerant or sensor problem; needs an EPA-certified technician.
• Auxiliary resistance constantly on in cold weather. Heat pump undersized or in fault. Auxiliary electric resistance is much less efficient and burns through the electricity savings.
• Refrigerant leak. EPA Section 608 certification required for repair and recharge. Don’t accept “topping off” as a long-term fix; find and repair the leak.
• Outdoor unit obstruction. Snow, leaves, or vegetation around the outdoor unit. Clear seasonally.
• Compressor short-cycling. Often refrigerant charge issue or thermostat misconfigured for heat pump operation.

Washington note

WA is one of the friendliest US climates for ASHPs. Mild Puget Sound winters keep CoP in the 3+ range most of the heating season. East of the Cascades — Spokane, Yakima, Tri-Cities, Methow — winter lows can drop into single digits, and a cold-climate ASHP variant is appropriate.

The rebate landscape in WA is unusually generous and unusually rotating. PSE has had a dedicated “electric resistance to heat pump” conversion rebate plus a general HVAC rebate; SCL has its own program; Tacoma Power and SnoPUD each run separate programs. Stack with federal IRA 25C and (income-qualified) HEAR Act and the out-of-pocket cost on a typical install can drop substantially. Verify current rebate amounts at PSE, SCL, Tacoma Power, or SnoPUD pages before quoting.

The refrigerant phase-down under the federal AIM Act is moving residential ASHPs from R-410A to lower-GWP options (R-32, R-454B, R-290) through 2024–2026. New equipment installed today may use a different refrigerant than what’s currently on a 10-year-old unit; service techs need to be trained for the specific refrigerant in your system.

For older WA hydronic homes, the air-to-water variant is the electrification path. A load calculation and an emitter assessment come first — old cast-iron radiators sized for 180°F supply may not heat the home at the 110–135°F supply an air-to-water heat pump delivers without supplementing with electric resistance backup.

FAQ

How well do air-source heat pumps work in Washington winters?

Very well in Puget Sound and western WA. Standard ASHPs maintain CoP above 2.5 nearly all heating season at typical winter lows of 25–40°F. East of the Cascades and in inland WA where lows reach single digits, cold-climate ASHP variants are the right pick — they keep CoP above 2 at 0°F or below.

How much does an ASHP cost installed in WA?

Ductless mini-split single-zone systems start around $4,000–$8,000. Ducted central ASHP whole-home installs run $12,000–$28,000 depending on house size and ductwork condition. Air-to-water hydronic ASHP installs $15,000–$35,000. Stacked WA utility rebates plus federal IRA 25C plus (income-qualified) HEAR Act can reduce out-of-pocket cost substantially.

Do I need a panel upgrade for an air-source heat pump?

Often yes for whole-home ducted systems, sometimes for ductless. A whole-home ASHP draws 30–50 amps; a 100-amp service running a gas-heated home may need an upgrade to handle the new circuit alongside existing loads. A ductless mini-split single-zone often runs on a smaller dedicated 15–30 amp circuit and may fit existing panels. A licensed electrician’s load calculation answers definitively. Panel upgrades run $2,500–$5,000 in WA.