# Heat Pump Central Air Conditioning in Seattle

> Central air in Seattle, done as a heat pump: cooling sized for an 83°F design day on your existing ducts, plus efficient winter heat. Compare it to central AC.

URL: https://seattleheatpumpco.com/central-air-conditioning-heat-pump/

Updated: 2026-10-21

Central air in Seattle is usually best done as a ducted heat pump: it cools the whole house through your existing ducts exactly as a central air conditioner would, then runs in reverse to heat the house in winter. [The Seattle Heat Pump Company](https://seattleheatpumpco.com/) plans central air and heat pump systems for houses in Seattle and King County, sized to a room-by-room load calculation, and the installation is carried out by licensed and insured contractors.

A lot of Seattle homeowners start out asking for air conditioning and end up with a heat pump. The reasons are mechanical, climatic and financial, and they are worth understanding before choosing either one.

## A heat pump is a central air conditioner that can run backward

A central air conditioner and a heat pump use the same refrigeration cycle and largely the same parts: an outdoor unit with a compressor and coil, an indoor coil on the furnace or in an air handler, and refrigerant lines between them. In cooling, both pull heat out of the house air at the indoor coil and release it outdoors.

The difference is a reversing valve and the controls around it. A heat pump can reverse the refrigerant flow, so the outdoor coil gathers heat from outdoor air and the indoor coil releases it into the house. An air conditioner cannot. Buy an air conditioner and you get summer cooling only, with the furnace doing all the heating. Buy a heat pump and the same equipment carries most or all of your heating too.

In summer you cannot tell them apart. A heat pump in cooling mode removes heat and moisture the same way an AC does, with the same efficiency metric (SEER2). The [SEER2 and HSPF2 guide](https://seattleheatpumpco.com/guide/seer2-hspf2-ratings-explained/) explains the cooling and heating ratings side by side.

## Why Seattle is adding cooling now

Seattle used to be a place where most houses did fine without air conditioning. The climate data shows why that changed. Sea-Tac averaged about 176 cooling degree days a year in the 1990s and about 337 a year from 2016 to 2025, roughly double. Days at 90°F or hotter went from zero to six a year in the 1990s to between two and 13 a year over the last decade. On June 28, 2021, the heat dome pushed Sea-Tac to 108°F, Seattle's all-time record.

Homeowners responded. The American Housing Survey found about 53% of Seattle-area homes had a primary air conditioning source in 2021, up from about 31% in 2013. Across Washington, NEEA's 2022 residential building survey still found 38% of single-family homes with no air conditioning at all, so plenty of houses are still deciding.

Meanwhile, the house already has a winter heating system, and it is often a gas furnace that also needs replacing within a few years. That timing is what makes the heat pump choice so common. The [Seattle heat pump guide](https://seattleheatpumpco.com/guide/seattle-heat-pump-guide/) covers how heat pumps handle the rest of the year.

## Sizing for an 83°F design day

Seattle's energy code requires heating and cooling loads calculated with ACCA Manual J and equipment selected with Manual S, no larger than needed to meet the load. For Seattle and Bellevue, the cooling design temperature is 83°F; Edmonds uses 82°F and Auburn 84°F.

The design temperature is not the hottest day on record. It is the condition the system should meet comfortably on a typical hot afternoon, accepting that a rare extreme like 2021 will push indoor temperatures up a few degrees. Sizing for 108°F would oversize the system for every other summer day.

Oversizing is a real cost in cooling. An air conditioner or heat pump removes moisture when the indoor coil runs cold for long enough that water condenses and drains. A unit too big for the house cools the air quickly, reaches the thermostat setting and shuts off before it has removed much moisture. The house hits 74°F and still feels clammy, and the frequent starts waste energy. Even in a summer climate as dry as Seattle's tends to be, short cycling leaves more moisture in the air than a properly sized system would.

Because a heat pump does both jobs, the load calculation checks cooling and heating together. In most Seattle houses the heating load is the larger one, so the system is sized to heat at 24°F and confirmed to cool at 83°F without being oversized for summer. Variable-speed equipment helps by running long, slow cycles at low output on mild days. The [Manual J sizing guide](https://seattleheatpumpco.com/guide/heat-pump-sizing-manual-j/) goes through the inputs.

## Heat pump or central AC: how the choice usually breaks

There is no universal winner, but the decision usually comes down to three questions.

**How old is the furnace?** If the gas furnace is new and you only want cooling, a central AC coil on that furnace is a reasonable choice. You can also put a heat pump coil on the same furnace and let the heat pump carry milder heating hours. If the furnace is near the end of its life, a heat pump with an electric air handler replaces both the furnace and the need for a separate AC.

**Which utility sells your electricity?** Inside Seattle, City Light's current residential rate is 13.38 cents per kWh, and a heat pump beats a 95% efficient gas furnace on energy cost once it averages a COP of about 2.2. On the Eastside, PSE electricity costs about 20.6 cents per kWh, and the break-even COP rises to about 3.4. That gap shows up in permit data: about 45% of Bellevue heat pump permits filed over the latest three years of records also listed a furnace, the pattern of a [dual fuel setup](https://seattleheatpumpco.com/dual-fuel-heat-pump/) that keeps gas for the coldest hours. The [heat pump versus gas furnace guide](https://seattleheatpumpco.com/guide/heat-pump-vs-gas-furnace-seattle/) runs the comparison.

**Which incentives apply?** City Light's contractor discount of $300 or $600 is for qualifying heat pumps, not air conditioners. Oil-heated Seattle homes can qualify for Seattle Clean Heat's $2,000 rebate on a qualifying heat pump. The federal 25C tax credit ended for equipment placed in service after December 31, 2025. The [Seattle rebates guide](https://seattleheatpumpco.com/guide/heat-pump-rebates-seattle/) has the current list.

A heat pump generally costs somewhat more than an air conditioner of the same capacity, because of the reversing valve, defrost controls and heating-rated components. Against that, it replaces some or all of the furnace's job and the energy it burns.

## Adding central air to a house with a furnace

If the house has a forced-air furnace, the ducts are already in place. The indoor coil sits in the supply plenum above the furnace, and the furnace blower pushes air across it. Two checks decide whether that works well.

The first is the blower. Cooling and heat pump heating both want roughly 350 to 450 cubic feet per minute of airflow per ton, often more than an older furnace's blower was set to deliver. A furnace with too small a blower, or one already struggling against restrictive ducts, limits what the new coil can do.

The second is the ducts. Ducts sized only for heating often have one small return and long, narrow runs to upstairs rooms. Cool air is denser than warm air and does not rise on its own, so upper floors that were warm enough in winter can be the hardest rooms to cool. Ducts that run through a hot attic gain heat on the way. A static pressure test and a look at the return path show what needs to change; the [existing ductwork page](https://seattleheatpumpco.com/heat-pump-existing-ductwork/) explains the measurements.

## Houses without ducts

Some older houses are heated by radiators, baseboard or a gravity furnace and never had ducts suited to cooling. Adding central air there means adding a duct system, which depends on whether there are practical routes through a basement, attic and closets. The [older homes page](https://seattleheatpumpco.com/heat-pump-older-homes/) covers that planning. Where there are no reasonable routes, ductless systems are often the more practical way to add cooling; the Seattle Heat Pump Company works on ducted systems, and the [ducted versus ductless guide](https://seattleheatpumpco.com/guide/ducted-vs-ductless-heat-pump/) explains the trade-offs.

## What central air costs in Seattle

A ducted heat pump on existing ducts that test as usable typically runs about $6,700 to $20,000 for Seattle-area projects and like-for-like replacement of an existing heat pump about $6,700 to $15,000. These are budgeting ranges, not quotes. Duct changes, a new air handler, an electrical upgrade and the outdoor unit's location move the number. The [installation cost guide](https://seattleheatpumpco.com/guide/heat-pump-installation-cost-seattle/) breaks down each driver, and the [cost estimator](https://seattleheatpumpco.com/tools/heat-pump-cost-estimator/) gives a range for your house.

Running costs follow the season. In summer, a heat pump and an air conditioner of the same efficiency use the same electricity. In winter, the heat pump's electricity replaces furnace gas. The [operating cost guide](https://seattleheatpumpco.com/guide/heat-pump-operating-cost-seattle/) shows how that plays out on City Light and PSE rates.

## What's included

- Cooling and heating load calculation for the whole house
- Duct airflow check, with attention to upstairs rooms
- Heat pump sized to the cooling load and checked against the heating load
- Coil on the existing furnace, or a new air handler
- Outdoor unit sited for Seattle's noise limits
- Line set, condensate drain and overflow protection
- Dedicated circuit and outdoor disconnect
- Thermostat set up for cooling, heating and staging
- Permits, inspections and startup commissioning in both modes

## Central air options on a forced-air house

|  | Central AC with gas furnace | Heat pump with gas furnace (dual fuel) | Heat pump with electric air handler |
| --- | --- | --- | --- |
| Summer cooling | Yes | Yes | Yes |
| Winter heat source | Gas furnace only | Heat pump most hours, furnace in the coldest | Heat pump, with electric strips as backup |
| Indoor equipment | Cooling coil on the furnace | Heat pump coil on the furnace | Air handler replaces the furnace |
| Fits best when | Cooling is the only goal and the furnace is new | Furnace has years left, or on PSE electric rates | Furnace is at end of life, or the goal is all-electric |

## Questions

### Should I get central AC or a heat pump in Seattle?

For most Seattle houses with ducts, a heat pump is the better buy. It cools exactly like a central air conditioner and also heats, which on City Light rates usually costs less than gas once the heat pump averages a COP above about 2.2. Central AC alone makes sense mainly when the furnace is new and cooling is the only goal.

### Can I add central air to my house if I have a gas furnace?

Yes. A heat pump or air conditioner coil sits on top of the furnace and shares its blower and ducts. The furnace blower needs enough capacity for cooling airflow, and the ducts need to carry it. With a heat pump coil, the same setup also gives you heat pump heating, with the furnace taking over in the coldest weather.

### What size central air system does a Seattle house need?

It depends on the house, not the square footage alone. Seattle's energy code requires a Manual J load calculation using an 83°F cooling design temperature and equipment no larger than the load requires. Oversized systems cool the air fast but remove less moisture, so the house feels clammy. Window area, shading and insulation all change the answer.

### Why are my upstairs rooms hot even with central air?

Usually airflow. Upper floors gain heat through the roof, and in many houses the ducts serving them are long, small or run through a hot attic. A heat pump cannot cool rooms it cannot get air to. A duct assessment shows whether the fix is sealing and insulating attic ducts, resizing a branch, adding a return, or zoning.

### Is it worth installing central air before the next heat wave?

If you plan to add cooling, spring is a more practical time than the first hot week, when demand for installs rises. Sea-Tac's cooling degree days roughly doubled between the 1990s and the last decade, and the June 2021 heat dome reached 108°F. A heat pump bought for cooling also pays its way in winter.