Updated October 2026. Most homes need a heat pump between 1.5 and 5 tons (18,000 to 60,000 BTU per hour), but the right size comes from a room-by-room load calculation, not square footage. In the Puget Sound area the heat pump should be sized to your home’s heating load on a design-temperature night (24°F at SeaTac, 17°F in Olympia). Home Comfort Alliance runs an ACCA Manual J load calculation on every heat pump installation.
Quick answer
- Typical range: 1.5 to 5 tons for a house.
- Size to the heating load at your design temperature, then check cooling.
- Get a Manual J; don’t copy the old unit’s size.
Tons, BTU and what “size” means
One ton of heat pump capacity is 12,000 BTU per hour. A heat pump’s output changes with the outdoor temperature: it is rated at 47°F and delivers less as it gets colder, which is why the cold-night capacity matters more than the label.
The square-foot rule of thumb, and why it misleads here
Manufacturer charts use about 20 to 30 BTU per square foot (Carrier, read October 2026). That is a cooling-led shortcut: it ignores insulation, windows and air leakage, and a 1970s rambler and a new code-built home of the same size can need very different equipment.
| Home size | Rule of thumb at 20 to 30 BTU per sq ft (Carrier) |
|---|---|
| 1,000 sq ft | 1.5 to 2.5 tons |
| 1,500 sq ft | 2.5 to 3.5 tons |
| 2,000 sq ft | 3.5 to 5 tons |
| 2,500 sq ft | 4 to 5 tons, or two systems |
Manufacturer rule of thumb, Carrier, October 2026. Your load calculation decides.
How cold does it get where you live?
Sizing starts with your outdoor design temperature, the cold-night temperature the system must handle. These are the code values from the 2021 Seattle Energy Code, Appendix C (heating and cooling, °F). Towns not listed use the nearest station.
| Station | Heating design (°F) | Cooling design (°F) |
|---|---|---|
| SeaTac | 24 | 83 |
| Bellevue | 24 | 83 |
| Redmond | 17 | 83 |
| Everett (Paine Field) | 21 | 79 |
| Tacoma | 29 | 82 |
| Puyallup | 19 | 86 |
| Olympia | 17 | 85 |
| Bremerton | 29 | 83 |
| Shelton | 23 | 85 |
| Bellingham | 19 | 78 |
| Port Angeles | 28 | 75 |
| Snoqualmie Pass | 6 | 80 |
Source: 2021 Seattle Energy Code, Appendix C. For how cold-climate models perform at 5°F, see cold-climate heat pumps.
Size to the heating load, not the cooling load
Western Washington homes need far more heating than cooling, so the heat pump is sized to the heating load at your design temperature, and cooling is checked against it. Natural Resources Canada’s sizing guide aims cooling capacity at about 80% to 125% of the cooling load.
Illustration, arithmetic only: heat loss rises roughly with the gap between indoor and outdoor temperature. With the house at 68°F, SeaTac’s 24°F design is a 44°F gap and Olympia’s 17°F design is a 51°F gap, so the same house needs about 16% more heating capacity in Olympia.
How a Manual J and Manual S work
A Manual J measures each room’s heat loss and gain from its walls, windows and their direction, insulation, air leakage, ducts and occupants. A Manual S then picks equipment whose real output at your design temperature matches that load, using the manufacturer’s performance data. Washington’s residential energy code (section R403.7) requires equipment to be sized with Manual S based on Manual J or another approved method (WAC 51-11R-40360). We run a Manual J on every installation and pull the permits.
What happens if a heat pump is too big or too small
| Too big | Too small |
|---|---|
| Short cycles: starts and stops often | Leans on backup heat on cold nights |
| Uneven rooms and clammy air in summer | Higher bills in cold snaps |
| Costs more to buy and can wear faster | Struggles to recover after a setback |
NYSERDA’s Clean Heat program notes problems start around 120% of the load and get severe above 150%.
Variable-speed heat pumps change the sizing math
Variable-speed (inverter) heat pumps run most efficiently at about 40% to 80% of their capacity (NYSERDA). So the maximum output must cover your cold design night, and the minimum output should be low enough to run steadily on our many 45°F drizzle days instead of cycling.
Ductless mini-splits: how many heads and what size
Ductless systems are usually sized room by room: one indoor head for each closed room or zone, each matched to that room’s heat loss, with sun, ceiling height and open floor plans changing the answer. As a Mitsubishi Electric Diamond Dealer, we lay out heads and the outdoor unit from the load calculation; see ductless mini-splits.
Backup heat and the balance point
The balance point is the outdoor temperature where your heat pump’s output equals your home’s heat loss. Below it, backup heat (electric strips, or a gas furnace in a dual fuel system) makes up the difference. We confirm how backup heat will work at the estimate.
Ducts, airflow and your electrical panel
Ducted heat pumps need roughly 400 to 500 cubic feet per minute of airflow per ton (U.S. Department of Energy), so undersized ducts can cap the size you can install. A larger system may also need more panel capacity; see electrical panel upgrades.
What to ask for on a heat pump quote
- The Manual J report or its load numbers.
- The design temperature used.
- The equipment’s capacity at that design temperature.
- Its minimum capacity, for variable-speed systems.
- How backup heat will work.
- Who pulls the permit.
More questions to ask: heat pump installer checklist. Some rebates require cold-climate or variable-speed equipment; see rebates.
How Home Comfort Alliance sizes your heat pump
Since 1977 we have installed heating systems across the Puget Sound area. NATE-certified technicians run a Manual J on every installation, and we are a Mitsubishi Electric Diamond Dealer, Carrier Factory Authorized Dealer and Lennox Premier Dealer. Get a price range in about 30 seconds with our instant quote tool, then a free in-home visit confirms the size and price. New installations come with a 3-year Comfort Shield labor warranty on new installations, kept active with yearly maintenance. First-year maintenance is included. Costs by size are in our heat pump cost guide.
Heat pump sizing FAQ
What size heat pump do I need for a 2,000 sq ft house?
The 20 to 30 BTU per square foot rule of thumb puts a 2,000 sq ft home at roughly 3.5 to 5 tons, but a Manual J load calculation can land higher or lower depending on insulation, windows and your design temperature. We size to the calculation, not the chart.
How many square feet does a 3-ton heat pump heat?
Common charts say roughly 1,200 to 1,800 sq ft at 20 to 30 BTU per square foot (Carrier). The real answer depends on your design temperature, insulation and windows.
Is it better to oversize or undersize a heat pump?
Neither. An oversized heat pump short cycles, controls humidity poorly and costs more. An undersized one leans on backup heat in cold snaps. A Manual J load calculation is how you hit the right size.
What happens if my heat pump is too big?
It reaches temperature quickly and shuts off, so it starts and stops more often, leaves some rooms uneven, removes less moisture in summer and can wear faster. Problems tend to start once capacity is about 20% above the load.
Should a heat pump be sized for heating or cooling in Washington?
For heating. Puget Sound homes are heating-dominated, so the heat pump is sized to the heating load on a design-temperature night, and cooling is checked against it.
What is a Manual J, and is it required in Washington?
A Manual J is a room-by-room heating and cooling load calculation published by ACCA. Washington’s residential energy code requires equipment to be sized with ACCA Manual S, based on a Manual J or another approved load calculation.
Can I replace my heat pump with the same size as the old one?
Not safely without checking. Homes change with new windows, insulation or additions, and older systems were often oversized. The old size is a clue, not the answer.
Is an online heat pump size calculator accurate?
It gives a rough range. Calculators use square footage and a climate zone, but they miss your windows, insulation, air leakage and duct losses, which a Manual J measures.
How many mini-split heads do I need?
Usually one indoor head for each closed room or zone you want to heat and cool, each sized from that room’s load. Open floor plans can sometimes share a head; we decide that from the layout at the estimate.
What size mini-split do I need for one room?
It depends on the room’s heat loss, sun, ceiling height and how open it is to the rest of the home. Small single-room heads start around 6,000 to 9,000 BTU; a room-by-room load calculation picks the right one.
Do I need backup heat with a properly sized heat pump?
Often a little, for the coldest nights. Ducted systems usually have electric backup or a gas furnace in a dual fuel setup. We confirm how backup heat will work at the estimate.
Does a variable-speed heat pump need to be sized differently?
Yes, in two directions: its maximum output must cover your cold design night, and its minimum output should be low enough to run steadily on mild, damp days.
Not sure what size you need?
We run a Manual J load calculation on every installation and price the system that fits your home, with rebate help and financing if you need it.
Get an estimate
A local comfort advisor will call you to set up your visit.
Sources: 2021 Seattle Energy Code, Appendix C; WAC 51-11R-40360 (R403.7); DOE, air-source heat pumps; NYSERDA Clean Heat Connect, do not oversize; Natural Resources Canada, ASHP sizing and selection guide; Carrier, heat pump sizing. Checked October 2026.