Seattle Heat Pump Sizing Calculator
Estimate the right heat pump capacity for your Seattle-area home using Manual J load calculation principles adjusted for the Pacific Northwest climate (Design heating temperature: 21°F; Design cooling temperature: 83°F).
Formulas Used
Envelope Heat Loss/Gain (each component):
Q = U × A × ΔT
- U = overall heat transfer coefficient (BTU/hr·ft²·°F)
- A = surface area (ft²)
- ΔT = indoor–outdoor design temperature difference (°F)
Infiltration Load:
Q_infil = 1.1 × CFM × ΔT where CFM = (Volume × ACH) / 60
Solar Cooling Gain:
Q_solar = SHGC × A_window × Peak Solar Irradiance × Orientation Factor
Internal Cooling Gains:
Q_internal = (Occupants × 250 BTU/hr) + (Floor Area × 3 W/ft² × 3.41 BTU/W)
Design Load with Safety Factor:
Q_design = max(Q_heat, Q_cool) × 1.15
Tons of Cooling/Heating:
Tons = Q_design / 12,000 BTU/hr → rounded up to nearest standard size (0.5-ton increments)
Estimated Annual Heating Energy:
kWh = (Q_heat × HDD × 24) / (ΔT_design × COP × 3,412)
Assumptions & References
- Seattle Heating Design Temperature: 21°F (99% design condition) — ASHRAE Fundamentals Handbook; Seattle-Tacoma International Airport weather station.
- Seattle Cooling Design Temperature: 83°F (1% design condition) — ASHRAE.
- Indoor Setpoints: 70°F heating / 75°F cooling — ACCA Manual J standard.
- Seattle Heating Degree Days: 4,424 HDD (base 65°F) — NOAA Climate Data.
- U-values are representative of typical construction for each era per ASHRAE 90.1 and Washington State Energy Code (WSEC).
- ACH (Air Changes per Hour): Poor = 0.8, Average = 0.5, Good = 0.35, Excellent = 0.20 — typical blower-door test results for Seattle housing stock.
- Peak Solar Irradiance: 200 BTU/hr·ft² — Seattle summer peak (NREL Solar Resource Data); orientation factor 0.5 applied for mixed exposures.
- Internal Gains: 250 BTU/hr per occupant (sensible) + 3 W/ft² lighting/appliances — ACCA Manual J Table 1.
- Safety Factor: 15% added per ACCA Manual J recommendation for equipment selection.
- Standard Equipment Sizes: 1.0–6.0 tons in 0.5-ton increments — common residential heat pump offerings (Mitsubishi, Daikin, Carrier, etc.).
- COP Estimates: 2.8 heating at 21°F / 3.5 cooling at 83°F — representative of modern cold-climate inverter-driven heat pumps (NEEP Cold Climate HP List).
- Methodology: ACCA Manual J Residential Load Calculation, 8th Edition; ASHRAE Handbook of Fundamentals.
- This calculator provides an estimate only. Actual sizing requires a full Manual J calculation by a licensed HVAC professional accounting for local shading, duct losses, orientation, and other site-specific factors.