Seattle WA Free Heat Load Calculator

Estimate heating, cooling, insulation losses, HVAC capacity, and Seattle energy costs for residential or commercial buildings using editable climate assumptions.

Preliminary engineering estimate — not certified Manual J

Calculation Results

Default values are ready. Select Calculate Heat Load to view results.

Heating Loss Breakdown

Cooling Gain Breakdown

Room-by-Room Results

Room	Area ft²	Heating BTU/hr	Cooling BTU/hr	Sensible	Latent

1. Location and Weather

Seattle ZIP / Microclimate

Building Type

Calculation Mode

Temperature Unit

Area Unit

Weather Source / Reference

Weather Reference Year

Outdoor Winter Design Temperature

Outdoor Summer Design Temperature

Indoor Heating Setpoint

Indoor Cooling Setpoint

Annual Heating Degree Days (°F days)

Annual Cooling Degree Days (°F days)

Starting outdoor temperatures and degree days are editable examples, not verified Seattle design conditions. Choose appropriate weather stations, reference periods and design percentiles before applying results.

2. Building, Airflow and Solar Inputs

3. Rooms and Thermal Assemblies

Enter exposed gross wall area including openings. Windows and doors are subtracted automatically. Roof and floor areas only count exposed boundaries. R-values use US units, while window and door U-factors use BTU/(hr·ft²·°F).

4. HVAC, Efficiency and Annual Costs

For annual heating, this tool scales UA by heating degree days. Annual cooling uses a simplified cooling degree-day approximation. Actual performance depends on weather, schedules, thermal mass, infiltration, equipment curves, and controls.

Formula Used

Conduction: Q = U × A × ΔT, or Q = A × ΔT / R. Infiltration: Q = 1.08 × CFM × ΔT. CFM: Area × Ceiling Height × ACH / 60. Latent: Q = 0.68 × CFM × humidity difference (grains/lb). Solar: Q = glazing area × incident solar × SHGC × shade and orientation factors.

Heat pump electrical usage ≈ thermal energy / (3,412.142 × COP). Heating energy ≈ UA × heating degree days × 24, adjusted for utilization and distribution loss. Convert BTU/hr to kW by dividing by 3,412.142. Convert cooling BTU/hr to refrigeration tons by dividing by 12,000.

How to Use This Calculator

Select Seattle ZIP and verify design weather data.
Enter building area, height, leakage, and ventilation.
Add rooms and exposed envelope surfaces.
Enter insulation values, window U-factors, and solar settings.
Select HVAC performance and utility tariffs.
Calculate, inspect warnings, and export reports.

About Heat Load Estimation in Seattle

Understanding Heat Transfer

Seattle buildings require heating during many cooler months. Windows, walls, roofs, and floors transfer heat outdoors. Insulation reduces this transfer by adding thermal resistance. Air leaks can create additional heating demand. Ventilation also exchanges conditioned air with outdoor air. Heat load estimates combine these energy pathways.

Heating and Cooling Conditions

A heating design temperature represents a selected cold-weather condition. A cooling design temperature represents a selected warm-weather condition. These values depend on weather stations and design percentiles. Verify official reference data before sizing equipment. Indoor thermostat settings determine the temperature difference. Larger differences generally increase heating or cooling demand.

Windows, Humidity, and Airflow

Solar radiation increases cooling demand through exposed glazing. Window orientation changes the timing of solar gains. Shading can reduce unwanted afternoon heat. Occupants, lighting, and appliances also add internal heat. Outdoor air sometimes introduces moisture into conditioned spaces. Latent cooling estimates account for moisture removal requirements.

Equipment Selection and Energy Costs

Heating capacity should match the calculated building demand. Heat pumps deliver different capacities at different outdoor temperatures. Manufacturer performance tables help evaluate cold-weather operation. Backup equipment may cover remaining winter loads. Oversized systems can create comfort and control problems. Annual energy estimates require more than peak loads. Degree days provide a simplified seasonal approach. Electricity tariffs convert estimated consumption into operating costs.

Using Results Responsibly

Separate room estimates reveal major heat loss sources. Improved insulation and glazing can lower energy requirements. Comparison calculations help assess potential building upgrades. Ventilation rates must still meet applicable requirements. Commercial buildings may need different calculation standards. Local rules and current code editions require verification. This estimator is educational rather than permit documentation. Professional load calculations remain important for final selections. Confirm Seattle requirements with qualified professionals before installation.

Example Insulation Data

Assembly R (US)	Equivalent U (US)	Illustration
R-5	0.2000	Low resistance
R-10	0.1000	Moderate resistance
R-20	0.0500	Higher resistance
R-30	0.0333	High resistance
R-40	0.0250	Very high resistance

Illustrative whole-assembly equivalents, not minimum requirements.

Frequently Asked Questions

Seattle References

Seattle Energy Code overview · Seattle energy code forms and official sizing resources

Weather defaults in this tool are illustrative rather than an authoritative weather dataset. No external weather service is queried. Verify the applicable climate design tables and jurisdiction before use.