Heat Load Calculator

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Heat Load Calculator

Calculate heating and cooling loads for your home room by room, or get a quick estimate by square footage.

Heating Load
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-- Tons
Cooling Load
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-- Tons

Load Breakdown

Per Room

Room Name Heating Load Cooling Load
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About This Tool

This calculator estimates residential heating and cooling loads using simplified Manual J principles. It helps homeowners and HVAC professionals determine how much heating and cooling capacity each room needs, based on the building envelope, insulation levels, windows, infiltration, and local climate conditions.

Sources: ACCA Manual J · ASHRAE · DOE

How to Use

  1. Select climate zone and enter building settings
  2. Add rooms with dimensions, walls, and windows
  3. Review heating and cooling loads with breakdown

Methodology

Heat loads are calculated using the fundamental heat transfer equation Q = U × A × ΔT, where U is the thermal transmittance (1/R-value), A is the surface area, and ΔT is the temperature difference between indoor and outdoor design conditions. Infiltration loads use Q = 1.08 × CFM × ΔT, where CFM is derived from the building volume and natural air change rate (ACH). Solar heat gain through windows uses ASHRAE peak solar factors by orientation and latitude. Design temperatures follow ASHRAE 99%/1% values for each IECC climate zone.

Sources: ASHRAE Handbook of Fundamentals · ACCA Manual J

Understanding Your Results

The heating load shows the maximum heat your home needs on the coldest design day. The cooling load shows the maximum cooling needed on the hottest design day. These "peak" values determine equipment sizing. The breakdown chart shows which components contribute most to the load. Large window loads suggest better glazing would help. High infiltration loads indicate air sealing opportunities. Use the total BTU/h values in the HVAC Sizing Calculator to select properly sized equipment.

Sources: ACCA Manual J · DOE Energy Saver

Sources

Practical Examples

Example: A 12×10 ft bedroom with 8 ft ceilings, one exterior wall (R-13), one window (12 sq ft, U=0.30), and average construction quality in Zone 5A (Chicago, heating design temp 1°F). Heating load: Wall 820 BTU/h + Window 234 BTU/h + Infiltration 393 BTU/h = 1,447 BTU/h total. This room needs about 1,450 BTU/h of heating capacity. For a 2,000 sq ft whole house in Zone 5A with average insulation: approximately 70,000 BTU/h heating and 40,000 BTU/h cooling — roughly a 3.5-ton cooling system and an 80,000 BTU furnace.

Load Calculation Tips

Windows are often the largest single contributor to both heating and cooling loads. Upgrading from double-pane clear to low-E with argon can reduce window loads by 40-50%. Air sealing (reducing infiltration) is one of the most cost-effective ways to reduce loads. Sealing attic bypasses, rim joists, and window/door gaps can cut infiltration loads in half. Don't forget that insulation works differently in heating vs cooling. Attic insulation matters most for heating loads (heat rises), while wall and window insulation matters more for cooling loads. When in doubt, use the room-by-room mode rather than the quick estimate. It accounts for differences in room exposure, windows, and position within the house.

Sources: DOE Energy Saver · ASHRAE

All calculations are performed locally in your browser. No data is sent to any server.

Frequently Asked Questions
What is a heat load calculation?
A heat load calculation determines how much heating and cooling capacity a building needs to maintain comfortable indoor temperatures. It accounts for heat flowing through the building envelope (walls, windows, roof, floor), air infiltration, solar heat gain through windows, and internal heat from occupants and appliances. The result, expressed in BTU/h, tells you the size of HVAC equipment needed.
What is the difference between heating and cooling loads?
Heating load measures heat loss from the building during winter — heat escaping through walls, roof, and windows to the cold outside. Cooling load measures heat gain in summer — heat entering through the envelope plus solar radiation through windows and internal heat from people and appliances. Cooling loads also include latent heat from humidity. The two loads are calculated separately because they peak at different times and often have different magnitudes.
What design temperatures should I use?
Design temperatures represent near-extreme outdoor conditions for your location. The heating design temperature (99% value) is exceeded 99% of the year — only 1% of hours are colder. The cooling design temperature (1% value) is exceeded only 1% of the year. This tool automatically provides ASHRAE design temperatures when you select your climate zone. For example, Zone 5A (Chicago) uses 1°F heating and 91°F cooling design temperatures.
How does infiltration affect heat load?
Infiltration is uncontrolled air leakage through cracks, gaps, and openings in the building envelope. Cold air entering in winter and hot air entering in summer both add to the heating and cooling loads. The infiltration load is calculated as Q = 1.08 × CFM × ΔT, where CFM depends on the building's air tightness. A tight home (0.15 ACH natural) may have 15-20% of its load from infiltration, while a leaky home (0.75 ACH) can have 40-50% from infiltration alone.
What factors determine a room's cooling load?
A room's cooling load includes: (1) heat conduction through walls, windows, ceiling, and floor based on R-values and temperature difference; (2) solar heat gain through windows, which varies by orientation — south and west-facing windows receive the most solar radiation; (3) infiltration of hot outdoor air; (4) internal heat gains from occupants (about 230 BTU/h per person) and appliances (kitchens generate significantly more); and (5) latent heat from humidity. South-facing rooms with large windows typically have the highest cooling loads.
How accurate is this compared to a full Manual J?
This tool uses simplified Manual J principles — the same fundamental heat transfer equations (Q = U × A × ΔT) and infiltration formulas (Q = 1.08 × CFM × ΔT). For typical residential buildings, results are within 10-15% of a full Manual J calculation. The simplifications include: using a single window orientation assumption for solar gain, default door and air film values, and estimated infiltration rates rather than blower door measurements. For permit applications or final equipment selection, a full Manual J calculation by a licensed professional is recommended.
What is the sensible heat formula?
The sensible heat formula for air is Q = 1.08 × CFM × ΔT, where Q is heat in BTU/h, CFM is airflow in cubic feet per minute, and ΔT is the temperature difference in °F. The constant 1.08 comes from the density of air (0.075 lb/cu ft) × specific heat of air (0.24 BTU/lb·°F) × 60 min/hr. For conduction through building surfaces, the formula is Q = U × A × ΔT, where U is the thermal transmittance (1/R-value) and A is the area in square feet.
How do I use these results for HVAC sizing?
Take your total heating and cooling loads (in BTU/h) to the HVAC Sizing Calculator to determine the right equipment size. Per ACCA Manual S, cooling equipment should be 100-115% of the design cooling load, and heating equipment 100-140% of the design heating load. The CFM value tells you the total airflow needed, which feeds into duct sizing with the Duct Size Calculator. For room-by-room results, use the Room Airflow Calculator to determine CFM per room and register sizes.