Heating Load Calculator

On a cold design day, heat flows from inside (warm) to outside (cold) through every surface and every crack. The diagram below shows the two main paths — conduction and infiltration — and how they split for different home sizes and climate zones.

Heating load is conduction (envelope U×A×ΔT) plus infiltration (1.08 × CFM × ΔT). Both scale with the indoor-outdoor temperature difference at design conditions.

Average insulation, 9-foot ceilings, 0.5 ACH. Older homes (1.0+ ACH) need ~25% more; well-insulated new builds (R-21 walls, 0.3 ACH) can drop 30–40%.

• Using the wrong design temperature — pulling the historical low instead of the 99% design
• Ignoring infiltration in tight new builds — even at 0.3 ACH, infiltration is 10–15% of load
• Forgetting duct losses in unconditioned attic/crawl — adds 15–25% to the system size
• Sizing a heat pump on the AHRI 47°F rating instead of the design-temp rating
• Not accounting for window orientation — north-facing windows lose more heat than south-facing
• Skipping the night setback assumption — design load assumes 24/7 setpoint, not setback

Every credible heating load calculation begins with two fixed numbers: the indoor setpoint (usually 70°F) and the 99% winter design temperature for your location. That design figure is not the record-cold reading the local news talks about — it is the temperature that is exceeded on the cold side only 1% of the hours in an average year, published in the ASHRAE Handbook of Fundamentals and used by ACCA Manual J, the residential load procedure recognized in the International Energy Conservation Code (IECC). The IECC also defines the climate zone map that the U.S. Department of Energy uses, so a home in IECC zone 5 (Boston, Chicago) sizes around a 5°F design day, while zone 7 (northern Minnesota) sizes near −15°F. Choosing the true 99% value rather than the historical extreme keeps equipment from being oversized for a handful of hours that happen once a decade.

The difference between indoor and outdoor design temperatures, written as ΔT, is the single multiplier that drives the whole calculation. A 75°F ΔT in a cold zone produces roughly twice the conduction loss of a 38°F ΔT in a mild zone, which is why an identical floor plan needs very different furnace capacity from one region to the next. That relationship is what the formula section above expresses as Q = U × A × ΔT.

A Manual J heating calculation treats the house as a set of separate surfaces, each with its own area and its own U-value, the reciprocal of the assembly R-value. You compute Q = U × A × ΔT for the ceiling, the above-grade walls, the windows, the doors, and the floor or slab perimeter, then sum them. Windows almost always dominate the per-square-foot loss: a single-pane window can run near U-1.0, while an R-21 wall sits around U-0.047 — more than twenty times tighter. If you want to convert insulation thickness into the U-factor the formula needs, the U-value calculator handles the R-to-U arithmetic for layered assemblies.

Below-grade surfaces behave differently. Soil a few feet down stays far milder than design air, so a basement floor loses comparatively little heat, which is why slab-floor conduction is often simplified or omitted. The slab-on-grade exception is the perimeter, where the edge of the slab meets outdoor air; perimeter heat loss is calculated per linear foot of exposed edge, and that edge is exactly where rigid perimeter insulation earns its keep. The table below shows how envelope losses typically distribute across surfaces for a mid-size home at a 70°F ΔT.

Conduction is only part of the answer. Cold outdoor air leaks through the envelope and must be reheated, a loss captured by Q = 1.08 × CFM × ΔT, where 1.08 is the sensible-heat air constant and CFM comes from the home volume times its air changes per hour (ACH), divided by 60. A blower-door test gives the real ACH; in its absence, 0.3 ACH suits tight new construction, 0.5 ACH an average house, and 1.0 ACH or more an older leaky one. ASHRAE Standard 62.2 sets the minimum ventilation that should accompany a tight envelope, so reducing infiltration does not mean starving the house of fresh air — it means delivering that air on purpose through balanced ventilation.

Two real-world adders frequently get left out. Ductwork routed through an unconditioned attic or crawlspace bleeds 15–25% of the delivered heat before it reaches the rooms, which is why ACCA Manual D duct design and proper sealing belong in the conversation. And the design load assumes a constant 24/7 setpoint; a deep night setback creates a morning recovery spike that an exactly-sized system cannot make up quickly, so many designers either skip aggressive setback or add a modest margin.

Once you have the total load, equipment selection follows ACCA Manual S. For a gas furnace, remember that the nameplate input BTU/hr is not the delivered heat — multiply input by the Annual Fuel Utilization Efficiency (AFUE) to get output, so a 80,000 BTU/hr input furnace at 96% AFUE delivers about 76,800 BTU/hr. Our furnace size calculator walks through that input-versus-output step, and the guide on what size furnace you need puts the numbers in context. Heat pumps demand the opposite caution: their capacity derates as it gets colder, so a unit rated 36,000 BTU/hr at the AHRI 47°F point may deliver only 24,000 BTU/hr at your design temperature. Always size a heat pump from the manufacturer's low-temperature performance table, not the headline rating.