Heating Load Calculator: Winter Heat Loss in BTU/h

The heating model estimates envelope UA from area x zone heating rate x insulation factor x max(1, ceiling height / 8), divided by (70 deg F - zone winter temperature). Conduction is then UA x actual indoor-to-outdoor delta T. Infiltration adds 1.08 x (area x ceiling height x natural ACH / 60) x delta T. The 1.08 constant assumes standard air near sea level. Zone temperatures are illustrative profiles; use local ASHRAE design weather for a full calculation.

Defaults: 2,000 sq ft, 9-ft ceilings, zone 4, average insulation, 70 deg F indoors, and 0.5 natural ACH. delta T = 70 - 18 = 52 deg F. Envelope load = 2,000 x 35 x (9 / 8) = 78,750 BTU/h. Leakage airflow = 2,000 x 9 x 0.5 / 60 = 150 CFM; infiltration = 1.08 x 150 x 52 = 8,424 BTU/h. Total = 87,174 BTU/h.

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.

The diagram uses the same screening model with its own controls: average insulation, 9-ft ceilings, 70°F indoors and 0.5 natural ACH. Its conduction share is an area-based estimate, not measured losses through the drawn surfaces. Changing the main calculator does not change these independent controls.

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.

Calculated screening totals use average insulation, 9-ft ceilings, 70°F indoors and 0.5 natural ACH. Column headings show the tool’s illustrative outdoor profile temperatures. Increasing ACH raises only infiltration; selecting better insulation reduces the estimated envelope term. Neither change has a universal percentage effect on total heating load.

• Using the wrong design temperature — pulling the historical low instead of the 99% design
• Ignoring natural infiltration or entering blower-door ACH50 directly — leakage load depends on volume, weather and leakage estimate
• Omitting ducts in an unconditioned attic or crawlspace — document insulation, leakage and location rather than adding a fixed percentage
• Sizing a heat pump on the AHRI 47°F rating instead of the design-temp rating
• Using window count without area or whole-window U-factor — orientation does not by itself change conductive U-value
• Confusing steady design heating load with setback recovery — discuss recovery time and controls separately

A heating load calculation starts with indoor and outdoor design conditions. A common residential indoor heating setpoint is 70°F. The outdoor 99% design temperature comes from local weather data rather than the historical record low: colder conditions occur for roughly 1% of annual hours in that data. A record extreme can lead to selecting equipment for a condition that seldom occurs, while an overly mild design temperature can understate winter demand. Use the station and design conditions required for the project, including elevation and local exposure where relevant. This calculator cannot accept a local outdoor temperature directly. It assigns illustrative zone profiles: zone 4 uses 18°F, zone 5 uses 5°F, and zone 7 uses −15°F. Those are site screening assumptions, not official weather values for every location in each IECC zone. Two cities in the same zone can have different winter design conditions. Obtain the local ASHRAE or ACCA design value for a complete calculation and document the indoor setpoint used.

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. Compute Q = U × A × ΔT for above-grade assemblies and use the appropriate ground-contact or perimeter method for foundations, then sum the terms. Windows commonly have higher per-square-foot conductive loss than insulated walls: a window with U = 1.0 loses 20 times as much heat per square foot as an assembly with U = 0.05 at equal ΔT. Nominal cavity insulation R-value is not whole-wall R-value: framing, films and other layers matter. 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 ground-contact losses require a different temperature basis from exposed walls, but this does not justify omitting foundation losses from a complete calculation. 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 is an above-grade arithmetic example at 70°F ΔT. These areas and U-factors are illustrative, not assumed shares of every home’s loss. Ground-contact floors require a separate method.

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 provides pressure-test leakage data, commonly ACH50, not natural design ACH directly. The tool’s 0.3, 0.5 and 1.0 natural ACH values are screening cases; converting measured leakage requires weather and building assumptions. 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 deliberate outdoor air through a designed ventilation system. That airflow and any heat recovery must be assessed separately from uncontrolled leakage.

Two installation and operation details need separate attention. Ductwork routed through an unconditioned attic or crawlspace can lose heat through conduction and leakage; the amount depends on duct area, insulation, temperature and sealing, 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 can lengthen morning recovery for equipment selected close to the steady design load, so recovery time, thermostat strategy and staging should be evaluated separately instead of adding an undocumented 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 an 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: capacity is model-specific at each outdoor condition. As an illustrative example, a unit delivering 36,000 BTU/h at 47°F might deliver 24,000 at a colder design point; that is not a universal derating rule. Always size a heat pump from manufacturer low-temperature performance, not the headline rating. Use the Manual S calculator to compare output with a verified load. Commonly cited furnace output limits are about 100–140% of heating load; applicable edition and equipment provisions govern.

This is not a measured U x A envelope calculation: its estimated UA stands in for walls, roof, floor, and windows. The BTU heating mode omits the separately added leakage term, which explains its lower result. Duct loss, deliberate outdoor ventilation, and heat-recovery performance require project-specific inputs. Do not add a blanket safety factor.

Convert heating load to furnace input.

ACCA Manual J is the residential load-calculation reference, Manual S covers equipment selection, and Manual D covers ducts. The area rates and adjustment percentages in this tool are site assumptions, not values supplied by those standards.

ACCA Manual S: residential equipment selection. For the load-calculation workflow, read how to calculate HVAC load.