Heat Load Calculator

Calculator overview

Inputs and outputs

This summary comes from the calculator's published input and output contract.

Inputs

Orientation: About this input

One row per glazing surface: enter its area, U-factor and solar heat gain coefficient. The compass orientation labels are FIXED and are read by row position, not by name -- row order sets which peak solar gain factor applies, so relabelling a row changes nothing. Enter each window's area on the row for the direction it faces. Areas and U-factors follow the unit system; the solar heat gain coefficient is a dimensionless fraction.

Default 5 rows

Column Range or allowed values

Orientation Not declared

Area At least 0

U-factor At least 0

SHGC 0 to 1
Outdoor Summer Design Temperature Conditional: About this input

The outdoor design temperature for cooling, in degrees Fahrenheit or Celsius according to the unit system, usually a hot percentile for the location.

Unit deg F Default 95
Number Of Occupants: About this input

The number of people the space is designed for, used for the sensible and latent occupant gains.

Unit people Default 4 Range At least 0
Latent Gain Per Person: About this input

The moisture heat each occupant adds, in British thermal units per hour, from respiration and perspiration. Around 200 is common for light activity. This entry stays in British thermal units per hour in both unit systems.

Unit Btu/h Default 200 Range At least 0
Lighting Load: About this input

The connected lighting power in the space, in watts, converted to a sensible cooling gain.

Unit W Default 300 Range At least 0
Outdoor Summer Relative Humidity Conditional: About this input

The outdoor relative humidity at the summer design condition, as a percent from 0 to 100, used for the latent cooling load.

Unit % Default 45 Range 0 to 100
Supply Air Temperature Difference: About this input

The design difference between room air and supply air, in degrees Fahrenheit or Celsius according to the unit system, used to convert the sensible load to the supply airflow required.

Unit deg F Default 20 Range At least 0
Unit System: About this input

Selects inch-pound or metric units for the inputs and results.

Default IP Allowed IP, SI
Sensible Gain Per Person: About this input

The sensible heat each occupant adds, in British thermal units per hour, the part that warms the air. Around 250 is common for light activity. This entry stays in British thermal units per hour in both unit systems.

Unit Btu/h Default 250 Range At least 0
Outdoor Winter Design Temperature Conditional: About this input

The outdoor design temperature for heating, in degrees Fahrenheit or Celsius according to the unit system, usually a cold percentile for the location rather than the record low.

Unit deg F Default 10
Safety Factor: About this input

A margin added to the final loads, as a percent. Enter 10 for a 10 percent safety factor. Keep it modest, since oversizing hurts comfort and efficiency.

Unit % Default 10 Range At least 0
Average Ceiling Height: About this input

The average interior ceiling height, in feet or metres according to the unit system, used with floor area to find the conditioned volume.

Unit ft Default 9 Range At least 0
Choose A Construction Enter The Area Override The Rvalue Only If You Know It Set Area To Zero To Skip A Row: About this input

One row per envelope surface: choose a construction type and enter its area, in square feet or square metres according to the unit system. The listed R-values are illustrative nominal whole-assembly figures, so read a result as an estimate rather than as your actual assemblies. Set area to zero to skip a row. The R override column replaces the tabulated R-value for its row when you enter one (in the active unit system's R units); leave it blank to use the table. A row with an area but no valid construction is excluded and the status names it.

Default 8 rows

Column Range or allowed values

Construction Wall, wood frame 2x4 with R13 batt, Wall, wood frame 2x6 with R21 batt, Wall, frame with R13 batt plus R5 continuous, Wall, uninsulated frame, Ceiling under vented attic, R38, Ceiling under vented attic, R49, Roof deck, unvented with R30, Floor over unconditioned space, R19, Floor over unconditioned space, R30, Basement wall, R10 continuous, Slab edge, R5 vertical, Door, insulated steel

Area At least 0

R override At least 0
Appliance Load: About this input

The connected appliance and equipment power in the space, in watts, converted to a sensible cooling gain.

Unit W Default 600 Range At least 0
Air Changes Per Hour: About this input

The infiltration rate, in air changes per hour, the number of times the conditioned air volume is replaced by outside air each hour.

Unit ACH Default 0.35 Range At least 0
Altitude Above Sea Level: About this input

The site elevation above sea level, in feet or metres according to the unit system. It lowers air pressure, which reduces the latent infiltration load.

Unit ft Default 0
Climate Preset: About this input

Selects a climate preset that fills the outdoor design temperatures and humidity. Override the individual values if you have local design data.

Default Custom (use the values below) Allowed Custom (use the values below), Very cold, illustrative, Cold, illustrative, Mixed, illustrative, Marine, illustrative, Hot-humid, illustrative, Hot-dry, illustrative
Indoor Summer Temperature: About this input

The indoor air temperature to maintain in summer, in degrees Fahrenheit or Celsius according to the unit system, the cooling setpoint.

Unit deg F Default 75
Indoor Winter Temperature: About this input

The indoor air temperature to maintain in winter, in degrees Fahrenheit or Celsius according to the unit system, the heating setpoint.

Unit deg F Default 70
Indoor Summer Relative Humidity: About this input

The indoor relative humidity target in summer, as a percent from 0 to 100, used for the latent cooling load.

Unit % Default 50 Range 0 to 100
Conditioned Floor Area: About this input

The conditioned floor area served, in square feet or square metres according to the unit system, used for volume, infiltration and per-area results.

Unit ft2 Default 2000 Range At least 0
Duct Loss Allowance: About this input

The extra load added for duct heat loss or gain, as a percent of the load. Enter 15 for a 15 percent allowance; use a smaller value for ducts in conditioned space.

Unit % Default 10 Range At least 0

Column	Range or allowed values
Orientation	Not declared
Area	At least 0
U-factor	At least 0
SHGC	0 to 1

Column	Range or allowed values
Construction	Wall, wood frame 2x4 with R13 batt, Wall, wood frame 2x6 with R21 batt, Wall, frame with R13 batt plus R5 continuous, Wall, uninsulated frame, Ceiling under vented attic, R38, Ceiling under vented attic, R49, Roof deck, unvented with R30, Floor over unconditioned space, R19, Floor over unconditioned space, R30, Basement wall, R10 continuous, Slab edge, R5 vertical, Door, insulated steel
Area	At least 0
R override	At least 0

Outputs

Heating Temperature Difference: About this output

The design temperature difference driving heat loss, in degrees Fahrenheit (degrees Celsius in SI): indoor winter minus outdoor winter design temperature.

Unit deg F
Indoor Humidity Ratio: About this output

The moisture content of the indoor summer air, in grains of water per pound of dry air, used for the latent load.

Unit grains/lb
Infiltration Airflow: About this output

The outside air entering by infiltration, in cubic feet per minute (litres per second in SI), from the volume and air changes per hour.

Unit cfm
Heating Load Per Floor Area: About this output

The heating load divided by floor area, in British thermal units per hour per square foot (watts per square metre in SI), a check against typical intensities.

Unit Btu/(h.ft2)
Heating Duct Losses: About this output

The added heating load from duct losses, in British thermal units per hour (watts in SI), from the duct loss allowance.

Unit Btu/h
Heating Envelope Conduction: About this output

The heating load from conduction through the envelope, in British thermal units per hour (watts in SI): total UA times the heating temperature difference.

Unit Btu/h
Heating Infiltration: About this output

The heating load from infiltrating outside air, in British thermal units per hour (watts in SI).

Unit Btu/h
Model Status: About this output

The overall check on your entries, shown above the results. It reads OK when the inputs are usable, NOT VALID with a reason when an entry makes the model meaningless, or CHECK with a reason when a result is valid but worth a second look. Read it before you trust the numbers below.

No unit declared
Total Heating Load: About this output

The whole-house design heating load, in British thermal units per hour (watts in SI), after the duct allowance and safety factor.

Unit Btu/h
Total Latent Cooling Load: About this output

The total latent cooling load, in British thermal units per hour (watts in SI), the part that removes moisture rather than lowering air temperature.

Unit Btu/h
Total Sensible Cooling Load: About this output

The sensible part of the cooling load, in British thermal units per hour (watts in SI), the part that changes air temperature rather than removing moisture.

Unit Btu/h
Total Cooling Load: About this output

The total cooling load, in British thermal units per hour (watts in SI), sensible plus latent, after the duct allowance and safety factor.

Unit Btu/h
Outdoor Humidity Ratio: About this output

The moisture content of the outdoor summer air, in grains of water per pound of dry air, used for the latent load.

Unit grains/lb
Sensible Heat Ratio: About this output

The sensible share of the total cooling load, as a fraction between 0 and 1. Lower values indicate a larger moisture removal duty.

Unit ratio
Supply Airflow Required: About this output

The supply airflow to carry the sensible cooling load at the supply air temperature difference, in cubic feet per minute (litres per second in SI).

Unit cfm
Cooling Infiltration Latent: About this output

The latent cooling load from removing moisture in infiltrating air, in British thermal units per hour (watts in SI), from the indoor and outdoor humidity difference.

Unit Btu/h
Cooling Infiltration Sensible: About this output

The sensible cooling load from infiltrating outside air, in British thermal units per hour (watts in SI), from the temperature difference.

Unit Btu/h
Cooling Load Per Floor Area: About this output

The cooling load divided by floor area, in British thermal units per hour per square foot (watts per square metre in SI), a check against typical intensities.

Unit Btu/(h.ft2)
Cooling Envelope Conduction: About this output

The sensible cooling load from conduction through the envelope, in British thermal units per hour (watts in SI).

Unit Btu/h
Conditioned Volume: About this output

The conditioned air volume, in cubic feet (cubic metres in SI): floor area times average ceiling height.

Unit ft3
Cooling Appliances And Lighting: About this output

The sensible cooling load from lighting and appliances, in British thermal units per hour (watts in SI), converted from the connected watts.

Unit Btu/h
Cooling Capacity: About this output

The total cooling load expressed in tons of refrigeration, at 12000 British thermal units per hour per ton (kilowatts in SI).

Unit tons
Cooling People Latent: About this output

The latent cooling load from occupant moisture, in British thermal units per hour (watts in SI): number of occupants times the latent gain per person.

Unit Btu/h
Envelope Ua Opaque: About this output

The heat transfer coefficient of the opaque surfaces, in British thermal units per hour per degree Fahrenheit (watts per kelvin in SI), the sum of each area divided by its R-value. Doors entered in the assembly grid are counted here.

Unit Btu/(h.F)
Envelope Ua Total: About this output

The total envelope heat transfer coefficient, in British thermal units per hour per degree Fahrenheit (watts per kelvin in SI), opaque plus glazing.

Unit Btu/(h.F)
Floor Area Per Ton: About this output

The conditioned floor area served per ton of cooling, in square feet per ton (square metres per kilowatt in SI), a rule-of-thumb check; a very low value suggests an oversized or high-load design.

Unit ft2/ton
Envelope Ua Glazing And Doors: About this output

The heat transfer coefficient of the GLAZING rows only, in British thermal units per hour per degree Fahrenheit (watts per kelvin in SI). Despite the name, doors are not included here: a door entered in the assembly grid is counted in the opaque figure.

Unit Btu/(h.F)
Cooling People Sensible: About this output

The sensible cooling load from occupants, in British thermal units per hour (watts in SI): number of occupants times the sensible gain per person.

Unit Btu/h
Cooling Solar Through Glazing: About this output

The sensible cooling load from solar gain through glazing, in British thermal units per hour (watts in SI), weighted by each window's orientation row.

Unit Btu/h
Cooling Temperature Difference: About this output

The design temperature difference driving heat gain, in degrees Fahrenheit (degrees Celsius in SI): outdoor summer minus indoor summer design temperature.

Unit deg F

What it is

The Heat Load Calculator estimates the whole-house heating and cooling loads for a home, so you can judge what size of equipment it needs. It reports the design heating load, the cooling load split into its sensible and latent parts, the cooling capacity in tons, the supply airflow required, and a breakdown showing which part of the building each load comes from.

You describe the building through an envelope grid of construction assemblies and areas, a window grid by orientation, the infiltration rate, the occupants and internal gains, and your indoor and outdoor design conditions.

This is not an ACCA Manual J calculation. It is an independent block-load model built from first principles. It is not equivalent to Manual J and does not satisfy a permit or an equipment warranty that requires one. It sizes for the whole house and does not size individual rooms, registers or branch ducts. Whole-house load calculators commonly differ from one another by plus or minus 20 percent, so equipment selection should rest on a full professional calculation.

Methodology

Purpose and model boundary

This model estimates a whole-building block heating load and sensible/latent cooling load from envelope areas, design temperatures, infiltration, glazing solar gain, occupants, and internal electrical gains. It also reports approximate cooling capacity and supply airflow.

This is not an ACCA Manual J calculation, permit document, equipment selection, or room-by-room distribution design. It is a transparent peak-load estimate for screening and comparison.

Inputs and units

The workbook supports IP and SI display/input arms. Switching Unit_System changes labels and interpretation but does not convert numbers already typed. Climate presets replace the typed outdoor design values unless Custom (use the values below) is selected.

Building inputs include conditioned floor area, average ceiling height, air changes per hour, occupants, and altitude. The opaque-envelope grid supplies construction type, area, and an optional R-value override; a nonblank override wins, otherwise the selected construction's typical whole-assembly R-value is used. Setting area to zero skips a row. The glazing grid supplies orientation, area, U-factor, and solar heat-gain coefficient.

Internal gains are appliance and lighting watts plus sensible and latent gains per person. Duct-loss allowance, safety factor, and supply-air temperature difference are percentage/temperature assumptions.

Governing relationships

The workbook converts selected inputs to an internal IP basis. Conditioned volume and infiltration airflow are

volume = floor area x ceiling height

and

infiltration cfm = volume x ACH / 60.

For each opaque component, U = 1/R and UA = area/R. For glazing, UA = area x U. Total envelope conductance is the sum of opaque and glazing UA values. Steady-state conduction is

q_conduction = UA x deltaT.

Heating infiltration is 1.08 x cfm x heating deltaT. Heating subtotal equals envelope conduction plus infiltration. The duct-loss allowance and safety factor are applied successively:

total heating = heating subtotal x (1 + duct fraction) x (1 + safety fraction).

Cooling solar gain is summed by orientation as area x SHGC x peak orientation factor. Occupant gains equal people times the entered per-person values. Appliance and lighting watts are converted to Btu/h. Sensible infiltration is 1.08 x cfm x cooling deltaT.

Outdoor and indoor humidity ratios use the ASHRAE saturation-pressure and moist-air relationships at the altitude-derived barometric pressure. Latent infiltration uses the positive humidity-ratio difference in grains per pound:

q_latent = 0.682 x cfm x MAX(outdoor grains/lb - indoor grains/lb, 0).

The 0.682 factor is derived in the workbook from air density, latent heat, minutes per hour, and grains per pound. Sensible and latent subtotals are each multiplied by the duct and safety factors. Total cooling is their sum; cooling tons are Btu/h / 12,000, sensible heat ratio is sensible / total, and supply airflow is sensible Btu/h / (1.08 x supply deltaT).

Calculation sequence

Resolve unit and climate-preset branches and convert the selected input arm to the internal IP basis.
Calculate building volume, infiltration airflow, design temperature differences, barometric pressure, and humidity ratios.
Resolve each opaque construction or R override and sum opaque UA; sum glazing UA and orientation-based solar gain.
Calculate heating conduction and infiltration, then apply allowances.
Calculate cooling conduction, solar, occupants, appliances/lighting, and sensible/latent infiltration, then apply allowances.
Derive cooling capacity, sensible heat ratio, supply airflow, and area-normalized diagnostics.
Convert outputs to the selected display system and evaluate Model_Status in the exact order below.

Outputs and interpretation

Primary results are total heating load, total cooling load, and cooling capacity. Sensible and latent subtotals expose the cooling composition. Envelope UA, conduction, infiltration, solar, occupant, and internal-gain results provide an audit trail. Supply airflow is based only on the sensible cooling load and the entered supply-temperature difference.

Load per floor area and floor area per ton are diagnostics, not sizing rules. They help identify implausible assumptions but cannot validate a building by themselves.

Validation and status logic

The workbook returns the first matching status, so a heating-temperature warning can precede the no-envelope error.

Condition	Returned status
Conditioned floor area is less than or equal to zero	`NOT VALID: floor area must be greater than zero`
Indoor winter temperature is not above outdoor winter temperature, producing no positive heating difference	`CHECK: no heating load, indoor winter temperature is not above outdoor`
Total entered envelope `UA` is less than or equal to zero	`NOT VALID: no envelope entered, add areas to the assembly grid`
A positive-area opaque row has no valid resolved construction/R-value	`CHECK: a row with an area has no valid construction selected; it is excluded from the envelope`
Outdoor humidity ratio is not above indoor humidity ratio	`CHECK: outdoor air is drier than indoor, latent infiltration load is zero`
Floor area per ton is below 300	`CHECK: floor area per ton is below 300, verify the inputs`
None of the preceding conditions applies	`OK`

Assumptions and limitations

The model is steady-state and peak-instantaneous. It does not simulate thermal mass, hourly weather, shading geometry, infiltration pressure, ventilation standards, duct location, diversity, latent storage, equipment performance curves, zoning, or room-by-room distribution. The orientation solar factors and construction R-values are illustrative workbook data. Real project assemblies, fenestration ratings, shading, and design weather must replace them.

Whole-building load calculators can differ materially because they treat solar gain, diversity, construction, and leakage differently. The model intentionally makes no claim of Manual J equivalence. SI results are converted from an IP calculation path; typed values do not auto-convert when the selector changes.

Restrictions and non-computing states

Floor area must be positive and the grids must yield a positive envelope conductance. Input checking rejects negative quantities and invalid humidity percentages before calculation. Blank opaque R override is supported and uses the construction lookup; zero or invalid resolved R on a positive-area row excludes that row and produces a CHECK. A nonpositive supply-temperature difference is outside the published limits for ordinary use and would prevent meaningful supply-airflow interpretation.

Errors and warnings

NOT VALID identifies missing building size or envelope information. CHECK preserves a computed estimate but flags a non-heating winter condition, excluded assembly row, zero latent infiltration, or unusually high capacity per floor area. An input rejection or a calculation-service or connection failure is not a workbook status.

References

The workbook reproduces no table, chart or text from ACCA Manual J, the ASHRAE handbooks, the IECC or any other standard. Design temperatures and R-values are inputs with illustrative defaults. The psychrometric relations it implements are cited below.

American Society of Heating, Refrigerating and Air-Conditioning Engineers. ASHRAE Handbook, Fundamentals, Chapter 1, Psychrometrics. Equations 3, 6 and 20, for barometric pressure with altitude and the humidity ratio from saturation vapour pressure. https://www.ashrae.org/technical-resources/ashrae-handbook
Wikipedia. Thermal conduction, for the steady-state relation used for envelope conduction. https://en.wikipedia.org/wiki/Thermal_conduction
Wikipedia. Humidity, section "Humidity ratio", for the moisture measure the latent load is built on. https://en.wikipedia.org/wiki/Humidity
Wikipedia. Solar heat gain coefficient. https://en.wikipedia.org/wiki/Solar_gain

Manual J is a trademark of the Air Conditioning Contractors of America and ASHRAE is a trademark of its owner. Neither endorses this workbook, and this is not a Manual J calculation.

Additional source notes migrated from Methodology

The workbook cites steady-state Q = UA deltaT, air-change infiltration, ASHRAE Handbook—Fundamentals Chapter 1 equations for barometric pressure and humidity ratio, and an orientation-based peak solar estimate. Its References sheet explicitly states that no Manual J or other proprietary standard table is reproduced and that the model is not endorsed by ACCA or ASHRAE.

Frequently asked questions

Can I use this instead of a Manual J calculation?

No. This is an independent block-load model built from first principles. It is not ACCA Manual J, is not equivalent to it, and does not satisfy a permit or an equipment warranty that requires one. Manual J is a trademark of the Air Conditioning Contractors of America, which does not endorse this tool. Use this for an early estimate and commission a full professional calculation before selecting equipment.

Can I enter my own R-value for an assembly?

Not on this page. The assembly grid takes a construction type and an area only, and the R-value always comes from the shipped table of illustrative values. The underlying workbook supports an override column, but it is not exposed here, so any R-value you supply will not be used. If your assemblies differ materially from the listed constructions, treat the result as indicative only.

Does renaming a row in the window grid change its solar gain?

No. The orientation labels are fixed and are matched to the solar gain factors by row position rather than by the label text. Row one takes the north factor, row two east, row three south, row four west and row five horizontal. Enter each orientation's glazing area on its own row, and do not rely on relabelling.

Why did my numbers go strange after switching between IP and SI?

Because switching the unit system changes how the values you already typed are interpreted rather than converting them. A floor area of 2000 entered as square feet becomes 2000 square metres when you switch to SI. Re-enter every numeric input after switching. Note also that the input and output descriptions on this page are written in inch-pound units throughout.

Why is my cooling load so much smaller than the heating load, or the other way around?

Usually because the two design temperature differences are very different. In the shipped example the winter difference is 60 degrees Fahrenheit and the summer difference only 20, so envelope conduction dominates heating while cooling is spread across solar gain, occupants, appliances and infiltration. Look at the breakdown rather than the totals to see which path is driving your result.

What does the sensible heat ratio tell me?

It is the sensible share of the total cooling load, as a fraction. A value near 1 means almost all the cooling duty is lowering air temperature. A lower value means more of the duty is removing moisture, which comes mostly from infiltration and occupants and matters for equipment selection in humid climates.

Should I trust the floor area per ton figure?

It is a rule-of-thumb check rather than a design value. It tells you whether the result is in the range you would expect for a house of that description, and the status raises a CHECK when it falls below 300 square feet per ton, which usually points at an input error or a genuinely high-load building. Do not size equipment from it.

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