HVAC Load Calculator: Heating and Sensible Cooling Estimate
An HVAC load is the rate of heat a building gains in summer or loses in winter. This calculator estimates a winter heating screening load and a summer sensible cooling screening load from envelope conductance, sensible air leakage, indoor and outdoor design temperatures, entered window solar gain, and entered internal sensible gains. It is intentionally more detailed than the site’s general AC Size Calculator, which addresses cooling capacity only.
The cooling result is not a complete total cooling load: latent humidity load and ventilation latent load are not included, and duct gains are not included unless the user manually represents them in an entered gain. The result is a transparent screening estimate in BTU per hour, not an ACCA-certified Manual J calculation, an equipment selection, or a substitute for field-verified construction data.
Heating Load and Sensible Cooling Screening Calculator
Enter project-specific values, calculate, then change an input to compare scenarios. The result stays in this page and sends no visitor data anywhere.
Estimated sensible cooling only: latent humidity and ventilation latent loads are excluded. Duct gains are excluded unless manually represented. This is not Manual J or an equipment selection.
What the calculator is doing
The envelope portion uses the steady-state relationship Q = U × A × ΔT. R-value is converted to U-factor as 1/R. Gross exposed wall area is the exterior perimeter multiplied by ceiling height; entered window and door area is removed from that wall area and calculated with its own U-factor. Ceiling and floor boundaries are calculated separately.
Air leakage uses building volume and the entered air changes per hour (ACH). Airflow is volume × ACH ÷ 60, and sensible heat transfer is estimated as 1.08 × CFM × ΔT. The 1.08 constant is the standard sea-level air sensible-heat approximation documented in U.S. Department of Energy material. High-altitude work requires an air-density correction.
For sensible cooling, the calculator adds the peak solar gain you enter, lighting and appliance watts converted at 3.412 BTU/h per watt, and your entered sensible people allowance. Every cooling term in this tool is sensible. It does not calculate moisture removal from indoor and outdoor humidity, latent ventilation load, or an automatic duct-gain allowance. It also does not invent orientation, shading, equipment, or occupancy correction factors.
How to enter useful values
- Exposed perimeter: count walls adjoining outdoors or unconditioned space, not party walls between conditioned rooms.
- Effective R-values: whole-assembly performance is preferable to the number printed on an insulation batt. Framing, gaps, compression, and thermal bridges reduce assembly performance.
- Window U-factor: use NFRC labels, approved plans, or verified product data when available.
- ACH: a blower-door result is more defensible than choosing a generic tightness label.
- Design temperatures: use the recognized local heating and cooling design conditions, not a record high or low.
Reading the heating and sensible cooling results
The larger displayed number is only the controlling screening value among the loads this tool calculates; it does not automatically dictate one equipment size. The sensible cooling value must be combined with a defensible latent calculation before it can be treated as total cooling load. A heat pump must then be evaluated against both complete design loads and its delivered capacity at the actual design conditions.
Dividing a complete cooling load by 12,000 converts BTU/h to nominal tons for comparison, but this calculator deliberately does not turn its partial sensible result into an equipment recommendation. Coil match, airflow, fan power, sensible heat ratio, humidity removal, and outdoor temperature affect delivered capacity.
Example: why the two loads differ
Consider a moderately insulated home with many west-facing windows. Winter conduction and infiltration may produce a 42,000 BTU/h heat loss. In summer, envelope gain plus window solar and internal sensible gains might produce a 35,000 BTU/h sensible screening result. That summer figure still needs project-specific latent, ventilation, and duct analysis before equipment selection. If the same home used a heat pump, its low-temperature performance and backup-heat strategy would also be checked.
What this estimator leaves out
A professional room-by-room load calculation accounts for orientation, shading schedules, construction layers, thermal bridges, latent humidity load, ventilation sensible and latent loads, duct location and leakage, diversity of internal gains, and each room’s share of the total. Those omitted terms are not hidden inside this tool. A professional calculation also verifies input values in the field; this screening tool cannot determine total cooling load, room airflow, duct sizes, or equipment size.
Request a professional calculation when replacing major equipment, changing windows or insulation, adding conditioned floor area, converting an attic or garage, solving persistent comfort or humidity problems, or choosing a heat pump near its cold-weather capacity limit. Review the homeowner checkpoints in What Is Manual J? and use Manual S equipment-selection principles after the load is established.
Common questions
Is this a Manual J calculator?
No. It applies public heat-balance relationships to user-entered data for screening. It does not implement the complete ACCA procedure or certify a design.
Should I add a safety factor?
The calculator adds none. Stacking undocumented safety factors promotes oversizing. Designers address uncertainty by verifying inputs and following the applicable load and equipment-selection procedures.
Why can the sensible cooling result be lower than the heating load?
Climate, envelope performance, solar exposure, and internal sensible gains affect the two seasons differently. This comparison still omits cooling latent load, so it cannot identify the final controlling equipment load by itself.