Formula Used
This calculator uses practical cooling load equations for early HVAC sizing checks.
- Wall load = Wall U value × Wall area × Adjusted wall CLTD
- Roof load = Roof U value × Roof area × Adjusted roof CLTD
- Glass conduction = Glass U value × Glass area × Temperature difference
- Solar gain = Glass area × SHGC × Solar factor
- Lighting load = Lighting watts × 3.412 × Lighting use factor
- Equipment load = Equipment watts × 3.412 × Equipment use factor
- Ventilation sensible load = 1.08 × Outdoor air CFM × Temperature difference
- Infiltration CFM = ACH × Room volume ÷ 60
- Latent air load = 0.68 × CFM × Moisture difference in grains
- Final load = Base load + Duct allowance + Safety allowance
- Cooling tons = Final load ÷ 12,000
How to Use This Calculator
- Enter project and room names for clear records.
- Add floor area, ceiling height, wall area, roof area, and glass area.
- Enter U values from drawings, product sheets, or code references.
- Use adjusted CLTD values for wall and roof heat gain.
- Enter indoor and outdoor design temperature and humidity values.
- Add people, lighting, equipment, ventilation, and infiltration values.
- Press the calculate button to view results above the form.
- Use CSV or PDF buttons to save the calculation summary.
Example Data Table
| Input |
Example Value |
Unit |
| Floor area | 500 | sq ft |
| Wall area | 620 | sq ft |
| Roof area | 500 | sq ft |
| Glass area | 120 | sq ft |
| Outdoor temperature | 95 | °F |
| Indoor temperature | 75 | °F |
| Ventilation | 160 | CFM |
| Infiltration | 0.35 | ACH |
Understanding ASHRAE Heat Load
A cooling load estimate explains how much heat a room gains during a design condition. This calculator follows common load parts used in air conditioning design. It groups heat from the envelope, solar exposure, people, lights, equipment, outdoor air, and infiltration. The result is shown in Btuh, tons, and kilowatts.
Why This Method Helps
ASHRAE style load work separates sensible and latent heat. Sensible heat changes dry bulb temperature. Latent heat comes from moisture. This split matters because a unit must cool air and remove water. A room can have a moderate sensible load, yet a high latent load when outdoor humidity is high.
Main Load Sources
Walls and roofs gain heat by conduction. Their load depends on area, U value, and a corrected temperature difference. Windows add conduction and solar gain. Solar gain uses glass area, SHGC, and a selected solar factor. Internal gains come from people, lights, and equipment. Electrical watts become heat by multiplying by 3.412.
Air Load Details
Ventilation brings planned outdoor air into the zone. Infiltration is unplanned leakage through cracks, doors, and gaps. The calculator estimates infiltration from air changes per hour and room volume. Sensible air load uses CFM and temperature difference. Latent air load uses the humidity ratio difference converted to grains of moisture.
Using The Result
The final load includes duct allowance and safety allowance. These additions help cover distribution loss, minor uncertainty, and future changes. They should not replace a detailed design review. Oversized equipment can short cycle and control humidity poorly. Undersized equipment may fail during peak weather.
Best Practice
Enter realistic areas and U values from drawings, product data, or code tables. Use design outdoor conditions for the project location. Check glass orientation and shading before choosing solar factor. Review the sensible heat ratio before selecting equipment. A low ratio means moisture removal needs attention. Compare several options before changing equipment size. Note every assumption beside the result. Small inputs can create large load changes. Keep records for later audits and design discussions. Also revisit inputs when occupancy or envelope details change. Use this tool for early sizing, comparisons, classroom work, and quick checks. Final designs should be verified by a qualified professional.
FAQs
What is an ASHRAE heat load calculation?
It is a cooling load estimate that separates heat gains from envelope, solar exposure, people, lights, equipment, ventilation, and infiltration. It helps size cooling systems during design checks.
Is this calculator suitable for final HVAC design?
It is best for early sizing and study work. Final HVAC design should include project drawings, local weather data, construction details, and review by a qualified professional.
What is CLTD?
CLTD means cooling load temperature difference. It adjusts simple conduction calculations for solar effect, mass, timing, and design conditions used in cooling load work.
Why does humidity matter?
Humidity creates latent load. The cooling system must remove moisture, not only lower air temperature. High latent load can affect comfort and equipment selection.
What does sensible heat ratio mean?
Sensible heat ratio compares sensible load with total load. A lower value means more of the total load comes from moisture removal.
How is infiltration estimated?
The calculator converts air changes per hour into CFM using room volume. This gives an estimated leakage airflow for sensible and latent load calculations.
Why add duct and safety allowances?
Duct allowance covers distribution loss. Safety allowance covers reasonable uncertainty. Very high allowances can oversize equipment, so use realistic values.
Can I download the result?
Yes. Use the CSV button for spreadsheet records. Use the PDF button for a simple printable summary of the calculated load.