HVAC Heat Load Calculator

Input Parameters

Room Length (ft)

Room Width (ft)

Room Height (ft)

Window Area (ft²)

Door Area (ft²)

Wall Exposure Factor (0 to 1)

Roof Exposure Factor (0 to 1)

Floor Exposure Factor (0 to 1)

Wall U-Value

Roof U-Value

Floor U-Value

Window U-Value

Door U-Value

Summer Outdoor Temp (°F)

Summer Outdoor RH (%)

Indoor Cooling Temp (°F)

Indoor Cooling RH (%)

Winter Outdoor Temp (°F)

Indoor Heating Temp (°F)

Solar Gain Factor (BTU/hr·ft²)

Shading Coefficient

Cooling Load Factor

Number of Occupants

Sensible Gain per Person (BTU/hr)

Latent Gain per Person (BTU/hr)

Lighting Load (W)

Equipment Load (W)

Ventilation Airflow (CFM)

Infiltration (ACH)

Heating Internal Gain Credit

Safety Factor

Reset

Example Data Table

Parameter	Example Value	Unit	Why It Matters
Room Size	30 × 20 × 10	ft	Sets floor area, wall area, and building volume.
Window Area	60	ft²	Influences solar and conductive heat gain.
Wall / Roof U-Value	0.08 / 0.05	Btu/hr·ft²·°F	Controls transmission through envelope surfaces.
Summer / Winter Outdoor Temp	95 / 35	°F	Defines design temperature difference for sizing.
Occupants	6	people	Adds sensible and latent internal loads.
Ventilation / Infiltration	180 / 0.30	CFM / ACH	Drives fresh-air and leakage load components.
Typical Adjusted Cooling	≈ 3.1	tons	Representative capacity after safety adjustment.

Formula Used

Envelope conduction: Q = U × A × ΔT. This is used for walls, roof, floor, windows, and doors.

Solar window gain: Q_solar = Window Area × Solar Factor × Shading Coefficient × Cooling Load Factor.

Ventilation and infiltration sensible load: Q_sensible = 1.08 × CFM × ΔT.

Ventilation and infiltration latent load: Q_latent = 4840 × CFM × ΔW, where ΔW is humidity ratio difference in lb water per lb dry air.

Internal gains: People sensible = Occupants × sensible gain per person. People latent = Occupants × latent gain per person. Lighting and equipment heat = Watts × 3.412.

Cooling total: Total Cooling = Sensible Cooling + Latent Cooling. Adjusted Cooling = Total Cooling × Safety Factor.

Heating net: Net Heating = max(0, Gross Heating − Internal Gain Credit). Adjusted Heating = Net Heating × Safety Factor.

How to Use This Calculator

Enter the room length, width, and height in feet.
Provide window and door areas, then enter envelope U-values that match the project construction.
Enter design temperatures and cooling humidity conditions for outdoor and indoor air.
Add solar, occupancy, lighting, equipment, ventilation, and infiltration values.
Set the heating internal gain credit and desired safety factor.
Press Calculate Heat Load to display the result block above the form.
Review the breakdown tables, then export the summary through CSV or PDF buttons.

FAQs

1. What does this calculator estimate?

It estimates room-level cooling and heating loads using envelope transmission, solar gain, occupancy, equipment, lighting, ventilation, and infiltration inputs.

2. Is this suitable for final equipment selection?

It works well for preliminary engineering checks and quick sizing. Final equipment selection should still consider local codes, zoning, duct losses, and full design procedures.

3. Why are both sensible and latent loads shown?

Sensible load changes air temperature. Latent load removes moisture. Cooling equipment must handle both to maintain comfort and indoor humidity targets.

4. What is a good safety factor?

A modest margin such as 1.05 to 1.15 is common for early checks. Excessive oversizing can reduce comfort, control, and operating efficiency.

5. How does infiltration affect the result?

Higher air leakage increases cooling sensible load, cooling latent load, and winter heating loss. Tight envelopes usually reduce equipment size and energy demand.

6. Why does the heating result subtract internal gains?

People, lights, and equipment release heat indoors. During winter, part of that heat offsets transmission and airflow losses, reducing the net heating requirement.

7. Can I use metric units here?

This version is arranged around imperial HVAC inputs. You can still use it after converting dimensions, airflow, temperatures, and U-values into the shown units.

8. What if cooling load appears when outdoor air is cooler?

Internal gains and solar gains can still create cooling demand. Envelope and airflow cooling components use only positive cooling temperature difference values.