Construction Calculator
Example Data Table
| Input | Example Value | Unit |
|---|---|---|
| Outdoor airflow | 5,000 | CFM |
| Summer outdoor condition | 95 DB, 55 RH | °F, % |
| Indoor condition | 75 DB, 50 RH | °F, % |
| Cooling supply condition | 55 DB, 90 RH | °F, % |
| Sensible recovery | 60 | % |
| Latent recovery | 45 | % |
Formula Used
Atmospheric pressure: P = 101.325 × (1 - 0.0000225577 × altitude meters)5.25588
Humidity ratio: W = 0.621945 × Pv / (P - Pv)
Moist air enthalpy: h = 0.24T + W × (1061 + 0.444T)
Total cooling load: Qt = 4.5 × CFM × density factor × Δh
Sensible cooling load: Qs = 4.5 × CFM × density factor × 0.24 × ΔT
Latent cooling load: Ql = Qt - Qs
Heating load: Qh = 4.5 × CFM × density factor × 0.24 × ΔT
Final load: Load = adjusted subtotal × (1 + safety factor)
How to Use This Calculator
Enter the total outside airflow in CFM. Add the project altitude when known. Enter summer and winter outdoor design conditions. Add indoor room conditions for the design reference. Enter desired cooling and heating supply temperatures.
Add sensible and latent recovery values if the DOAS unit uses an energy recovery device. Add fan heat, duct gain, duct loss, and safety factor. Choose whether to include humidification. Press the calculate button. Review cooling tons, heating load, moisture removal, and load per CFM.
Construction Design Notes
Why Peak DOAS Load Matters
A one hundred percent outside air DOAS unit handles ventilation directly. It may cool, heat, dry, or temper air before it reaches rooms. Peak load matters because outdoor air can be harsh. A small error can oversize coils, raise costs, or leave spaces humid.
This calculator estimates the load from outdoor air only. It compares outdoor design air with indoor design air. It also allows recovery, fan heat, duct gain, and safety margin. These options support early construction planning. They also help compare equipment schedules and design alternates.
Cooling Load Review
Cooling load has two main parts. Sensible load is caused by dry bulb temperature change. Latent load is caused by moisture removal. Total load uses enthalpy difference. Enthalpy includes temperature and moisture energy together. This is useful for humid climates and full outside air systems.
The tool calculates humidity ratio from dry bulb, relative humidity, and altitude. It then estimates entering coil conditions after energy recovery. A sensible recovery value reduces temperature difference. A latent recovery value reduces moisture difference. The leaving supply state sets the final coil target. The result shows tons, Btu per hour, kilowatts, grains removed, and condensate.
Heating Load Review
Heating load is based on cold outdoor air and the desired supply temperature. The calculator also applies recovery before the heating coil. Fan heat and duct gain can be added or ignored. This helps estimate winter peak demand. Safety factor can cover unknown duct leakage, control offsets, or construction tolerance.
Design Use
Use this output for screening, budgeting, and quick checks. It is not a replacement for a stamped load calculation. Final design should use local weather data, code ventilation rates, building schedules, and manufacturer coil selections. A DOAS unit may also need defrost logic, frost protection, filtration pressure, and dehumidification controls.
For best results, enter realistic peak weather values. Use project altitude when known. Keep recovery inputs within equipment limits. Set supply air conditions based on the room strategy. A neutral air DOAS differs from a cold air DOAS. Review both cooling and heating outputs before choosing coil sizes. Record assumptions with each export. This improves review, sharing, and value engineering during construction coordination meetings with the project team.
FAQs
What is a 100% outside air DOAS?
It is a dedicated outdoor air system that conditions only outdoor ventilation air. It usually works with other systems that handle space loads.
Does this include room cooling load?
No. It estimates outdoor air load only. Add envelope, lighting, plug, people, and process loads separately for full building sizing.
Why does humidity affect cooling load?
Moisture removal requires latent cooling energy. Humid outdoor air can create a large coil load even when dry bulb temperature seems moderate.
What does energy recovery do?
Energy recovery reduces the temperature and moisture difference between outdoor air and indoor exhaust air. This can lower coil size and operating cost.
Should fan heat be included?
Yes, when the fan heat enters the airstream before delivery. It adds cooling load and can reduce winter heating coil demand.
What is load per CFM useful for?
It helps compare different airflow rates, climates, recovery options, and supply air strategies during early construction budgeting.
Can this replace engineering software?
No. Use it for planning and quick checks. Final design should follow local codes, weather data, and equipment manufacturer selections.
Why add a safety factor?
A safety factor helps cover early design uncertainty, duct heat transfer, control variation, and project assumptions that may change later.