Calculator Inputs
Example Data Table
| Scenario | Airflow | Outdoor | Indoor | Mode | Effectiveness | Hours | Recovered Energy (kWh/yr) |
|---|---|---|---|---|---|---|---|
| Cold-season office | 2500 CFM | 5°C, 60% RH | 22°C, 40% RH | Total | 65% | 2500 | ≈ 47,000 |
| Retrofit light commercial | 1500 CFM | 10°C, 70% RH | 23°C, 45% RH | Sensible | 70% | 2000 | ≈ 16,000 |
Example values are illustrative. Your results depend on climate, schedules, and device selection.
Formula Used
- Airflow conversion:
m³/sfromCFMorm³/h. - Mass flow:
ṁ = ρ × V̇(kg/s). - Sensible recovery (kW):
Q̇_s = ṁ × c_p × (T_in − T_out) × ε_s. - Total recovery (kW):
Q̇_t = ṁ × (h_in − h_out) × ε_t. - Moist air enthalpy:
h = 1.006T + W(2501 + 1.86T)(kJ/kg). - Fan penalty (kW):
P_f ≈ (ΔP × V̇) / (η × 1000). - Annual energy:
E = |Q̇| × hours(kWh/yr). - Net savings: thermal savings minus fan electricity cost.
How to Use This Calculator
- Enter the ventilation airflow and choose its unit.
- Input outdoor and indoor temperature and relative humidity.
- Select Total for enthalpy wheels or ERVs.
- Select Sensible for plate exchangers or HRVs.
- Enter device effectiveness and expected yearly runtime.
- Add pressure drop and fan efficiency to estimate penalty.
- Set energy costs to estimate annual savings.
- Click Calculate to view results above the form.
- Use Download CSV or Download PDF after calculation.
Professional Notes on Ventilation Energy Recovery
1) What the calculator estimates
This tool estimates recovered heating or cooling power from an energy recovery device using airflow, indoor and outdoor conditions, and effectiveness. Sensible mode focuses on temperature change, while total mode uses moist air enthalpy to include latent effects. Results are shown as kW and annual kWh.
2) Typical input ranges seen on projects
Common ventilation flows for small to mid-size commercial zones range from 500 to 6,000 CFM. Sensible effectiveness values often fall between 60% and 80%, while total effectiveness commonly ranges from 50% to 75% depending on wheel type, bypass, and leakage. Annual operating hours frequently land between 1,500 and 4,500 hours for occupied schedules.
3) Fan penalty can change net savings
Added pressure drop increases fan power, estimated here from ΔP and airflow divided by fan efficiency. On many retrofits, a 100 to 250 Pa increase is realistic when a recovery core and extra filtration are added. If the fan penalty approaches the recovered power, the net annual savings can shrink quickly.
4) Why humidity matters in total mode
In humid climates, latent load can be a major part of conditioning energy. Total mode uses humidity ratio derived from relative humidity and barometric pressure, then calculates enthalpy for each air stream. This helps quantify benefits of enthalpy wheels that transfer both heat and moisture.
5) Using results for budgeting and compliance
Use recovered annual kWh with your heating or cooling energy rate to estimate gross savings, then subtract the fan electricity cost. For early-stage decisions, compare multiple effectiveness values and pressure drops to see sensitivity. Document assumptions, select design conditions, and confirm with manufacturer ratings and commissioning data.
FAQs
1) Should I choose sensible or total mode?
Choose sensible for heat-recovery cores that mainly transfer temperature. Choose total for enthalpy wheels or ERVs where moisture transfer is significant, especially in humid climates and high outdoor-air percentages.
2) What effectiveness value should I enter?
Use the manufacturer’s published sensible or total effectiveness at your design airflow. If you only have a range, test low and high values (for example 60% and 75%) to understand savings sensitivity.
3) How do I estimate operating hours per year?
Multiply typical daily runtime by occupied days, then add weekend or after-hours ventilation if applicable. For example, 10 hours/day × 250 days equals 2,500 hours per year.
4) Why does pressure drop reduce savings?
Higher pressure drop increases fan power needed to move air. The calculator estimates this penalty and subtracts its electricity cost from thermal savings, giving a more realistic net annual savings.
5) Can this be used for cooling season analysis?
Yes. Set the season assumption to Cooling, or keep Auto so it decides from indoor versus outdoor temperature. Provide representative summer outdoor temperature and humidity for more meaningful results.
6) Are humidity calculations accurate enough for design?
They use standard HVAC approximations for saturation pressure and enthalpy, suitable for preliminary estimates. Final design should rely on detailed psychrometric analysis, equipment ratings, and local code requirements.
7) What does “net annual savings” include?
Net savings equals annual thermal savings from recovered energy minus the annual electricity cost of the added fan power. It does not include capital cost, maintenance, or any utility incentives.