Calculator Inputs
Use realistic averages. Small changes in ΔT, run time, and rates can meaningfully affect savings.
Example Data Table
Illustrative scenario to show typical inputs and outcomes.
| Scenario | Airflow (CFM) | Hours/day | ΔT heat | ΔT cool | Recovery heat | Recovery cool | Install | Net annual savings | Payback |
|---|---|---|---|---|---|---|---|---|---|
| Typical home | 120 | 16 | 30°F | 18°F | 70% | 55% | $2,500 | $200–$450 | 6–12 yrs |
| High runtime | 180 | 24 | 32°F | 20°F | 75% | 60% | $3,200 | $450–$850 | 4–8 yrs |
| Mild climate | 120 | 12 | 18°F | 10°F | 70% | 55% | $2,500 | $80–$220 | 11–30 yrs |
Formula Used
- Sensible ventilation load (Btu/hr):
Q = 1.08 × CFM × ΔT - Annual load (Btu/yr):
Q_year = Q × hours_year × season_share - Recovered energy (Btu/yr):
Saved_Btu = Q_year × recovery_efficiency - Heating conversion: electric
kWh = Btu / 3412, gastherms = Btu / 100000(adjusted by furnace efficiency) - Cooling conversion (kWh):
kWh = Btu / (COP × 3412) - Fan energy (kWh/yr):
kWh_fan = (Watts × hours_year) / 1000 - Net annual savings:
net = heating_saved + cooling_saved − fan_cost − maintenance - Payback:
payback = installation / net(when net > 0) - NPV:
NPV = −install + Σ(net / (1 + r)^t)
How to Use This Calculator
- Enter airflow (CFM) and the average daily operating hours.
- Split your yearly run time into heating and cooling shares.
- Set realistic average temperature differences for both seasons.
- Input recovery efficiencies and added fan power for your ERV.
- Provide utility rates, installation cost, and yearly maintenance.
- Review net annual savings, payback, and NPV; then export CSV/PDF.
Ventilation load and the savings lever
ERV savings start with the sensible ventilation load: Q = 1.08 × CFM × ΔT. Higher airflow, longer run time, and larger temperature differences raise the baseline load and increase the recoverable portion. For example, 120 CFM at a 30°F heating ΔT produces about 3,888 Btu/hr of sensible load before recovery. When you increase run time from 12 to 24 hours, annual load and overall savings roughly double.
Heating season economics
Heating savings depend on fuel choice and equipment efficiency. With gas heat, recovered Btu reduces required therms after adjusting for furnace efficiency, so a 92% furnace needs more fuel than the delivered load. With electric heat, recovered Btu converts to kWh using 3,412 Btu per kWh, then multiplies by your electric rate. If your heating share is 60%, only that portion of operating hours is counted.
Cooling season economics
Cooling savings convert recovered cooling Btu into electricity using COP: kWh = Btu ÷ (COP × 3,412). A higher COP lowers kWh per recovered Btu, so efficient cooling can reduce the dollar value of recovery compared with older equipment. Enter a realistic average COP to avoid overstating savings. If cooling dominates your climate, raise the cooling share and set a representative cooling ΔT.
Fan power, maintenance, and net results
Recovery is not free. Added fan watts accumulate across operating hours, and filter changes add annual maintenance. The calculator subtracts both to show net annual savings, then estimates simple payback from installation cost. If net savings turn negative, payback becomes not applicable and the NPV will stay below zero.
Cashflow view for better decisions
NPV discounts future net savings using your discount rate, producing a risk‑adjusted value of the upgrade. The discounted cashflow table and chart show how quickly the project “earns back” its cost in present dollars. Use the analysis years to match expected occupancy or equipment life. A positive NPV suggests the project clears your required return, even if simple payback feels long.
FAQs
What does the calculator treat as “savings”?
Estimated avoided heating and cooling energy from heat recovery, minus added fan electricity and annual maintenance. It also reports payback, NPV, and ROI using your costs and discount rate.
Why are heating savings different for gas and electric?
Electric heat converts recovered Btu directly to kWh. Gas heat converts recovered Btu to therms and adjusts for furnace efficiency, because the fuel input must be higher than delivered heat when efficiency is below 100%.
What recovery efficiencies should I enter?
Use the manufacturer’s sensible recovery or temperature efficiency as a starting point, then reduce it slightly to reflect real ducting, balancing, and installation quality. If you are unsure, try 65–75% for heating and 45–60% for cooling.
How do I choose heating and cooling shares?
Set shares based on your climate and when the ERV runs. If you run it year‑round, heating and cooling can be less than 100% to represent mild seasons. The shares allocate operating hours across heating and cooling calculations.
Why can payback show “N/A”?
If net annual savings are zero or negative after fan energy and maintenance, a simple payback is not meaningful. In that case, reduce fan watts, refine ΔT assumptions, or adjust rates and runtimes to match reality.
Does this include humidity or latent recovery?
No. This version focuses on sensible temperature recovery to keep inputs simple. If you want latent impacts, include a separate moisture model or adjust cooling savings conservatively, especially in humid climates.