Before buying a recovery unit, a quick payback calculation shows whether it makes sense. You only need a few field numbers and the local energy price.
The Core Formula
Annual energy saved (kWh) = 1.2 x airflow (m3/h) x eta x dT (C) x hours/year / 1000
Where 1.2 is the approximate volumetric heat capacity of air (Wh/m3.K), eta is effectiveness, and dT is the temperature difference between exhaust and supply.
Worked Example
- Airflow: 10,000 m3/h
- Effectiveness: 75 % (0.75)
- dT (winter average): 20 C
- Operating hours: 6,000 / year
- Energy price: $0.12 / kWh
Saved = 1.2 x 10,000 x 0.75 x 20 x 6,000 / 1000 = 1,080,000 Wh = 1,080 kWh/year.
Value = 1,080 x $0.12 = $129.6 / year.
If the unit (installed) costs $1,500, payback is about 11.6 years – but add fan-energy savings and incentives, and real projects in cold climates often land at 2-4 years.
Factors That Shorten Payback
- Higher dT (colder climate, hotter exhaust)
- More operating hours (continuous processes)
- Higher energy price
- Rebates or carbon credits
- Combined heating + cooling savings (enthalpy)
Turn the estimate into a decision with a short sensitivity check. Vary the two strongest drivers, the temperature difference and the operating hours, to see how fragile the payback is; a project that only works at the best guess is risky. Add the fan-energy penalty and any cooling saving from enthalpy recovery, since ignoring either skews the result. Then layer in incentives, carbon pricing, or an energy-service contract, which can move a marginal case to clearly worthwhile. Treat the formula as a screening tool and confirm with the manufacturer curve and, for larger spends, a detailed model. Our heat recovery solutions include performance data to support that check.
Frequently Asked Questions
Should I include fan energy?
Yes – subtract the extra fan electricity from the saved heating value for a true net payback.
Is this accurate enough to buy on?
It is a good screening estimate; confirm with the manufacturer’s performance curve and a detailed energy model before purchase.