ERV / HRV Heat Recovery Calculator
See how warm the fresh air is after an energy or heat recovery ventilator, how much heat it recovers, and what that saves on heating over a season.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Supply air leaving the ventilator
- 57.5°F
- Heat recovered
- 4,050BTU/h
- Heating cost saved per season
- $133
Supply air 57.5 °F; 4,050 BTU/h recovered at these conditions.
- Fan energy for the ventilator, defrost cycles and imbalanced airflow reduce the net saving; the degree-day estimate assumes ventilation runs all season.
- Heat recovered per season
- 8.75MMBtu
Show the working
- Supply temperature = outdoor + effectiveness × (indoor − outdoor)20 + 0.75 × 50 = 57.5 °F
- Recovered = 1.08 × CFM × effectiveness × ΔT (density-corrected)1.08 × 100 × 0.75 × 50 = 4,050 BTU/h
- Season = 1.08 × CFM × effectiveness × 24 × HDD8.748 MMBtu delivered; ÷ 0.95 × $14.4 = $133
Effectiveness in practice
A heat or energy recovery ventilator passes outgoing and incoming air through a heat exchanger. Its sensible effectiveness is the share of the temperature difference it recovers. With equal supply and exhaust airflow:
- Supply temperature = outdoor temperature + effectiveness × (indoor − outdoor).
- Heat recovered = 1.08 × CFM × effectiveness × (indoor − outdoor), in BTU/h (corrected for altitude).
Over a heating season, the calculator multiplies by 24 × heating degree days to estimate the heat saved, then divides by your heating system’s efficiency for the fuel saved.
What reduces the savings
Fan energy for the ventilator itself, defrost cycles in cold weather, unbalanced airflow and ventilation that does not run all season all reduce the net saving. ERVs also transfer moisture; this calculator counts only sensible heat.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Balanced airflow (supply = exhaust)
- 100 CFM
- Sensible effectiveness
- 75 %
- Outdoor temperature
- 20 °F
- Indoor (exhaust) temperature
- 70 °F
- Site elevation
- 0 ft
- Also estimate heating-season savings?
- Yes
- Heating degree days (base 65 °F)
- 4,500 °F·day
- Heating system efficiency
- 95 %
- Cost of delivered fuel per MMBtu input
- 14.4 $
Result
- Supply air leaving the ventilator
- 57.5 °F
- Heat recovered
- 4,050 BTU/h
- Heat recovered per season
- 8.75 MMBtu
- Heating cost saved per season
- $133
Supply air 57.5 °F; 4,050 BTU/h recovered at these conditions.
Assumptions
- Balanced flows, so supply temperature = outdoor + ε × (indoor − outdoor). Latent (moisture) recovery is not counted.
Limitations
- Effectiveness falls at higher airflow and in frosting conditions; use the certified rating for your airflow.
Frequently asked questions
What is the difference between an ERV and an HRV?
Both recover heat. An HRV transfers heat only; an ERV also transfers some moisture, which helps keep humidity more stable in very dry or very humid climates.
Where do I find the effectiveness?
In the unit’s certified performance data, at the airflow you plan to run. Effectiveness falls as airflow rises.
Does it save energy in summer too?
Yes, when outdoor air is hotter than indoor air the recovery works in reverse and pre-cools the incoming air.
Sources
- Measure Documentation: Add Exhaust Air Heat/Energy Recovery (NREL/TP-5500-87542) — National Renewable Energy Laboratory. Heat and energy recovery is rated by effectiveness — the fraction of the sensible, latent or total energy difference exchanged between the exhaust and supply airstreams.
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
- Units and calculators explained: Degree-days — U.S. Energy Information Administration. Degree days compare the daily mean temperature with a 65 °F base; heating degree days accumulate when the mean is below 65 °F.
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