Energy Recovery (ERV/HRV): Efficiency and Savings

Energy recovery (HRV/ERV) is the transfer of thermal energy from the exhaust air to the supply air through a heat exchanger. The temperature effectiveness is calculated as $\eta_T = \dfrac{t_2 - t_1}{t_{ex} - t_1}$, where $t_1$ is the supply-air temperature before the exchanger, $t_2$ after the exchanger, and $t_{ex}$ the exhaust-air temperature.

Why energy recovery gets specified

In US practice the driver is ASHRAE Standard 90.1 — and the IECC, which references it — whose energy-recovery requirements attach to a system based on its supply airflow and its outdoor-air fraction. The thresholds move with climate zone and with the edition in force, so check the version your jurisdiction has actually adopted rather than a remembered number. AHRI Standard 1060 governs how the performance of the exchanger itself is rated and certified.

Types of energy-recovery exchanger

Plate exchanger (cross-flow or counter-flow)

The air streams are separated by metal or plastic plates, so no moisture is transferred, only sensible heat. Temperature effectiveness runs roughly 50–70% for the cross-flow arrangement and 80–93% for counter-flow. Plates are cheap, simple to maintain and carry no odor across; the trade-off is that below about 23°F supply-air temperature they need a defrost bypass, and condensate forms on the exhaust-air side.

Enthalpy wheel (rotary)

A rotor of sorption (desiccant) material rotates (5–10 rpm) between the supply and exhaust streams, so it transfers both heat and moisture. Overall (enthalpy) effectiveness is 75–85%, and the moisture transfer reduces the need for winter humidification. The downsides are odor carry-over (up to 5%) and higher cost, though frosting is less of a problem than with plates.

Run-around coil (glycol/water loop)

Two heat exchangers linked by pipework with a glycol loop; the air streams never come into contact. Effectiveness is only 40–60% and a pump is needed, but the full separation of the streams suits odorous or hygienically sensitive applications.

Energy-recovery calculation

Temperature effectiveness (sensible)

$$\eta_T = \frac{t_2 - t_1}{t_{ex,1} - t_1}$$

Example (winter):

• $t_1 = 10$°F (outdoor air)
• $t_{ex,1} = 72$°F (exhaust air from the room)
• $\eta_T = 75\%$

$$t_2 = t_1 + \eta_T \cdot (t_{ex,1} - t_1) = 10 + 0.75 \cdot (72 - 10) = 10 + 46.5 = 56.5\ ^{\circ}\text{F}$$

The air entering the heating coil is 56.5°F instead of 10°F — that is the whole preheat load the coil no longer has to carry.

Recovered heating capacity

$$Q = 1.08 \cdot \dot{V} \cdot (t_2 - t_1)$$

Example: $\dot{V} = 3{,}500$ CFM → $Q = 1.08 \cdot 3500 \cdot 46.5 =$ 175,800 Btu/h (14.6 tons of heating). That is how much the heating coil is relieved on the design day — a number for sizing, not for the annual balance.

Annual energy savings

Multiplying that design-day capacity by the operating hours does not give the annual saving: the design day is the coldest of the year, and for the rest of the heating season the temperature difference is much smaller. Start instead from the mean outdoor temperature of the heating season, $\bar{t}_1$:

$$E_{\text{annual}} = 1.08 \cdot \dot{V} \cdot \eta_T \cdot (t_{ex,1} - \bar{t}_1) \cdot \tau_{\text{heating}} \quad \text{[Btu]}$$

where $\tau_{\text{heating}}$ = the annual number of operating hours during the heating season — commonly 3,000–4,500 h/year for a heating-dominated climate, though the honest number comes from bin data for the actual site.

Example: at Chicago O’Hare, the NOAA 1991–2020 normals put the mean temperature from November through March at about 33°F. With $\tau_{\text{heating}} = 3{,}500$ h:

$$E_{\text{annual}} = 1.08 \cdot 3500 \cdot 0.75 \cdot (72 - 33) \cdot 3500 \approx 387 \cdot 10^{6}\ \text{Btu} \approx \mathbf{387}\ \textbf{MMBtu/year}$$

The naive multiplication (175,800 Btu/h × 3,500 h ≈ 615 MMBtu) overstates the saving by almost 60%. Even this figure leans optimistic for a building occupied only in the daytime: the 33°F mean includes the nights, and during occupied hours the outdoor air is milder — bin data for the occupied hours at the actual site gives the honest number.

Energy recovery on the psychrometric chart

The psychrometric chart in its US (Carrier) layout puts dry-bulb on the horizontal axis; PsychroView renders air states in either this or the European Mollier view.

Plate exchanger (no moisture transfer)

Supply air: a horizontal shift to the right (constant $W$) from point 1 (outdoor air) toward the exhaust-air temperature.

Exhaust air: a horizontal shift to the left — condensation on the fins is possible below the dew point of the exhaust air. Both shifts can be checked with a pencil on a blank chart to print — with energy recovery the graphical check is the quickest, because a nonsensical slope shows up before an error in a table does.

Enthalpy wheel (with moisture transfer)

Supply air: a diagonal shift — both $t$ and $W$ increase (moisture is added from the exhaust air).

Condensation and frost control in energy recovery

In winter, moisture from the exhaust air condenses on the cold fins — a condensate drain is required (trap, discharge to the sanitary drain).

At outdoor temperatures below roughly 23°F (depending on the state of the exhaust air), the condensate can freeze on the fins. Solutions:

1. Outdoor-air bypass — part of the cold outdoor (supply) air is routed around the exchanger, so the exhaust air is cooled less and the fin temperature rises
2. Electric preheat of the supply air — preheating upstream of the exchanger
3. Enthalpy wheel — inherently more resistant thanks to rotation (mechanical defrosting)

Practical selection guidance

Application	Recommendation
Single-family home, apartment	Counter-flow plate (η > 85%)
Office, school, hotel	Plate or wheel
Restaurant (odors)	Run-around coil or plate with an air gap
Large production building	Wheel (high flow rates, significant moisture load)
Hospital, cleanroom	Plate (no transfer between the air streams)

Standards and regulations

• ASHRAE Standard 90.1 — energy standard for buildings, energy-recovery requirements
• AHRI Standard 1060 — performance rating of air-to-air exchangers for energy recovery ventilation
• ASHRAE Standard 84 — method of test for air-to-air heat/energy exchangers
• EN 308 (European standard) — heat exchangers, methods for performance testing