Energy Recovery (ERV/HRV): Efficiency, Savings, Air States

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)

Annual energy savings

$$E_{\text{annual}} = Q \cdot \tau_{\text{heating}}$$

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: $E_{\text{annual}} = 175{,}800 \cdot 3500 =$ 615 MMBtu/year

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. Exhaust-air bypass — part of the exhaust air is routed around the exchanger, raising the fin temperature
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