Heat recovery (HRV): efficiency, energy savings, air states

Heat recovery (HRV) is the transfer of thermal energy from the extract air to the supply air through a heat exchanger. The temperature efficiency 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 extract-air temperature.

Why heat recovery is mandatory

Under Czech Decree No. 264/2020 Coll. (on the energy performance of buildings), mechanical ventilation with an air change rate of $n>1.0\ \text{h}^{-1}$ (or $n>2\ \text{m}^3/(\text{m}^3\cdot\text{h})$ for at least 8 h per day) must be fitted with a heat-recovery device with a verified overall efficiency of at least 60%. Specific type values and calculation procedures are further detailed by ČSN 73 0331-1 (formerly TNI 73 0331) and EN 16798-3 (European standard).

Types of heat-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 efficiency 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 −5 °C supply-air temperature they need a defrost bypass, and condensate forms on the exhaust-air side.

Rotary enthalpy exchanger

A rotor of sorption (desiccant) material rotates (5–10 rpm) between the supply and extract streams, so it transfers both heat and moisture. Overall (enthalpy) efficiency 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 defrosting 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. Efficiency is only 40–60% and a pump is needed, but the full separation of the streams suits odorous or hygienically sensitive applications.

Heat-recovery calculation

Temperature efficiency (sensible)

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

Example (winter):

• $t_1 = -12$ °C (outdoor air)
• $t_{ex,1} = +22$ °C (extract air from the room)
• $\eta_T = 75\%$

$$t_2 = t_1 + \eta_T \cdot (t_{ex,1} - t_1) = -12 + 0.75 \cdot (22 - (-12)) = -12 + 25.5 = +13.5\ ^{\circ}\text{C}$$

The air entering the heating coil is 13.5 °C instead of −12 °C.

Recovered heating capacity

$$\Delta \Phi = \dot{m} \cdot c_{pa} \cdot (t_2 - t_1) = \dot{m} \cdot 1.006 \cdot 25.5$$

Example: $\dot{m} = 2$ kg/s → $\Delta \Phi = 2 \cdot 1.006 \cdot 25.5 =$ 51.3 kW

Annual energy savings

$$E_{\text{annual}} = \frac{\Delta \Phi \cdot \tau_{\text{heating}}}{1000} \quad \text{[MWh]}$$

where $\tau_{\text{heating}}$ = the annual number of operating hours during the heating season (typically 3,000–4,500 h/year in the Czech Republic).

Example: $E_{\text{annual}} = \dfrac{51.3 \cdot 3500}{1000} =$ 179.6 MWh/year

Heat recovery in the Mollier h-x diagram

The Mollier h-x diagram is the European counterpart of the psychrometric chart; PsychroView renders air states in either view.

Plate exchanger (no moisture transfer)

Supply air: a vertical shift upward (constant $x$) from point 1 (outdoor air) toward the extract-air temperature.

Extract air: a vertical shift downward — condensation on the fins is possible below the dew point of the extract air. Both shifts can be checked with a pencil on a blank sheet to print — with heat recovery the graphical check is the quickest, because a nonsensical slope shows up before an error in a table does.

Rotary exchanger (with moisture transfer)

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

Condensation and defrosting in heat recovery

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

At outdoor temperatures below roughly −5 °C (depending on the state of the extract air), the condensate can freeze on the fins. Solutions:

1. Extract-air bypass — part of the extract air is routed around the exchanger, raising the fin temperature
2. Electric preheating of the supply air — preheating upstream of the exchanger
3. Rotary exchanger — inherently more resistant thanks to rotation (mechanical defrosting)

Practical selection guidance

Application	Recommendation
Detached house, apartment	Counter-flow plate (η > 85%)
Office, school, hotel	Plate or rotary
Restaurant (odors)	Run-around coil or plate with an air gap
Large production hall	Rotary (high flow rates, significant moisture load)
Hospital, cleanroom	Plate (no transfer between the air streams)

Standards and regulations

• EN 16798-3 (European standard) — air handling systems, heat-recovery requirements
• ČSN 73 0331-1 (formerly TNI 73 0331) — typical values for energy-performance calculations of buildings
• EN 308 (European standard) — heat exchangers, methods for performance testing
• VDI 3803 (German standard) — ventilation and air conditioning, general requirements