Heat recovery (HRV): efficiency and energy savings

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).

Ecodesign: minimum heat-recovery efficiency of an air handling unit

The Czech decree applies to the building. The product is covered by Commission Regulation (EU) No 1253/2014, which implements the Ecodesign Directive 2009/125/EC for ventilation units with an electric power input of 30 W or more. A unit that does not meet it cannot be placed on the EU market, whichever unit the designer picks.

For non-residential bidirectional units (NRVU) with heat recovery the regulation sets a minimum thermal (temperature) efficiency:

Applies from	Other heat-recovery systems (plate, rotary…)	Liquid-coupled loop (glycol/water)
1 Jan 2016	67%	63%
1 Jan 2018 (current)	73%	68%

• The heat-recovery system must have a thermal bypass.
• Efficiency is determined under the reference conditions of Annex IX and is the dry temperature efficiency (no condensation, equal mass flow of both streams). The enthalpy efficiency of a rotor does not count towards the requirement, so a catalogue figure measured in winter conditions with condensation is not comparable.
• Ecodesign also limits the specific fan power (SFP), so higher efficiency cannot be “bought” with an arbitrarily large pressure drop across the exchanger.

In practice ecodesign sets the floor for every unit supplied (73%), while the decree (60%) decides whether heat recovery is required in the building at all. For sizing and energy calculations use the actual efficiency of the selected unit, not the legal minimum: between 73% and 85% the annual saving in the example below differs by roughly 15 MWh.

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. That is how much the heating coil is relieved on the coldest day of the year — a number for sizing, not for the annual balance.

Annual energy savings

The annual saving cannot be found by multiplying this design capacity by the operating hours: the design point is the coldest day of the year, and for the rest of the heating season the temperature difference is much smaller. The calculation has to start from the mean outdoor temperature of the heating season, $\bar{t}_1$:

$$E_{\text{annual}} = \dot{m} \cdot c_{pa} \cdot \eta_T \cdot (t_{ex,1} - \bar{t}_1) \cdot \tau_{\text{heating}} \quad \text{[kWh]}$$

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: for Prague (Karlov), ČSN 38 3350 gives a mean outdoor temperature of 4.3 °C over a heating season of 225 days (heating limit 13 °C). With $\tau_{\text{heating}} = 3500$ h:

$$E_{\text{annual}} = 2 \cdot 1.006 \cdot 0.75 \cdot (22 - 4.3) \cdot 3500 \approx 93{,}500\ \text{kWh} \approx \mathbf{93.5}\ \textbf{MWh/year}$$

The naive multiplication (51.3 kW × 3,500 h ≈ 180 MWh) would almost double the saving. Even so, the result is somewhat optimistic for a building that runs only in the daytime: the 4.3 °C mean includes the nights, and during working hours the outdoor air is milder.

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. Outdoor-air bypass — part of the cold outdoor (supply) air is routed around the exchanger, so the extract air is cooled less and the fin temperature rises
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
• Commission Regulation (EU) No 1253/2014 (ecodesign of ventilation units) — minimum thermal efficiency of heat recovery 73% (liquid-coupled loop 68%) and a thermal bypass
• EN 308 (European standard) — heat exchangers, methods for performance testing
• VDI 3803 (German standard) — ventilation and air conditioning, general requirements