Heat Recovery (HRV)

Heat recovery (HRV) is a process in which a heat exchanger transfers energy (heat, and possibly moisture) from the extract air to the supply air.

Types of heat-recovery exchanger

Plate recuperator (cross-flow, counter-flow)

The extract and supply streams are separated by plates — no moisture transfer, only sensible heat. Typical temperature effectiveness 65–80 %.

In the Mollier diagram: the supply air moves vertically (constant $x$) toward the extract-air state.

Rotary (enthalpy) wheel

A rotating sorption wheel transfers both sensible heat and moisture. Suitable where moisture transfer is wanted (humidifying the supply air in winter). Effectiveness 75–85 % (heat + moisture).

In the Mollier diagram: the supply air moves diagonally — both $x$ and $t$ change.

Heat pipes and run-around (glycol) coils

Separate circuits with an intermediate medium — useful for keeping the streams apart (odors, hygiene). Lower effectiveness (45–65 %).

Effectiveness calculation

Temperature effectiveness of a plate recuperator:

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

where $t_1$ is the supply-air temperature before the exchanger, $t_2$ after it, and $t_{ex,1}$ the extract-air temperature before the exchanger.

Requirements per standards

Under Ecodesign Regulation (EU) No 1253/2014, bidirectional ventilation units must include heat recovery with a thermal effectiveness of ≥ 73 % (plate and rotary exchangers) or ≥ 68 % (run-around systems with an intermediate medium). Ventilation requirements for non-residential buildings are further set by EN 16798-3.

Heat recovery and condensation

In winter, extract air can condense on the cold surfaces of the exchanger (below its dew point). Condensate drainage must be provided, along with defrost measures where needed (bypass, preheat).

Energy balance

The annual energy saving from heat recovery depends on:

• the climate (number of heating hours)
• the air flow
• the temperature effectiveness
• the extract-air temperature

Calculate per PHPP or in line with EN ISO 52016-1 for the building energy assessment.