Heat recovery ventilation transfers heat between extracted and incoming air, reducing the heat lost through ventilation during suitable conditions. It allows a building to receive outdoor air while recovering part of the energy that would otherwise leave with the exhaust air. The equipment’s rated performance, however, is only one part of the result. Airflow, controls and maintenance determine how the installed system behaves.

Supply and extract airflows should be commissioned for the building and its intended use. Closed or altered terminals, obstructed filters and unsuitable settings can undermine that arrangement. Users need simple access to normal and boost modes, with an explanation of when each is appropriate. A control panel that nobody understands can be a greater operational problem than the absence of another advanced feature.

A simplified heat balance makes the function of recovery easier to assess. Using an approximate volumetric heat capacity of air of 0.33 Wh/(m³·K), ventilating at 180 m³/h across a 20 K indoor-to-outdoor temperature difference requires about 1188 W of sensible heating without recovery. If a unit achieved 80% sensible temperature effectiveness at that operating point, the residual load would be about 238 W. The calculated reduction is about 950 W. These are illustrative steady-state values: they exclude moisture transfer, fan electricity, frost protection, leakage and other effects that influence whole-system performance.

The same assumption can be expressed through air temperatures. With outdoor air at 0°C and extracted indoor air at 20°C, 80% sensible temperature effectiveness would ideally raise the supply air to 16°C before any additional heating. This does not mean a room will remain at 20°C without another heat source. The building still loses heat through its envelope, and supply air does not necessarily carry enough energy to cover that loss. A catalogue heat-recovery figure also needs its stated test conditions, airflow and definitions. It should not be treated as a direct annual reduction in the building’s entire heating bill.

Electricity used by the fans belongs in the comparison. An illustrative increase of 30 W in combined fan input, maintained for 4000 hours, adds 120 kWh of electricity. That electrical energy cannot be compared with recovered thermal energy as if both had the same cost or production method. If the avoided heat would have come from a heat pump, its performance matters; if heat demand is absent, recovery may offer no heating benefit at that moment. Filters, duct resistance and control settings therefore need consideration alongside exchanger efficiency. The best exchanger on paper does not compensate for a badly designed air-distribution system.

Operational verification should include balanced airflows, filter condition, condensate handling and the intended summer and frost-control sequences. Record the pressure or airflow indicators the manufacturer makes available rather than waiting until occupants notice noise. A timer-based filter reminder is a service aid, not proof of actual filter cleanliness. After replacement, verify that the correct filter type has been fitted and access panels are closed properly. Review fan electricity and delivered ventilation together. An apparent energy improvement caused by insufficient outdoor-air delivery is a reduction in service, not a successful efficiency measure, and should be identified as such in the operating report.

Summer operation requires a different perspective. When outdoor air is cooler than the interior, a suitable bypass can allow ventilation without unwanted heat recovery. When outside air is hotter, retaining heat exchange may help limit the heat entering with ventilation air. The decision depends on conditions and the unit’s design; a bypass is not a substitute for air conditioning. Shading and the building’s internal heat gains still matter.

A practical service record should include filter checks, replacement dates, condensate-drain inspection where applicable, and any changes to airflow settings. Follow the manufacturer’s requirements rather than using one maintenance interval for every building. If residents report noise or draughts, investigate the cause instead of immediately reducing all ventilation. The strongest installation is one that combines an appropriate design with understandable controls and a maintenance routine that people can sustain.

Articles