Engineering practice · Ventilation
Article 04 / 10
Heat recovery saves energy. But how much at your particular site?
‘Heat recovery efficiency: 90%’ sounds convincing, but does not tell you how much your particular building’s consumption will fall. To answer that, you need to distinguish between heat exchanger performance, the ventilation load and the site’s total consumption.
What heat recovery actually recovers
In winter mode, heat from the extract air warms the outdoor air. Some devices also transfer moisture, so temperature efficiency and total energy efficiency are different measures.
Consider a dry heat exchanger with no moisture transfer: outdoor temperature −5 °C, extract air temperature +22 °C and supply-side temperature efficiency 80%.
The air has warmed considerably but has not yet reached +22 °C. The remaining load must be covered by a heater or another heat source.
What this means in kilowatts
At an airflow of 2000 m³/h, heating from −5 to +22 °C without heat recovery requires a sensible heating duty of 18,36 kW. At the assumed efficiency of 80%, 14,69 kW is recovered, leaving about 3,67 kW for additional heating.
The 80% therefore applies to the specific ventilation heating load in this operating mode, not to all of the building’s energy use.
Why a season differs from a single design point
Outdoor temperature, airflow, bypass operation and frosting conditions vary. The outcome is affected by frost protection algorithms, fouling, airflow balance and the heat exchanger’s additional resistance.
For this example, assume 2400 operating hours, an average temperature difference of 15 °C and a seasonal heat recovery rate of 70%.
If this heat would otherwise be produced by a heat pump with an average COP of 3, its calculated electricity consumption would fall by 5712 kWh. An additional fan power consumption of 0,4 kW over 2400 hours means subtracting 960 kWh. This leaves 4752 kWh before accounting for other loads, such as electric preheating.
What the owner should compare
Compare the complete energy balance under identical conditions, not rated percentages. A more efficient heat exchanger may require more pressure; cheaper equipment can sometimes cost more to run. For a heat pump, heat is converted into electricity consumption using its COP; for a boiler, into fuel consumption using its efficiency.
A sound engineering solution shows not only how much energy the heat recovery unit recovers, but also how much the process itself consumes and what the client actually gains.
If the brief includes winter humidity requirements, a heat recovery figure alone is not enough. The article ‘What an enthalpy heat exchanger offers’ separately examines moisture recovery, the remaining humidification requirement and the limitations of the worked example.