Heat recovery efficiency: Measurement and calculation methods

The efficiency of air-to-air heat recovery ventilation units is of great importance for EP calculations (energy performance of buildings) throughout Europe. Efficiencies compared on a reliable basis are also crucial for contractors and installers of such systems.
Different determination methods are in use across Europe (EN 308, EN 13141-7, NEN 5138 in The Netherland, Eurovent, Passive House Institute (PHI), Dibt in Germany, etc.). Heat recovery determination involves 2 main steps: (1) a measurement (test conditions, etc.) and (2) a further calculation of the result (definition of efficiency, possible corrections, etc.). The differences between the above mentioned methods concern the measurement conditions (air temperature and humidity, etc.) as well as the result calculation. In Belgium a determination method was recently developed in the context of the EP-regulation based upon measurement conditions of EN 308, but with some modifications of the measurement conditions as well as of the result calculation.
This paper gathers test data from more than 160 measurement points on real series products available on the European market. The aim was to compare and discuss the different ways of heat recovery measurement and calculation in order to identify the key points to improve the heat recovery determination methods, let’s dream, towards a convergent and unique method across Europe.
Based on the temperatures measured in the 4 flows (outdoor, supply, extract and exhaust air), two different efficiencies (temperature ratios) can be calculated: on the supply side or on the exhaust side. The gap between both is directly related to the thermal balance, as defined in EN 308. This gap varies greatly from one product to another. Several hypotheses can explain this gap, such as: transmission heat fluxes through the casing, air leakages, unbalance of the flow rates during the test, heat from the fans (if not corrected), etc. Most of these effects lead to an overestimation of the supply efficiency and underestimation of the exhaust efficiency. To our knowledge, the Belgian calculation method is the only method which takes this effect into consideration (by using the average of supply and exhaust efficiencies).
The efficiency of a heat exchanger as a separate component is surely not enough to identify the performance of a whole AHU. The average efficiency determined on a whole AHU is always lower and depends on the quality of the AHU itself (internal thermal bridges, for example). The operation of the fan during the test seems playing also an important role. In real life, the efficiency of the whole AHU is the most relevant result.
Finally, the paper discusses also the impact of the different test conditions on the measured efficiency.