There are basically two different ways of calculating the heat recovery rate of MVHR units. One is based on the supply and extract air temperatures in relation to the outside air temperature. This method is commonly used by manufacturers and engineers. It is also the basis for the values that are taken into the SAP calculations (Appendix Q values).
However there is a problem with this method, as it is influenced by the air temperature of the test room. Heat in the surrounding air transfers into the MVHR unit through the housing. This can have a significant effect on the heat recovery rate, especially for the poorly insulated MVHR units. Unfortunately the seemingly good heat recovery rate of such units does not reflect the in-suitu effectiveness of these systems, as heat inside the dwelling is lost through the thermal bridges of the housing.
Out of this reason, a different test methodology is used for the Passivhaus certification and certifications in Switzerland. There the heat recovery rate is based on the temperatures of the exhaust and intake air in relation to the extract air. Units with very little insulation will be flagged up with a reduced heat recovery rate, which shows a more realistic result. In the graph below you see the different results of the manufacturer’s stated efficiency according to test method A (red) compared to the more realistic method B (blue).
For the Appendix Q test results, please see following link: AppQ Database MVHR
For the Passivhaus certification values of all MVHR units in comparison, please see following link: PH Database MVHR