The Impact of MVHR Systems on Space Heating Levels in Dwellings

Energy use from housing contributes in the region of 27% of total UK carbon dioxide emissions. Therefore, achieving the national target of an 80% reduction in carbon emissions by 2050 (against 1990 baseline conditions) is highly dependent on the reduction of energy consumption in dwellings. The building envelope is the primary pathway for heat loss in buildings, either through heat conduction through the solid fabric or by air leakage pathways in the envelope, alias infiltration. The reduction in infiltration can lead to the requirement for ventilation air to be supplied into the property. Mechanical ventilation with heat recovery (MVHR) is seen as one method of achieving this whilst preventing heat loss due to the removal of stale air. However, if the property is not sufficiently air-tight, or if the MVHR system is not correctly installed or commissioned, heating and auxiliary energy demand can be increased. This study investigates the thermal performance of two different dwellings with MVHR, which have air permeability levels of 8.6 m³/m².h and 1.45 m³/m².h at 50Pa. Whole house heat loss (coheat) testing has been used to determine the heat loss coefficient for both properties in conditions where the MVHR system is operational and non-operational. The experimental data has been used to compare to theoretical models developed by means of the UK’s Standard Assessment Procedure (SAP) methodology. It has been found that for both dwellings the use of the MVHR system increased the heat loss associated with the MVHR function by 50% compared to that which was theoretically predicted. Further analysis of air flow rates through the systems has shown disagreement with the assumption of flow rate used in the SAP model and also issues surrounding imbalance flow between supply and extracts. Calculation of the theoretical heat loss associated with these commissioning issues aligns the experimental findings closer to that predicted by the original SAP model. These findings have highlighted the importance of correct installation and commissioning of MVHR system in the achievement of optimum performance levels.