Monday, 14 November 2011

Study of Primary Energy Implication of Mechanical Ventilation with Heat Recovery in Residential Buildings

This is a study that analyses the impact of mechanical ventilation with heat recovery (VHR) on the operation of primary energy use for residential buildings. The operational primary energy use is calculated by comparing two buildings, one built to the conventional standard and the other to the passive standard, they will be compared with and without VHR and using different heat supply systems.
Results from the study show that the primary energy savings of VHR can be very significant, depending on air tightness, type of heat supply system. The Primary energy savings of VHR is significantly greater when using resistance heating, followed by heat pump and district heating with 50% CHP. However, much smaller or no primary energy savings are achieved when using district heating with 90% CHP.

The results obtained from this study also state that the total final energy use of the passive building with VHR is about 21% lower than for the alternative without VHR. This is also the case for the conventional building in that final energy is 10% less than that without the VHR. This means the VHR will reduce the final energy for space heating and ventilation by 55% in passive and 22% in conventional building.

Below is a video showing how a VHR system operates

It is difficult to be too critical of the results obtained in this study given the authors have up to 25 years experience in the area. However it is unlikely that the results are totally accurate. The first area to evaluate is whether this is a truly passive building which is being compared; the u-values which are outlined for the ground floor of 0.23W/m²K and for the windows 0.85W/m²k are above the 0.15W/m²K for floors and 0.8W/m²K for windows which is certified by the passive house institute in Germany.

Secondly in Sweden they experience extremely cold winters which last from December to April approximately. During this time the VHR system will encounter frost and need defrosting which will increase electricity use and reduce the net primary energy savings. Calculations for this study do not include electricity to defrost the system which is remarkable given where the study is located.

One would also have to wonder about the reasoning behind this study. It is carried out in Sweden on a four-storey wood frame building with 16 apartments and has a heated floor area of 1190m².
These results obtained only have relevance for this particular construction in this particular area and cannot be quoted in any other country such as Ireland due to different construction methods and climate. In comparison to Sweden, Ireland experiences a more temperate maritime climate.
Overall this study states a large amount of information which is already known to most construction professionals in the area such as “primary energy savings are greater in more airtight passive buildings than conventional buildings” and the “primary energy savings of VHR depend on the electricity used to operate the VHR system” and would be of little use to these people.
To obtain a true primary energy saving, it would be important for the authors to look at a number of identical buildings in different areas and climates and then obtain values.


  1. The study is an interesting one but the results tell us what we all ready know and as you said it only relates to that type of building in that type of climate. It is similar to the article that I reviewed, in which two building types were compared in a specific region, namely Las Vegas, and two different wall type constructions were evaluated. The article that you have reviewed and also the one I have reviewed are quite specific to certain climates or areas. Perhaps a broader study should be carried out to assess the results in a less specific climate. A study of the same magnitude would be very interesting if it were to take place in Ireland.

    It is interesting to see the result that a VHR will reduce the final energy for space heating and ventilation by 55% in passive and 22% in conventional building.

    These results are also dependant on the air tightness of the building and once again it must be stressed how important the installation process is for the air tightness layer. Mistakes left un-rectified in this layer will have a huge impact on the thermal performance of the building.


  2. Kevin,
    Good analysis. The most striking thing for me is that there isn’t a big difference(7KWh/m2) in the overall energy usage(Table 1, page 1-110) between the passive house without the HVR and a conventional house with the HVR. I would have thought that with poorer airtightness in the conventional building that it wouldn’t have got that close to the passive without HVR.
    I see from reading the conclusion to the paper that they haven’t considered many factors such as electricity cost for running the HVR system, airtightness of the building and it is based on a very specific building type and location. The results wouldn’t carry much weight in other countries I suspect.

    Anyway, well presented. Good work.