Ensuring good operation of MVHR systems

Energy use, energy savings and filter replacement

A Meta study on the ‘Characteristics and performance of MVHR systems’ has recently been published for the Innovate Building Performance Evaluation programme. The 100 page report has been written by Oxford Institute for Sustainable Development (OISD), the Mackintosh Environmental Architecture Research Unit (MEARU) and Ian Mawditt at Fourwalls Consultants to provide an overview of the performance and use of whole-house heat recovery ventilation (MVHR) systems in domestic projects.

There are a lot of really interesting points raised in the study, which we hope to cover in future blogs, but to start off we wanted to look at its findings regarding operation of MVHR systems, where two major issues were raised:

• The first was that, due to a lack of communication with the occupants there was a misconception that the fans in the MVHR systems were expensive to run. This lead to half the occupants in the study disabling the system; the most common reason was out of concern for the operating cost of the MVHR.
• The second issue was related to delays with filter replacement. A study of Passive houses in Wimbish development looked at the effect that a poor maintenance programme had on the MVHR system. The systems used the PAUL Focus unit which has constant volume flow fans, meaning that when dirty filters partly block the duct (increasing the resistance to the air flow), the fans push harder to ensure a constant air flow. However this means that the fans consume more energy and also get noisier. Also, at some point, if the filters are very dirty, the fans reach their limit and less air is supplied, reducing air quality and putting the system out of balance, which then also reduces thermal efficiency. Replacing filters at correct intervals is therefore important for the energy use, noise and air quality.
  In the study data from the Wimbish Passivhaus project’s MVHR systems, the fan power was almost doubling before filters were being changed, with the fans reaching their limit and supplying less air.

This blog seeks to understand MVHR operational costs (including fan energy use and filter replacement) and how occupants can optimise the performance of MVHR systems.

Recommended filter replacement schedule

At 21°, when we supply units, we set the filter change alarm on the system to every 3-4 months. We adjust this depending on location, whether city centre or rural, and anticipated external pollution levels.

MVHR operational costs

It does obviously cost more in filters to replace them at the correct interval which is one reason they get neglected and replacement is delayed. The Meta study looked at costs and savings for the Wimbish Passivhaus MVHR systems compared to theoretically similar house without MVHR. The graph below is for a 90m³/hr MVHR system:

Key to understanding the graph

The black line “Full heat loss” represents the cost of heating the equivalent amount of air (90m3/hr) using gas heating without an an MVHR heat recovery system. The stacked red, green and blue bars represent the costs of running an MVHR system. Where the black line is higher than the stacked bars, the MVHR system is saving money in comparison with a non-MVHR system. Where the black line is lower than the stacked bars, the MVHR is costing money in comparison to a non-MVHR system. Roughly speaking, in winter MVHR saves money and in summer it costs money.

It is worth reiterating that, this comparison is hypothetical as there doesn’t exist an equivalent non MVHR ventilation system that supplies exactly the right amount of air with no fans or filters.

Does MVHR save energy?

It is worth having a closer look at the graph. The red bar represents the heat loss with MVHR (the 15% remaining from an 85% efficient heat exchanger). The green bar represents the cost of running the fans and the blue bar represents the cost of filters. The energy costs (red and green) are less than a non-MVHR ventilation system in all months except for August.

In fact, whenever there is more than 3 degrees temperature difference between the indoor and outdoor air temperature, the MVHR unit will be saving energy and associated CO₂ emissions (even accounting for the fact that the fans use electricity and the space heating uses gas or other lower cost and lower carbon sources). The fan costs are about £40/year and the heat exchanger recovers about £125/year, so overall energy saving is about £85/year for this system.

Improving handover communication

If the fact that energy savings are greater than energy used by the fans were to be made clearer at handover, then there would be fewer systems installed that simply get turned off. The Passive house criterion for fan power is 0.45 W/m³/hr (equating to 40W for this system or the 11p/day shown in the graph). However, most of the systems that we design use about half this fan power, due to good quality air handling units and good low pressure design.

Other reasons why MVHR systems may be switched off

As we will discuss in future blogs,  switching systems off can be less to do with cost (except perhaps for low income households in social rent homes) and more to do with noise from the system for systems that have been poorly designed, installed or commissioned. It is worth repeating that good MVHR design, installation and commissioning is crucial to good MVHR performance.

Air Quality and MVHR

The filters are important to keep the heat exchanger and ductwork clean and functioning efficiently and so could be seen as part of MVHR’s routine maintenance costs, like the costs for having a gas boiler serviced once a year. The filters also perform a vital role in improving indoor air quality. The dirt that collects on the intake filter is all prevented from entering the house. Filters could, and perhaps should, be regarded as an added cost for clean filtered air, at about £2/week. Good indoor air quality is of course particularly valuable if the occupants suffer from asthma or hay-fever or the building is in an area of high air pollution.

In our next MVHR blog we will look at further findings from the Meta study including the PROS and CONS and cost benefits of different ventilation types.

Anna Marie Byrne, MVHR Design Engineer & Certified Passive House Designer, 21°