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

MVHR Filters

At Green Building Store, 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 Designer and Certified Passivhaus Designer, Green Building Store

Anna Marie Byrne, MVHR Design Engineer & Certified Passive House Designer, Green Building Store www.greenbuildingstore.co.uk

18 responses to “Ensuring good operation of MVHR systems”

  1. dennis merrigan says:

    I have a PAUL system What about the use of standby mode as opposed to Fan1 when the left is left empty for a couple of weeks?

    • We don’t recommend leaving the unit on standby for any significant length of time for a couple of reasons:

      Firstly, the building needs ventilating at a low level, even when it is unoccupied. One of the reasons for ventilating is to keep pollutants such as VOCs (volatile organic compounds) at safe levels and also keep the humidity levels under control. VOCs are continously off gassed from furniture, carpets etc. A low continuous level of ventilation ensures that the building fabric is maintained in good condition and when the building is occupied again, the air is of good quality.

      Secondly, if air is not passed through the heat exchanger, under certain circumstances it can develop mould growth. It is very difficult to clean this off and could result in a new heat exchanger being required if very bad.

      I hope this answers your question, if not, give us a ring to discuss.

      • dennis merrigan says:

        Thank you very much for your reply. Very helpful. I will continue leaving the fan on level 1 when building is unoccupied for any period of time.

  2. Tony Hamer says:

    The main reason for using a MVHR unit for us that it gives a better air quality and living in a damp area of the UK (Cornwall) we have no mould growth in our home, where previous to sealing the building and using MVHR to ventilate the property we have none!! Yes, filters cost money, there are reliabilty issues with some units, but definitely worthwhile fitting in any modernised/updated building.

  3. Ian says:

    filters also take too long to buy and I feel I am being ripped of, as no one is completing on the cost of them.

    It is about time there was a standard filter that fitted all makes of MVHR units, that I could just pick up at my local B&Q or buy from Amazon.

    • Ian, I agree. Its almost like the printer purchase dilemma: is the real profit for the printer manufacturer contained in the cost of the propriety ink cartridges, the printer itself being just the way to get you to spend money on massively overpriced ink? There are filter-less options available based on distributed MVHR units (fine wire or Lunos, for example) that might be cheaper in the long run. They also have application in places where maintenance is usually ignored, like local authority housing or schools. Added to that is the very high efficiencies they seem to be able to achieve, some are even Passive House certified.

      • We are entirely sympathetic to the argument about printers and ink. We use it a lot when selling our cartridge-less Airflush waterless urinal systems. We would challenge, however, the suggestion of MVHR filter “profiteering”.

        In our view MVHR systems need servicing in the same way (and for the same reason) that your vacuum cleaner needs servicing. Air carries dust, moisture and grease. And that is just the internal air. If it is not filtered the detritus will be caught in the machinery.

        We would very much like to hear from people who have successfully used an unfiltered MVHR unit. However, at the moment, we do not see any alternative. And just to reiterate, the cost of MVHR filters for a typical home remains at around £2/week.

    • My assumption has been that as MVHR becomes increasingly commonplace that the market will provide. In the US one can pick up a generic furnace filter at virtually any supermarket.
      On the other hand, maybe in the same way that Dyson now offers a filterless vacuum perhaps we can hope for innovation to come to our rescue and save us from the cost and inconvenience of having to replace the filters.

  4. M Bakley says:

    Very interesting blog, but should we not be asking why we are putting in MVHR in the first place? It costs at least £146 a year to run if using the above figures, and also uses energy and requires regular maintenance. My experience of living in a energy efficient home of the 90’s in our relatively mild climate was that very little energy was used for heating, even in midwinter. We had to keep the windows ajar all year around to prevent overheating. I believe with better window design, including top lights which can provide fresh air without draughts, safety or security risks, would be a better way to ventilate well-insulated houses.

  5. Peter Chisnall says:

    Are there no MVHR units that have re-usable filters, e.g. can they be rinsed out under a tap, in a dishwasher or blown clear?

  6. Rob says:

    I have a miaco unit and i wash out the filters, dry them and put them back, not sure if this is a good idea. The filters themselves are just pieces of filter fabric that i’m sure you can buy and cut up yourself rather than buy precut pieces.

    I don’t think our system was designed, installed or commisioned very well, but it works well enough

    • MVHR Dept says:

      The filters supplied by some MVHR unit manufacturers are made of a fleece material so could in theory be carefully washed and re-used however we don’t think that this would be a good idea for the following reasons:

        The fleece material on filters is usually cheap and will degrade.

        Washing doesn’t remove all the particulate, the filter material has fibres with a rough surface that catch some of the very small particles, these will mostly not wash out.

        Once dust or other particles gets into a heat exchanger it will be very difficult to remove, this will restrict air flow, reduce heat exchange efficiency and could eventually lead to a replacement heat exchanger being required (we’ve only ever seen this happen once when no filters were fitted). For those MVHR units fitted with constant volume fans then the power consumption and noise levels may increase as the fans compensate for the reduced airways. Washing heat exchangers is difficult and relatively ineffective.

      We do specify a kitchen extract grill with a fleece filter that can be washed a few times, this will stop grease and dust from the kitchen getting into the system and prematurely clogging up the extract filter which is not normally washable. The fleece filter on the kitchen is there to catch the grease/oil droplets/vapour. Fleece is very effective at this as the oil/grease is sticky.

  7. Anna Marie, thanks for bringing this to a wider audience.
    One of my points you didn’t bring out was that MVHR is really an enabler, and one ought not to be trying to carry out a standalone justification – the original chart was on the basis of ‘if you insist then this is what the analysis might look like’. The MVHR study states “Of course, we must bear in mind that the quantity of heat required has been hugely reduced by the Passivhaus, for which we need ventilation. Having saved several hundred pounds a year on heating, we should not quibble over spending a small portion of this on providing fresh, healthy air.”

    • Hi Martin,
      Nice to hear from you. Thank you for your comment. Yes, you’re right the chart was just theoretical and misses out the bit about all the heat loss savings through PH level air tightness. I originally wrote a longer blog going into more detail on that and we’ve split it to form a separate post so I’ll go into detail there. I’m not sure what you mean about MVHR being an enabler? Can you explain?

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