Five most common problems with Mechanical Heat Recovery (MVHR) Units

Modern homes are shifting towards increasing levels of airtightness in order to secure heat-saving benefits. With increasing levels of airtightness comes a need for a ventilation strategy. For Passivhaus, this means an MVHR system.

Heat from the home is lost in two ways:

  1. through the fabric elements as it passes through our walls, windows, doors, floors and roofs
  2. through holes, draughts, open windows and doors through air movement

UK Building Regulations on home airtightness are terrible

The government building regulations on airtightness are pretty bad. They let our country’s house-builders build homes with an A4 piece of paper-sized hole in the building fabric. That’s the equivalent of leaving a window wide open constantly, all year round – even if it’s minus 20 DegC outside. Try and keep your home warm when all of the heat can escape out of an open window.

It’s almost impossible, and it’s very expensive to try, as people that buy new builds in England will know.

Building Regulations allow new homes a maximum of 10 air-changes per hour at 50 Pascals of pressure (the equivalent pressure of a light wind blowing against the house).

This means that every six minutes all of the air in your home is replaced with new, cold air from outside. It’s terrible, and that’s the standard of house-building in this country in 2021.

Aren’t airtight homes stuffy and smelly?

Not if you install some form of mechanical ventilation.

If you have an airtight home up to Passivhaus standards, a mechanical ventilation heat recovery system (MVHR) is vital.

An MVHR is not a new piece of technology. You already use one everyday in your car. When you turn on the heater in a car it brings in fresh, cold air from outside and passes it over the engine, exchanging heat, through a filter and into the car as warm, fresh air.

A home MVHR system does the same thing. It extracts warm air from the rooms that create heat and moisture from occupants (kitchens, bathrooms and media rooms) and it passes in little pipes through cold, filtered air from outside to warm that air up. That warm, clean air goes into the supply rooms (living rooms, bedrooms and dining rooms).

How an MVHR exchanges warm air with cold

 

This means that 90 per cent of the heat created in your home stays in your home. That’s one of the ways that Passivhaus saves up to 90 per cent on household heating bills.

One block of flats in London hasn’t turned their heating on in winter for two years because the MVHR is so efficient.

So what can go wrong with an MVHR system?

Local councils are on a drive to promote Passivhaus amongst its private-public housing projects, partly for the environment, but also because it saves the council money in heating bills in the long run (which they often pay for).

However, badly performing Passivhaus multi-development projects that aren’t designed/installed correctly have been showing up over the last few years.

Mould, damp and increases in asthma problems are being reported, which sounds like humidity problems; ie, the flats are struggling to remove the moist air created by its occupants. Both UK Building Regs and Passivhaus demand minimum levels of ventilation in rooms like bathrooms and kitchens because the danger of lingering moisture is so serious to human health. MVHRs have to be designed so that they operate efficiently in clearing out moist air, but also aren’t too noisy.

To do this, MVHR designers calculate total pressure drops in the system as air moves through ducts around the building. Too many twists and turns in the ductwork and the air will struggle to move through into the rooms where it needs to be.

The five most common issues with MVHR systems are:

  1. Poor initial MVHR design (no proper airflow rate calculations, so the fan isn’t optimised to push and pull air through the property)
  2. Cheap ducting that is easily crushed or torn, or a cheap MVHR unit – stay away from the very cheap ones, they do nothing.
  3. On site deviations from the MVHR design (the system has been designed properly but the installers have changed duct runs, or not sealed the ducts properly so air can escape, or they’ve accidentally or negligently crushed areas of ductwork so that air escapes before it can get to the rooms)
  4. The MVHR hasn’t been commissioned properly (so each room has not been individually balanced to ensure proper airflow rates to each room)
  5. Filters haven’t been changed (In smoggy, dirty London the filters will have to be changed every 6 to 12 months or they’ll get clogged and the system won’t work effectively)

It’s a great shame that public schemes have had issues with the MVHR system, because when the MVHR system goes right it provides clean, warm fresh air to the home quietly and effectively – and with 90 per cent plus efficiency, that’s the future of home heating.

My aim is to help raise the standards for MVHR systems in UK homes, so if you want to discuss your build project incorporating MVHR, or you’d like a sizing and specification proposal for your own home for a ventilation system that is optimised for health and comfort, please contact me via the Contact Page or submit your details here.

Patrick

4 Comments

  1. I have recently moved into a shared ownership property where I have had a co come round to look at mvhr system and rates are very low and the last tenant had not ever changed or cleaned the filter. It has a flat roof and there seems to be no access to the loft and the housing association are stating I would have to have someone create access to look at the problem! How can it be legal to install this in not create access to it?

    • Hi Sharon, we spoke via email at Patrick [at] heatspaceandlight.com, but in short, this needs to be looked into as it doesn’t sound like a compliant set-up. Access for servicing must be made available for all ventilation systems, as we would for a gas boiler or heat pump.

  2. I am concerned that the ventilation system can stop working— unnoticeably , because they are so quiet– for several reasons, e.g. yo go away for 3 weeks and by the time you have come home there is mold everywhere

    Because the homes are so airtight , humidity can not escape.
    The other issue is that it is a money pit , as it is a 24 hour lifetime electricity user .
    The third problem is that if the heat exchanger does not work, you will not know then you are pumping out warm air , needing to reheat the cold air that has been sucked from the outside

    • Hi Amrita,

      I can answer all your concerns and I hope then you will feel better about MVHRs going forward.

      Firstly, if the MVHR breaks or there is a power cut whilst you are away from the home, there will not be mould everywhere. This is because it is us as occupants who create water vapour in our homes by washing, cooking and drying clothes (20 litres of water vapour a day for a typical four-person family). A vacant property does not generate any water vapour, so no mould will form whilst you’re away.

      Although the MVHR is near-silent, you will be able to tell when it’s turned off because the air will start to feel stuffy, and you’ll notice way before any mould occurs. The MVHRs I personally work with have very few warranty claims – I can count five out of 500 in the last four years, and they were fixed easily and quickly by the manufacturer. For the most part MVHRs are very simple, stable technologies.

      The energy to run the two high performance MVHR fans is the equivalent of running two LED light bulbs – so it’s not very expensive to run long term in terms of energy usage either – 10 to 20p a day.

      Finally, the heat exchanger is passive – there are no working parts that could break, it’s simply running the warm exhaust air from the home through the heat exchanger and warming the fresh, incoming filtered air. Compare this to all other forms of ventilation, which bring in cold air from outside through trickle vents.

      I hope the above helps. Please let me know if you have any other concerns and I’ll be happy to respond.

      Best wishes,

      Patrick

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