The problems with Mechanical Heat Recovery (MVHR) Units explained

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 2016.

PassivHaus demands a maximum of 0.6 air-changes per hour at 50 Pascals of pressure. This means that every one hour and forty minutes the air in your home will have escaped, a much more manageable and gradual process in keeping your heated air in the home.

Aren’t airtight homes stuffy and smelly?

Only if you don’t 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?

Camden Council is 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.

However, the council is about to pay around one million pounds in compensation because new residents are so angry at how badly their “PassivHaus” apartments are performing at their flagship Chester-Balmore estate in Highgate, London, they’re moving out.

Chester Balmore estate sales sign

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 issue at Chester-Balmore sounds like an MVHR problem, and probably stems from one of four of the most common issues with MVHR systems:

  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. 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)
  3. The MVHR hasn’t been commissioned properly (so each room has not been individually balanced to ensure proper airflow rates to each room)
  4. 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)

Chester-Balmore-estate has MVHR problems

It’s a great shame that such a public scheme has had issues with its 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 efficiency, that’s the future of home heating.





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