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:
- through the fabric elements as it passes through our walls, windows, doors, floors and roofs
- 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 2023.
If you would like to learn more about airtight homes and the relationship with MVHR systems I’ve written about that here.
Aren’t airtight homes stuffy and smelly?
Not if you install some form of mechanical ventilation (with or without heat recovery).
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.
Our nose is also a kind of biological heat exchanger in winter. Cold dry air is breathed in and the nose warms it safely via the warm air breathed out.
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 outdoor air to warm that air up. That warm, clean air goes into the supply rooms (living rooms, bedrooms and dining rooms).
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.
This video shows the energy saving in action in a residential property we retrofitted one of our MVHR systems into:
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 like ourselves 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:
- Poor initial MVHR design (no proper airflow rate calculations, so the fan isn’t optimised to push and pull air through the property). Our MVHR design team completes the ventilation calculations bespoke to each project every time.
- Cheap ducting that is easily crushed or torn, or a cheap MVHR unit – stay away from the very cheap ones, they do nothing. Our company has banned flexi-duct from our projects, forever.
- 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). At HSL we quality check every system at commissioning to ensure the design has been followed and works.
- The MVHR hasn’t been commissioned properly (so each room has not been individually balanced to ensure proper airflow rates to each room). We won’t sign off a system until we’ve confirmed it’s working as designed.
- 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). We remind our clients to change filters, and offer the cheapest pricing for new filters that we believe is available.
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 it is definitely a future technology more homes need.
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.