Five Minute Guide to... Mechanical Ventilation with Heat Recovery
five-minute guide to Sustainability expert Tim Pullen explains the pros, cons and costs — and why a MVHR system is essential to an airtight home
Sustainability expert Tim Pullen explains the pros, cons and costs — and why a mechanical ventilation with heat recovery (MVHR) system is essential in an airtight home
Ventilation is necessary for a healthy living environment and a mechanical ventilation system may be the answer — with or without the heat recovery bit. Sellers of such systems – the good ones, at least – will tell us that we need an ‘airtight’ house for the system to be effective. But ‘airtightness’ is not an absolute. It is measured on a sliding scale and there are points on that scale where a mechanical ventilation with heat recovery (MVHR) system will work and points where it will not. An effective ventilation system will be part of an overall design and it can be argued that it is an essential box to be ticked in the design of an energy-efficient house.
How Does MVHR Work?
There will be an air-handling unit, normally in the attic or in a plant room, and ducting to each room. Some of that ducting will draw out stale air and other ducting will replace it with fresh air. The heat recovery bit is where the warm, stale air is passed through a heat exchanger in the air handling unit and the heat extracted is then used to warm the incoming fresh air.
The ducting will tend to be 100mm to 150mm in diameter, depending on the size of the house. The warmed incoming air also needs to be insulated, to make sure the heat gets to the right place, which could increase the diameter to 250mm. Clearly ducting can’t be put just anywhere and needs to be properly designed in.
Retrofitting to older properties is possible but there are some problems: hiding the ducting within the fabric of the building is usually problematic and older properties tend to be less airtight. Retrofitting is also more expensive than installing a MVHR system in a new build, so it could well be that the cost of overcoming the aesthetic problems and getting the building sufficiently airtight outweighs the potential benefit.
The Importance of Airtightness
Part L1A of the Building Regulations (which concerns itself with the conservation of fuel and power in new builds) requires a maximum airtightness of 10m3/hr/ m2 at a pressure of 50 Pascals (Pa) in new homes. Good practice in the UK is said to be 7m3/hr/m2 and the Scottish Building Regulations require that as the maximum. (See page 208 for more on achieving compliance with Part L of the Building Regs).
Passivhaus is said to be the best energy efficiency building standard currently available and requires an airtightness of 0.8m3/hr/m2 at 50Pa. There is no question that airtightness is a key factor in achieving thermally efficient houses. Airtightness stops heat escaping from the house and stops cold air entering (in the form of draughts).
MVHR works by extracting air from a room and replacing it with a similar volume of warmed air. If we imagine a situation where the room is very leaky, the extraction will draw in cold, outside air, outweighing the value of the warmed air being introduced. So clearly there is a level of airtightness at which MVHR will start to be effective. That is generally accepted to be 5m3/hr/m2. The more airtight the house, the more effective the MVHR system will be, and again it is generally accepted that 3m3/hr/m2 level is necessary to see real benefits. It has to be said that most MVHR system suppliers do not advertise this, as achieving that level of airtightness, even in a new build, is neither cheap nor easy.
Are they Noisy?
This is largely a cost issue. Flexible ducting is cheaper but has a rippled internal surface. That increases resistance to air pressure and thereby creates noise — not a lot, but enough to notice. Rigid ducting is more expensive but has a smooth internal surface. Even these will not be silent but the noise level is so low as to be inaudible.
“The more airtight the house, the more effective the MVHR system will be”
Are MVHR Systems Efficient?
Figures of 80% to 90% will be bandied about, which sounds impressive but what those figures actually mean is more difficult to establish. The implication is that 80% or 90% of the heat in the extracted air is transferred to the fresh, incoming air. What it does not tell us is how much heat is being extracted. Typically (but not always) the heating system design for a new build will ignore the heat recovered by the MVHR system, mainly because the occupier has the option to turn it off. The heating system has to be designed to meet the whole heating demand, with the MVHR contribution often taken as a bonus.
How Much Does an MVHR System Cost?
The price varies with the house. For a standard four bedroom house the system itself is likely to cost upwards of £3,000. But the installation cost could easily double that, depending on the room layout and the difficulty of installing the ducting. The savings in terms of the heating bill will have a direct relationship to the airtightness of the building. At 10m3/hr/m2 (Building Regs standard), the system simply will not work. At 5m3/hr/m2 it starts to work but heating bill savings are likely to be less than 5% per year. At 3m3/hr/m2, savings could get to 20%.
Are There Any Alternatives?
Trickle vents and extract fans are essentially the baseline standard to comply with Building Regulations. They are also a cause of draughts. Continuous extract ventilation provides continuous, very low level extraction from kitchens and bathrooms. This needs relatively poor airtightness to work effectively. Positive input ventilation is a single unit which typically sits in the loft — fresh air is pumped into the house, forcing stale air out of the gaps and cracks (that should be sealed to get a reasonable level of airtightness). Passive stack ventilation uses the same principle as a chimney to draw out stale air, and trickle vents, or similar, to allow in fresh air. Proprietary systems can also come with heat recovery. However, this is dependent on the wind to work well.
Thermal insulation standards are only increasing, along with the airtightness needed to achieve those standards. And with that comes the need for better controlled ventilation that can’t possibly be achieved with trickle vents and extract fans. Adding heat recovery to that system is still an option rather than a mandatory requirement at the moment, but whether that remains true as we move closer to Passivhaus as a standard, is an open question.