MVHR: The Beginner’s Guide

There are numerous considerations those looking to build energy-efficient, or even Passivhaus, homes need to make in order to ensure their home is as efficient as possible. One such consideration would be the specification of systems, such as MVHR (Mechanical Ventilation with Heat Recovery), which will help to ensure that the efficiency requirements are met.

What is MVHR?

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 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.

Can You Retrofit an MVHR System?

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. 

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.

Pros and Cons of MVHR
Pros

  • A controlled ventilation system
  • Better air quality
  • Lower humidity and no condensation
  • Can be used to deliver heat throughout the house and eliminate the need for underfloor heating or radiators
  • Homogenous atmosphere — similar temperatures throughout the house

Cons

  • Can’t have open fires or woodburners with internal air supply (requires a dedicated external or direct air suppy)
  • High capital cost
  • Achieving required levels of airtightness also has a cost
  • Another maintenance job, as filters need regular replacement
  • Homogenous atmosphere — we might not like similar temperatures throughout

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.

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%.

Alternatives to MVHR

  • 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

Main image: Neil Gourlay and his wife Mary have built a truly sustainable home that features an MVHR system and was built using as many locally-sourced materials as possible

Link Source