Andrew Farr from Green Building Store explains what qualities mechanical ventilation with heat recovery (MVHR) should bring to a house. He also shares how to make sure you get a system that is properly designed, installed and commissioned.
Interview with Andrew Farr
Andrew Farr’s background is a mixture of mechanical engineering and building work, having done multi-material fabrication for a number of years. When Chris Herring, the founder of the Green Building Store, was developing their Mechanical Ventilation Heat Recovery Department he invited Andrew to join them as they had previously worked together on various building projects and as Andrew is a fluent German speaker.
HRV and MVHR are the Same Thing
HRV (Heat Recovery Ventilation) and MVHR (Mechanical Ventilation with Heat Recovery) refer to one and the same thing. Andrew clarifies that there are various other acronyms out there, but the broad idea is that you are ventilating and recovering the heat energy out of the ventilation air.
In a Balanced Ventilation System the Same Quantity of Air is Supplied as is Extracted
An MVHR system can be a pragmatic way of delivering very good air quality into a building. The Air Handling Unit (AHU) or the Mechanical Heat Recovery device of the MVHR system draws air into the building from the outside, filters it, takes it through a heat exchanger and supplies the clean air into the habitable spaces of the house, usually bedrooms, living rooms, studies etc. A completely separate ducting system extracts air out of the foul air rooms (toilets, bathrooms, kitchens etc.) through the other side of the same heat exchanger and exhausts it out of the building. This enables a balanced ventilation system where you are supplying exactly the same quantity of air into the building as you are extracting out of it.
MVHR systems may not be essential for every project, however, they are recommended for very airtight buildings with an airtightness of 3ach @50pascals or lower. For an MVHR system to be energy and cost effective, the airtightness of the house should be below 1ach.
Getting the Design of the MVHR System Right – Sound Thresholds and Air Flow Rates
The criteria set out for MVHR design by Building Regulations and Code for Sustainable Homes in the UK are primarily intended to create an energy efficient system. This has led to a common occurrence of noisy MVHR systems in the mainstream building industry owing to their poor design.
Andrew argues that in the case of a house, the primary criteria should be comfort and good air quality. A well designed MVHR System does not impact on the consciousness of the people who are living in that building. It should not be the first thing you notice when you walk in, nor the last thing that you notice when you're going to sleep.
To achieve this, the main criteria of the design should be to deliver an inaudible – in everyday activity – ventilation system which generates less than 24 dBA of sound at a distance of more than one meter away from the air valve (The human threshold of hearing is somewhere around 24 to 27 dBA). With the amount of noise that the ventilation unit is going to create, the location of the MVHR unit itself is also crucial. Positioning the MVHR unit close to the kitchens and bathrooms, where people are used to hearing noise, is more suitable than locating it close to a bedroom.
Another important criteria is to deliver exactly the right amount of air into the building. Over ventilating will tend to dehumidify the air in winter and also use excess energy. While under ventilating will result in unpleasant odours and humidity remaining in the building.
The Traits of a Good MVHR Unit
Green Building Store has the franchise for selling PAUL Ventilation units (PAUL is a German company) which they believe are some of the very best on the market as they are very well manufactured. The ventilation unit itself emits a relatively low frequency which doesn’t travel as well through walls, as compared with other units. The fan has a constant volume function to continually deliver exactly the same amount of air through both sides of the heat exchanger – as set up at the time of commissioning. This is extremely important, as during the yearly cycle of a ventilation unit there are various things that happen which change the resistance to the air moving through both the intake and the exhaust ducting systems.
One of the major influences on the performance of the MVHR system are the filters on the ventilation unit. A filter on the intake prior to the heat exchanger protects the heat exchanger from the very fine debris that can be drawn into the air stream. There is also a filter on the extract prior to the heat exchanger. These filters will get dirty and provide resistance to the air-flow at different rates depending on what the pollution load is. In the case of a house being situated close to a main road, carbon particulates will block up the very fine filters resulting in the intake filter becoming dirtier sooner than the extract filter. To maintain a balance between both air-flows, the fans will increase their effort against the filter resistance in order to maintain the same volumes of air. Thus having a fan with a constant volume function is very crucial to maintaining the heat recovery efficiency of the ventilation unit.
Maintaining the Efficiency of the MVHR System
Modern ventilation units are very similar in their tested performance (the one against which they will be certified). However, there is a very real difference between the unit’s individual tested performance and that of its performance embedded in the building. The efficiency of the MVHR system depends on two important factors:
- maintaining the balance between the two air flows, and
- the design of the ducting associated with the MVHR system
If the resistance to the air moving through the ducting system is high, then the fans will have to exert a lot of effort in order to move the air. Consequently they will make more noise and use more energy, which in turn will cost more. The energy efficiency of the MVHR system is the efficiency of the whole system, including the ducting and everything associated with it, and not just on the efficiency of the MVHR unit alone.
Round and Rigid Ducting Works Best
The most important criteria when considering what type of ducting to use is which type of ducting will allow the air to move through most easily and offer the least resistance. Round ducting is the most efficient in this case, as it offers the best surface area to volume ratio – the surface area is smaller than the volume of air. Rigid ducting is sealed by design, not by the addition of mastics and tapes as in the case of flexible ducting and is less likely to fail.
Invest in a Well-designed MVHR System from the Get Go
The design of the ducting system should be of paramount importance in order to deliver a ducting system that you will want to live with. Ducting systems should be designed to reduce pressure loss and to attenuate the associated sound. If it is not designed properly and especially in larger houses, some air valves could end up being very noisy because they've got too much pressure on them, and in other rooms the air valves are not getting sufficient air out of them. A well designed system with rigid ducting and appropriate sound attenuation however will come at a premium cost. Rather than going for a cheaper poorly designed system, Andrew’s advice is to invest in a well-designed system from the outset that won’t have to be replaced after a year.
Quality of MVHR Design Currently in the UK
Up until now the industry has seen experienced commercial mechanical and electrical installers bringing their knowledge from commercial spaces into domestic MVHR installations, where the requirements for sound are not the same.
In an office, you probably won't notice a bit of fan noise in the room because you're busy, there are computers on or you are talking to people. But when you're at home and you're working on your desktop PC, the running fans and hard drive generate such a constant background noise, that when you turn it off, you realise the blissful peace that follows.
According to Andrew, an MVHR system should be giving you this blissful peace all the time despite the fact that it's running, and if it's not, then it's not designed to quality. Andrew believes that in the next few years the quality of delivery of MVHR systems in the UK will improve and that more people are now beginning to understand what the appropriate criteria for MVHR design in domestic buildings is.
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