Business Development Manager, Andrew Nash, examines the conflict between the design of modern urban apartments and the spec of ventilation in an article for H&V News Magazine, January 2017 edition.
The design of modern urban apartments too often comes into conflict with the scope of the ventilation – not only the use, location and size but also installation method of the MVHR systems being specified. We’re essentially asking too much of the system and then wondering why it’s noisy.
View the January 2017 edition of H&V News here!
Problem one - overheating
The design of modern apartments often leads to overheating. The location of city-centre apartments is often in problematic zones: next to or above railway lines, next to busy roads and roundabouts. Air pollution and noise pollution can mean that windows cannot be opened to control temperature or to provide purge ventilation. So the overheating, which is already a problem with building design, becomes a greater issue.
While building regulations drive us towards super-sealed buildings that lose little energy through infiltration, marketing trends have encouraged properties with large glazed areas.
In the middle of summer, you have to run the MVHR system at elevated speeds to move air through the property faster to reduce the risk of overheating. Given the percentage of glazing in some modern developments, it’s like ventilating a high-rise greenhouse.
The scale of the problem varies according to the direct the apartment faces. A north-facing apartment will suffer less than a south-facing one. If you can’t open a window due to traffic noise outside then you must run the unit at elevated airflow – up to 150l/s – which is not sensible.
Low-e glass is designed to maximise solar gains, thus minimising the amount of energy required to heat the building. But the process cannot be controlled and, while free heat in winter is desirable, in summer it leads to a tendency for buildings to overheat. Lightweight construction is more cost-effective to build but lacks thermal mass required to ‘even-out’ internal temperatures through the seasons – no bricks, no exposed concrete.
Criterion 3 of Part L of the Building Regulations states that even if a building has comfort cooling, it should not be used to control overheating problems. This stipulation is designed to stop people designing buildings that overheat.
Criterion 3 aims to stop architectural or interior design aesthetics from over-riding building performance and to prevent designers from going for form over function. If they have no solar shading and /or have too much glazing, they get really hot. Full-height glazing brings huge overheating risks. Although the regulations have put a stop to it, there are still lots of apartments built six or seven years ago that have serious overheating problems.
The problem is that you can’t put cooling into affordable housing. With comfort cooling eliminated as a solution, either on the grounds of cost or due to planning restrictions, the only option left is to increase the ventilation rate.
Ventilation rates to mitigate overheating should not be confused with ‘purge ventilation rate’ in Part F. This is primarily designed to allow rapid ventilation to deal with things such as the smells resulting from burnt toast or a high concentration of VOCs (paint fumes) when redecorating.
Part F requires a minimum purge ventilation of four air changes per hour, with openable windows in the living room and bedroom to provide natural ventilation. But in relation to overheating, this is not intended to be a solution.
Problem two – noise pollution
Noise pollution is often an issue with some apartments where the windows cannot open because of traffic noise. In addition, low-level apartments are often affected by security measures that limit window openings and prevent them from achieving the 1/20th of the floor area called for in Part F for purge ventilation.
Affordable homes or the affordable element of larger high-end developments generally occupy the lower floors, where security limits window openings or are closer to noisy areas or roads. Given that this type of property almost never has comfort cooling, there is little alternative to using the MVHR system to control overheating.
Adding a ducted or wall extract fan to boost extract air-flow will fail to solve the problem. Firstly, it is likely to increase noise in providing the required ventilation rate, often exceeding the external noise levels. Secondly, extract must be balanced with an equivalent supply.
The most cost-effective means to increase the size, speed and/or capacity of MVHR unit. While the SAP calculation will indicate to designers than an apartment is at risk of overheating, thermal modelling will be required to establish the full extent of the risk and the airflow required to mitigate the problem.
A range of one to three air changes per hour in habitable rooms can be shown to reduce the risk of overheating below the guidelines called for in CIBSE Guide B.
This has implications in respect of meeting noise criteria, these being lower in living rooms and bedrooms. Living and bedrooms are usually adjacent to the façade to offer the best views and therefore at the greatest risk of overheating.
Problem three – vibration
Recently, I’ve visited a number of apartments where transmission of vibration from wall-mounted MVHR units to adjacent rooms has been reported. The problem in all cases was not really the wall-mounted MVHR unit, but the structure to which it was fixed.
What designers and installers are currently doing is rigidly fixing ventilation plant directly to the back of a light, flexible partition and then being surprised when vibration can be felt from the other side.
Both the mass and flexibility of wall materials have an effect on vibration transfer. Brick walls have high mass and don’t flex, whereas 13mm plywood has relatively low mass and flexes a lot.
Fixing an MVHR system to a partition wall creates a ‘sub-woofer’ effect where the MVHR effectively replicates a hi-fi speaker. In the same way as a hi-fi speaker creates sound by moving air in waves at different frequencies, an MVHR unit rigidly fixed to a flexible sheet (a lightweight partition wall) will send out waves of very low frequency. These are very much felt as heard and usually in the room that the services cupboard backs on to. Boxing in around a wall-hung MVHR couples it even tighter to the structure, ensuring that 100 per cent of the vibration is transferred to the structure , thus further increasing noise levels.
Nuaire has now started stipulating what the wall construction should be for our units in a bid to avoid this. I’ve seen jobs recently where units have been boxed in and this has made it instantly worse. The unit needs to be able to move around and the walls need density.
Whilst increasing the rigidity of the supporting partition wall with a 25mm sheet of MDF will provide a much-needed increase in mass, the best way of insuring that all vibration transfer is eliminated is to fit a wall mounting bracket with rubber AV mounts of appropriate deflection.
These can be installed in advance of the MVHR system, or retrofitted after noise problems are detected.
The MRXBOX95AB-WH1-1Z is a fully-acoustically treated unit. Designed to operate at moderate speeds in the normal backgrounds and boost modes, it can be run at near maximum speeds to control overheating while still meeting the acoustic criteria. This is critical to solving the problem. There are many units on the market that can run at multiple speeds to meet different ventilation needs, but any unit not acoustically treated will exceed the noise criteria when running at near full speed.
The 1Z solution places the unit together with a 4-way attenuator with AV-mounts and flexible duct connections, all with an acoustic enclosure.