As improvements to energy standards have led to greater levels of air tightness in buildings to reduce uncontrolled heat loss, a consequence is that the ventilation provision in modern homes now has to be designed. In current building regulations guidance, airtight buildings (between 3 and 5 m3/hr/m2 @50 Pa) require mechanical ventilation. One method for achieving this is through the use of dMEV systems. These are an increasingly popular strategy due to the relatively low cost and ease of integration.
However, some concerns have been expressed about the ability of these systems to provide whole house ventilation, particularly in the light of changes to the guidance for trickle vents. To investigate these issues the Scottish Government Building Standards Division (BSD) commissioned a study that would examine the real world performance of modern homes with dMEV systems. The study undertook a survey of 223 homes to ask occupants about their knowledge and operation of their ventilation system, and a subset of these homes were monitored to examine the actual ventilation performance and determine the factors that affect this. A further study was undertaken in a selected dwelling to experiment with different ventilation strategies using dMEV to identify key factors.
The survey found that although there was good awareness of the presence of ventilation provisions, there was a lack of knowledge regarding how these systems were controlled. Many households did not know how to boost the ventilation rate in the dMEV system (or didn’t feel the need to do so), and a lack of engagement with trickle vents was clear.
The monitoring found that over 50% of homes appeared to have poor ventilation overnight (where carbon dioxide levels exceeded 1,000ppm for the majority of the time), and that bedrooms were a particular cause for concern. There were a number of variables that affected this. These included the nature of the trickle vents, the window coverings, the path between the room and the dMEV (including the door opening or undercut, and the arrangement of the home) and the installation and performance of the system. Essentially homes with shorter, more open paths for air movement performed better, but rooms which relied on more remote dMEV systems frequently had poor ventilation.
Inspection of the monitored homes found a high number of installed dMEV systems (42-52% - depending on location) were sub-optimal (exceeding recommended airflow rates by >15%), or non-compliant with the guidance (17-48%). Flow rates were highly variable, sometimes this was due to the system setup and commissioning, but some systems had provision for occupant control. Given that bedroom doors were often closed (41%) due to occupant preference or fire requirements, the strategy relies on door undercuts but these were undersized in 20% of properties.
There were a number of homes (51%) where trickle vents were installed in wet rooms with dMEV systems. Whilst this may improve the efficacy of extract and
moisture control in these rooms, this undermines the ability of the system to assist with ventilation in more remote rooms.
Whilst dMEV systems in ensuite bathrooms provided the best outcomes for adjacent bedrooms, problems with systems being disabled were encountered in 56% of homes and the predominate problem was one of noise. The physical monitoring found a much higher incidence of this than the overall survey.
In the test house a number of different ventilation scenarios were tested over a week. The bedroom with an ensuite performed reasonably well, but the bedroom which relied on a remote dMEV only achieved good ventilation when the windows were open at night. The next best scenario was when the occupants left the bedroom door open overnight. Subsequent modelling suggests that air flows from door undercuts are less effective.
The findings would suggest that whilst there are some situations where a dMEV system can assist with the ventilation provision of modern airtight homes, the ability to act as a whole house system is limited, particularly in larger more complex layouts, and where ventilation loads are high.
Although trickle ventilation provision in habitable rooms did not appear to be a major determinant of carbon dioxide concentrations in the monitored dwellings, these results should be interpreted with caution, given the small sample size and large number of confounding variables identified. It is likely therefore, that the impact of reduced area of trickle ventilation was overshadowed by other key components such as air flow pathways, pressure differentials, dMEV extract rates etc. As such, the system as a whole requires careful design, taking into account the house layout, paths for air movement (including undercuts and pass vents), the nature of the mechanical system, and consideration of remote rooms. The system will only be effective when these are optimised.
Issues to consider therefore include:
o Better design of ventilation strategies using dMEV as a component of the
system and accounting for other key components o A need for pass vents between rooms, fire protected where required, ideally at
higher levels o Better standards for commissioning and testing in use o Improved standards for noise for as-installed systems o Better design of occupant interfaces of mechanical systems, in particular boost
modes and occupant control elements o Better advice and information for occupants about the ventilation system, its
optimal use, and requirements for maintenance o Fall-back strategies for ventilation, where mechanical systems may fail or
become sub-optimal over time o Direct extract ventilation for non-flued gas appliances o The development of performance standards for ventilation rates that can be compared with in-use data, and that provide an alternative means of compliance