Why is mechanical ventilation failing?
The very success of the industry in delivering airtight buildings and, even, exceeding standards, is throwing into sharp relief the failings of mechanical-ventilation systems, especially in homes — leading to their value being questioned. Mike Smith explores the issues.
The rapid adoption of airtightness testing and the ability of the industry to achieve the right result first time in 89% of tests is one of the success stories of the UK construction industry over the past decade. The BSRIA Compliance team tested over 10 000 dwellings and 720 non-dwellings in 2012 and found the average dwelling airtightness value was 4.89 m3/h.m2 envelope area at 50 Pa (against a maximum regulatory value of 10 m3/h.m2).
From a standing start in 2006, builders have grasped the importance of airtightness testing as a proxy for quality of construction and the contribution good airtightness makes to energy efficiency. Testing is rigorous, robust and, arguably, now at a very low economic price. It has respectability provided by UKAS accreditation for non-dwellings testing, the training of testers and, in the case of dwelling testing, registered testers through the Airtightness Testing & Measurement Association.
The mantra should be ‘build tight, ventilate right’. As fabric standards improve, driven further by the 2013 Building Regulations, the role of passive and mechanical ventilation systems increases.
Unfortunately in the world of unintended consequences, we are seeing dwellings achieving better airtightness than the designer intended, which means less air leakage and associated energy waste — but this is only useful if the designed-in ventilation systems can cope. In a nutshell the infrastructure supporting domestic ventilation engineering has not developed at the same pace as the improvement in building airtightness.
There is significant current activity to help remedy this problem but we are on the back foot with increasing numbers of poor installations and the questioning of the value of mechanical ventilation solutions.
The systems we are talking about are not complex, but they are sensitive to errors. What is missing is not so much the technology or science but the widespread creation and adoption of proper codes of practice. Mechanical ventilation (MV) systems and the more complex MV heat-recovery (MVHR) systems have to be site tested to ensure they are extracting and supplying appropriate amounts of ventilation. During compliance testing BSRIA is seeing two main kinds of problems.
The first is the performance of the specified equipment in a given situation, i.e. that the fan is correctly selected to match both the actual application and the inherent system losses that the system components will introduce. In simple terms this comes down to understanding the resistance characteristics of ductwork and its routing and the resistance of terminal units both inside and out. There is a widespread misunderstanding that ventilation fan outputs are usually quoted with outputs measured in ‘free air’. In reality they have to overcome backpressures from fittings. Even where kits are bought, we see alternative terminal units used — usually to meet architects’ demands for aesthetics.
The second is the actual installation of the associated ductwork, with very poor understanding of the dramatic effect on performance that can arise from bad workmanship.
In a recent case BSRIA found approximately a metre of flexible ductwork that had been stuffed into the cavity wall for a straight-through-the-wall installation that is about 300 mm thick. An additional 100 mm dogleg had been introduced on site to match the actual positioning of a porch structure. The result was a lot of fan noise with almost zero movement. The fan, when bench tested with zero back pressure, had a performance of 22 l/s, the designed performance including the ducting was 20 l/s; however, the actual performance was 5 l/s.
As part of the ‘catch up’ in dealing with the rapid rise in the use of domestic ventilation we have identified that the act of measuring MVHR performance using published guidelines will give false results if the correct equipment or correction factors are not used. There is an easy remedy but not widely used at present. The automatic volume flow meter with pressure compensation — more commonly known as a ‘powered diff’ will provide an instantaneous and accurate value.
A more common hooded anemometer will impose a back pressure on the terminal, ducting and fan under test, so the readings must be corrected (post use) specifically for both the anemometer model and the actual fan under test. More detail on this can be found in BSRIA’s ‘Domestic ventilation systems — a guide to measuring airflow rates — BG46/2013’.
And all of this is compounded by a lack of thinking regarding operational needs, limited controls, and poor instructions to the user, especially on what maintenance is required to keep performance at its peak.
So, airtightness demands have led to unforeseen consequences and something of a reaction against the use of mechanical ventilation. What then can be done to avoid making the same mistakes on other systems and concepts?
With fabric issues now largely dealt with in the Building Regulations, it is likely that new focus will fall on the efficiency and operation of the MEP services in dwellings. If modelling and measuring the thermodynamics of a brick wall is difficult, imagine how complex a multi-valent heating system is going to be! And before being put into use, these complex integrated systems will need commissioning and possibly proving as well.
The Zero Carbon Hub has recognised that we will need to devise new test methods and regimes that, for example, will evaluate how the performance of solar-thermal collectors meets expectations when linked with the ground-source heat-pump system that serves hot-water generation, underfloor heating and thermal storage — in concert with a biomass boiler or room heater.
Before regulation stimulates the market we need to have good-practice guidance and proven on-site commissioning and test processes in place.
This work is urgent and needs significant central support. With the next revision of Part L expected for 2016 – this time aimed at achieving zero (or nearly) carbon homes, time is not available to embark on a protracted negotiation with innumerable and varied industrial interests.
Certainly industry’s support will be available but only for a properly directed and centrally funded programme.
If we fail to put into place a mechanism to improve the on-site verification of performance of new systems, we will only have ourselves to blame for the next set of well publicised ‘failures to launch’ and consequent setback of achieving national aims.
Mike Smith is the director of engineering at BSRIA.
