Official guidance on single room heat recovery systems could lead to £millions being wasted on ineffective ventilation systems. John Davison explains why.
Energy saving, energy efficiency, no one can avoid these two issues at the moment. Not just in the ventilation sector but in just about every aspect of life. Government directives, regulations, initiatives and guidance abound; directing manufacturers to comply and encouraging consumers to ensure they are as ‘green’ as possible.
As a result the rise in the availability of energy saving ventilation products has been matched by a variety of claims made about their performance. The problem with some of these claims is the criteria and methods used to determine the energy performance and efficiency credentials of products. Some of the data presented to highly reputable bodies, such as the Energy Saving Trust (EST), is misleading to say the least. Worse still these figures have in some cases been accepted and used to form the basis of guidance to users.
One such example is single room heating recovery systems. Data supplied by some manufacturers to EST, upon which approvals are based, is technically flawed. Independent tests have proven that these systems simply do not offer any benefits whatsoever in the UK climate. If they are used in social housing £millions of pounds of taxpayers’ money could be wasted.
At the heart of the problem is the British weather. Most tests on the efficiency performance of ventilation equipment are carried out to ‘free (still) air’ i.e., with no wind resistance, and yet figures available from the MET office show that the average wind speed in the UK is about 5 m/sec – sufficient to render single room HRV systems ineffective.
Ventilation systems specialist Airtech commissioned independent tests from BeataFlow Limited, a leading British ventilation/airflow R&D company, to examine a typical single room HRV unit of the type known to have been approved by EST. Test results revealed that – even on boost – with an external wind speed of 5 m/sec no air whatsoever was extracted by the HRV unit. In boost mode with an external wind speed of 7 m/sec – a wind state that occurs frequently in the UK – the air flow through the unit was actually reversed and would be blown back into the room unless it were (unrealistically) hermetically sealed. In trickle mode with an external wind velocity of 1 m/sec, i.e. barely a breath of wind, almost no air was extracted: at wind speeds of no more than 3 m/sec the air flow was again reversed and blown backwards.
Quite rightly the EST bases its energy efficiency rationale on energy consumption and single room HRV units appear to perform well in this respect because energy calculations take into account claims that heat is recuperated and returned to the room. Total energy figures generally reflect the cost of running the unit less the value of the heat returned.
However, for this to be realistic air has to pass in both directions through a heat recovery unit, otherwise known as a heat exchanger, before any heat recovery can take place. The problem is that in typical British wind conditions no air is actually expelled through the heat exchanger because the average wind speed in the UK blows it back.
In reality, heat recovery during a 24 hour period would be negligible. Cold air would often be blown into the dwelling and this would be exacerbated by the air intake mechanism within the unit. Furthermore the HRV unit would be consuming electricity 24 hours a day.
With no extraction and no heat recovery it would be wise not to consider single room HRV systems either as energy efficient or to have any significant impact on reducing condensation.
Unfortunately the BRE (Building Research Establishment), the highly respected industry organisation charged with a responsibility for offering expert opinion, has advised the EST in a recent Energy Efficiency Best Practice in Housing (EEBPH) scheme and approvals will flag up single room HRV units as being energy efficient.
If specifiers, installers and consumers follow this guidance there is a real danger that a great deal of money will be wasted on installing inappropriate ventilation systems that are not as energy efficient as claimed and, indeed, could even result in increased heating bills for residents. The problem could be especially hard felt by families suffering fuel poverty. On top of this the incidence of damage to health and housing through condensation is likely to increase.
To avoid potentially costly and damaging criticism the ventilation industry should seriously question the performance and energy efficiency of single room HRV units.
Not a true picture
Bruce Denness, former professor of engineering at Newcastle University, who carried out the tests on a typical single room heat recovery unit at BeataFlow Limited, on the Isle of Wight said: “It would appear that the current way of describing the air and heat exchanging capabilities of so-called heat recovering mechanical ventilators do not present a true picture of the devices’ actual performance under normal operational conditions in which they are faced by even a moderate wind simply because standard test procedures have hitherto required only that their performance in still air be measured.” The tests show that:
Still air does not represent the normal British wind condition
The average British wind blows at about 5 m/sec (a. moderate breeze)
When approaching a building the wind can blow at various angles
The deleterious effects of the wind normally increase with wind speed
Air movement through heat recovery ventilators is not uniform
Non-uniform movement challenges manufacturers’ claims for air exchange rate
Propelled air circulation in rooms is not uniform
Actual propelled air circulation does not appear to remove all stale air effectively
Some wind approach angles reduce, eliminate or reverse air expulsion
Reduced air exchange can negate the effectiveness of heat recovering ventilators with respect to: – the removal of moisture and stale air – heat recovery • Reduced air exchange occurs most often on “Trickle” settings
“Trickle” settings are used 23 hours per day • Clogging of filters from normal use will aggravate poor air exchange
Heat recovering ventilators are relatively expensive vis-à-vis extract only vents