Sustainable Homes Scotland
MVHR / HRV - What is it?
Thermal energy efficiency in buildings boils down to mainly three ingredients:
Design, Insulation and air tightness
With increasing levels of air tightness however a variety of new problems occurred, such as high humidity and the associated fungus and mould growth, bad air quality and unhealthy levels of pollutants, mainly from carpets. Building control has recognised the fact that houses which perform in a pressure test lower than 5 m3/(h m2) at 50Pa need some sort of mechanical ventilation.
Experience shows that quality conscientious builders in Scotland and the UK in general can achieve these levels down to 3 m3/(h m2) at 50Pa without special design for air tightness.
Controlled Ventilation was invented as an answer to these problems and meanwhile it a well researched technology.
Such systems do not only extract air from rooms which normally would have extractor fans: kitchen, bathrooms, WC’s and utility rooms, but also provide fresh air into all living spaces, such as bedrooms, living & dining areas, studies, etc. With the air moving slowly from supply rooms through hallways and stair cases into the extract rooms, it is made sure that all areas of the building are ventilated.
Unlike the common mechanical extract systems (extractor fans), which intermittently extract not only the used air, but also the heat energy in it, HRV systems recover the heat in the air before it is exhausted outside and warm with it the fresh air which is introduced into the building. Instead of active heating methods, like heat pumps or air conditioning, it pre-warms the fresh air by passive means.
What a HRV system does and what not
Is it a heating system?
The main purpose of MVHR systems is to provide fresh air in relatively airtight buildings. Although it minimises the heat losses with the heat recovery function, it is not a heating system as such. However MVHR systems can be combined with small heaters in the supply air.
What is the difference to air conditioning?
Air conditioning (AC) systems operate mainly on the re-circulation of air and heating or cooling it bit by bit with every cycle. This is the reason why the windows should be kept shut when AC systems are in operation. Such heating or cooling is done actively with the use of relatively much electrical energy. Special attention to hygiene has to be paid as the air circulates in the system.
HRV systems operate on a completelt different level: The extracted air is not re-introduced into the building and the warming is done passively via a heat exchanger. A certain limited amount of cooling is provided when a summer bypass is installed, but the effect is not reaching the levels of AC systems.
Can I still open my windows?
Yes, you can! HRV systems do not operate as air conditioning do. Therefore the opening of windows does not upset the system. In the cold months of the year you will find that the opening of windows is not necessary as the air quality is good enough.
Will it be noisy?
The noise of rattling extractor fans is something very disturbing, especially if it would be on a continuous basis in your dwelling. Therefore good HRV systems are designed to be inaudible in the normal operational mode; in bedrooms the level is even to be below 25dB.