The Clever Heat Exchanger
One of the little understood advantages of a balanced heat recovery ventilation system is the energy savings it provides.
The following chart from the Passive House Institute shows standard ventilation in blue and heat recovery ventilation that has been assessed as 85% efficient, in yellow. The addition of heat recovery ventilation means that the space heating demand has been significantly reduced and the colder the climate, the greater the energy savings.
This is due to the heat exchanger which acts much like your own lungs to warm the incoming fresh air using energy recovered from the outgoing stale air. The sketch below shows how a Mechanical Ventilation with Heat Recovery system (MVHR) utilising a cross flow heat exchanger operates. below it (for comparison) is a counterflow heat exchanger that is much more efficient due to the greater area of plates.
1. Cool, fresh air from outside is drawn inside via a filtered fan to remove dust, dirt, allergens and other fine particles.
2. At the same time, warm stale air is drawn via a filtered fan from the kitchen, laundry and bathrooms.
3. The warm stale air warms the plates inside the heat exchanger as it passes over them to be exhausted out and away from the home. As it cools, the air releases vapour as liquid, which is drained away via a condensate drain.
4. Without the incoming fresh air mixing or coming into contact with the outgoing air, it is warmed by the plates and distributed to the bedrooms and living areas.
5. Filtering the incoming air helps people with respiratory issues such as asthma. Filtering the outgoing air, reduces the build-up of dust within the system.
PASSIVE HOUSE PARTICULARS
COMFORT – 16°C supply air temperature at -10°C outside air temperature
EFFICIENCY – the whole system to be at least 75% efficient, not just the heat exchanger, whilst using lightbulb like power of 0.45Wh/m3 for fans and control unit, standby losses max 1W
CONTROL – Must be quiet; 35 decibels in the room housing the unit and not feel draughty at any supply outlet.
COMMISSIONED – The system must be properly balanced and commissioned.
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