Mechanical ventilation efficiency depends on the performance of the whole system, not just the heat recovery unit. While Mechanical Ventilation with Heat recovery (MVHR) systems can recover a high proportion of heat from extract air, real-world efficiency is also shaped by fan energy use, ductwork design, airflow resistance, commissioning, system balancing, filtration and maintenance. To deliver good indoor air quality while limiting energy use, ventilation systems should be correctly designed, installed and maintained, with verified product performance data supporting reliable specification and selection.
The efficiency of mechanical ventilation systems depends on several factors. Heat recovery from stale extract air is an important part of MVHR performance, and heat exchanger efficiencies are often quoted at around 70% to over 95%, depending on the product and test method. However, this figure alone does not represent whole-system efficiency. Installed performance is also influenced by fan energy use, ductwork design, air resistance, system balancing, commissioning, controls, filtration and ongoing maintenance.
Many modern mechanical ventilation systems use low-energy, high-efficiency fans, often with electronically commutated motors and backward-curved centrifugal impellers. Even with efficient fan technology, any unnecessary resistance to airflow will increase fan power consumption and reduce overall system efficiency.
Ductwork should ideally be designed using rigid or semi-rigid materials, with routes kept as short, direct and smooth as possible. Flexible ductwork should be minimised and used only where necessary, for example to accommodate slight misalignment or provide acoustic separation at terminals or the ventilation unit. Where flexible ducting is used, it should be fully extended, securely fixed and installed in line with the manufacturer’s instructions and applicable guidance to avoid crushing, sagging or sharp bends.
Any ducts connected to the outside, or passing through cold spaces such as uninsulated lofts, should be insulated to reduce the risk of condensation and avoid potential damage to fans, filters and surrounding building fabric. Filters, air inlets and discharge grilles should also be checked and maintained regularly to prevent dirt build-up and airflow restriction.
On the supply side of the fan, the ductwork should be designed around the layout of the property, with the minimum practical length and number of bends. Duct bore should be at least compatible with the fan spigot and correctly sized for the required airflow rate in each part of the dwelling. Undersized ducts, excessive bends or poorly selected terminals can all increase pressure drop, noise and fan energy consumption.
Extract air should be taken from areas where moisture, odours and pollutants are generated, such as bathrooms, kitchens and utility rooms. It is also important to ensure that replacement air can enter or move through the dwelling as intended. For extract-only ventilation, this may be provided through appropriately sized background ventilators, such as trickle vents, air bricks or other purpose-designed openings. MVHR systems, however, rely on a balanced supply and extract strategy, together with good fabric airtightness and suitable air transfer routes between rooms. These transfer routes are commonly provided through door undercuts or other transfer arrangements, sized in line with current applicable guidance.
In any ventilation system, restricted airflow on either the extract or supply side will force the fan to work harder, increase energy use and reduce overall efficiency. Correct design, installation, commissioning and maintenance are therefore essential to ensure that the system delivers the intended airflow rates and performs efficiently in real conditions.
No. Heat recovery efficiency is important, but it is only one part of overall ventilation performance. Fan power, ductwork resistance, system balance, installation quality, controls, filters and maintenance all affect how efficiently the system performs once installed.
Poorly designed ductwork can increase pressure drop, restrict airflow and make fans work harder. Short, direct duct routes, correctly sized ducts, limited bends and well-maintained filters help reduce energy use, noise and performance issues.
Eurovent certified MVHR systems, AHU fans and ductwork systems give specifiers, installers and building owners greater confidence in declared performance data. Through independent third-party certification, Eurovent Certified Performance programmes verify key performance characteristics, helping users compare products more reliably and select systems that support indoor air quality and energy efficiency.
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