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HEATING, VENTILATION & SERVICES
solutions, which can help remove excess heat. These systems are compact, low-noise, and suitable for installation in all habitable rooms. They offer on-demand operation or automatic switching, ensuring effi cient performance aligned with occupant needs.
MVHR & MVHR DX
Its core objective is to protect occupant health and wellbeing, not to guarantee thermal comfort, with two overarching requirements: limiting unwanted solar gains and ensuring excess heat can be removed from the indoor environment. Crucially, Part O prioritises passive measures fi rst, reserving mechanical cooling only when necessary. There are two calculation methods: the Simplifi ed Method focuses on limiting solar gains and ensuring adequate natural ventilation by providing prescriptive criteria for glazing areas, window openings, and building orientation. It is suitable for sites that are not constrained by noise or air pollution. The second is Dynamic Thermal Modelling (CIBSE TM59): this is a performance-based approach used when simplifi ed compliance is not possible. This method requires professional input and modelling, especially when building in ‘high-risk’ zones like London. A noise and air quality assessment will help confi rm if a Simplifi ed or Dynamic model is needed to comply with Part O. When it comes to removing excess heat, Part O lists acceptable methods in order of preference, starting with opening windows, followed by ventilation louvres, mechanical ventilation, and fi nally mechanical cooling. When natural ventilation options are restricted, mechanical ventilation provides a valuable solution with a number of different types of technology available.
Extract ventilation
Where windows cannot be relied upon for ventilation, modern extract systems, such as specially developed unitary fans with built- in temperature sensors and complete inline
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Now a core part of low-carbon building design, the latest Mechanical Ventilation with Heat Recovery (MVHR) units deliver coolth or heat recovery effi ciencies of up to 93%, making them ideal for year-round performance. Crucially for overheating mitigation, modern MVHR systems can come with intelligent automatic 100% summer bypass capabilities, offering free cooling, allowing cool night-time air to fl ow through the home when conditions permit. These systems also provide excellent indoor air quality through high-grade fi ltration – an increasingly important consideration in urban developments with high external pollution. However, when free cooling via an MVHR summer bypass alone is insuffi cient to mitigate overheating throughout the year, architects can consider all the benefi ts of MVHR systems with integrated ‘DX’ cooling modules. Featuring intelligent controls that automatically switch between heat recovery, summer bypass, and active cooling via the DX cooling module, the units continuously measure internal and external temperatures to maintain comfort thresholds effi ciently. This helps ensure homes remain cool, comfortable, and energy-effi cient. The intelligent DX cooling module can activate on-demand or automatically to prevent overheating. Another new mechanical cooling option available to architects is a combined MVHR unit plus air source heat pump. This offers renewable hot water with a ‘free’ cooling solution, monitoring comfort temperatures defi ned by the user and when cooler outdoor air is not suffi cient to cool the space via the 100% summer bypass, the system automatically sends a signal to the heat pump to request cooling. This will then combine the air fl ow from the MVHR and the cool exhaust air of the heat pump via built-in dampers. This type of system offers the highest level of comfort and functionality all year round, while complying with Part O and TM59 while being extremely energy effi cient.
Steve Pearce is Vent-Axia product manager ADF MAY 2025
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