PHAM NEWS | MARCH 2026 34 Ventilation & Heat Recovery


Overheating in PDR


conversion projects


Permitted Development Rights (PDR) conversions are creating a growing workload for HVAC installers, but also exposing signifi cant overheating risks driven by regulatory gaps, building fabric limitations and design constraints, all highlighting the need for ventilation-led mitigation strategies and earlier technical engagement, says Nuaire’s Andrew Nash.


P


ermitted Development Rights (PDR) were introduced to accelerate housing delivery by allowing offi ces and


other commercial buildings to be converted into homes without full planning permission. Since their introduction in 2013, more than 100,000 new dwellings in England have been delivered through change-of-use schemes, and many installers will now be familiar with the growing volume of this type of work. While these projects can off er


strong commercial opportunities for the HVAC sector, they also bring a set of technical challenges that diff er signifi cantly from both traditional new build and standard refurbishment. Chief among these is overheating. Many PDR conversions are showing signs of poor thermal performance in warmer months, and installers are increasingly being called upon to resolve comfort complaints.


Regulatory gaps One of the defi ning characteristics of PDR conversions is that they are not subject to the full scope of Building Regulations. Critically, they are exempt from Part O, which addresses overheating in residential dwellings and aims to protect the health and wellbeing of occupants by reducing the occurrence of high indoor temperatures. Instead, developers are required to meet only the minimum ventilation standards set out in Part F. While Part F ensures that


Andrew Nash Divisional manager – residential, at Nuaire


a basic level of fresh air is delivered, it does not address solar gains, internal heat build-up or the cumulative eff ect of modern lifestyles on internal temperatures. The result is a growing number of apartments that are technically compliant but struggle to remain comfortable during hot weather. For installers, this often means inheriting problems that originate in the building fabric and layout rather than in the installed system itself.


Change-of-use issues The core challenge with PDR projects is that they often repurpose buildings designed for


a completely diff erent occupancy. Former offi ces were not built to house people 24/7. They typically feature large glazed facades, minimal external shading, and restricted natural ventilation. In densely built-up urban heat islands, where many of these buildings are located, opening windows – if they can be opened that is - may not be an option due to noise, air pollution, or security risks. These environmental constraints eff ectively render passive ventilation strategies unworkable. Part O prescribes maximum


glazed areas on facades (depending on orientation), encourages the use of solar shading and glass with lower g-values to limit solar gains, whilst setting minimum openable window areas so that excess heat can be passively ventilated away. But in the case of PDR schemes, this requirement is not mandatory, leaving developers to comply only with the bare minimum ventilation requirements stipulated by Part F.


MVHR solution MVHR systems off er a practical and eff ective approach to meeting Part F and can also assist in improving thermal comfort. MVHR not only preconditions


MVHR, such as the MRXBOX Hybrid Cooling System from Nuaire, can improve air quality and provide limited passive cooling in commercial property converted into residential housing


air and fi lters out pollutants but also provides summer bypass functions which can be used in combination with elevated airfl ows to provide free cooling when external temperatures are favourable. These systems can


 Former offi ces were designed with extensive glazing, limited shading and poor ventilation, making them ill-suited for residential occupancy


help maintain acceptable indoor temperatures year-round and are still regarded as passive means in Approved Document O. From an installer’s perspective,


MVHR off ers a familiar, space- effi cient solution that supports Part F compliance while delivering tangible improvements in comfort.


When MVHR isn’t enough Where standard MVHR may not suffi ce, installers may need to consider alternative approaches that sit between standard ventilation and full air conditioning, such as a Nuaire Hybrid Cooling unit which works in tandem with our MRXBOX MVHR, which could potentially assist in mitigating overheating. Such systems activate when


indoor temperatures exceed a threshold – typically 23°C – and introduce pre-cooled fresh air to help maintain occupant comfort, though the potential eff ectiveness of such a system should be assessed through dynamic thermal modelling. From a user’s perspective, it operates seamlessly and discreetly, avoiding the cost, complexity, and visual impact of conventional air conditioning systems.


Installer input Ideally, overheating risk should be assessed at design stage using dynamic thermal modelling, particularly on larger or more complex PDR schemes. In reality, installers are often brought into projects later. Even so, understanding the underlying causes of overheating allows installers to set realistic expectations and propose solutions that are appropriate to the building. As awareness of overheating


grows, housing providers, developers and managing agents are increasingly looking to HVAC contractors for guidance, not just installation. Installers who can explain why a PDR apartment struggles to stay cool and how ventilation-led solutions can help are well placed to add value and diff erentiate themselves. ◼ phamnews.co.uk/326/55


CUT TO THE CHASE


 PDR conversions deliver housing but bypass full Building Regulations


 Overheating is a growing issue in converted commercial buildings


 Part O overheating rules do not apply to PDR schemes


 Part F ventilation compliance does not address thermal comfort


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