Overheating in Homes

Overheating is no longer a rare issue—it’s a growing public health risk. The introduction of Part O was driven by clear evidence of widespread heat related discomfort in UK homes.

Ventilation plays a crucial role, but MVHR systems need to do more than recovery heat and effectively purge excess heat in the warmest months

Are Part O ventilation solutions simply bigger MVHR units that provide more airflow?  

Part O Key Statistics: 80% of homes overheat

80% of UK homes experienced overheating

A 2022 study by the University of East London found that 80% of UK homes experienced overheating during recent heatwaves—up from just 18% in 2011.This isn't just discomfort - it's a design and compliance failure.


What causes overheating?

Overheating is typically caused by a mix of design and environmental factors, including:

  • South or west facing glazing without shading
  • Lack of cross ventilation or secure night time openings
  • Undersized ventilation systems or ducting to handle performance
  • High internal heat gains from people and appliances
  • The urbanm heat island effect

In some cases, new apartments have recorded internal temps exceeding 28°C for more than 10% of occupied hours which is well above comfort guidelines.

What causes overheating?

Part O of the Building Regulations was introduced to reduce the risk of overheating in new residential dwellings. With homes becoming more airtight and summers growing hotter, overheating is a growing health risk — especially in flats and urban developments.

Key aims of Part O:

  • Limit unwanted solar gain
  • Ensure effective removal of excess heat
  • Support health, safety, and comfort of occupants

Applies to:

  • New build homes including flats, houses and student housing

Approved Document O Overheating

MVHR Design for Part O Compliance

Part O Ventilation solutions typically result in whole house ventilation systems like MVHR, especially in urban apartments, single-aspect flats, or noisy locations.  This ventilation strategy has becomes essential for compliance and is usually dynamically modelled using CIBSE TM59 for required performance and part O compliance. 

Optimising MVHR systems for overheating prevention means they must do more than recover heat, and effectively purge excess heat in warmer months.  Purge ventilation Part O requirements and how MVHR can deliver the required rates is a balance at the design stage - there's aiflow rates, ducting size and noise that need to be balanced effectively;

ResiDUCT™ by Pinnacle

MVHR Part O Compliance 

Meeting Part O (overheating) compliance isn’t just about upping your MVHR unit’s capacity, it’s about ensuring your ductwork can actually deliver the extra airflow. Simply increasing duct diameter isn’t always practical in tight residential spaces it can be costly, bulky, and hard to install

A typical ceiling void in UK housing is only ~150mm deep, yet compliance often demands duct sizes equivalent to 180–200 mm  Hybrid ducting solutions—combining rigid metal trunks for main runs with PVC or semi-rigid pipes for branches, help strike a balance between airflow performance and install practicality

How does ResiDUCT™ help?

  • Low profile: Offers standard profiles (300 × 100mm ≈ 180 mm dia, 300 × 125 mm ≈ 200 mm dia) that fit typical ceiling voids while supporting higher flow rates 
  • Metal-to-PVC connections: Designed to simply connect to typical PVC ducting without compromising airflow 
  • Engineered for air tightness: Low-resistance profiles with integral stiffeners and socket seal joining means leakage is dramatically reduced for installed performance
More Information
ResiDUCT™ by Pinnacle

FAQs

Does Part O apply to existing homes or extensions?

No. Part O only applies to new residential buildings, including flats, houses, and student housing. It does not apply to extensions, conversions, or non-residential buildings.

Does Part O set a maximum indoor temperature?

No. Part O doesn’t set a fixed maximum temperature. Instead, it uses thermal modelling (e.g. CIBSE TM59) to assess overheating risks, such as indoor temperatures exceeding 26°C for more than 3% of occupied hours

What is a summer bypass in an MVHR system?

A summer bypass allows the MVHR unit to divert warm air around the heat exchanger, so it doesn’t recover heat during warmer months. This helps expel hot indoor air and bring in cooler outdoor air to reduce overheating.

What type of homes require dynamic thermal modelling for compliance?

Flats, single-aspect dwellings, or homes in urban and high-risk areas usually require dynamic thermal modelling (CIBSE TM59) rather than the simplified method, to accurately assess and mitigate overheating risk.

Why is duct size important in MVHR design?

Larger ducting (e.g. 125–160mm) allows for higher airflow with less resistance and noise. Smaller ducts may cause pressure build-up and force the MVHR unit to work harder, increasing energy use and reducing comfort.
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