Positive pressure ventilation

[edit] What is positive pressure ventilation?

Positive pressure ventilation (PPV), also referred to as positive input ventilation (PIV), is a ventilation system that focusses on supplying fresh air to a building, as opposed to full air conditioning with supply and extract. The system supplies fresh (sometimes pre-warmed) air into a building, creating a positive pressure, which can drive stale or moist air out through the building fabric. It has been considered as a remedy for issues such as condensation, damp and even mould that can occur in some buildings.

[edit] How is positive pressure ventilation created?

This servicing approach can be achieved by installing a single unit below a roof (see image above left) that draws in fresh, warmed internal air from the attic space and drives it into the occupied areas within the insulation line (middle image above). Alternatively, it can be achieved by central fan integrated supply (CFIS), a central unit that pulls fresh air into a building which might be warmed by the space and supplied into the occupied areas (right hand diagram).

Systems vary in their degree of complexity and function, potentially including intake ducts to help preheat the incoming fresh air, in a similar way to mechanical ventilation heat recovery (MVHR). However MVHR units tend to be fully balanced systems with the same number of extract ducts as supply ducts, aiming to achieve neither positive nor negative pressure but a balance. Likewise full air conditioning with air handling units aims to balance supply and extract. MVHR systems rely on high level fabric performance in terms of heat loss and air tightness, this avoids issues with drafts, cold spots and cold surfaces.

It is important to note that although there are some similarities to MVHR, the two systems are very different in that one runs at constant positive pressure, whilst the other is in balance, relying on careful design and performance of the building envelope. Also, in MVHR systems the heat from stale warm moist air is retained and used to reheat incoming fresh air, which means the heating energy already invested in that air gets recycled, reducing the amount of new heat energy required. In a PPV system the air effectively leaks out of the building fabric along with the heating energy invested in it, and the new fresh air needs to be preheated using a new energy source.

[edit] When was positive pressure ventilation developed?

Positive pressure ventilation was developed in the 1970's to deal with poor performance in existing buildings built to older fabric performance standards, in particular to tackle issues of poor ventilation and moisture build up. The idea was that where a building envelope does not ventilate enough or has cold areas and moisture issues, pumping fresh air into the space will, over time, dilute the stale air by forcing most of it out through the fabric. This maintains a better internal air quality without costly upgrades to the envelope.

[edit] Mould risk

In a building that has low levels of insulation and air tightness as well as poor ventilation and in some cases poor heating, there can be stale air within the building, cold spots in the fabric and drafts that cause higher levels of moisture and dampness which can lead to mould and air quality issues. Mould can form when excess moisture in the air comes into contact with cold surfaces, such as at windows or poorly insulated walls. Growth of mould is likely to be worse in winter and exacerbated by poor ventilation and lack of heating.

In such a scenario some arguments can be made in favour positive pressurisation to ensure fresh air is constantly supplied, thus helping drive moist air out through the poorly performing fabric. However as an approach this might be seen as a way of dealing with the symptoms of a poorly performing space rather than dealing with the root cause - which is the poorly performing fabric.

In 2020 the issue of poor indoor air quality (as a result of poor ventilation and fabric performance) lead to the tragic death of a young boy in Rochdale, UK. The coroner ruled that the boy died from a respiratory condition which was caused by mould in a one-bedroom flat managed by a housing association. The issue was discussed in Parliament and Michael Gove followed with various comments in November 2022. The Property Care Association (PCA) then called for more stringent enforcement of the Building Regulations to tackle the problem of damp and mould in homes, of which its members said they were seeing a “significant rise”.

[edit] Related articles on Designing Buildings

• Active pressurisation.
• Air change rates.
• Airflow
• Airtightness of energy efficient buildings.
• Design of isolation rooms for infection control.
• Effective ventilation in buildings.
• Pressurisation of Closed Heating and Cooling Systems (BG 82/2022).
• Pressurisation in buildings.