Polypipe discusses how to approach ‘green’ ventilation systems in refurbishments as more people look to renovate their existing properties

With house prices increasing and the cost of moving considerable, more and more people are looking to refurbish their existing property to turn it into a home more suited to their needs and lifestyle.

For a number of homeowners and some social housing providers, this is also the ideal opportunity to incorporate ‘green’ technologies and enhance the sustainability of their property. There are many well-documented approaches to improving a property’s energy retention, but the question raised time and time again is: can I fit a heat recovery system in an existing property?


A mechanical ventilation with heat recovery (MVHR) system efficiently pre-warms fresh filtered air drawn into a building using heat extracted from stale air leaving the building − up to 95% of waste heat can be recovered through this method. The fresh warm air is then distributed around the living areas of the home.

The MVHR units are often relatively compact and designed for either loft or wall mounting; the latter can be conveniently hidden away in a cupboard. Finding space for the unit isn’t normally an issue in a refurbishment project.

Nor is there usually any challenge with the duct runs connecting to the outside of the property as these can terminate either through the roof or wall. The challenge is where to put the internal ducting.


Ducting for MVHR systems is normally Ø125 or 150mm (or the equivalent in rectangular) so it can be difficult to hide the duct behind trunking or fit between tight spaces. The ducting system itself also needs to be carefully designed to maximise air flow and reduce air leakage, which means avoiding too many sharp bends. All this has an impact on where the duct runs go, but homeowners are unlikely to want ducting on show in their newly refurbished property.

There are options for concealing the ducting behind bedroom wardrobes or cupboards, but it is not always possible in every room, so this needs to be given serious thought. If however, the refurbishment is a major one, then you can build risers and accommodate the ducting that way. 

Alternatively, you could use a Radial semi-rigid ducting system. Unlike traditional branch-based ducting systems, Radial systems allow each room vent to be served by single or twin duct runs which connect directly to the central distribution system, evenly splitting the air from the MVHR unit.

As an example, Polypipe’s Domus Radial system is suitable for small spaces as it uses Ø75mm semi-rigid ducting and has an ultra slim 125mm deep manifold which can easily be fitted between loft joists or in tight spaces. The small manifold size also allows for greater location flexibility, removing the need to place the manifold adjacent to the centralised appliance.

If you can accommodate the ducting in the property being refurbished, you still need to consider whether it will be effective, in relation to the degree of air tightness of the property in question. For the large majority of older properties, draughts are a way of life.

Air permeability

MVHR systems work by using the heat contained in the atmosphere in a building; if all warm air is exiting through the gaps under doors or around windows, there’s nothing left for the MVHR system to use. 

So, it’s essential you check the property’s air leakage or permeability.

Building Regulations sets the allowable maximum air leakage at 10m³/h. Approved Document F (Ventilation) of the Building Regulations classes 5 as a leaky property. For MVHR to work effectively, the leakage rate should be ≤3. Anything above that is questionable as to whether you will reap any benefits from the system. This isn’t unachievable in a refurbishment project, but MVHR systems are more effective in relatively new properties or ones that are undergoing a major refit, rather than a single room refit.

MVHR systems have been on the market for some time, but they have only recently become prevalent with the move towards greater sustainability.

With many households now deciding to stay put and refurbish their properties, this sector is also looking to benefit from the energy saving and health qualities MVHR systems can bring.  But before recommending an MVHR system as part of a refurbishment, you must take into account the possible duct runs and what needs to be done to reduce the building’s air leakage. Only then will you be able to install a truly effective MVHR system.