MVHR: sourcing the essential resource

EnviroVent’s Larry Soper gives his top tips on specifying the correct MVHR (Mechanical Ventilation with Heat Recovery) system to provide fresh air to your air-tight property

Self-builders are increasingly choosing MVHR as their preferred choice for ventilating a new home. MVHR offers a sustainable long-term solution while generating good indoor air quality and energy savings.

What is Mechanical Ventilation with Heat Recovery ?

All new build properties require adequate ventilation, which means specifying a system which will remove any excess moisture and prevent condensation and mould growth. Extract fans and centralised extract ventilation systems can be inefficient and reduce the energy efficiency of a dwelling. However, MVHR addresses the issue by recovering the heat from the extracted stale, moist air within a property’s kitchen, bathrooms and ensuites. The energy from this extracted air is transferred through a high efficiency heat exchange cell to fresh filtered air and is re-supplied to the habitable areas of the home.

A typical MVHR system comprises a heat recovery unit containing a high efficiency heat cell. This is typically installed in a utility room, cupboard or loft space and is connected to the habitable and wet rooms in the property via a network of ducts, which open into the rooms via extract or supply valves in the ceiling.

What are the benefits of MVHR?

For many new properties, MVHR is the most efficient form of ventilation currently available. The right system, correctly installed, will take care of any condensation problems and will help reduce the cost of heating. MVHR naturally provides all-year-round fresh, filtered air. By maintaining an optimal indoor climate and removing condensation, MVHR not only saves energy but can also reduce wear and tear on paint, wallpaper and furnishings, reducing maintenance costs. Having good indoor air quality also helps to reduce symptoms of asthma and other respiratory illnesses, which can be triggered or worsened in humid, moist conditions where dust mites can be prevalent.

Get the sizing right

It is essential that the MVHR unit is sized correctly for your property. The type of MVHR unit selected depends on the size of the property, the number of rooms, ‘wet rooms’ and occupants. An oversized unit may be inefficient and use more power; an undersized unit will be noisy, inefficient and consume too much power. On a very large property the use of multiple units is recommended because it can provide a quieter and more energy efficient system with improved heat recovery.

The sizing of the duct is also important. A duct that is too small for the airflow rate will result in high resistance and velocity of air creating noise issues. A reputable manufacturer will be able to advise on the optimum duct size for a given system.

Good installation is essential

The Building Research Establishment (BRE) recently conducted a study into the installation of MVHR units. Their findings suggest that as many as nine out of ten heat recovery units in UK homes have been incorrectly installed, and that many systems required subsequent changes to air inlet valves, ducting or insulation.

It is therefore essential to get the installation of an MVHR system right, from the start, by working with a competent and certified ventilation installer. The most efficient unit on the market will perform worse than the least efficient unit if it is poorly installed. The efficiency promised by a particular unit can only be reproduced in the installed environment if it is properly implemented as part of a well-planned system.

Correct commissioning of the system is also crucial to ensure that the right airflow is delivered, and that the system is properly balanced.


In choosing an MVHR unit, noise pollution and air quality in the area must be considered. That’s because MVHR draws in air from outside the building, transferring heat from the outgoing stale air to the new fresh filtered supply air. Obviously, if the air from the outside is polluted with particulate matter then this could be introduced into the house. MVHR units come with filters that may be capable of filtering particles up to PM10 or PM1. However, in some scenarios, greater filtration may be required.

Filters help to protect the heat exchange unit as well as ensuring consistently clean air inside the property. Over time, filters can clog with dirt and other substances. A clogged filter can no longer effectively clean the air, and, just as importantly, the heat recovery system will be unable to run at maximum efficiency.

The worse the pollution outside, the more important a filtration system will be, and the more often filters will need changing. If filters are simply allowed to clog up and to be left clogged, the heat recovery system will not be able to maintain its performance.

Choose high quality ducting

A heat recovery system is only as good as the ductwork installed. It can be false economy to specify the most expensive type of MVHR unit and install it with sub-standard ducting. When ducting is selected and installed correctly, it improves and prolongs the efficiency of the whole system, leading to long-term low maintenance. On the other hand, a ventilation system that has issues with the ducting, which can range from ‘slump’ of flexible types, through to inadequate jointing mechanisms, is always going to underperform. Poorly installed ductwork can potentially damage the ventilation unit and the fabric of the building. It is worth ensuring that the ducting is as high quality as the ventilation unit to avoid any long-term issues.

EnviroVent offers a complete system design, supply and installation service. The company can ensure that a suitably sized unit is specified and the customer is provided with a detailed breakdown of costs.

Larry Soper is technical training manager at EnviroVent