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What to Look for When Specifying an MVHR Product


By EnviroVent Jun 05, 2019

Property developers and planning managers are increasingly looking to MVHR, Mechanical Heat Recovery Ventilation, as the ventilation system of choice for new build properties. MVHR provides clear benefits in efficiency, energy savings, and long-term cost, but in order to realise those benefits the right decisions need to be made at the outset, about which system to invest in and how to install it. 

MVHR is a somewhat more technical system than simple extractor fans, passive stacks or mechanical extract without heat recovery, and initial installation can be more expensive. That means there’s a bit more to know about MVHR, a few more decisions to be made, and the potential for significant expense if things go wrong at the planning stage.

What is Mechanical Heat Recovery Ventilation?

All new-build properties require sufficient ventilation: that is, a system which works to remove any excess moisture by creating airflow. Typically, ventilation works by removing some of the air from a property and replacing it with air from outside. However, this system has a serious downside, which is that along with the moisture and stale air from within a property, traditional ventilation systems also remove heat, severely reducing the property’s energy efficiency and increasing the cost of heating.

Mechanical Heat Recovery Ventilation addresses that shortcoming by recovering the heat from any air and moisture extracted from a property. A typical MVHR system comprises a heat recovery unit, ideally installed in a central location, which is connected to every room in the property via a network of ducts, which open into the rooms via ceiling or wall vents.

What are the benefits of MVHR?

In suitable properties, MVHR is the most efficient form of ventilation currently available, and it’s the most energy efficient too. The right system, correctly installed, will take care of any condensation problems and substantially reduce the cost of heating and cooling buildings. Heat recovery helps to maintain a steady internal temperature, significantly reducing demand on air conditioning and central heating with their associated financial and environmental costs. Furthermore, MHVR is a very low-noise and non-intrusive ventilation system, reducing noise pollution and associated stress for residents.

In addition, MHVR naturally provides all the benefits of any good ventilation system. A property with an MHVR system will enjoy year-round fresh, dry air, making it far less vulnerable to damp and damp-related conditions such as black mould. By maintaining an optimal interior climate and removing condensation, MHVR not only reduces energy bills but also reduces wear and tear on paint, wallpaper and furnishings, reducing maintenance costs. The steadiness of the interior climate may also have health benefits for residents. 

Specifying an MHVR product: the challenges

Heat recovery systems are relatively site-specific. It’s important to choose the right system for a property and to ensure that it’s installed correctly. The Building Research Establishment recently conducted a study into the installation of MHVR units. Their findings suggest that as many as 9 out of 10 heat recovery units in UK homes may have been imperfectly installed. Researchers found that many systems required subsequent changes to air inlet valves, ducting or insulation.

Needless to say, altering an MHVR system once installed, let alone re-installing altogether, is expensive, and it’s an entirely unnecessary expense. By focusing on a number of key aspects of MHVR, these errors can be ruled out at the planning stage.

Check that MHVR is suitable for your property

Most new build properties are highly suitable for MHVR, but it’s best not to take suitability for granted. While the ventilation system should perform well regardless of a property’s airtightness, optimum heat recovery performance requires an airtightness of less than 5ach (air changes per hour). At 5ach or above, heat recovery ceases to be cost effective. The closer a property is to 0ach, the more effective an MHVR system will be. To a certain extent this is simple common sense: if your building is leaking heat, there’s relatively little point in retaining the heat extracted by ventilation alone. In smaller homes, your unit must be able to extract a minimum of 13L/s from the kitchen, 8L/s from each bathroom or utility area, and, of course supply air at the same rate. For larger buildings, multiply the usable floor area to achieve a minimum ventilation rate of 0.5 air changes per hour.

Heat recovery systems are dependent on effective filtration. Filters help to maintain the efficiency of the heat exchange unit as well as ensuring consistently clean air inside the property. Over time, filters clog with the dirt and other substances they filter out. 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. It’s exactly the same as a hoover: when the filter clogs, it can’t clean. 

The first question for developers is how much filtration is required. The UK Pollution Map is a great resource for checking the levels of air pollution in your area. The higher the pollution, the more important your filtration system will be, and the more often filters will need changing. The second question is about building use. How can you ensure that your building’s occupants know how to change filters, and that they’ll make the change as often as necessary? If filters are simply allowed to clog and left clogged, your building may as well not have a heat recovery system at all.

Check that MHVR is suitable for your building’s occupants

In other ways too, a Mechanical Heat Recovery Ventilation System will require some upkeep by building occupants. Unlike an extractor fan or a stack ventilation system, an MHVR is essentially a white good, a piece of technology that rewards careful use and maintenance. An important question for planners and developers is how to ensure that building occupants know how to use the system properly. If occupants are vacating the property for an extended period — for example to go on holiday — they need to know how to turn it off. Similarly, adjustments may occasionally be required, such as in unseasonable weather, which is becoming increasingly common. 

This consideration also affects installation. MHVR systems typically incorporate a control panel and/or thermostat. These need to be located somewhere convenient and easy to access, for instance a central hallway. In an airtight building, an MHVR unit is the single crucial element controlling internal climate. Unless they know how to use it, occupants will be left powerless to control temperature.

Evaluate MHVR as a system, not a unit

This may be the single most important thing for developers to bear in mind as they plan and install a heat recovery system. While the unit is the expensive and technical part of the heat recovery system, it is only a part, and as such it is only as good as the system as a whole. Ducting, filtration, insulation and installation are crucial factors which can make or break the efficiency of an MHVR system. 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.

First of all, the unit needs to be the right size for your property. There can be no compromise on this. It may be tempting to install a pricier, more efficient unit and save on costs by downsizing: for instance, installing a top-of-the-range unit designed for a three-bedroom house in your four-bedroom new build. However, this decision would cost more than efficiency. The unit would struggle and may even become noisy. Indeed, you should choose a unit which can ventilate your property at less than 70% efficiency. Your unit should not be working at 100% all the time.

Choosing the right venting system is also crucial. The important factors are noise transfer, airtightness and diameter. Venting should be the right size for your unit.

Check that your system will achieve compliance with building regulations

Mechanical Ventilation with Heat Recovery (MVHR) is covered under System 4 of the Building Regulations, Approved Document F. MVHR systems are required to provide at all times the minimum whole building ventilation rate. This can be calculated by determining the whole building ventilation supply rate and the whole building extract rate. Approved Document F provides required airflow rates for MVHR systems.

Install with care

MVHR units are compact, and can usually be sited in a cupboard, attic or ceiling void, although best practice suggests that the unit should be installed within the building’s heat envelope. The most compact units can fit into a space the size of a kitchen cupboard. Aside from space, the most important consideration is noise. If properly installed, MVHR units are not noisy, but common sense is advised: a unit directly above a bedroom might disturb a light sleeper. Another noise factor is ducting: if the ducts are not the right size for the volume of air they handle, they will cause noise.

As we saw from the BRE’s research, most MVHR installations are imperfect. For that reason, it’s highly recommended that developers work with professional installers who are specialists in MVHR. EnviroVent provide expert installation for a wide range of MVHR units and systems.