04 Feb 2020

Considerations when choosing an MVHR system

Larry Soper, Technical Training Manager at EnviroVent, offers his advice on specifying the correct MVHR (mechanical ventilation with heat recovery) system.

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Self-builders are increasingly choosing MVHR as the preferred choice for their new home. MVHR offers a sustainable long-term solution whilst delivering healthy indoor air quality to the home and energy savings.

What is MVHR?

All new-build properties require adequate ventilation, which means specifying a system which will remove any excess moisture and create good indoor air quality. Extract fans and centralised extract ventilation systems remove heat, which reduces the home’s overall energy efficiency and can increase the cost of heating. However, MVHR addresses the issue by recovering the heat from the extracted stale, moist air within a property’s kitchen, bathroom and en-suites. The energy from this extracted air is transferred through a high-efficiency heat exchange cell to fresh filtered air and 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 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 and the most energy-efficient too. The right system, correctly installed, will take care of any condensation problems and may reduce the cost of heating. MHVR naturally provides all-year-round fresh, filtered air. By maintaining an optimal indoor climate and removing condensation, MHVR 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 worsen in humid, moist conditions where dust mites can be prevalent.

Get a professional opinion

Most new-build properties are well-suited to MHVR, but it is important to have ventilation specified from a specialist to make sure the right system is specified.

While the ventilation system should perform well regardless of a property’s airtightness, optimum heat recovery performance requires an airtightness of less than 5m³/(h.m²) @50Pa. At 5m³/(h.m²) @50Pa or above, heat recovery can be less cost-effective. The more airtight the property, the more effective an MHVR system will be. It makes sense that if your building is draughty, there is relatively little point in retaining the heat extracted by ventilation alone. Depending on the size and location and air permeability of the property, calculations are used to ensure sufficient airflow throughout the dwelling.

Get the sizing right

The MVHR unit must be the right size for your property. The type of MVHR unit selected should depend on the size of the property, the number of rooms, wetrooms and the number of occupants. An oversized unit may be inefficient and use more power; an undersized unit will be noisy, inefficient and consume too much power. The use of multiple units on a very large property 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, like EnviroVent, will be able to advise on the optimum size for a given system.

Check that your system will be compliant 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 and the whole building extract ventilation rate. Approved Document F provides required airflow rates for MVHR systems.

Good installation is essential

The Building Research Establishment (BRE) 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 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 fully trained and qualified 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.

Filtration

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 maintain the efficiency of 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 be left clogged, the heat recovery system will not be able to maintain its performance.

Functionality

A homeowner should take into consideration how they want to control their ventilation system as some MVHR units, like energiSava, are available with smart apps, which make it easier to set up and operate the ventilation system via an ioS or Android phone. Modern MVHR units also have humidity sensors, CO2 sensors or manual boost switches which can make them operate more effectively.

Use 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.

EnviroVent offers a complete system design, supply of unit 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.

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