Mechanical Ventilation with Heat Recovery - NEW

Good ventilation is essential for the modern home. Larry Soper offers advice on specifying the right system.

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Most new-build properties are well suited to MVHR, or to give it its full name, mechanical ventilation with heat recovery. However, it is important to have ventilation specified by a specialist to make sure the system is right for you. While MVHR 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 MVHR will be. If your home is draughty, there is little point in retaining the heat extracted by ventilation alone. Depending on the size, location and air permeability of the property, calculations are used to ensure sufficient airflow throughout the dwelling.

The more airtight the property, the more effective MVHR will be

Get the sizing right

The MVHR unit must be the right size for your property. The type of MVHR unit should be appropriate for the size of the property, the number of rooms, ‘wet’ rooms and the number of occupants. An oversized unit may be inefficient and use more power; an undersized unit will be noisy, inefficient and use too much power. Multiple units on a large property are recommended because a quieter and more energy-efficient system can be achieved with improved heat recovery.

The size of the ducting is important

The size of the ducting is also important. A duct that is too small for the airflow rate will result in high resistance and velocity of air, causing noise issues. A reputable manufacturer will be able to advise on the optimum size for a given system. Check that your system complies with Building Regulations. 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

A study into the installation of MVHR units by the Building Research Establishment suggests that as many as nine 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 right, from the start, by working with a qualified ventilation installer. The most efficient unit on the market will perform worse than the least efficient unit if it is poorly installed.

A correctly specified and fitted MVHR system can provide a healthy living environment, while saving on heating bills.

The efficiency promised by a manufacturer can only be achieved if a unit 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.

Efficiency is achieved from a unit that is part of a well-planned system

Filtration is essential

Noise pollution and air quality in the area must be considered when choosing an MVHR system. That’s because MVHR draws in air from outside the building, transferring heat from the outgoing stale air to the new fresh filtered air supply. 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 these are simply allowed to clog up, the heat recovery system will not perform properly.

Functionality

A homeowner should take into consideration how they want to control their ventilation system as some MVHR units are available with smart apps, which make it easier to set up and operate via a smartphone. 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. It can be false economy to install the most expensive unit with substandard ducting. Selected and installed correctly, ducting 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 duct work can potentially damage the ventilation unit and the fabric of the building.

A heat recovery system is only as good as the ductwork

Larry Soper is technical training manager at MVHR specialists EnviroVent


This Beginner's Guide to MVHR is from the September 2021 issue of SelfBuild & Design magazine.

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