An increasing number of landlords and homeowners are looking to Mechanical Ventilation with Heat Recovery (MVHR) to improve ventilation in their properties, while also increasing energy efficiency and reducing cost. Not every home is suitable for MVHR (more on that below), and the cost of installing an MVHR system varies depending on the size and age of the property. However, where MVHR is an appropriate ventilation solution, it provides long-term energy cost savings. For property-owners interested in implementing MVHR, there are a number of things to consider as you plan and install your system.
When assessing whether MVHR is right for you, it’s useful to know how the system works. A typical MVHR installation centres on a heat exchange unit. Several sets of ducts connect to the unit. The first set extracts air from each wet room in the property, usually through simple grille vents, and directs it to the heat exchange unit, where the heat from the air is “recovered” before the stale air is expelled outside the house. The second set of ducts retrieves fresh air from outside, filters out pollutants and allergens, and then directs this air to the heat exchange, where it is heated with the energy extracted from the expelled air before being piped through the house to the habitable rooms. The heat recovery system means that the house does not have to be reheated with every air change, resulting in big savings in cost and increased energy efficiency.
It’s probably self-evident that this system can only be effective in a property with a relatively high degree of airtightness. If there are many constant sources of incoming air, such as passive ventilation inlets or draughty windows, the benefits of heat retention are reduced, since heat will be dissipating within the house before the system can recover it. Most experts agree that cost-effectiveness for an MVHR system requires a minimum airtightness of 5m³/(h.m²) @ 50Pa
Ideally, MVHR should be built into a property, but it’s also perfectly possible to install an MVHR system in an existing property. The cost of such an installation is extremely variable. To establish a cost for your property, seek expert advice.
Another factor to bear in mind is filtration. The MVHR’s filtration system doesn’t only benefit occupants, it also ensures the smooth and efficient functioning of the unit. Depending on where your property is located, it may require more or less filtration. The UK Pollution Map can give you an idea of air quality in your area. This factor is especially relevant for landlords. You need to be sure that your tenants know how to change the filters in their MVHR system, and how often. If filters are left clogged, the system will provide ineffective ventilation, and heating costs will rise.
Unlike passive forms of ventilation, like stack ventilation, an MHVR requires some ongoing supervision by occupants. Although using an MHVR system is extremely straightforward, landlords need to ensure that there is a handover in which tenants are advised how to locate the controls . If tenants don’t know how to use the system, It may render the system useless, damage the system or building fabric.
Heat recovery ventilation systems need to be carefully tailored to each property., MHVR requires expert installation and planning. It’s wise to involve ventilation specialists at the planning stage. A recent study by the Building Research Establishment found that as many as 9 out of 10 MHVR systems had to be altered after their initial installation. Needless to say, these alterations can be extremely costly and disruptive. The best way to ensure that your installation is perfect first time round, is to take the time to plan accurately.
Mechanical Ventilation with Heat Recovery (MVHR) is addressed in System 4 of the Building Regulations, Approved Document F. Like any other ventilation system, MVHR systems are required to provide the minimum whole building ventilation rate at all times. You can calculate the required ventilation rate for your property by first establishing the whole building ventilation supply rate and the whole building extract rate. Different ventilation systems must achieve different minimum airflow rates. Approved Document F provides required airflow rates for MVHR systems. For most residential properties, your unit must be able to extract at least 13L/s from all kitchens and 8L/s from any bathroom or utility room.
Many homeowners make the mistake of thinking that a smaller, higher-efficiency unit will do the same job as a larger, lower-efficiency unit. This is sadly not the case. The most expensive, top-of-the-rage unit in the world will struggle if it’s installed in a property larger than it was designed for. A struggling unit is much less energy-efficient, and too-small units are also the leading cause of an MVHR system becoming noisy. It’s best to opt for a unit that can achieve the required airflow in your property working at about 50-60% of its maximum for that system. A unit that has to work at 80% most or all of the time will quickly become less effective.
While the right MVHR unit is essential, any MVHR system is only as good as its weakest component. Unless careful attention is paid, the weakness in your MVHR system is likely to be your ductwork, the intake and exhaust terminals, or the room vents. Using the wrong sized ducting, or ducting made of the wrong material, can cause noise or even breakdown in the system. Ducting should also be completely airtight, or efficiency is lost. The position of vents and terminals can significantly affect the performance of your system.
Flexible ducting should be avaided. Short lengths of flexi-ducting may be used to connect the unit and valves to rigid ducting, and particular attention should be paid to sealing these connections.
Silencers may be necessary, and some form of insulation is always a good idea. Any ducting in unheated areas (i.e. loft space) must be insulated. The extra cost of insulation pays for itself in increased energy efficiency.
As far as possible, ducting should be kept within the property’s thermal envelope (i.e. within the insulation layer), and where ducting has to extend beyond the envelope it should be thoroughly insulated.
Ideally, these terminals should not be close to one another. It’s especially important that the exhaust terminal not be located too closely below the intake terminal, as the exhaust air is likely to be warmer than external air and will rise to the intake terminal. Horizontal distance is less significant. Even at a distance of 5cm, cross-contamination is negligible.
The position of the intake vent is particularly important, because this is the source of the air occupants will breathe. Where possible, it should be sheltered from the wind, especially if the property is downwind from any source of pollution. Intake terminals must not be located near flu’s or chimneys.
It seems like common sense, but it’s often ignored: extract terminals in the bathroom should be as close as possible to the shower or bath and away from the internal door. Anything else greatly reduces efficiency. Although the heat exchange unit can be tucked away, it shouldn’t be inaccessible: occupants may need to access it from time to time. An accessible attic is suitable, but a cupboard is ideal.
As found in the BRE’s report above, the majority of MVHR installations are carried out poorly. In the long run, it’s far more cost effective to involve experts from the beginning, and especially at the installation stage.
EnviroVent provides expert advice and installation for a wide range of MVHR units and systems. The first step is to arrange a free home survey; then a local ventilation expert will be in contact with you to discuss any issues that you are facing, then our expert will arrange a convenient time in order to pop over to your home so they can carry out the free survey.Arrange a Free Survey
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