Installing an MVHR unit is very achievable. However, you do need to know what to look out for. An MVHR system (mechanical ventilation with heat recovery) ensures a healthy indoor climate by continuously supplying fresh air and extracting stale air, while recovering the heat from the extracted air. That's efficient, but it does require the MVHR unit installation to be done correctly. Whether you want to install an MVHR system yourself as a DIY enthusiast, or simply want to understand how connecting an MVHR unit works: in this blog we take you through everything you need to know, step by step. From the right mounting position and electrical connection to duct selection, sound insulation and commissioning your MVHR installation.
In this blog you'll read about, among other things:
For many DIY enthusiasts, installing an MVHR system is very achievable. You don't need a certified installer for the installation itself, but some technical knowledge is required. Think of basic knowledge of electrics, working with ventilation ducts and a good understanding of an installation manual. If you have that knowledge, you can install, connect and commission an MVHR unit without needing to call in a professional. Still in doubt? Then it's smart to contact our specialists beforehand. They are happy to think along with you.
Depending on the type of MVHR unit, it can be mounted in four positions:
Which positions are possible for your MVHR unit can always be found in the installation manual.
Every MVHR unit is connected via a plug. Two versions are possible, depending on the type of unit:
A Perilex plug has five connections:
Via the switching wires, you can control the MVHR unit with a fixed, wired three-speed switch (speed 1, 2 and 3). This requires a direct wired connection between the switch and the Perilex socket.
An earthed plug has three connections: a live wire, a neutral wire and an earth wire. This is the most common version and is usually controlled via a wireless RF remote control or, for more modern units, via an app or smart home integration.
Tip: Not sure which plug type your MVHR unit has? Check the unit's specifications.
Every MVHR unit has four connections: two on the indoor side and two on the outdoor side. The location of these connections on the unit varies by type and version. Below you'll find an overview of the most common versions and what each connection does.
* Depending on the type, this may vary slightly. Always refer to the installation manual of your MVHR unit.
Tip: Determine in advance which type of MVHR unit and which connection version best suits your situation. Need help? Our specialists are happy to assist you with the choice.
For an MVHR system, rigid ducts made from PVC, EPP, or galvanised steel are preferred. Flexible ducting is useful for the final connection to a valve, but it creates more resistance and should never be used as a full alternative. Keep ducts as straight and short as possible and minimise the number of bends. Each bend increases resistance (Pascal) and reduces system efficiency.
Are the ducts routed through an unheated space such as a loft? Then both the supply and extract ducts must be vapour-sealed and insulated. Because an MVHR unit recovers up to 95% of heat, the exhaust air leaving the unit is also relatively cold — which can cause condensation on the duct. Finally, ensure airtight connections at all joints. Leaking ducts will directly reduce the efficiency of your MVHR installation.
An MVHR unit has an average lifespan of 15 to 20 years. Time for a replacement? Good news: in most cases, the existing ductwork can be reused. However, check in advance whether the connection sizes of the new unit match the existing ducts, and whether the capacity meets the current ventilation calculation.
Before dismantling the old unit, always photograph the wiring and connections — this makes connecting the new unit much easier. Connect the acoustically insulated hoses to the indoor connections and the thermally insulated hoses to the outdoor connections (supply and extract to outside). Only restore power once everything is correctly connected. After installation, the unit always needs to be recommissioned. More on that below.
After installation or replacement, an MVHR system always needs to be commissioned. This means setting the airflow rates (volume flows) per room so they meet the minimum requirements of Approved Document F of the Building Regulations, and ensuring that supply and extract air are in balance. If this balance is disrupted, the system operates inefficiently and indoor air quality can deteriorate.
The correct order is: first set the total airflow on the MVHR unit itself, then adjust the individual vents per room. Important: adjusting one vent affects the airflow of all other vents in the system. Commissioning requires a professional flow meter and the necessary expertise — this is not a job you can do yourself without the right equipment. A recommission is also required after a thorough duct clean or following any renovation work.
For the connections of an MVHR unit, you use two different types of ducts, each with its own application:
Acoustic duct — is used on the indoor side, from the MVHR unit to the living spaces and wet rooms. The inner foil contains small holes that create a strong sound dampening effect, preventing the noise of the unit from entering the home through the ducts.
Thermal (insulated) duct — is always used on the outdoor side, from the MVHR unit to the roof or facade (intake and exhaust). The insulation serves two purposes: it prevents condensation on the outside of the duct and keeps the air temperature stable. The latter directly affects the efficiency of the heat recovery — the less heat that is lost before the heat exchanger, the more heat is recovered.
Tip: Using thermally insulated ducts on the outdoor side is not only sensible, but also a manufacturer's requirement for maintaining the warranty. Often mentioned in the fine print of the installation manual.
On the indoor side, an MVHR unit is always connected with sound attenuators, to prevent the noise of the unit from entering the home through the ducts. You can choose between rigid and flexible sound attenuators — below we explain the difference.
Rigid sound attenuator — attenuates significantly more sound than a flexible variant, offers less resistance and is easy to clean. The downside: it is not flexible, and therefore requires sufficient straight space in the duct.
Flexible sound attenuator — ideal for smaller spaces or awkward corners due to its flexibility. Slightly cheaper to purchase, but attenuates less sound, offers more resistance and is not easy to clean.
Tip: Do you need a sound attenuator longer than 1.2 metres? Then an acoustic duct is also an option. Do bear in mind that during a future duct cleaning, the acoustic duct will need to be replaced — it does not survive the process.
Crosstalk attenuators — are small attenuators that you place directly in the duct, in front of the valve. They prevent sounds and voices from room A being audible in room B via the ventilation ducts. Crosstalk attenuators are a useful addition, but not a replacement for a good sound attenuator.
Installing roof penetrations involves more than just drilling holes. In addition to proper insulation and a watertight finish, you also need to consider the distance between the supply and exhaust outlets. This distance is not arbitrary — it is determined by a dilution factor calculation.
The dilution factor is a measure of the quality of the incoming air supply and is governed by Approved Document F of the Building Regulations. It indicates what percentage of the extracted, contaminated air may enter the fresh air supply.
The calculation depends on the type of roof or façade, the airflow rate in m³/h, and the chosen location of the supply and exhaust outlets. Always take this into account during installation. If you don't, there is a risk that the MVHR unit will draw back in the contaminated air it has just expelled. This can lead to poor indoor air quality and health issues. Find out more about dilution factor calculations and other ventilation standards in our blog on ventilation calculations.
MVHR units are available in different versions, each with their own controls and functionalities. Most modern systems can be controlled wirelessly via an RF remote control or a wall control. Additionally, there are more and more systems with extended features, such as:
Installing an MVHR system yourself is very achievable for the motivated DIY enthusiast, as long as you know what to look out for. With the right preparation, the right materials and optionally our free installation advice, you'll install a system that functions well for years to come.
Do you have any questions? Or would you like more information about a specific MVHR unit from our range? Our specialists are happy to help you via WhatsApp, chat, email or phone.
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At Ventilationland, we think it is important that we can always be reached and answer your questions as quickly as possible. Therefore, you can reach us in several ways, including via chat or WhatsApp. We are happy to help!
