This section has been written by the Heat, Space and Light team based on many years of experience helping home-owners, architects and building contractors to design and install their ideal MVHR system.
If your question isn’t answered below, or if you would like to discuss an MVHR system for your own project, please contact us directly.
What is an MVHR system?
A Mechanical Ventilation with Heat Recovery system (MVHR) provides ventilation with heat recovery to a building, whether a home, office or school. We need mechanical ventilation in modern buildings as energy efficiency regulations and a desire for comfort and healthy indoor air quality means homes are better insulated and airtight than historically.
An MVHR provides fresh, filtered outdoor air to the living areas and bedrooms of a home, and extracts warm, humid air from bathrooms, utility and kitchen areas – in equal measure.
As the warm air passes within the heat exchanger the warmth is given to the cold winter incoming air, warming it up to almost indoor ambient air temperatures. That means the heating system in the home doesn’t need to work as hard to keep the home environment warm in winter, saving energy and increasing comfort.
The air itself never mixes. The waste, damp air is exhausted outside to ensure the indoor air of the home is clear of pollutants such as CO2, chemicals and moisture which can cause respiratory issues. It is replaced in balance with oxygen-rich, fresh and filtered outdoor air.
Who manufactures MVHR units?
The biggest and in our opinion best MVHR suppliers are Zehnder, BRINK and Lindab. We specify Passivhaus Institute-accredited (PHI) MVHR units as standard for our projects as they are proven the quietest and most efficient on the market. You can search the PHI MVHR database by brand here.
Will an MVHR save money on heating?
An MVHR system recovers useful heat that would normally be lost via draughts, trickle vents and extractor fans. Instead this useful heat is passed to the incoming supply air at levels we require for healthy indoor air, which means the heating system does not have to work as hard.
It is much more energy efficient and comfortable than opening a window or using trickle vents in winter.
Is an MVHR expensive to run?
No, an MVHR cost pennies per day to run. Here’s an image taken from one of our projects showing that the MVHR is able to warm the air from 1.5 DegC to 22 Degc for only 16W of power usage – about 0.4kWh per day.
Does the property need to be airtight for an MVHR to work properly?
Making the home as airtight as possible during the construction will have health, comfort and energy saving benefits, because lots of heat is lost through draughts and cracks. The more airtight the home, the better the MVHR will work, and the more heating saved.
Research by the Passivhaus Trust has demonstrated that a well-designed MVHR system will operate efficiently at even low levels of airtightness though – you can read more about it here.
As you renovate or build a home to modern standards, it will inevitably become more airtight.
If ventilation isn’t increased in lockstep with this it may cause mould and condensation issues. With an MVHR you can remove window trickle vents and fill in drafts/air leaks as much as possible whilst still guaranteeing fresh, tempered air. Heat, Space and Light Ltd also provides pre- and post-construction airtightness testing services that can be combined with MVHR commissioning – find out more here.
Does an MVHR system make noise?
An MVHR should be designed properly to be near-silent, and should not be distracting or audible above ambient noise. We specify the quietest MVHRs in the industry, and we add additional sound attenuation to further mitigate any potential for noise.
Our careful design ensures that ducts and valves are correctly sized to prevent excessive noise from fast air movement, and our clients tell us that they are surprised by how near-silent our installed MVHR systems are – please read our Google Reviews for genuine client feedback.
What are the disadvantages of MVHR?
If an MVHR system is poorly specified or using cheap materials the disadvantages are that it may be noisy, difficult to control and decrease home comfort, health and air quality.
I’ve written more about the disadvantages of MVHR here if you’d like to see the detail.
Does an MVHR run all the time?
Yes, even at night, so it’s crucial that it’s been designed and specified correctly – that’s our job at HSL. MVHR systems should never be turned off once operational because there’s a risk the damp air could mould in the heat exchanger core. Instead, most have a holiday mode to use very small amounts of electrical power.
How large is the MVHR unit and associated ductwork?
Our typically-specified MVHR unit measures 850 x 725 x 570mm (HxWxD), which we normally liken to about the size of a dishwasher for a 150m2 – 350m2 property. Smaller and slimline MVHR units are also available – we will carefully specify the most-suited MVHR system for your project.
250mm is required below the MVHR unit, and approximately 450mm above for ductwork. The front panel requires around 1m of free area for future maintenance access.
Ducting to rooms will be either 75mm or 90mm external diameter and we will work hard to use hidden space so that boxing or ceiling voids are not necessary. More information about the MVHR system is available here.
Where should we put the MVHR unit?
Our recommendation is to place the unit as close to an external wall or roof as possible, because the ducts to outside will carry very cold air and can therefore be a “cold bridge”. The MVHR should be within the warm envelope (ie, insulation) of the home.
Our design team will help you to work out the optimal location of your MVHR system.
Will an MVHR help cool my home in summer?
An MVHR system is designed to be very low but constant and healthy levels of air. It is best not to rely on it for heating or cooling of the air, but see it mainly for ventilation.
However, it will exhaust out warm indoor air in summer using its summer bypass mode. More is explained on that process here.
During our commissioning site visit at the end of the MVHR installation we will also explain to our clients the techniques and strategies to make the MVHR keep the home as cool as possible.
Will an MVHR help with condensation and damp?
Yes, an MVHR will better modulate the Relative Humidity (RH) of air in the home by removing excess humidity and replacing it with tempered air. Damp is a complex issue and improving ventilation is only one part of it – please find out more here.
Does an MVHR still need a kitchen cooker hood?
Yes, it is entirely separate to the MVHR. A cooker hood deals with airborne grease from cooking that we wouldn’t want to enter the MVHR ductwork. It can be recirculating or ducted to outside, but please contact us to clarify which version you should use.
Can I have a fireplace or stove?
A traditional open fireplace is not compatible with a low energy home or MVHR, as it requires a cold air feed in the walls to supply oxygen to the fire. However, a room-sealed wood-burning stove with a dedicated air feed is fine, but please contact us to clarify which version to use.
Do MVHR systems deal with outdoor pollutants?
Yes, MVHRs will filter out pollen, carbon dust, insects and other particulates. The only external pollutants they do not filter are nitrogen oxides (NOx), which are a gas produced by diesel fumes. Additional NOx filters are available – please contact us for more information.
What’s the difference between branch and radial ductwork layouts?
For most residential homes we would recommend a radial layout for your project as they use a smaller diameter, ultra-airtight duct that is optimised for acoustics, air movement and future maintenance compared to steel duct.
For offices, schools and community centres we would normally recommend large-bore steel ductwork in a trunk-and-branch layout.
More detail on the differences between radial and branch systems is here.
How does air move between rooms?
10mm door undercuts from finished surface for a 760mm-wide door are required for all ventilation system types (not only MVHR) in Building Regulations Approved Document F, as this allows air to move between rooms even with doors closed. If the door is wider this can be reduced, as the definition for the opening is 7,600mm2.
Will the external ducts penetrate through the roof or the wall?
Whether roof or wall depends on the location of the unit and your preference. Two 150mm – 250mm diameter openings spaced at least 1.5m apart for the intake and exhaust are needed, but we can also design grilles with diverters and cowls to place them next to each other without short-circuiting the incoming and outgoing air – information on combined grilles here.
Where should valves be placed in rooms?
For good airflow, valves should be 400mm from any adjacent surface (eg, wall or ceiling). Extract valves are placed at high level to capture rising warm air, and supply valves can be placed at high, low or floor level as preferred.
Are there any different valve types for the rooms? Can we paint them?
There are many different options for valves, at varying prices, but we do recommend the standard valves as they are optimised for airflow, noise and comfort. They come in white or stainless steel but can be powder coated in any colour.
What maintenance/servicing is required?
Filters must be checked every 6-12 months and either cleaned or replaced. Valves and external grilles should be cleared of dust during routine cleaning, and the unit itself should be serviced after five years. More information available here.
Does an MVHR remove dust?
Good MVHR units have filtration to prevent outdoor carbon dust, pollen and flies into the home, which can lower dust levels in the home. MVHR systems also extract from bathrooms and kitchens and will remove towel fluff, dust and airborne grease also, where it’s captured in removeable filters.
Can I open windows with an MVHR?
Yes, you absolutely can. The benefit of MVHR is that in summer or winter if there’s an outdoor noise, pollen, pollution or security issue you can close the windows tightly and rely on the fresh flow of ventilation from the MVHR system.
If you prefer to open your windows in summer you absolutely can, and the MVHR can be set to extremely low levels of power consumption. We at HSL also have a strategy for passive summer cooling via the MVHR unit controls that we provide to our clients.
Can an MVHR have active cooling?
Yes, although cooling via the MVHR is limited by the airflow rate. Alternate MVHRs with heat pumps which allow for air cooling are available – please speak to us to find out more.
What do we need for Building Control sign-off?
A certificate showing the system has been correctly installed and balanced by an MVHR technician with calibrated measuring equipment is needed and will be issued by our team at the final stage of the project once the system has been successfully commissioned.
What kind of warranty is available for an MVHR system?
The standard warranty from the MVHR unit supplier we specify is two years for the unit and electrical items (ie, controls), which extends to six years if the unit is registered and official filters are used.
Heat, Space and Light Ltd can supply filters in packs of 10 F7 and 10 G4 types, which should last 5-10 years depending on usage.
What is the VAT rate for an MVHR system?
Generally 20% as standard, although for a new build the VAT should be zero VAT rated or reclaimable. For unoccupied homes it could be set at 5% VAT.
Do you install MVHR?
Installation is not included and should be led by a competent person – we can help find one of our recommended installers for you, or help you save money by completing your own self-installation.
What building works in connection are needed for an MVHR system?
We will inform of any building works required to accommodate the MVHR system as part of our design process (eg, location of external penetrations for intake/exhaust outlets).
In terms of plumbing, a 32mm connection with a Hepvo dry trap to a waste water pipe is required below the unit. If an enthalpy exchanger is supplied by our team this condensate drain is not needed and both drains below the MVHR can be capped. This is because an MVHR system with an enthalpy exchanger can recover moisture as well as heat.
In terms of electrical requirements for an MVHR, the unit must be installed on 230V~ 50Hz mains connection. The fuse rating incl/excl pre-heater is a standard 13 amp fused spur. Although the unit is delivered with a standard household plug, a fused spur should be installed by your electrical contractor.
How long does it take for you to complete an MVHR design?
It normally takes one to six weeks for our detailed ductwork layout design and calculations to be completed, but please allow as much time as possible. Ducting should be installed before any other trades services.
When during the building stage should an MVHR system be designed?
Once the plans for the project are relatively set / frozen, eg, once planning has been accepted, then this would be a good time to start detailed MVHR design. This will allow as much time as possible to prepare the design, order the materials and set a date for first fix installation.
Do you require detailed drawings or plans?
Ideally yes, but we can also design ductwork layouts even with simple sketches or estate agents’ plans. The best case is to ensure that all drawings, sections and elevations are made available to allow for a faster design review. Hand-drawn plans are fine, but may require more time to design to.
Do I need building control permission to install an MVHR system?
We assume that, as applicable, the relevant Building Control, Fire, Planning and Conservation authorities have been notified in advance of works relating to the MVHR, and authorisation where necessary has been approved for all aspects of the system.
The MVHR system should be permissible under the premise of improving works to the ventilation, but it’s worth checking with your local authority, particularly if permission will be needed for the external intake and exhaust vents.
What fire regulations are needed for an MVHR system?
Again, it is vital to check with your local authority what fire regulations are required but generally additional fire-rated items will be needed where the ductwork penetrates a fire barrier/partition, or where a building is above a certain height from street level. We can advise on strategies for ensuring compliance with the fire regulations for your MVHR system.
As mentioned, if you have any other questions about MVHR, or want to discuss your own project, please contact us directly.
What is the lifespan of MVHR?
An MVHR should last 10 to 20 years if maintained regularly. We specify only MVHR units which are built modular – ie, easy to change parts if they fail, and from companies with long track records in the industry.