MVHR Guide: Everything You Need To Know About MVHR

 

 

MVHR is increasingly popular in new-build properties – both residential and commercial. But why?

Every building needs proper ventilation. To keep conditions inside comfortable and healthy for people. And even to maintain the integrity of the building, by keeping mould and other threats at bay.

The purpose of ventilation is to remove stale or polluted air from indoor spaces and replace it with fresh air from outside. This reduces the concentration of indoor pollutants such as carbon dioxide, moisture and volatile organic compounds, which can have negative effects on human health and comfort. Our bodies need a constant supply of oxygen, which is only possible when there is proper ventilation.

If a building is left without ventilation, pollutant levels will inevitably build up, even if there are no occupants in a building to produce chemicals or humidity by cooking or running water. Moisture will find its way in and allow mould and damp to thrive, damaging the building.

So while the need for ventilation is obvious, what’s so special about MVHR? And exactly what is it?

What is MVHR?

Ventilation systems have existed for centuries. Opening windows and doors is one way to ventilate a space. If stale air is allowed to leave via one opening and fresh air enters through another, you’ve created a cross-flow ventilation system. 

 

However, this is still a rudimentary system, and one that isn’t practical when temperatures outside are too cold. Vents in roofs, foundations and other locations are an improvement, but still lack the power required to ventilate rooms quickly or reliably. Mechanical ventilation was invented to solve this problem.

 

Extractor fans are one form of mechanical ventilation. Whether it’s wall-mounted in a bathroom or accompanied by a hood in a kitchen, they generally need turning on and shutting off. 

 

Then there are positive pressure ventilation systems. They typically employ a fan unit positioned in a hallway or landing, for access to multiple rooms. As a result, they can ventilate more than one room in the house. But one flaw in this set-up is that if any window is opened, air will naturally leave the building that way, rather than ventilating other rooms. Also, this system brings cold air in from the loft space. So, unless an expensive heater is involved, this cold air can create problematic moisture in gaps and cracks, potentially affecting the building’s fabric.

 

The most complete ventilation solution is MVHR (meaning mechanical ventilation with heat recovery). This is a type of ventilation system that uses mechanical means to supply fresh air to a building while simultaneously extracting stale air – and humidity – from the building. The system works by using a heat exchanger to transfer heat from the outgoing air to the incoming air, which helps to reduce the energy required to heat or cool the building. MVHR systems are usually used in homes and other buildings that are highly airtight, and particularly those that require a greater level of air quality and energy efficiency.

 

 

How does MVHR work?

An MVHR system brings fresh air into habitable areas of a house or office, without letting the heat escape. Centralised MVHR systems ventilate a whole building, using a large heat recovery unit. While decentralised MVHR systems are also known as single-room MVHR, because they just ventilate one particular room. Although several decentralised systems can work together to ventilate a whole building.

Whether it’s centralised or decentralised, MVHR has numerous health benefits and keeps spaces free of mould and other harmful growths – all while providing a cost-effective, more sustainable ventilation method.

 

Here’s how a centralised MVHR system works:

 

1. Fresh air is drawn into the building from outside, through a filter. There are even some units with two filters on the inlet.

 

2. The stale air from inside the building is simultaneously extracted through another vent, primarily from ‘wet rooms’ like bathrooms and kitchens. This removes pollutants, dust and other impurities – as well as humidity in the air.

 

3. Both the incoming and outgoing air streams pass through a heat exchanger (except in summer). Heat from the outgoing air is transferred to the incoming air stream, via a filter. This way, the incoming air is preheated in winter, reducing the load on heating systems – and reducing bills.

 

4. The fresh air is then distributed to different rooms in the building through semi-rigid ducting.

 

5. The process repeats itself, constantly supplying fresh air to the building and removing stale air, while recovering heat from the outgoing air stream. By continuously supplying preheated air into living areas and extracting contaminated air from wet rooms, a whole-house ventilation system is created.

 

As carefully designed and intuitive as an MVHR system is, for the building occupants, it’s simple to operate and low maintenance. They don’t have to turn fans on and off or tinker with temperature controls: the system does all that automatically. A demand control function detects when moisture or extra carbon dioxide is being produced and boosts the level of ventilation.

MVHR works to maintain a healthy, energy-efficient working or living environment, often without the occupants ever having to think about it. It’s an exceptionally modern solution to an age-old problem.

 

 

When is MVHR recommended?

To keep any type of building in working order – and habitable – it’s vital to have proper ventilation in place. MVHR is just the most efficient, most modern way to ventilate a space. That’s why so many newly-constructed buildings feature it as standard. Plus, with new-builds being designed to stay airtight and as energy efficient as possible, intensive ventilation is even more necessary.

 

Making buildings airtight is the logical way to make them as energy efficient as possible, particularly in colder climates. In the UK, it’s unthinkable to build a new structure without proper insulation. Not only does proper insulation reduce heating costs, but it’s also much more environmentally friendly. Energy efficiency is now so highly valued that EPC ratings appear on every property listing.

 

But there are downsides to this ultra-airtight approach to construction. When we take gaps and draughts completely out of the equation, opportunities for natural ventilation are squeezed to almost zero. Occasionally opening windows and doors doesn’t provide adequate ventilation, but it does let colder air in when it’s cooler outside. Then comes the additional cost of reheating the home or workplace.

 

When the building in question is a large one, the cost savings from MVHR are obviously more significant. And when it’s a hospital or school, cost isn’t even the main consideration. Air quality is. 

 

The Covid-19 pandemic highlighted the issue of air quality in schools, hospitals and retirement homes. This is an issue where MVHR can make a profound difference. Indeed, if any building is occupied for long periods of time – as our homes are – the case for improving air quality becomes even more compelling.

 

A common problem that schools, hospitals and similar buildings have is age. It’s easier to install whole-house ventilation systems like MVHR when it’s been planned from the start. It can be awkward to fit all the ducting required into existing spaces, especially in older buildings designed long before modern ventilation systems were envisaged. New-build properties lend themselves better to MVHR, meaning that some people and businesses are deterred from installing MVHR in older buildings. However, it’s still worth evaluating the costs of retrofitting MVHR and similar systems against the savings in utility bills and, of course, the health benefits.

 

How is MVHR installed?

Since centralised MVHR is such a comprehensive, whole-house system – with hardware like ductwork running throughout a residential or commercial property – there are several steps to installation. Installation is much more complex than it is for a single-room solution like an extractor fan, for example.

 

The exact process will always vary from building to building, because every space is different. For example, retrofitting MVHR will sometimes require a completely different approach to installing MVHR in a new-build property.

 

The process will also depend upon the exact MVHR system you install – and how you intend to use it – but here are the typical steps for installation of a centralised system.

 

 

1. Site survey and assessment Conducting an in-depth examination of the site or the architectural drawings is crucial. It could be that MVHR is not even a suitable solution for the property. 

The size and layout of the building should be carefully analysed, as well as where the windows and doors are situated. Then you can determine the best locations for the MVHR unit and the ventilation ducts. It’s obviously vital that air extraction and air intake points are chosen carefully and the two types of vents aren’t in close proximity. Plus, they should be positioned away from doors, so they can ventilate the whole room before the air leaves it.
2. System design Only after a thorough site survey – or analysis of drawings – can the MVHR system design begin. There are several variables to consider, including the particular MVHR unit, the placement of the ventilation ducts, and the type of filters and controls needed.

3.Installation of the MVHR unit Now you’ve established the ideal type of MVHR unit, it’s time to fix it in place. A plant room, utility room or kitchen is often used, keeping the system out of sight, yet easily accessible for maintenance. Even though the room where it’s kept doesn’t need to be habitable, the temperature should still be above freezing, to keep the unit working properly. MVHR units need to be placed inside a building’s thermal envelope.
4. Installation of the ventilation ducts

In a centralised MVHR system, ventilation ducts are placed throughout the building, typically in the ceiling voids or walls. The ducts are sized and positioned to provide adequate ventilation to each room, while minimising heat loss.
5. Commissioning and testing Before you can hit the ON button on the new MVHR system, you need to commission and test it, to make certain it functions properly. Commissioning means balancing the system so that the extract flow rate is equal to the air inflow rate. Individual valves for each room will also be checked, to make sure enough stale air is leaving and adequate fresh air is coming in. And the controls and sensors must also be tested, so that airflow changes with conditions on a daily basis – or when demand requires it.

 

Can you retrofit MVHR?

Yes, you can retrofit an MVHR system, but it can be both costly and time-consuming. Much depends upon the size and age of the property, as well as the level of complexity involved. But whatever the property in question is like, it requires careful planning and real expertise to select the most appropriate systems, ducting and positioning of components.

 

MVHR ductwork simply doesn’t fit comfortably into some buildings. It might not be possible, for instance, to hide the ducting away in ceiling voids or the walls. In some properties, the cost of doing this could be prohibitive. Semi-rigid ducting, with its flexible coils, is one possible solution to this issue.

 

Airtightness is another issue in older buildings. Drafts and natural ventilation are usually common in period properties, in particular. If a building cannot be made sufficiently airtight, even the best MVHR system won’t function correctly. Heat will simply escape before the system can recover it and reuse it.

 

So they might not be right for each and every building, but MVHR systems are successfully retrofitted to older properties every day. Plus, when a carefully-designed retrofit is carried out, the energy savings between a draughty old property and one with modern ventilation are substantial.

 

That’s the key to successful retrofits: expert design.

 

Ductwork has to be routed so that it’s effective, well-concealed and not needlessly complicated. The MVHR unit selected must be the right one. Otherwise, it might end up doing too much work, and become noisy or ineffective as a result.

 

When considering ventilation solutions for an existing property, the initial site survey and assessment is absolutely vital. It can be the difference between going ahead with an unsuitable MVHR installation and achieving a ventilation solution that’s both cost-effective and dramatically improves air quality.

 

 

Why is airtightness important when it comes to MVHR?

The whole principle of MVHR relies upon a building being airtight. Otherwise, there’s no control over the flow of air and the system simply won’t work as it’s supposed to. 

 

If there are any passive ventilation devices, or even a draughty window or door, you’ll lose heat – instead of allowing the system to recover it. MVHR systems can only take heat from air that passes through them. They can’t take anything from air escaping through the fabric of a building or through vents.

 

Having MVHR without a sufficiently airtight building creates a situation where the system is drawing in cold air, without enough warm air to heat it up because it’ll be leaking out. Plus, gaps like vents will be letting cold air in at the same time. That can lead to problems with condensation and mould.

 

Airtightness is why MVHR and new-build properties are such a natural match: being airtight keeps heat in and unwanted cold air out. The more airtight a building is, the more effective the MVHR system will be – and the greater the efficiency savings it can make. 

 

It’s not just the airtightness of the building that’s central to MVHR though. It’s also vital that the system itself is absolutely airtight. Ducting must be completely free of any small gaps or weaknesses. If there’s any place where air can escape from, inefficiency will be the result.

 

Installing an MVHR system can dramatically change a working or living environment – and even the health of those inside it. However, the system needs to be chosen carefully. 

 

Opting for an unsuitable one can mean you’re stuck with a potentially costly system that is inefficient, unreliable or both. Then the property could experience poor quality or even mould or damp issues. The consequences of choosing the wrong MVHR system can be long term, for the building, its occupants and your finances.

 

So, how do you make sure you get the right MVHR system? There are a few factors to consider.

 

 

Property size and requirements

 

The larger a property – and the greater the number of occupants – the more powerful its ventilation system must be. It’s vital to determine the airflow for your property, figuring out how much air you need your MVHR system to supply and extract. At a minimum, Part F of UK building regulations requires a general ventilation rate of 0.3 litres per second per metre squared of floor area.

 

It’s also crucial to calculate the system pressure once the duct system has been designed – as this will inform the choice of MVHR unit.

 

 

Noise levels

 

Having a well-designed MVHR set-up, with an efficient, not-overstrained unit helps to keep noise levels down. Airtight ducting has the same effect. But even the most thoughtful set-up won’t help if the MVHR system is an inherently noisy one.

 

Some MVHR units need an attenuation box to keep noise down, which often comes as an extra expense. So it’s important to consider how much breakout noise is coming from a unit if you want to avoid that additional cost.

 

Plus, any attenuation box will go around the MVHR unit, requiring extra space. If that isn’t an option, you’ll need an MVHR unit with built-in attenuation. 

 

It’s particularly important to seek out a relatively quiet unit if you’ll be positioning it where it might disturb occupants. If, however, the unit is placed in a room away from people, noise levels won’t be quite so important.

 

 

Ease of maintenance

 

For an MVHR system to operate properly, it needs maintenance. Otherwise, air quality and performance will suffer, and the system itself might break down. Mould and bacterial growth can even result from poor maintenance.

 

So it’s critical that MVHR components can be easily accessed for regular cleaning and maintenance. Any user should be able to clean and replace filters relatively easily. Some units will help them with indicator lights signalling when maintenance is needed. 

 

Fans may require a professional to clean them, but they should also be removable. In addition, it’s important that the condensing water outlet can be checked, ensuring that it’s not clogged.

 

 

What ducting should you use for MVHR?

As with every component of an MVHR system, the ducting you use needs to consider the whole system and your ventilation requirements. The structure of the building, its layout, the space available for ducting, your energy efficiency aims and the airflow rate should all be taken into account.

 

In the past, some installers might have used flexible ducting since it’s cheap and easy to install. However, it’s also more vulnerable to damage and attracts more dirt. New regulations mean its use is no longer permitted.

 

Semi-rigid ducting, on the other hand, offers greater durability, while still being simple to fit. Some semi-rigid ducting even comes with an anti-static coating to stop dust collecting and/or antibacterial lining to prevent bacteria building up inside.

 

Plus, unlike rigid ducting, its semi-rigid counterpart allows for a radial ducting configuration. Fully rigid ducting uses a branch system, with everything coming off a central branch. But radial ducting means several pipes can come off the main unit, requiring less pressure to push the air through. With lower fan speeds come less noise and greater energy efficiency.

 

Of course, above all else, it’s vital to make sure that your ducting is suitable for your particular application – while meeting standards for air quality, insulation and fire safety. You’ll need a qualified HVAC professional to verify this.

 

 

How should MVHR ducting be installed?

 

Whenever it’s practical to do so, ducting should be installed within a building’s thermal envelope, alongside existing insulation. When ducting extends outside the envelope – into an unheated loft space, for example – it’ll need its own insulation. This prevents heat loss when air travels through unheated rooms – as well as condensation inside ducting when cold air travels through a warm room. 

 

Another consideration is noise. To counter the inevitable noise arising close to ventilation ducts, silencers can be fitted to ducts, usually downstream of the heat recovery unit.

 

Plus, it’s absolutely vital to make sure that the ducts chosen are the right size for your system and airflow. With a duct diameter that’s too small, excessive speed in the system will always create noise.