Tag: HRV

HPH360: Why MVHR maintenance and servicing is important – with Richard Hollas

Richard Hollas from 21 Degrees explains why regular MVHR maintenance and servicing is essential, what it entails, and we accompany him step-by-step as he services a unit.

Interview with Richard Hollas

Richard Hollas, a commissioning and service engineer for 21 Degrees (formerly Green Building Store), details the process of servicing an MVHR unit, using Ben’s system as a case study. 21 Degrees supply a range of products and services for Passivhaus and low energy building projects. The team can take you through the whole process for MVHR systems; including the design, supply, installation, commissioning and servicing, and emphasise the importance of each step in this process.

There is a difference between MVHR maintenance and servicing

Richard starts by outlining the difference between the maintenance practices we can do as homeowners and the need for servicing by an engineer. MVHR maintenance should include changing filters every 6-9 months, cleaning air vents and possibly cleaning the heat exchanger every 2 years. An MVHR system should be serviced by a qualified engineer every 3-4 years to prevent issues like increased noise levels, condensation, and reduced airflow. A service will inspect the sensors and fans, remove any contamination, check air flow rates and make any adjustments to ensure the system is working efficiently.

Changing the extract filter in the kitchen

It should be easy to access your MVHR unit

Ben’s unit is in his garage and Richard explains that this is the case for about 20% of MVHR units. The best location is in a utility room or plant room with space around it. This means it’s always simple to maintain or service the machine. Putting an MVHR unit in a loft is not ideal because it will be out of the way. You also don’t want them to be in a bedroom or other main living space as there will be some low-level noise from their operation.

A service is also an opportunity to double-check the installation

Richard explains that during the first full MVHR service he will check that the unit has been installed correctly. A common problem that arises is that the Armaflex insulation has not been installed. This is an extra layer of insulation that goes over the ComfoPipe, which is required by regulation, and so would need to be added.

MVHR service: step by step

Richard uses Ben’s MVHR system as a case study to take us through the service process:

1. The initial system checks

Richard begins the service by looking at the status of the system in the installer menu. He checks to see if the fans are going at different speeds, which would indicate a blockage somewhere in the system. The fans on Ben’s system are working well. He then puts the system in service mode, which stops the fans, moves the bypass flaps to the right position to allow him to open the unit.

2. Examining the inside of the unit

With the front off, Richard uses his torch to examine the inside for contamination and notices that the sensors are dusty, but otherwise everything looks good.

Richard points out the bypass flaps, which are activated by the external temperature. At a certain temperature, these flaps come down, so that the air from outside doesn’t hit the heat exchanger and come straight into the house. As these are modulating ones, they will partially open and so allow the air to mix together. There is also a preheater for frost protection, if the temperature goes down to zero this will come on to stop the unit frosting up inside. The main box is the heat exchanger itself.

Alongside this is the circuit board, that is controlling the fans, bypass flaps and other elements, so it all works together. Richard needs to access the board to detach the preheater for inspection. The preheater can become clogged with insects, creating a blockage which will then affect the air flow. Even though the unit is sealed, sometimes contamination will occur. When changing your filters make sure the unit is on filter change mode, which turns the fans off to ensure that debris is not pulled off the filters into the machine.

Contamination on the extract fan

3. Cleaning the supply fan and extract fan

Although the supply fan will get a clean it’s usually in a really good condition already, whereas the extract fan will almost always have contamination. It is important to remove the extract fan for cleaning so that debris doesn’t drop into the bottom of the unit. Bottle brushes are ideal for cleaning the fan as most of it is dry flaky material that can easily be removed.

4. Testing the condensate drain

A small amount of water is put into the condensate drain to ensure it is draining freely. Moisture is coming into the unit from our day to day living and needs to travel through it, hit one side of the unit and then drain away. Water damage on one side of the unit would indicate a blockage.

5. Cleaning the sensors

Richard explains that the sensors are monitoring conditions, such as temperature and moisture and that it is really important to keep them clean. If they build up with dust they may give false readings (or even trigger an error message) which will affect the efficiency of the unit. For example, if the sensor that monitors external temperature is giving the wrong information it will affect how the bypass flaps are working and impact the preheater for frost protection.

The heat exchanger is removed and cleaned

6. Cleaning the heat exchanger

The heat exchanger is a box that the air flows through, so providing it is kept free of contamination it will keep working. A regular service will prevent a build up within the heat exchanger and ensure that it is easy to remove for cleaning. Richard describes how a heat exchanger left for 5-10 years may become so contaminated that it is almost welded into place and won’t come out at all. If that happens a new unit will be needed.

7. Resetting the system

Once the fans and heat exchanger are clean they can be re-inserted, and everything can be plugged back into the circuit board. When the front cover is back on, the filters can be changed and the unit can then be switched back on. The first thing it will show is an error triangle as it carries out an analysis. Richard explains that it will check the bypass, the sensors and preheater to ensure it has been reconnected properly and that there are no faults. This will also happen any time the unit switched off and then on again.

Richard and Joe check an extract valve

8. Checking the air flow and vents

Richard checks all the vents for contamination around them and for excessive dust in the ducts. The air flows are also checked to ensure they are meeting building regulations. Richard is able to remove the vents and valves for cleaning without changing the original valve settings, as these have been set with locking nuts and are very difficult to change by accident. They should therefore be the same as when they were commissioned.

Bathroom extract fans often have the most contamination

The extract valves can get a build-up of contaminants in the kitchen from cooking and the bathroom from towels and skin but any contamination generally settles around the valves and doesn’t enter the ducting system. Richard has used a camera down ducting on several occasions to check for contamination and each time it just looks like brand new, as the filters are working to remove any bits before they reach the ducting.

Richard points out how the supply valves can have a ring of dust around them, but that it is not coming out of the vents. It is to do with the air pressure, as the air comes out of the vents and meets the air in the room the dust settles and this can easily be wiped off with a duster.

Richard uses an anemometer to check the air flows in each room to make sure the air flow is meeting building regulations. He can feel that the air flow is good and the readings show that Ben’s unit is achieving 50 cubic metres in the kitchen on speed 2, which is what it needs to achieve at speed 3. This shows that the unit is doing well and there is good ventilation.

Richard can also check his readings against the commissioned figures. If one valve is the same it is likely that they are all correct and with locking nuts in place there should never be a need to recommission them. If you didn’t have locking nuts in place and someone altered one valve, say from 5 mm to 10mm it would change the air flow and so affect all the other valves.

The extract valves are the most important ones when you’re measuring air flow during a service, because things could build up and potentially cause a blockage further down. This is unlikely to happen with the supply vents, because air comes through the filters first. If Richard doesn’t have the commissioning figures, then he will be looking to hit building regulations and make sure the system balances out.

If you’re one of our Hub members, Chris Herring explains how to get the best value from high performance windows

MVHR can automatically deal with excessive humidity

Richard is sure that Ben will not have any problems with condensation as his Zender Q unit has a humidity protection setting, which will have been put on during commissioning. This means that if humidity reaches 75% in the house the fans will gradually speed up to remove the excess moisture automatically.

A service is an opportunity to recap information with the homeowner

Richard tells us how it can be hard for customers to take in all the information about their MVHR system at the start. They may just have had a heat pump fitted, under floor heating and a whole range of new products. He suggests having an MVHR service after the first 12 months, so everything can be checked and he can go through all the workings when it is less stressful. This develops a relationship and also gives homeowners the chance to ask questions.

Without regular servicing the unit may start to develop problems

When the service schedule is neglected it is likely that contaminants will build up. This could cause a conflict in the machine. For example, the fans might speed up to clear the air but this would then activate the noise guard, which is there to protect the house from noise. This would result in your fans slowing down again, and you may start to notice condensation. The unit is then not running at optimum efficiency and not performing well. It is much better to establish a regular maintenance and service programme to ensure your MVHR system works well for as long as possible.

Find out more

Visit the website of 21 Degrees

What makes a well designed MVHR system?

How do you specify high performance windows?

Two radical retrofits that use an internal wall insulation strategy

Low energy barn conversions

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September 11, 2024
HPH299 : Is mechanical ventilation just something else to go wrong? – with Ben Adam-Smith
Ben Adam-Smith shares his own experience of living with a mechanical ventilation system. While he’s clocked up a few error messages on the unit and had more problems than most, he explains why he wouldn’t want to go back to natural ventilation.

MVHR is a key component of low energy buildings

In case you’re wondering what MVHR is, well, it stands for mechanical ventilation with heat recovery.

As buildings become increasingly airtight, a good ventilation strategy becomes all the more important.

But there’s a clever twist: by controlling the ventilation you can recover the heat when you want to (transferring it to the fresh incoming air).

Hopefully that’s clear, but if not we made a documentary about airtightness a few years ago!

Since this documentary was made I have moved into my own Passivhaus.

Error messages are warnings

I am a big fan of MVHR (pun intended) but that doesn’t mean I’ve had the best relationship with my MVHR unit.

I’ve had 3 sets of errors on the machine in the two and a half years I’ve had it.

The first errors might have been in connection with a silencer that was retrofitted a year after we moved in. I suspect the machine was clever enough to notice something had changed. The errors resolved themselves!

About a year later, an external pressure message cropped up a number of times.

The fan status readings on fan speed two which should be 200m3/hour are:
Supply fan – Speed 550rpm, Duty 15%, Flow 93% ( a noticeable drop in air flow)
Extract fan – Speed 1900rpm, Duty 48%, Flow 0% ( air flow can be detected from the extract air valves when a piece of tissue is held up to them )

Error messages can be reset

Interestingly, these errors can be reset while you’re investigating the cause… but they usually return, even if that is weeks later.

Apparently there was a known issue with the humidity sensors on my particular unit. This was changed and again it was business as usual.

However, at the beginning of the year a new error cropped up!

Nobody likes seeing a flashing red light on their control panel because it means something needs fixing (or at least investigating).

At this particular point we’d been getting night-time temperatures between -2C and -5C, which is the coldest we’ve experienced since we built the house.

When the temperature warmed up a little I was able to reset the error and everything would return to normal (or until I could get the machine looked at).

However, that trick didn’t work the next day.

The machine completely died and I couldn’t get it to come back on. There was also a light burning smell inside the house.

I had a couple of concerns at this point:
- How long will it be until I get my machine back up and running?
- Might I get a build up of mould somewhere in my system if I can’t get it fixed soon?
Fortunately when it comes to mould, I was quickly reassured by my friends in the know.

So, now whilst the problem was investigated we had to survive without the machine.

With no MVHR running, we resorted to opening windows!

Not that I’d choose to do this experiment but it was quite interesting to see the impact of living in a Passivhaus with no MVHR running.

Firstly, the house remained nice and warm inside (the insulation has gone nowhere). Perhaps the heat wasn’t as even; I don’t know. But it wasn’t a terrible experience.

It was however, a right faff to have to open the windows every few hours… not to mention the heat we must have wasted… and the family complaining at having the windows open when it was freezing outside.

Perhaps the most interesting impact of no MVHR was the instant lingering smells. My wife noticed that each room had its own smell! And it was definitely harder to shift cooking smells.

Eliminate the simple stuff first

On a chat with Steve Clarke at Green Building Store trying to diagnose the problem, we double-checked that power was getting to the unit.

Embarrassingly I hadn’t even checked the trip switches. In fact, we’d managed to cover them up with a shelving unit so that I’d forgotten they were even there!

With the machine humming to life, I had a massive sense of relief. Whatever the issue is it’s not quite so urgent when the system is at least on.

The preheater needs to be replaced

It didn’t take long to diagnose the problem (the burning smell was the preheater working for the last time) but British wisdom of leaving the EU meant the new preheater took several weeks to get here.

The warranty period on MVHR units is typically 2 years

For an essential item for Passivhaus dwellers, this does seem short.

The clock also starts ticking when you buy it.

Mike Coe, the self-builder behind Portree Passivhaus, bought his unit in advance and subsequent delays have meant it will already be out of warranty.

My own unit was out of warranty, too, but the manufacturers kindly supplied the new part free of charge.

I installed my new preheater!

With Steve Clarke from Green Building Store on the phone, I was expertly talked through what I needed to do.

Buying through a company with good customer service was a great move.

We’ll have to wait for more cold weather to check it has done the job.

‘MVHR needs to be as reliable as a fridge’

I was interested to see whether other people had had trouble with their units and so I posted up on social media.

I don’t believe many Passivhaus owners have actually had problems, which is a good sign. As Fionn Stevenson says ‘good quality assurance is essential’.

Andy Simmonds, Chief Executive of the AECB, stressed it is such an important piece of kit. The emphasis needs to be on durability, reliability and repairability.

He likened an MVHR unit to a fridge.

Having bought a new fridge when I built the house I have noticed that even fridges have got more complicated.

In the past it’s normally the plastic door seal that has failed before anything else, but on the new more complex models water dispensers get jammed on and ice trays freezer up, and don’t get me started on the plastic water filters!

I would not go back to ‘natural ventilation’

Despite the inconvenience of investigating different error messages, my MVHR unit has never stopped! Yes, I was a bit slow to check the trip switches, but if there’s a next time I won’t make the same mistake.

If the machine is still operating, you haven’t got a big problem

If your machine is not working at all – i.e no ventilation whatsoever – then I would really be looking for a speedy repair.

As with anything in life that breaks, it normally takes a little while to fix it.

So it would be lovely to have the option of a 24-hour repair service in the years to come.

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March 24, 2021
HPH096 : Is Natural Ventilation Fit for Purpose? – with Kara Rosemeier from Passive House Academy New Zealand
Kara Rosemeier from Passive House Academy New Zealand looks at why natural ventilation leads to little control of ventilation. She also shares why the Passivhaus approach is the logical step forwards.

Interview with Kara Rosemeier

After starting out in physiotherapy, Kara Rosemeier has spent most of her working life doing building physics. Currently she is the Director of the Passive House Academy New Zealand (no affiliation with the Irish Passive House Academy), a board member of the Passive House Institute New Zealand, a building physics consultant and a part-time lecturer at a university.

The Term ‘Natural’ is Not Helpful When Describing Materials

Kara believes that people’s desire to build with natural materials is misguided because there are problems defining the word natural.

Kara admits that she may be particularly sensitive to the connotations of the term natural being non-heterosexual and fending off abuse such as ‘it’s not natural’.

She says: “I question the division between natural with this connotation of healthy, good, desirable and unnatural meaning unhealthy, bad and unwanted.”

Relating this to buildings there are many contradictions. Are buildings natural at all? Are people – and the things we do – natural? Kara concludes everything we do is natural.

Natural Materials Can Emit Pollutants Too

Kara recalls one house she monitored where the walls and floors were built from rammed earth. The homeowners had had a strong desire to build with natural materials yet the radon concentrations in this house were three times the maximum recommended by the WHO. Although Kara can’t say with certainty that the rammed earth was the culprit it is quite likely as radon is emitted from decaying materials and soil.

Kara continues: “Wood, even completely without glues, is emitting formaldehyde. Not many people are aware of that. And timber dust is a potent carcinogen, so it’s not like nature tells you anything about whether a material is natural or whether a material is healthy.”

Therefore our efforts should be on materials and construction that does not cause, or at least minimises, harm to people and fellow creatures. For this, information on production processes, ingredients and performance indicators is essential. The label natural does not substitute all this.

Natural Ventilation Requires Constant Human Intervention

Every hour we need about 30m³ of fresh air per person.

If the building envelope is so leaky that no human intervention is necessary on a calm day it follows that the home will be horribly draughty on a windy day!

Kara makes the analogy of this being like punching holes in your water pipes to occasionally have a shower. In other words, you wouldn’t do it.

To provide adequate ventilation windows and doors must be opened at regular intervals.

Again, the term natural falls down. Kara points out: “If we go with the – well in my view more or less meaningless – distinction that natural is free from human intervention, then natural ventilation is as unnatural as it gets. Free running is a much better term if you translate this to not being mechanically pressurised.”

The Stack Effect is Not a Reliable Driver for Air Exchange

Kara says that with a low rise building in a temperate climate, the stack effect is not a reliable driver for air exchange.

It requires either a large temperature or height differential. Without mechanical help the wind is the main driver for air exchange, but wind is not steady.

For the pressure that’s necessary to affect air exchange it also matters where on the façade the wind impacts.

Kara says: “There’s at least a windward and leeward side of a building and you’ll have positive and negative pressures on parts of the façade as a result. But you don’t know where is what at any given time so whether opening a window will affect how many cubic metres of air exchange. Without constant pressure you don’t know what’s happening really.”

Indoor Environments are Full of Pollutants

It is not just the materials used in the construction of a house that contribute to the indoor environment. Anything that’s in your house – from furniture, decorations, utensils, cleaning products, etc. – will alter it.

Kara recalls being given a reusable textile shopping bag at her local supermarket. Although it looked nice (and seemed natural!), she soon became aware of the terrible smell and had to leave it to air on her balcony for a month.

Without monitoring equipment, it’s impossible for the homeowner to gauge indoor air quality. There could be particles such as nitrous dioxide or carbon monoxide in the air and nothing perceptible to the senses.

Dilution is the Best Way to Control Indoor Air Quality

While there are various approaches that would improve indoor air quality, such as extracting pollutants where they are emitted before they can spread through the house, ultimately dilution is the best solution.

Kara says: “When you bring in about 30m³ of fresh air per person every hour then you can be reasonably certain that your contaminants are diluted to a level where they are not harmful.”

A Tale of Two Living Rooms

Monitoring of CO₂ levels in 2 new 3-bedroom houses in Auckland over a week in winter. Both houses were inhabited by 4 people and a pet. Only the house with a balanced ventilation never exceeds the 1,000 ppm threshold, and shows a fairly consistent indoor air quality. This is in stark contrast to the free-running house, which relies on natural forces and someone opening the windows. CO₂ readings here are strongly fluctuating, and far exceeding the recommended threshold at peaks.

You Can Filter Incoming Air with a Pressurised Ventilation System

One key benefit of building an airtight house and using a pressurised ventilation system is that the outdoor air can be filtered before it enters the house. So it won’t matter whether you live next to a busy road or in the middle of the countryside, you will still have fresh air.

Passivhauses typically use an F7 filter, which is very fine and filters out almost everything that could be harmful.

CO₂ Concentrations Often Indicate the Presence of Other Pollutants

The CO₂ levels in a building are relatively easy to monitor. This is helpful for another reason as well because common indoor air contaminants correlate with these CO₂ concentrations.

*In some of Architype’s Passivhaus schools, the teachers report that their pupils are much more attentive in the afternoons compared to when they were in their old traditionally-built schools.

Growing Plants Inside Doesn’t Necessarily Improve Air Quality

Ben was interested whether growing plants inside houses does anything to the indoor air quality.

While Kara says some research concludes that it helps with indoor air contaminants like volatile organic compounds or formaldehyde it will not help with CO₂ levels (because plants will be emitting CO₂overnight).

Using Natural Ventilation Will Not Lead to a Comfortable House

Kara concludes that natural ventilation is not fit for purpose if your purpose is a comfortable and healthy indoor environment.

It also makes little sense in terms of energy when heat or coolth is lost unnecessarily to the outside.

Find Out More

Passive House Academy New Zealand

Follow the Passive House Academy New Zealand on Twitter

Transcript

Download a transcript of the interview with Kara Rosemeier.

Film: Walter’s Way, London’s Self-Built Street

If you have a spare 10 minutes this is well worth a watch. It’s a documentary short on architect Walter Segal, reflecting on his life and how he left his mark on self build in the UK.

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August 26, 2015
HPH087 : Assuring Quality When Building a House – with Andrew Peel from Peel Passive House Consulting
Andrew Peel from Peel Passive House Consulting explains why the quality assurance procedure is at the heart of the Passivhaus standard. He also feels the process can be useful to improve any building, even if not striving for the standard.

Interview with Andrew Peel

Andrew Peel is a low energy building consultant based in Toronto, Canada. It was when he was doing a masters thesis on renewable energy that he decided to veer more towards energy efficiency: “I felt that it made more sense, was easier, more effective to reduce our energy consumption than worry as much about how do we change the way that we produce energy.”

After accepting a thesis position at The Passive House Institute in Darmstadt, Germany, he then worked at Building Research Establishment (BRE) in England, before returning to Canada. He’s now involved in Passivhaus certification, consultancy and training.

Which Passivhaus Standard Do North Americans Use?

Just to confuse anyone looking to reach the Passivhaus standard (Passive House standard) in North America there are two options!

One is the 19-year-old international standard, which is supported by groups such as New York Passive House (NYPH) and the Canadian Passive House Institute (CanPHI).

The other has been recently introduced by Passive House Institute US (PHIUS), a breakaway group that felt that lower energy prices (amongst other things) in North America made the international standard less viable.

Andrew feels the feud has been detrimental overall because the vast majority of practitioners just want to build decent, high performance buildings.

Quality Assurance is About Getting the Building That Was Designed!

With most construction there is a performance gap between what the designs say will get built and what actually gets built.

So the quality assurance procedure (at the heart of the Passivhaus standard) is really a guarantee that what gets built is what was intended. Thus the new building will perform as anticipated.

Quality Assurance is a Rigorous Process

When construction professionals go through the Passivhaus quality assurance process for the first time it can come as a shock!

It is about validating all their calculations, assumptions and any of the data they collect on equipment.

Andrew gives an example of the performance of windows and how they are rated. In North America this is done through the National Fenestration Rating Council and so the tests and simulations carried out are different to those in Europe. That means that while the basic model can be modified to meet the European standards ultimately how they represent those results are simplified. So they can’t just be used directly in Passivhaus assessments. A lot of work is required to ensure that the figures that are being used in the Passive House Planning Package (PHPP) are appropriate.

Two Tests Give a Good Indication of the Quality of Construction

1. An air pressure test, better known as a blower door test, checks the airtightness of the building and reveals how leaky it is.

2. Ventilation commissioning requirements ensure that the system is operating as designed, and that the required amount of air is being supplied or extracted from the various rooms.

These two tests are good indicators of the quality of the construction and the installation of the systems.

Just through the documentation submitted and the airtightness result it will be clear whether a lot of care has been taken in the construction of the building.

Andrew Recommends a Two-Stage Review to Achieve Passivhaus Certification

An initial design stage review aims to resolve any issues that might jeopardise certification.

Once the building is complete the remainder of the documentation would be submitted and reviewed along with any changes.

This would include full set of plans (floor plans, elevations, section drawings), the window schedule, the ventilation design, the airtightness strategy, window performance figures, performance documentation from other equipment etc.

Too Much Construction is Sloppy

Andrew reflects on how the Toronto skyline has changed over the last 15 years – skyscrapers now dominate.

Unfortunately many of these buildings perform poorly with high energy demands, comfort issues in both the summer and winter, over-sized heating and cooling systems and so on.

However, the world’s first Passivhaus certified high-rise building was unveiled in Germany in 2013. This shows that things could be different.

The RHW.2 office tower in Vienna is Passivhaus standard (copyright MVAHA)

The Quality Assurance Process Can Help Improve Any Building

Even if you don’t want to go to the full extent of Passivhaus, using the quality assurance procedure is likely to result in a far better building.

Andrew gives an example of one of his clients in ‘cottage country’ who were about to build to code but asked for some help. Through simple measures Andrew was able cut their energy demand in half, and it wasn’t even going to be much extra work. The couple were amazed!

Most Homeowners Do Not Realise What is Possible

The majority of people are still unfamiliar with the Passivhaus standard and the benefits that building to it can afford.

As part of the Canadian Passive House Institute, Andrew says it’s their mandate to educate the public, professionals, builders etc. that there is a better way.

Find Out More

Peel Passive House Consulting

Canadian Passive House Institute

Transcript

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Watch Regen Media’s Latest Video

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April 6, 2015
HPH059 : What Qualities Should an MVHR System Bring to a Low Energy Building? – with Andrew Farr from Green Building Store
Andrew Farr from Green Building Store explains what qualities mechanical ventilation with heat recovery (MVHR) should bring to a house. He also shares how to make sure you get a system that is properly designed, installed and commissioned.

Interview with Andrew Farr

Andrew Farr’s background is a mixture of mechanical engineering and building work, having done multi-material fabrication for a number of years. When Chris Herring, the founder of the Green Building Store, was developing their Mechanical Ventilation Heat Recovery Department he invited Andrew to join them as they had previously worked together on various building projects and as Andrew is a fluent German speaker.

HRV and MVHR are the Same Thing

HRV (Heat Recovery Ventilation) and MVHR (Mechanical Ventilation with Heat Recovery) refer to one and the same thing. Andrew clarifies that there are various other acronyms out there, but the broad idea is that you are ventilating and recovering the heat energy out of the ventilation air.

In a Balanced Ventilation System the Same Quantity of Air is Supplied as is Extracted

An MVHR system can be a pragmatic way of delivering very good air quality into a building. The Air Handling Unit (AHU) or the Mechanical Heat Recovery device of the MVHR system draws air into the building from the outside, filters it, takes it through a heat exchanger and supplies the clean air into the habitable spaces of the house, usually bedrooms, living rooms, studies etc. A completely separate ducting system extracts air out of the foul air rooms (toilets, bathrooms, kitchens etc.) through the other side of the same heat exchanger and exhausts it out of the building. This enables a balanced ventilation system where you are supplying exactly the same quantity of air into the building as you are extracting out of it.

MVHR systems may not be essential for every project, however, they are recommended for very airtight buildings with an airtightness of 3ach @50pascals or lower. For an MVHR system to be energy and cost effective, the airtightness of the house should be below 1ach.

Getting the Design of the MVHR System Right – Sound Thresholds and Air Flow Rates

The criteria set out for MVHR design by Building Regulations and Code for Sustainable Homes in the UK are primarily intended to create an energy efficient system. This has led to a common occurrence of noisy MVHR systems in the mainstream building industry owing to their poor design.

Andrew argues that in the case of a house, the primary criteria should be comfort and good air quality. A well designed MVHR System does not impact on the consciousness of the people who are living in that building. It should not be the first thing you notice when you walk in, nor the last thing that you notice when you’re going to sleep.

To achieve this, the main criteria of the design should be to deliver an inaudible – in everyday activity – ventilation system which generates less than 24 dBA of sound at a distance of more than one meter away from the air valve (The human threshold of hearing is somewhere around 24 to 27 dBA). With the amount of noise that the ventilation unit is going to create, the location of the MVHR unit itself is also crucial. Positioning the MVHR unit close to the kitchens and bathrooms, where people are used to hearing noise, is more suitable than locating it close to a bedroom.

Another important criteria is to deliver exactly the right amount of air into the building. Over ventilating will tend to dehumidify the air in winter and also use excess energy. While under ventilating will result in unpleasant odours and humidity remaining in the building.

The Traits of a Good MVHR Unit

Green Building Store has the franchise for selling PAUL Ventilation units (PAUL is a German company) which they believe are some of the very best on the market as they are very well manufactured. The ventilation unit itself emits a relatively low frequency which doesn’t travel as well through walls, as compared with other units. The fan has a constant volume function to continually deliver exactly the same amount of air through both sides of the heat exchanger – as set up at the time of commissioning. This is extremely important, as during the yearly cycle of a ventilation unit there are various things that happen which change the resistance to the air moving through both the intake and the exhaust ducting systems.

One of the major influences on the performance of the MVHR system are the filters on the ventilation unit. A filter on the intake prior to the heat exchanger protects the heat exchanger from the very fine debris that can be drawn into the air stream. There is also a filter on the extract prior to the heat exchanger. These filters will get dirty and provide resistance to the air-flow at different rates depending on what the pollution load is. In the case of a house being situated close to a main road, carbon particulates will block up the very fine filters resulting in the intake filter becoming dirtier sooner than the extract filter. To maintain a balance between both air-flows, the fans will increase their effort against the filter resistance in order to maintain the same volumes of air. Thus having a fan with a constant volume function is very crucial to maintaining the heat recovery efficiency of the ventilation unit.

Maintaining the Efficiency of the MVHR System

Modern ventilation units are very similar in their tested performance (the one against which they will be certified). However, there is a very real difference between the unit’s individual tested performance and that of its performance embedded in the building. The efficiency of the MVHR system depends on two important factors:
1. maintaining the balance between the two air flows, and
2. the design of the ducting associated with the MVHR system
If the resistance to the air moving through the ducting system is high, then the fans will have to exert a lot of effort in order to move the air. Consequently they will make more noise and use more energy, which in turn will cost more. The energy efficiency of the MVHR system is the efficiency of the whole system, including the ducting and everything associated with it, and not just on the efficiency of the MVHR unit alone.

Round and Rigid Ducting Works Best

The most important criteria when considering what type of ducting to use is which type of ducting will allow the air to move through most easily and offer the least resistance. Round ducting is the most efficient in this case, as it offers the best surface area to volume ratio – the surface area is smaller than the volume of air. Rigid ducting is sealed by design, not by the addition of mastics and tapes as in the case of flexible ducting and is less likely to fail.

Invest in a Well-designed MVHR System from the Get Go

The design of the ducting system should be of paramount importance in order to deliver a ducting system that you will want to live with. Ducting systems should be designed to reduce pressure loss and to attenuate the associated sound. If it is not designed properly and especially in larger houses, some air valves could end up being very noisy because they’ve got too much pressure on them, and in other rooms the air valves are not getting sufficient air out of them. A well designed system with rigid ducting and appropriate sound attenuation however will come at a premium cost. Rather than going for a cheaper poorly designed system, Andrew’s advice is to invest in a well-designed system from the outset that won’t have to be replaced after a year.

Quality of MVHR Design Currently in the UK

Up until now the industry has seen experienced commercial mechanical and electrical installers bringing their knowledge from commercial spaces into domestic MVHR installations, where the requirements for sound are not the same.

In an office, you probably won’t notice a bit of fan noise in the room because you’re busy, there are computers on or you are talking to people. But when you’re at home and you’re working on your desktop PC, the running fans and hard drive generate such a constant background noise, that when you turn it off, you realise the blissful peace that follows.

According to Andrew, an MVHR system should be giving you this blissful peace all the time despite the fact that it’s running, and if it’s not, then it’s not designed to quality. Andrew believes that in the next few years the quality of delivery of MVHR systems in the UK will improve and that more people are now beginning to understand what the appropriate criteria for MVHR design in domestic buildings is.

Find Out More

Green Building Store

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Transcript

Download a transcript of the interview with Andrew Farr.

Passivhaus in Plain English

We are very excited about the fact that Elrond Burrell has launched his own blog! And we expect big things from it. Titled ‘Passivhaus in Plain English & More‘ it has already been noticed by Lloyd Alter from Treehugger.

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June 25, 2014