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Hello all!


We're considering installing dMVHR units in our Victorian ground floor flat in London. We do not have a roof/loft or space in the ceiling to do a centralized MVHR system.


The flat is currently very well ventilated with high ceilings and original vents in all the rooms, and we have no damp or condensation problems. We are lucky to have a decent size garden at the back of the flat, overlooking greenery and trees. There is a train that passes behind the garden about 25 meters away.


Our living room, kitchen, and bathroom face the garden. However, both our bedrooms face a busy main road (lots of cars and buses) which is 6 meters away.


My wife suffers from respiratory issues and is allergic to pollen, dust and mold.


All the literature we've come across so far mentions that MVHR systems 'extract stale air from inside the home while pulling fresh air in from the outside'. Given that the air outside our home is pretty polluted we have the following question:

If we install dMVHR units in our flat (initially in the bedrooms, and two years later, in the living room), while this may filter out pollen, will it bring in more polluted air, including vehicle emissions or harmful gases from the busy road inside the home?


And, if the system is running 24/7, will it bring more of the outside air at a faster rate than the flow that comes through the current vents?


In other words, are we going to save on heating bills but increase the pollution in our home?


Many thanks!

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All the air that's in your flat has come from the outside, so bringing in more will just be the same level of pollution.

You say it is well ventilated already so whats the point, unless you can filter the incoming?

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3 hours ago, chromaticmind said:

original vents in all the rooms

What does that mean in reality?


Have you looked at cascade MVHR, such as blu Martin or Fresh-R? Or a home brew version?

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5 hours ago, chromaticmind said:

If we install dMVHR units in our flat (initially in the bedrooms, and two years later, in the living room), while this may filter out pollen, will it bring in more polluted air, including vehicle emissions or harmful gases from the busy road inside the home?

It's the filters that do the work of reducing pollution in the home. If you want to reduce the size of the pollutants being filtered out, use a finer filter.





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PIV with some good filters would work. 


You can make your own pretty cheaply. 






You'll need a few bits of ducting too.


Suck in front the garden and allow the air to be expelled via the bedroom vents. 


No heat recovery but at least you'll have filtered fresh air. 


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22 hours ago, chromaticmind said:

flat is currently very well ventilated with high ceilings and original vents in all the rooms

As others have said, what does that actually mean?


In the 'olden days' buildings relied on a fire (usually open) to draw air up a chimney, that air was replaced by uncontrolled leaks in the building fabric.

Since we got rid of open fires (though there is still a lot of people that think they are the greatest thing invented) and fitted heating systems, we have relied on those original, uncontrolled fabric leaks to ventilate our properties.

As building standards and techniques have improved, generally our airtightness has got better.  This has caused a problem with internal humidity becoming higher and causing mould growth and general air stuffiness.

To get around this more ventilation, but more controllable, has been introduced.  This is usually though window trickle vents and air bricks.

While this change was happening, there was little regard to energy usage, so we fitted larger heating systems.

This seemed to work in the majority of homes, but, and this is an important but, if it is not used correctly i.e. heating on for only a few hours a day, a yoyo affect happens with the temperature rising a falling, which changes the relative humidity.  This in turn changes the dew point temperature and the water vapour that is naturally in the air (from people, cooking, washing, drying clothes and pot plants), this water vapour can now condense to liquid water on cold surfaces and cause an environment that is ideal for mould growth.

So just changing the air may not cure all the problems.

To get around the problem, many people fit mechanical ventilation in bathrooms and kitchens.  These units generally are oversized and shift a lot of air.  1 litre per second of air is converts to 4.5 kg of air per hour.  Cheap fans can easily shift 30 l/s.


Now the trouble starts when a fan is fitted with no regard to the original leakiness of the building.  Because replacement air has to enter the building somewhere, and will take the path of least resistance, it is possible to create cold spots around these leaks, this is where mould will starts.  This is not really a problem if they are visible and they can be dealt with on a regular basis, but if they are hidden, they become a problem as the air movement that is naturally there will distribute the mould spores.  It is those spores that are the problem.

This is why 'positive pressure' systems are often fitted, they are designed to blow outside air (I will not call it clean air) into a building in a controlled fashion.

That new air needs to find a path out the building, and it does it via the holes in the building fabric.  Often this is not a problem, but occasionally it can exit into an area what can cause problems and then condense, creating a mould problem that can start to rot the fabric of the building.

To get around this, modern houses are designed to be airtight, and then only known exit points all the inside air to leave (you can read up on vapour control layers and wind tight membranes, it is a huge subject and very misunderstood).


That is a bit of background (and unedited for clarity).

So just pumping in air, or extracting air, may not solve the problems on its own, and can cause energy usage to increase.

To reduce energy usage, mechanical extraction is often fitted with heat recovery (heat is the old word for energy).  These help keep bills lower as they can recover approximately 80% of the energy that would be lost in the expelled air and reintroduce it into the replacement air (again, I hesitate to say clean air).

The trouble is that it does nothing to change the existing leakiness of the building, so those thermal losses have to be added to whatever losses the MVHR (mechanical ventilation and heat recovery) losses are.  If that is not taken into account when designing a system then performance can seem disappointing.


If you want to deal with this problem, then the first thing you need to do is get an air tightness test done and fix all the leaks you can.  This can be expensive, but does give you a known base to work from, and solves all those small places that mould can grow.

Then you can fit just about any mechanical ventilation system and start to see benefits.

As you are in London, which is a big place with varied air quality, the type of filters used becomes important.

It is relatively easy to filter out pollens, but fungal spores are a lot smaller, so harder to filter out, gases like Nox are smaller still,  PM10 to PM1 are similar sizes sores, so virtually impossible to mechanically filter.

You have to stop fungus growing and being distributed indoors.


All seems a bit doom and gloomy, and expensive, but you can make improvements.


(I suffer really badly from allergies and they virtually disappeared, apart from seasonal hayfever and to animals, when I moved to 2 miles from the Cornish coast from Hertfordshire)


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