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Ventilation Systems

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1. An Introduction to Whole House Ventilation
2. PIV - Positive Input Ventilation
3. MEV - Mechanical Extract Ventilation
4. MVHR - Mechanical Extract Ventilation Heat Recovery
5. Installing a Centralised Ventilation System


1. An Introduction to Whole House Ventilation

As the name suggests, Whole House Ventilation is the air conditioning of an entire property or dwelling rather than traditional air treatment of a single room as is commonly undertaken by an extractor fan.

Whole House Centralised Ventilation

This concept has been used in commercial buildings such as hotels, office buildings and gyms for nearly fifty years because of course, installing individual extractor fans in each room would be impractical, prohibitively expensive and would not work anyway due to the necessarily long duct runs. 

In the last decade, however, driven by the need to conserve energy and build greener, more efficient homes, this technology has been developed for domestic dwellings. Part L of the 2010 Building Regulations, which dictated that carbon emission levels should be reduced by a further 25% compared to 2006 levels, and the introduction of the DER (Dwelling Emission Rate) which measures the total CO2 emission of a house or flat, compelled manufacturers to design more energy efficient ventilation methods. 

At the same time, as construction techniques and materials improved, buildings have become more air tight. Whilst this has obvious benefits in relation to preventing draughts and heat loss it has had the unwanted effect of inhibiting the in-flow of fresh air – leading to the build-up of moisture and pollutants.

In answer to this, Part F ‘Ventilation’ of the 2010 Building Regulations encourages the air treatment of the whole dwelling such that air quality is maintained at all times throughout the building: 4.2 of Part F states:
“The key aim of the requirement of Part F1(1) is that a ventilation system is provided that, under normal conditions is capable of limiting the accumulation of moisture, which could lead to mould growth, and pollutants originating within a building which would otherwise become a hazard to the health of the people in the building.”

Centralised Ventilation v Decentralised Ventilation
The theory behind Whole House Ventilation is fairly simple: A single large extraction unit is situated in a convenient central location – maybe the loft, garage or cupboard. Ducting connects this unit to extract points, (usually terminated by a wall, ceiling or floor grille) in multiple rooms. Air is drawn from these rooms, transported back into the unit and expelled through one main exhaust pipe to the outside world.

The extract points usually include wet rooms, bathrooms, toilets and the kitchen. However, each installation is different so extract points can be situated in any room where stale air is a potential problem such as utility rooms, gyms, workshops, even bedrooms etc.

2. PIV – Positive Input Ventilation

PIV works in the opposite way to normal extraction. Rather than sucking stale air out of a building, PIV takes fresh air from the outside and sends it into the building thus forcing stale air out through existing outlets - such as air bricks, keyholes, open windows and gaps around doors etc. 

PIV works on the principle that, no matter how well a house is built, no building is fully airtight. However as building techniques have improved it has become necessary to measure the air permeability of new buildings to ensure that it does leak enough air for PIV to be used.

This is not a common problem, however since PIV is most often specified in older buildings due to the fact that it requires less ducting and vents and is thus far more easily retrofitted. New builds tend to employ either MEV or MVHR..which you will meet in our next two sections:

3. MEV – Mechanical Extract Ventilation

The system described above is known as MEV – Mechanical Extract Ventilation, sometimes referred to as cMEV – the ‘c’ standing for ‘centralised’ so as to distinguish it from dMEV in which the ‘d’ signifies ‘decentralised’ and refers to single room units.

Continuous Ventilation v Intermittent Extraction
Apart from being centrally located, the other distinguishing feature of an MEV unit is that it operates continuously. This is quite different from a standard extractor fan seen in many bathrooms which operates only when someone uses the bathroom and is thus classified as ‘intermittent extraction’. 

Continuous ventilation typically occurs at lower air flow rates of between 50 and 60m3/hr than intermittent extraction which is usually between 80 and 90m3/hr. This continuous air flow rate is sometimes referred to as the ‘Trickle’ rate. Most MEV units feature a higher air flow speed known as the ‘Boost’ rate to deal with increased moisture or pollutants for example during shower time. Commonly – though it does depend on the specification and functionality of the MEV unit – the switch to Boost is handled automatically by use of integral humidistat. No human control or intervention is required.

4. MVHR - Mechanical Extract Ventilation Heat Recovery

So MEV is a convenient and cost effective way of extracting stale air and moisture from multiple rooms and entire dwellings. The consequence of removing air is that it must be replaced and since no normal dwelling is completely sealed, air from the outside flows in. 

All well and good but of course this results in fairly serious heat loss – Not a problem if you live in Barbados but in temperate climates as ‘enjoyed’ in the UK this can have a significant impact on heating bills and carbon emissions.

In order to mitigate this unwanted heat loss a system was developed that returns heat to the building. MVHR stands for Mechanical Ventilation with Heat Recovery and again, the theory is fairly simple: Warm air leaving the building is passed through a heat exchange unit containing a heat cube, manufactured from materials with a high propensity for thermal storage and conductivity. The fresh cold air from outside is simultaneously passed through the heat exchange unit and is thus warmed.

How Does MVHR work?

Does MVHR Work?
The efficiency of modern MVHR units is surprisingly high, with the best known models manufactured by Vent Axia, Envirovent and Silavent achieving heat recovery of up to 90%. 

Typical Features of Modern MVHR Units
1. Summer Bypass: This feature disables the heating function by diverting the fresh intake air around the heat exchange cell so that it is not warmed when being returned to the dwelling. This helps to lower indoor temperatures during summer months.
2. Frost Protection: When outside temperatures fall below zero condensate carried by the incoming cold air can sufficiently cool, then freeze on and damage the heat cell. Frost protection works by limiting the incoming airflow so that the outgoing warm air maintains the temperature of the heat cell.

5. Installing an MEV or MVHR unit

Requiring a network of ducting, termination in multiple rooms and electrical wiring, centralised ventilation units are only installed in new build projects or buildings undergoing major refurbishment. It would be impossible to retrofit such a system without serious disruption to the internal fabric of the building.

It is important to remember that an MEV or MVHR unit is just one (yes, rather crucial) part of the ventilation system. The other main constituent is the duct system - get this wrong and any money and time spent on getting the perfect unit will be wasted. There are a couple of duct systems available which offer different benefits:
Rigid Ducting Flat 204x60mm Flat 204mm x 60mm Rigid Ducting used in Branch Ducting Scheme
The original type of ducting used for whole house centralised ventilation systems, offering the principle benefit of low air resistance as the internal surface is completely smooth. This facilitates optimal operation of the MEV / MVHR unit. The main drawback is that in a real world situation it can be awkward and costly to install, involving many joints as the installer gets round, over and under existing building features like joists, water pipes, electrical wiring and tight spaces.

 Branch Ducting System Layout for MVHR System
Semi Rigid Ducting 75mm Semi Rigid Ducting used in Radial Ducting Scheme
Semi Rigid Ducting is now becoming very popular as it addresses the problems rigid ducting meets in installation. Indeed, some specialists state that it saves up to 75% time (and cost) on install. It does offer greater air resistance as the internal bore is not completely smooth. However this could well be offset by the fact that it requires considerably less joints than a flat duct system.

Radial Ducting Network instllation

Of course the principle difference between the two types of whole house ventilation unit is that MEVs only remove air from a building whilst MVHRs both remove and return air which requires both an extract and intake duct scheme as shown above.

g3 air filters

The obvious problem with taking fresh air from the outside and distributing it throughout a dwelling is that in urban UK areas the 'fresh' air is far from pure. Indeed, in some heavy traffic areas of London, air quality regularly drops below WHO guidelines. Pumping polluted air into a building entirely defeats the point of a whole house ventilation system. 

In urban installations, therefore, it is usual for filters to be used on the intake valve which collect particulates before they enter the duct system. The filters will need regular replacement - the period of which depends of course, on the quality of the surrounding environment.

A number of different filter types are available. Here are the most common:
G3: G Filters are for coarse duct particles ≥ 10 μm such as pollen. 
M5: M Filters are for fine duct particles ≥ 1 μm such as dry cement dust, entering low demand areas such as workshops, storage or garages
F7: F Filters are also for fine duct particles ≥ 1 μm such as accumulated carbon​​​​​​​​​​​​​​, entering high demand areas where people are often present.
E10: E Filters are for micro particles ≥ 0,01 μm such as tobacco smoke​​​​​​​​​​​​​​, entering specialist areas such as laboratories and hospitals.

Our offering of MEV and MVHR units:

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