With various acronyms for mechanical ventilation products on the market, let’s take a look at the different system types available and delve into what they do:
MVHR - Mechanical Ventilation Heat RecoveryHeat recovery ventilation systems not only extract air but also introduce fresh air into a building. By utilising heat sink exchangers, they glean the heat from the exiting air and use it to warm the incoming fresh air from outside, reducing heat loss and heating costs. As constant flow systems, MVHR units have the added benefit of helping to reduce internal pollutants whilst also controlling the replacement air flow, creating a gentle and balanced cyclical ventilation current throughout your home. They can be very effective at controlling humidity levels as well as filtering the incoming air, with many MVHR units coming with replaceable filters that can cover a wide range of external pollutants and allergen protection. Retrofitting MVHR systems into existing buildings is possible and relatively easy for rooms directly below your loft space but it can be harder to install ducting to ground floor areas in multiple floor buildings outside of renovation works. For new developments and major re-developments however, MVHR duct runs can be planned into the design and laid before building completion to hide them from view.
MVHR systems are available as single room units as well as whole house centralised systems. Both whole house systems and single room units have boost options, raising the extract/intake speed by a combination of wired or wireless boost switching and/or humidistat sensor to remove humidity faster or allow manually controlled purging:
Whole house systems generally sit centrally in a loft or service room with ducting incorporated into the building structure during construction, with specific runs for extraction or supply running to and from the main system hub unit from different areas of the building. There are only two duct runs installed to the outside, one to exhaust the stale air and one to supply fresh air to the hub unit.
Single room units are more common for retrofitting to existing buildings or into rooms with no easy access for ducting from a centralised unit. As the name implies, these units service a single space continuously extracting and supplying air either through a single entry/exit point heat exchanger or utilising a two-duct configuration for units that can sit in ceiling voids. The majority of these units simultaneously extract and supply air but some work on an alternating directional flow principle that draws out stale air, collecting the heat before switching the fan polarity and pulling in air, warming it from the previously collected warmth.
MEV - Mechanical Extract Ventilation
Constant flow extraction is an excellent way to improve indoor air quality but having multiple units installed around your home not only adds more holes for external venting but raises your electrical usage per individual unit installed. Consolidating your extraction into a single central hub system not only saves electrical energy but also allows for a more aesthetically pleasing single external venting point. MEV units are ideally situated in a central position in the home - usually in a loft space or service room - which allows ducting to be run from multiple areas of your house to it. Once the extracted air reaches the MEV unit, it is consolidated and vented out of the building in a single duct run to the outside world.
MEV units can be boosted from their continuous trickle extraction rate by two methods; most units will have either a wired or wireless boost switch option to allow for manual purging, others may also incorporate a built-in humidity sensor within the unit that tracks the moisture content in the air being extracted. This will automatically boost the extraction rate if any of the incoming ducts supply air with a higher percentage of humidity present above the set trigger level set on the sensor, returning to the trickle speed automatically once the humidity level drops again below the trigger point.
Retrofitting MEVs into existing buildings is possible and relatively easy for rooms directly below your loft space but it can be harder to install ducting to ground floor areas in multiple floor buildings outside of renovation works. For new developments and major re-developments however, MEV duct runs can be planned into the design and laid before building completion to hide them from view.
dMEV - Decentralised Mechanical Ventilation
Similar to MEV systems, dMEV fans are constant flow units that are especially good at managing damp and mould-inflicted rooms. Unlike MEV, these units are single room systems and generally take the place of a traditional intermittent extractor fan. dMEVs come with various functionality including humidistats, over-run timers and PIV (Passive Infra-Red) sensors and are generally axial fans with a few utilising centrifugal fan extraction. Although dMEV units are 4”/100mm units, due to the continuous flow function, they are governed by a different set of criteria in the UK building regulations (current issue Part F1[1.22]) and are suitable for both kitchen and utility room extraction. dMEVs are an excellent option for downstairs areas that cannot be connected to an MEV system in an existing building where ducting runs cannot be retrofitted and are also excellent for pairing with PIV systems to create controlled cyclical ventilation. Both quiet and energy efficient, dMEV units are quickly becoming the fan system of choice for bathrooms, taking over from traditional intermittent units due to their improved protection against humidity and ease of retrofitting as replacements due to their similar footprint and wiring configurations.
PIV - Positive Input Ventilation
Introducing fresh, clean air into properties is an increasingly important necessity. Houses built post-1970 have been designed to be more airtight to reduce the loss of heat from the building to reduce energy costs and to limit the amount of cold air that can make its way into the structure. These techniques have been improved over the years making many new builds the most airtight domiciles to date. Although this has a positive effect on heat retention, it has had a detrimental effect on indoor air quality and higher humidity content. Finding a way to balance heat retention while introducing fresh air into housing needs to ensure that the incoming air can displace the poor-quality air present in the building while minimising the displacement of heat. PIV systems are designed to force fresh, filtered air into a property causing a positive pressure imbalance in the building. This in turn forces the already present stale air and humidity out through the fabric of the building. In more modern builds where building design has improved to limit the loss of heat through the structure, pairing a PIV system with dMEV fan units in bathrooms, kitchens and utility rooms can create a controlled cyclical flow of air through your home, displacing the stale air and humidity and replacing it with fresh air from the PIV system.
PIV units are typically loft mounted with a single venting point into a central part of your home - usually above a landing or stairwell, to allow the incoming air to make its way to all parts of your home. These units pull air in from your loft space through large filters to remove particulates and other pollutants before forcing the air into the habitable part of the building. Many of these units incorporate a heating element to warm the incoming air through winter months which will disengage automatically once the weather improves. The units will also often incorporate an automatic summer bypass which will stop the unit from pushing in air from your loft if the air exceeds a certain temperature to stop uncomfortable hot air being blown in during summer months.
For flats, apartments and buildings without loft access, there are options for PIV systems that can be ducted through an external wall to pull the air in before releasing the air into the home. These units are still ideally centrally situated in the building so finding hidden routes for ducting is usually the preferred installation technique.
Intermittent Ventilation
Probably the best known and most prolific type of mechanical extraction, intermittent fans have been in use in the UK since the introduction of electricity at the end of the 19th century, becoming popular in households from the mid-20th century onwards. Intermittent units, as the name suggests, run only when activated, usually at a single speed. They can come in a wide range of sizes and styles with extraction rates suited to different applications. The most common axial wall or ceiling mounted household units usually are available for bathrooms in 4”/100mm diameter, utility rooms in 5”/125mm diameter and kitchens in 6”/150mm diameter with each size also delivering at least the expected minimum extraction rate for these spaces as determined by UK building regulations. A range of functionality options are available such as over-run timers, humidistat sensors, built-in pull cords and PIR (passive Infra-Red) movement sensors.
Intermittent inline units are also available, again in various diameters starting at 4”/100mm. Inline units are designed to be housed within a ducting run in loft spaces or service rooms out of sight and are therefore not designed to be aesthetically pleasing. But not being tied down to looks allows them to house much larger and therefore more powerful motors, giving inline units a far superior extraction rate. As an example, most 4”/100mm wall/ceiling mounted axial fans average extraction around 80-100m3/h whereas, 4”/100mm inline units average between 250-300m3/h. Inline units come in two basic options: standard on/off models or units with built-in over-run timers. Additional accessories can be installed alongside the standard on/off models such as remote humidistat/timer modules, stand-alone over-run timer circuits and variable speed controllers.