Decentralised – Mechanical Ventilation with Heat Recovery (d-MVHR)
~ unit cost £720 + installation £480 = £1200
Good ventilation is essential for the health and well-being of both a building and its occupants.
However, appropriate and effective vents are more often absent and present likely to be dysfunctional, obsolete or simply unnecessary.
More often, it is the vent itself that is making things worse.
- Mechanical Ventilation with Heat Recovery – MVHR
- Mechanical Extract Ventilation – MEV
- Internal Weather
Related… on another page
- We’ll look at the existing vents and their problems before considering more effective solutions.
- We’ll identify some historical vents and their purpose with examples of passive and active vents commonly fitted in our homes.
- We’ll ask if the vent is functioning correctly and operated appropriately to achieve its purpose and objective.
- We’ll critique the installation; is it ducting successfully to the outside; within less than a metre, insulated to prevent condensation and fitted with an air-tight back draught shutter to prevent air-ingress when not in use.
- We’ll consider examples of successful vent installations and ventilation strategies.
- Controlled ventilation can improve indoor air quality and reduce energy costs.
What is it? does it work and is it used appropriately?
- Active ventilation – kitchen and bathroom extractors.
- Passive ventilation – cross ventilation, stratification and convection.
- Obsolete vents – appropriate or inappropriate, to be improved or removed.
Cooker hood – fitted to an internal wall, the air is recirculated.
Extracting moisture generated from cooking a hood captures moisture from over the hob extractor unit can not easily be ducted to the outside and would fail to ‘push’ air the distance required.
- Located directly above the cooker an extractor hood extractor is excellent at capturing moisture and grease from the hob.
- Is it on an outside wall and does it connect within less than a metre?
- Does the extractor have a backdraught shutter to prevent unwanted ingress?
Cooker hood extractor – fitted to the external wall but not connected to the outside
Cooker hood extractor – fitted to an external wall and connected to the outside. However, without a backdraught shutter, cold air ingress occurs when not in use.
Bathroom extractor – fitted to a ‘landlocked bathroom and closed off with a plastic bag and a rubber band
Without a good backdraught shutter, mechanical bathroom extractors are a source of cold air ingress leading to problems. When not in use cold air constantly pours in from the outside making the bathroom cold during the heating season.
The bathroom is cooled by the draught all day and will require substantial warming to raise the room temperature back up to a comfortable level ready for the room to be used. The large thermal mass of the walls is slow to warm making it more likely the walls will be cold when the room is next used. Warm moist air will condense onto surfaces. When condensation evaporates away the walls will be further chilled the walls.Mould growth is likely to occur.
This bathroom was improved by closing off the dysfunctional vent and appropriately operating a window until a functional ventilator could be installed.
Why – How – What
Obsolete, inappropriate and ineffective.
Obsolete historical vents continue to make our homes unnecessarily draughty. It is a common misconception that these vents are there for good reason, in some way useful or even essential when in truth they are not.
Passive ventilation and the path of least resistance. Identification, purpose, troubleshooting.
Why – Preventing uncontrolled air-ingress/egress
When air change is required it should be possible to open a window, operate a Mechanical Extractor Vent or set up some form of a predetermined autonomous system to regulate air change. ‘You need some draughts’ is not correct, unimpeded air ingress/egress is just as likely to cause dampness as not ventilating appropriately. Good ventilation is controlled ventilation.
Wind sucks from high to low pressure. Passive ventilation is achieved for example by opening a window on opposite sides of a building. The same mechanism can be used to cross ventilate a roof or floor void. Typically a subfloor is ventilated via vents positioned on opposite sides.
How – Identification
Inappropriate, misplaced or obsolete passive vent. Examples are given.
Many of these historical vents are passive, none mechanical and often resemble an air-brick. They are not to be confused with external air bricks that ventilate timbers in the sub-floor or attic voids outside of the thermal envelope.
Historical passive vents that pass through the thermal envelope into the living space of the home were installed for a number of different reasons that are now be considered obsolete or inappropriate.
Likewise historical mechanical vents, installed with good intentions are most likely to be ineffective or dysfunctional and a source of cold air ingress exacerbating the problems they were installed to resolve.
Examples are given…[IMAGE]
For the purpose of this page, an air-brick is not a vent
Notes: Air-bricks are found outside, at ground level, ventilating the sub-floor void by allowing air to pass underneath the house; they do not pass through into the thermal envelope into the home.
Air bricks are a Victorian invention introduce around the same time as rising damp, the damp-proof course as well as gas-lighting in the home.
Tuberculosis vents and the path of least resistance.
This vent was installed to ensure the room had an uninterrupted air supply, a requirement for the installation of a gas fire. The old inefficient gas fire and a back boiler were removed over 30 years ago, the vent remained. This example is of a style and age that could be asbestos.
Larder or cold-store
This is example is also a “hit and miss” vent.
What – Materials and equipment
Notes: Foam applicator gun, papier-mâché. P3 particle mask
“In non-licensed tasks to remove asbestos, the HSE states that the RPE should have an Assigned Protection Factor of 20 or more. Wearers should use a disposable respirator to standards EN149 (type FFP3) or EN1827 (type FMP3), a half-mask respirator (to standard EN140) with a P3 filter or a semi-disposable respirator (to EN405) with a P3 filter. This equipment is only suitable for short-duration work. Licenced, high-risk work for long periods of time may require fan-assisted or air-fed respirators to protect against asbestos dust.” –
Foam applicator gun – see voids
Half-mask respirator with P3 filter to standard EN140 – Screwfix £17.99
National Asbestos Helpline
When a mechanical extraction vent is powered down or switched off uncontrolled air-ingress/egress will occur unless a good backdraught shutter is in place. Back-druaght shutter are seldom in place and when in place they are seldom effective.
Why – effective back-draught shutters
Mechanical ventilation is an active process, the air is pushed or pulled by a powered mechanism. A mechanical vent can move air in contrary to its passive or natural direction, from high to low pressure.
How – built-in, inline or external
What – type and purpose
Neoprene back-draught shutter – Eleconline £9.97
Regular Mechanical Extract Vents can not ‘push’ air much more than a metre.
Why – Identifying the objectives
Wind sucks, balancing pressure differences.
How – Making a plan
Exit strategy, wall, ceiling, centralised or decentralised.
What – Materials and equipment
Short, smooth, rigid, flexible, Insulated?
The following projects are a means of understanding the objectives, developing transferable skills and applying the principles required to achieve successful results. They are not intended as a comprehensive how-to instruction manual.
Basic – Envirovent d-MVHR – £350 – £450
300 400 500 600
Also – https://www.expandingfoamtape.co.uk
Discussion…- New post pending: Envirovent survey and installation service
The Envirovent is an SR-MVHR is an excellent affordable solution and made in the UK. However, contacting Envirovent direct for a survey from there network of installers is not without some challenges. So far, those from our network that have contacted `Envirovent have very politely had their rationale for the SR-MVHR challenge and have been encouraged to undertake the following two alternative solutions that are not as efficient but could be described as a more reliable ‘damp’ solution. Envirovent seems to work to the assumption that the occupant is not able to understand and operate their own homes to advantage and instead of removing responsibility from the occupant and favouring a less efficient and more expensive solution. This is contrary to the no-cost and low-cost approach advocated by Neighbourhood Construction.
Firstly, Envirovent will always advocate installing a Positive Input Ventilation, PIV unit. This pulls cold air in from the attic increasing heating demand. Because the house then has a positive air pressure air will escape out where previously a cold draught might have been felt coming in. The assertion made by Envirovent that air pulled in from the attic is warmer is absurd if the attic is properly insulated, solar gain is likely to be unreliable and more likely to increase VOCs or circulate fibres from Mineral wool loft insulation.
Secondly, the Cyclone wall mounted rapid extractor. This time the unit pulls air out or the room very effectively. However, this will cause cold air to be pulled in elsewhere, once again making the house cold.
Envirovent certainly has a range of product that can improve indoor air quality and deduce moisture ‘damp’ in the home. However, the blanket approach they apply neither empowers the individual to understand how to operate their home nor improves energy efficiency. You’d be as well to turn the heating up and open the windows.
Those who have proceeded with SR-MVHR contrary to the advice of Envirovent have been very satisfied with the result. The SR-MVHR unit effectively pulls air in at the same time as pushing it out without any of the problems described above and does so very efficiently.
When direct access through an outside wall is not possible and longer ducting is required.
Doors and windows
Extractors commonly cause uncontrolled air ingress/ egress when not in use. Cold air ingress into a room will make surfaces cold and leading to the manifestation of damp
Without an effective back-draught shutter, cold air ingress can cause problems with moisture and mould, exacerbating symptoms that the installation of the vent was once installed to resolve.
Don’t beware the temporary measure. This 100mm diameter hole is allowing unimpeded air ingress into this bathroom. This air-ingress is creating cold spots with the capacity to attract warm moist air. The vent is exacerbating the problem it was installed to resolve.
Envirovent – neoprene backdraught shutter
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