Single Room – Mechanical Ventilation Heat Recovery
~ unit cost £360 + installation £360 = £720

Page summary…

Good ventilation is essential for the health and well-being of both a building and its occupants.

However, vents are often absent and when installed either passive and active vents are commonly found to be inappropriate; dysfunctional, ineffective, obsolete or simply unnecessary. More often it is the vent itself that is making things worse.

We’ll look at the problems associated with common existing vents before considering a variety of better solution.

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On another page

  • Voids
  • Doors
  • Windows



Aims – Kitchen extractors are most often not connected to the outside and it is unlikely a good backdraught shutter is fitted to prevent unwanted air ingress when not in use. Neither kitchen nor bathroom vent can push air more than 1.5 metres. If it’s being pushed up a chimney or out via an attic it is most likely to be causing more harm than good and damp will manifest all around the home.

Objectives – More often than not vents are dysfunctional and obsolete. inappropriate or ineffective they either need to be replaced with a functional solution or may simply need to be closed off altogether.

Hypothesis – Removing inappropriate historical vents will reduce damp, lower heating costs and improving comfort for practically no-cost. Installing appropriate functional vents will further reduce damp, lower heating costs and improve comfort for low-cost



What is it, does it work and is it used appropriately?

  • We’ll identify some historical vents and their purpose; examples of passive and active vents commonly fitted in our homes.
  • We’ll ask if the vent is functioning correctly and being operated appropriately to achieving its purpose and objective.
  • We’ll critic 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 look at successful passive and active ventilation strategies to determine the objectives before considering some simple installation projects for kitchens, bathrooms, centralised and decentralised whole-house solutions.


  • Kitchen and extractor/ cooker hood.
  • Bathroom extractor.
  • Misc vents – trickle vents in windows, night latches and obsolete vents. Do they need repairing or removing?

Passive ventilation (none mechanical)

  • Air-bricks
  • Roof-vents
  • Doors and windows (see doors and windows)



Historical vents

Historical vents

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, 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…



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.

Un-louvered vent

Un-impeded air-vent

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.

Gas appliances

Larder or cold-store


Historical vents

Fireplace vent

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




Back draught shutters

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. Backdruaght 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 backdraught 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?

Ducting supplies




The following projects are intended as 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.



Mechanical Ventilation Heat Recovery (MVHR)

MVHR changes the air without losing all of the heat. Air is extracted out at the same time as fresh air is pulled in. The ingress and egress air is passed through a heat exchanger. The air does not meet or mix but the warmth from the outgoing air is captured by the heat exchanger and passed to the incoming air.



WhyFunctional and efficient

MVHR provides air-change while reducing heat loss and because air is brought in at the same rate that air is expelled the pressure is balanced.

HowAlways read the instructions

Mechanical Ventilation Heat Recovery can be either ‘decentralised’ or ‘centralised’.

A decentralised Mechanical Ventilation Heat Recovery system (d-MVHR) can be a stand-alone unit installed into a single room, referred to as SR-MVHR, or multiple units working independently or working together via a control system. This works in the same way as having independent electric room heaters rather than a centralised heating boiler that distributes to room radiators. Installation cost can be lower.

A decentralised system is a simple and effective solution for kitchens, bathrooms, however, an outside wall is required.

If it is not possible to install a wall-mounted d-MVHR unit such as in a ‘landlocked’ bathroom a small MVHR unit may be a better option. This unit is located outside of the room, extract moist air from the kitchen and bathroom, exchange it with the outside and returns fresh air to another room such as the bedrooms.

A centralised Mechanical Ventilation Heat Recovery system (c-MVHR) is a single device located centrally, perhaps in an attic, from where it connected via ducting to multiple rooms. It is normally an extensive whole house system that is more appropriately installed into new-build properties, due to the need for concealed ducting throughout the home. It is, therefore, less invasive and more affordable to install new rather than retrofit into existing homes.


What – Materials and equipment

Which solution for which scenario…


Envirovent d-MVHR – £350 – £450

Blaunberg d-MVHR – £477.27

Envirovent c-MVHR

Blaunberg c-MVHR



- 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.

decentralised Mechanical Extractor Vent – (dMEV)

When direct access through an outside wall is not possible and longer ducting is required.





Backdraught shutter

Vents and voids
Plastic bag and a rubber band.

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



Uncontrolled ventilation and dysfunctional extraction

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