Continuous Mechanical Ventilation – Getting It Right for Tenants
The challenge of applying dMEV to existing social housing
Continuous mechanical extract ventilation, or dMEV, is becoming an increasingly familiar solution in residential housing.
But when it comes to existing social housing, getting the ventilation strategy right is about much more than simply selecting a fan and installing it.
The dwelling, the available extract points, the required airflow, the ductwork, ongoing maintenance and, importantly, the tenant’s experience of the system all need to be considered.
This raises a fundamental question:
Can we provide the ventilation the dwelling requires, through the extract points available, while keeping the system tenant acceptable?
That question becomes particularly important in existing homes.
Unlike a new development, where a ventilation strategy can be designed into the building from the outset, social housing providers are often working with properties that were constructed many years ago, with limited options for where mechanical extraction can be installed.
A typical three-bedroom home, for example, may have only:
- one kitchen;
- one bathroom; and
- two mechanical extract points.
Yet those two extract points may need to provide the continuous ventilation required for the whole dwelling.
The whole-dwelling ventilation challenge
Using the three-bedroom example in Approved Document F for England, the whole-dwelling continuous ventilation rate is 31 l/s, subject also to the applicable floor-area criterion.
Importantly, that 31 l/s is not a requirement for one individual fan.
It is the whole-dwelling ventilation rate, with the available continuous extract points collectively providing the required ventilation.
And this is where the practical challenge begins.
If two conventional 100 mm dMEV fans are each operating at around 10–12 l/s continuously, the combined extract rate would be approximately:
20–24 l/s
against a whole-dwelling rate of:
31 l/s
The issue isn’t necessarily that dMEV doesn’t work.
The issue is that the number of available extract points, the airflow capability of the equipment and tenant acceptability all must work together.
Simply increasing fan speed isn’t necessarily the answer.
A ventilation system that delivers the required airflow but is not tenant acceptable may not deliver the intended outcome in practice.
Residents must live with these systems every day.
That makes “tenant acceptable” an important part of ventilation design and product selection – not simply an afterthought.
What does “tenant acceptable” actually mean?
We can design systems around airflow calculations, fan curves and regulatory requirements, but the system is ultimately installed in someone’s home.
If a resident considers the fan too noisy, intrusive or disruptive, there is a real risk that they may switch it off, interfere with it, or find ways to reduce its operation.
It isn’t enough to ask:
“Can the fan achieve the required airflow?”
We should also be asking:
“Can it achieve that airflow while remaining tenant acceptable?”
That becomes particularly important when attempting to achieve higher continuous ventilation rates through a small number of extract points.
Continuous and intermittent ventilation are not the same thing
It is also important to recognise that moving from intermittent extract to continuous mechanical extract represents a change in ventilation strategy.
Intermittent extract fans are commonly associated with what Approved Document F describes as Natural Ventilation with Background Ventilators and Intermittent Extract Fans.
The strategy relies on background ventilation through ventilators, air transfer through the dwelling and intermittent mechanical extraction when moisture production is elevated.
The fan is therefore not intended to provide the dwelling’s entire ventilation requirement continuously.
Continuous mechanical extract is fundamentally different.
With dMEV, the extract fans operate continuously at a background rate, providing a permanent level of mechanical ventilation, with higher rates available when additional ventilation is required.
The ventilation rates associated with the two strategies therefore cannot simply be compared on a like-for-like basis.
When moving to dMEV, the question changes from:
“Can the fan extract enough air when moisture is being produced?”
to:
“Can the dwelling continuously extract the required volume of air through the available extract points, while remaining tenant acceptable?”
For existing social housing, that is a significant change in the way the ventilation system needs to be considered.
Existing homes create different challenges
Continuous mechanical extract fans were commercially available in the UK well before they became established within Building Regulations guidance.
A significant regulatory milestone came with the 2006 edition of Approved Document F, when continuous mechanical extract was formally recognised as System 3, including systems using individual room fans.
Since then, continuous mechanical extract has become increasingly established across residential development and refurbishment.
But applying the principles of continuous mechanical extract to an existing property is not simply a case of replacing an intermittent fan with a continuous one.
The dwelling needs to be considered as a whole.
What extract points are available?
What airflow can realistically be achieved?
What is the resistance of the ductwork?
How will air transfer take place?
And can the system operate continuously without becoming unacceptable to the tenant?
These are particularly important questions where there may be only one kitchen and one bathroom available as mechanical extract points.
There isn’t necessarily one answer
There are different ways of approaching the challenge.
The Vortice AER dMEV range, for example, includes both 100 mm and 150 mm versions, providing designers with additional flexibility where the building, ductwork and installation allow.
The range also incorporates adjustable speeds and constant-flow functionality to assist with commissioning and maintaining airflow against system resistance.
But perhaps the more important question is this:
Should we start with the fan, or should we start with the dwelling?
Rather than asking how much airflow a conventional dMEV fan can provide, perhaps we should first establish what the existing home requires and then select the most appropriate ventilation solution.
Designed around the existing-homes challenge
This is where Fan for Life takes a different approach.
Fan for Life has been developed specifically with the social-housing environment in mind.
It can provide a continuous background airflow of 23 l/s from a 100 mm unit, providing considerably more ventilation capacity from a single extract point than many conventional 100 mm dMEV installations.
That changes the calculation.
Returning to our three-bedroom example, two Fan for Life units operating at 23 l/s could provide:
23 l/s + 23 l/s = 46 l/s
against a whole-dwelling rate of:
31 l/s
That doesn’t mean every dwelling requires two Fan for Life units operating at 23 l/s.
The point is that the additional performance provides the designer with considerably more flexibility when dealing with existing homes where there may be only one kitchen and one bathroom available as mechanical extract points.
And importantly, the objective isn’t simply to extract more air.
It is to provide the required ventilation in a way that is effective, practical and tenant acceptable.
But airflow is only part of the social-housing equation
This is where I believe the conversation around dMEV needs to develop.
A ventilation fan in a new development is primarily viewed as part of the building specification.
Once that property becomes part of a social-housing portfolio, however, the requirements change.
The landlord has to consider:
- ongoing maintenance;
- component failure;
- engineer attendance;
- access;
- replacement costs;
- resident disruption;
- fault diagnosis;
- evidence of system operation; and
- the long-term performance of the ventilation asset.
Many conventional dMEV products have understandably been developed around the new-build market, where the primary objective is to deliver the specified ventilation performance as part of the completed development.
The existing social-housing market has a different requirement.
The fan isn’t simply installed and forgotten.
It becomes a maintained asset within a housing provider’s portfolio.
Designed for the whole life of the asset
This is another area where Fan for Life takes a different approach.
Its modular construction means that key components can be replaced without replacing the complete fan assembly or disturbing the mains wiring.
That has the potential to reduce maintenance time, minimise resident disruption and reduce whole-life maintenance costs.
But there is another significant difference:
data.
Fan for Life incorporates integrated data tracking, allowing housing providers to retrieve operational information through USB, including up to 23 recorded parameters.
For a landlord, this provides something that many conventional extract fans do not provide:
information about what has actually happened to the ventilation asset.
Has it been running?
Has there been a power interruption?
How has it been operating?
Has there been a fault?
What has happened between installation and the next maintenance visit?
That information can become particularly valuable when dealing with damp and mould investigations, maintenance decisions and the increasing need for landlords to demonstrate that ventilation systems are being appropriately maintained and operated.
From replacing fans to managing ventilation assets
This is perhaps the biggest change I see coming in residential ventilation.
The question should no longer simply be:
“Which fan are we going to install?”
For existing social housing, the questions should be:
What ventilation does the dwelling actually require?
How many extract points do we have?
Can we achieve the required whole-dwelling airflow?
Will the system remain tenant acceptable?
How will it be commissioned and verified?
How will it be maintained over its lifetime?
And what information will the landlord have about its performance?
These are very different questions from simply replacing an old fan with a new dMEV product.
The opportunity for existing social housing
There is no doubt that decentralised continuous mechanical extract has an important role to play in residential ventilation.
But existing social-housing stock presents a different challenge from a new development.
We are dealing with buildings that may have been constructed decades before continuous mechanical extract became a recognised ventilation strategy.
We may have only one kitchen and one bathroom from which to achieve the required whole-dwelling ventilation rate.
We have residents who quite reasonably expect ventilation to be effective but also tenant acceptable.
And we have landlords who increasingly need to demonstrate not simply that a product was installed, but that homes are being properly maintained and that ventilation is performing as intended.
That is why I believe the next generation of ventilation products for social housing needs to deliver more than airflow.
It needs to combine:
Performance.
Tenant acceptability.
Maintainability.
Whole-life value.
And evidence.
For existing homes, that changes the conversation from:
“Installing a fan”
to:
“Managing a ventilation asset.”
And that is exactly where I believe Fan for Life has been designed to make a difference.
