A recent live social housing refurbishment project has reinforced a question I raised in my previous article: is MVHR automatically the right ventilation strategy for every refurbishment project?
This isn’t an article arguing that MVHR is a bad ventilation system.
It isn’t.
When correctly designed, installed, commissioned and maintained, MVHR can provide excellent continuous ventilation while recovering heat that would otherwise be lost.
The question I increasingly find myself asking is something slightly different:
Is MVHR always the most appropriate solution for the building in which we are trying to install it?
A recent review of an MVHR design for a large social housing refurbishment project has brought that question into sharp focus.
When the theory meets the existing building
The project is a typical 1960s Tower Block.
The proposed strategy was to install individual MVHR systems within each apartment, with the units located in storage cupboards adjacent to the kitchens.
On paper, this appears entirely reasonable.
The units are accessible, there is a logical location for the equipment, and in some apartments, the proximity of the kitchen and bathroom makes the ductwork relatively straightforward.
But then you start looking at the detail.
And this is where the difference between specifying MVHR and designing an effective MVHR system becomes important.
In some of the apartments, the internal bathroom and WC are located further away from the MVHR unit.
This introduces longer duct runs and requires ductwork to cross the central hallway.
More importantly, the proposed supply air terminals were located above the entrance doors to the rooms.
That immediately raised a concern.
Airflow isn’t just about litres per second
One of the most important lessons I have learnt during more than 30 years working in residential ventilation is that ventilation performance cannot be judged simply by looking at the airflow rate at the terminal.
You can deliver the correct number of litres per second and still fail to ventilate the occupied space effectively.
With MVHR, supply air needs to be introduced into habitable rooms in a way that encourages the air to move through the occupied space before transferring towards the extract areas.
The location of the supply terminal therefore matters.
Industry guidance recognises this principle, with NHBC guidance recommending that low-velocity air valves are positioned on the opposite side of the room from internal door openings to create cross-ventilation within the room.
That makes perfect sense.
If the supply valve is positioned immediately above or adjacent to the doorway, there is a potential for the incoming air to take the easiest route towards the transfer path.
The system may be achieving its design airflow.
But is it ventilating the room effectively?
That’s the question we should be asking.
The danger of designing the system around the equipment
This experience made me reflect on something I discussed in my previous article.
We often talk about MVHR in terms of its headline advantages:
- Heat recovery
- Energy efficiency
- Continuous ventilation
- Good indoor air quality
- Reduced heat loss
All of those benefits are valid.
But they are benefits of a properly designed and operated system.
They aren’t automatically delivered simply because an MVHR unit has been installed.
The system also needs:
- Correct ductwork design
- Appropriate air terminal locations
- Correct airflow rates
- Effective air transfer routes
- Proper commissioning
- Regular servicing and maintenance
- Filter replacement
- Long-term management by the landlord
And this becomes particularly important when we start putting MVHR into existing social housing buildings.
Perhaps we need to start with the building – not the technology
This is where I believe the conversation needs to change.
Instead of asking:
“How do we install MVHR in this building?”
perhaps the first question should be:
“What ventilation strategy will provide the most effective, reliable and maintainable ventilation for this building over its lifetime?”
Those are not necessarily the same question.
A 1960s building undergoing refurbishment has very different characteristics from a new-build dwelling specifically designed around MVHR.
The building structure already exists.
The room layouts already exist.
The service risers already exist.
The ceiling voids are already determined.
The locations of kitchens and bathrooms are fixed.
And the landlord will ultimately be responsible for managing and maintaining the ventilation systems throughout their operational life.
These constraints should influence the ventilation strategy.
What about MEV?
This is why I believe continuous mechanical extract ventilation deserves much more consideration in refurbishment projects.
A correctly designed MEV system takes a fundamentally different approach.
Rather than mechanically supplying and extracting air through a network of ductwork, replacement air enters through the habitable rooms and is drawn through the dwelling towards the wet rooms, where continuous mechanical extraction is provided.
In simple terms:
Habitable rooms → transfer routes → wet rooms → mechanical extract → outside
There is no heat recovery.
That is an obvious disadvantage compared with MVHR.
But there are also potential advantages.
The system can be considerably simpler.
There is less ductwork.
There are fewer components.
Installation can be less intrusive.
Maintenance is simpler.
There are no MVHR filters to replace.
And the system establishes a clear airflow path through the dwelling.
For a refurbishment project, those factors matter.
The landlord maintenance question
This is particularly important in social housing.
When we specify an MVHR system, we aren’t simply selecting a piece of equipment and handing responsibility to the resident.
The landlord has an ongoing responsibility for the building services within its housing stock.
That means the whole-life implications of MVHR need to be understood at the point the system is specified.
Who will service the units?
How frequently will they be serviced?
Who will replace the filters?
How will access be provided?
How will failed components be identified and replaced?
How will the landlord demonstrate that systems continue to perform as intended?
And, perhaps most importantly:
What will this cost across the entire housing stock over 15, 20 or 25 years?
These are not minor considerations.
A relatively modest annual maintenance cost becomes significant when multiplied across hundreds or thousands of properties and many years.
For example, if a landlord has 1,000 properties with individual MVHR systems, even an annual maintenance cost of £150 per property represents £150,000 every year before considering major component replacement or exceptional repairs.
Over 20 years, that is £3 million.
The actual figures will obviously depend upon the equipment, servicing regime and contractual arrangements, but the principle remains.
Whole-life cost matters.
The energy argument needs to be broader
The most common objection to MEV is obvious:
“But you’re throwing the heat away.”
Yes.
Compared with a correctly functioning MVHR system, MEV does not recover heat from the extracted air.
But I think we need to be careful about assuming that this automatically makes MVHR the more energy-efficient solution in real-world social housing.
The comparison should consider the whole system.
That means looking at:
Heat recovered by MVHR
versus
Electrical energy consumed + maintenance + filters + servicing + installation complexity + actual system performance + whole-life cost.
The landlord also needs to consider the performance of the system over its entire operational life rather than simply the performance demonstrated when it is first commissioned.
A system that theoretically recovers a significant amount of heat but subsequently becomes poorly balanced, inadequately maintained or compromised by installation constraints may not deliver the performance anticipated at design stage.
Conversely, a simpler ventilation system that operates continuously, reliably and effectively may provide a very good real-world outcome.
The damp and mould question
There is another reason why this debate matters.
The ultimate objective of a residential ventilation system should not be to install a particular piece of equipment.
It should be to provide effective ventilation that supports a healthy indoor environment.
Social housing providers are rightly focused on reducing condensation, damp and mould.
That means we need ventilation systems that work effectively in the real world.
Not just on a drawing.
Not just at the point of commissioning.
And not just when the system is new.
For me, the key question is therefore:
Which ventilation strategy provides the greatest probability of delivering effective, continuous ventilation throughout the dwelling for the next 15–20 years?
That is a much more useful question than simply asking which system has the highest theoretical energy efficiency.
I am not anti-MVHR
I think this is important to make clear.
I am not suggesting that social housing providers should stop using MVHR.
There are many situations where MVHR is absolutely the right solution.
But I do think we have reached a point where MVHR can sometimes be treated as the default answer whenever energy efficiency and ventilation are being considered together.
And that concerns me.
MVHR is a ventilation strategy – not a specification requirement.
The strategy should be determined by the building, the installation constraints, the ventilation requirements, the maintenance regime, the landlord’s long-term objectives and the whole-life economics.
Sometimes that will lead to MVHR.
Sometimes it may lead to MEV.
And in other circumstances, another solution may be more appropriate.
The question I would ask before signing off the design
Having recently reviewed a live MVHR design for a social housing refurbishment project, this is the question I would put to any project team:
“If we weren’t starting with MVHR as the preferred solution, would we still choose MVHR after comparing all the available ventilation strategies?”
If the answer is yes, that’s fine.
But let’s demonstrate why.
Compare the systems.
Look at the airflow strategy.
Look at ventilation effectiveness.
Look at installation.
Look at energy.
Look at maintenance.
Look at landlord costs.
Look at resident experience.
Look at commissioning and long-term performance.
And, critically, look at whether the system can realistically be installed and maintained to the standard required.
Only then can we confidently say that we have selected the right ventilation strategy.
Final thought
The most energy-efficient ventilation system on paper isn’t necessarily the best ventilation system for a particular social housing building.
The best ventilation system is the one that can be correctly designed, correctly installed, effectively commissioned, reliably maintained by the landlord and continuously operated to provide the intended ventilation performance for the people living in the building.
Perhaps it’s time we stopped asking:
“MVHR or MEV?”
And started asking:
“What does this building actually need – and what can the landlord realistically maintain for the next 20 years?”
That’s where good ventilation design should begin.
Got a refurbishment project where MVHR is already the assumed solution?
Before the design is finalised, let’s challenge the assumption.
At Vortice, we can review the proposed ventilation strategy and provide an objective comparison of MVHR vs MEV, looking at compliance, ventilation effectiveness, installation, energy, landlord maintenance and whole-life cost.
Sometimes MVHR will be the right answer. Sometimes it won’t.
The important thing is making that decision based on the building – not simply the technology.
If you’re involved in a social housing refurbishment project, drop me a message. I’d be happy to have an initial conversation.
