Most MVHR problems trace back to decisions made months before installation, not faults in the equipment itself. Ducting routed too late, units sized against guesswork rather than calculation, and vent positions chosen for convenience rather than performance all quietly set a system up to underperform. Catching these issues at concept stage costs nothing compared to fixing them once walls are closed up.
Why Do MVHR Systems Underperform Even When Correctly Installed?
MVHR systems underperform even when correctly installed because the design decisions made before installation often lock in limitations that no amount of careful fitting can fix. A unit installed exactly to specification will still struggle if the ducting route it feeds was never sized for the airflow required.
This is one of the most common frustrations contractors report, since the installation itself passes inspection while the system still fails to deliver expected performance.
The root cause almost always sits earlier in the process, often at concept or planning stage, long before a contractor ever picks up a duct.
What Design Mistakes Happen Before Ducting Is Even Considered?
The biggest early mistake is treating unit selection as separate from building layout, choosing a unit based on general specification rather than the actual airflow and room configuration of the project. This disconnect creates problems that only surface once ducting design begins in earnest.
Architects sometimes finalise room layouts and ceiling heights before confirming ventilation strategy, leaving little space for correctly sized ducting later. Our MVHR system design best practice guide covers how to sequence these decisions so ventilation strategy informs layout rather than fighting against it.
How Does Poor Ducting Planning Cause Long Term Problems?
Poor ducting planning causes long term problems by forcing air through routes that are too long, too narrow, or bent too sharply, all of which increase resistance and reduce the heat recovery efficiency a unit can actually deliver. These issues often go unnoticed until occupants complain about noise or poor airflow months after handover.
Our MVHR ducting size guide explains how to calculate correct duct dimensions against room requirements, avoiding the common trap of using a single duct size throughout a building regardless of actual demand. Vent positioning compounds these issues further, and our article on MVHR vent positioning covers how badly placed terminals can undo good ducting design entirely.
Contractors following drawings without querying obviously tight or convoluted routes often end up installing a system that was already compromised on paper.
Do Contractors Overlook Anything Specific During Installation?
Yes, contractors sometimes overlook the difference between a system's rated performance and its actual site performance, assuming that following the drawings exactly guarantees the specified outcome. Site conditions such as unexpected obstructions or last minute layout changes can force compromises that nobody circles back to check against the original design intent.
This is particularly common on projects where ducting decisions get revised on site without updating the compliance documentation to match. Our breakdown of Part F ventilation regulations covers the baseline requirements that any on site changes still need to satisfy, regardless of what was originally drawn.
Noise is another area where installation shortcuts show up quickly. Our guide on how contractors reduce noise in commercial ventilation systems covers how rushed installation decisions can introduce noise issues that proper planning would have avoided.
How Does Poor Coordination Between Architects and Contractors Cause Failures?
Poor coordination causes failures when architects specify a system without checking buildability, and contractors install without flagging design issues back to the design team before work proceeds. Both sides often assume the other has already resolved potential conflicts, which means problems only surface once work is too advanced to change easily.
This is especially costly on developments where similar layouts repeat across multiple units, since one uncaught design flaw multiplies across the whole site. Our guide to specifying MVHR for large multi zone residential projects covers how to build in coordination checkpoints that catch these issues before they scale.
Regular communication between design and installation teams during the early ducting phase catches far more issues than reviewing drawings in isolation ever will.
Are There Common Comparison Mistakes Made at Specification Stage?
Yes, a common mistake is comparing MVHR against alternatives like MEV, PIV or traditional extractor fans purely on unit cost, without factoring in how each system performs against the specific building's airtightness and layout. Choosing the wrong ventilation strategy at this stage causes problems no amount of careful ducting design can fully resolve later.
Our comparison of MEV vs MVHR for different project types and our guide on PIV vs MVHR both help clarify which system actually suits a given building rather than defaulting to whichever option seems simplest upfront. For projects weighing MVHR against simpler extraction, our piece on MVHR vs traditional extractor fans in new homes covers the practical differences that specification decisions should be based on.
Explore eFans' MVHR Range for Reliable Project Outcomes
Browse the full heat recovery units to compare specifications across brands, or explore the whole house MVHR range for centralised systems suited to larger projects. For smaller interventions, the single room MVHR collection offers compact solutions, while the Vent-Axia and Zehnder heat recovery ranges give architects and contractors strong brand specific options to avoid specification related failures from the outset.
Frequently Asked Questions
Can MVHR Failures Be Fixed After a Building Is Occupied?
Yes, MVHR failures can often be fixed after occupation through retrofitting silencers, replacing undersized ducting sections, or adjusting fan speed settings, though this is more disruptive and costly than correcting issues at design stage. Occupied buildings also limit how much invasive work can realistically happen without disturbing residents.
How Early Should MVHR Design Be Considered in a Project Timeline?
MVHR design should ideally be considered at concept or early design stage, alongside decisions about room layout, ceiling heights and structural openings. Leaving ventilation strategy until later in the process significantly limits the options available for correct ducting and terminal placement.
Does Using a More Expensive MVHR Unit Prevent Design Related Failures?
No, a more expensive unit does not prevent design related failures, since even high specification equipment underperforms if the ducting, terminal positions or airflow calculations behind it are wrong. Correct design matters more than unit price for avoiding these issues.
What Is the Most Common Site Issue That Causes MVHR Underperformance?
The most common site issue is ducting routed through unexpectedly tight voids, forcing installers to use smaller ducts or extra bends than originally specified. This quietly reduces airflow and heat recovery efficiency compared to what was modelled in the original design.
Who Should Be Responsible for Checking MVHR Design Buildability Before Installation?
Responsibility for checking buildability typically sits jointly between the architect specifying the system and the contractor or ventilation specialist installing it, with clear communication between both essential before work begins. Projects with a dedicated mechanical and electrical consultant often catch these issues earlier than those without one.