Designing MVHR for Overheating
As more homes are designed to meet Part O and net zero targets, overheating is becoming one of the most difficult performance gaps to close.
We often talk about solar gain, glazing ratios, and ventilation rates, but there’s one aspect of the product specification that’s too often overlooked: The Ductwork!
A MVHR system isn't just about the unit, but the ducting - this can essentially mean the difference between compliance without having to compromise too much
Why ductwork matters for overheating ventilation design:
The most common overheating complaints in MVHR-equipped homes come down to:
• Inadequate airflow (especially purge ventilation)
• Noise when fans ramp up
• Warm air recirculation or lack of cooling effect
In almost every case, undersized, over-complicated, or high-resistance ductwork is a root cause.
The Hybrid Ducting Solution for Overheating with MVHR
The challenge:
Overheating ventilation rates are high. Realsitically this means larger ductwork, often up to 200mm/ Modern homes have limited space for services and especially in ceiling voids, therefore this size of ducting isnt feasible, especially on a large scale. .
The solution:
Hybrid ducting systems that combine PVC flat ducting and galvanised steel or aluminium round ducting offer the best of both worlds:
• PVC low-profile ducting (e.g. 204x60mm or 220x90mm) fits within tight ceiling voids or wall zones
• Metal round ducting used for risers, plantrooms, and high-friction runs
• Improved airflow efficiency through smart layout, fewer bends, and larger cross-sections
• Better noise control due to smoother transitions and lower fan strain
• Easier on-site installation and airtight connections using combination adaptors
Supporting MVHR Performance in TM59 and Part O
TM59 overheating compliance depends on more than just having MVHR - it depends on having the right airflow rates, especially during purge mode.
Here’s how hybrid ductwork helps:
• Maintains 4 - 6 air changes per hour (ACH) when required
• Enables night-time ventilation with reduced sound transfer
• Fits in restricted ceiling zones without losing volume
• Reduces system pressure loss, so fans run quieter and more efficiently
• Supports summer bypass and night cooling modes to actively reduce indoor temps
Real-World Applications
We’ve used hybrid ductwork designs in:
• Urban apartment blocks where ceiling voids are restricted to 150mm
• Passive and net zero homes where ventilation systems must be invisible but powerful
• Retrofit projects where space is limited but overheating risk is high
The result? Better thermal comfort, quieter operation, and fewer complaints from residents.
Our Services
We support developers and designers with:
• MVHR layout design and duct specification
• Hybrid ducting schedules and pressure loss calculations
• Coordination with ceiling/joist/structural zones
• Full TM59 and Part O overheating compliance reports
Learn more about our services here.
Take Away
Your MVHR unit might be the brains of the system—but the ductwork is the bloodstream. If it’s undersized or poorly designed, you won’t get the flow needed to combat rising temperatures.
Don’t let overheating be a ductwork problem. Design it right from the start.
Talk to us today about hybrid ducting design
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