Radial ducting has quietly become the default distribution method for modern whole-house ventilation in the UK — and for good reason. Every Building Regulations-compliant MVHR (Mechanical Ventilation with Heat Recovery) or MEV (Mechanical Extract Ventilation) installation depends on getting fresh, filtered air to every room without leaking pressure, transmitting sound between spaces, or wasting fan energy on unnecessary friction. Traditional branch ducting — one large trunk stepping down through tees to reach each room — struggles on all three counts, especially in retrofits where you're threading pipework through joists that were never designed with ventilation in mind.
The Casavent Radial Ducting range at AIZO Quality Heating pairs anti-microbial, anti-static 75mm and 90mm semi-rigid duct with an EPP (Expanded Polypropylene) trunk system and full manifold hardware — a complete, thermally-insulated distribution kit designed to plug directly into a Vectaire MVHR unit and satisfy Building Regulations Part F (ventilation) and Part L (energy) from day one.
What is Radial Ducting?
In a radial layout, a central manifold sits close to the MVHR unit. From that manifold, a single semi-rigid duct runs uninterrupted to every supply and extract point in the home — the kitchen, each bathroom, each utility room, each bedroom. There are no in-line tees, no branch collectors, no reducers. Each room effectively has its own private airway back to the fan.
The semi-rigid duct itself is a corrugated polyethylene pipe with a smooth internal bore, typically 75mm or 90mm in diameter. The corrugations let it bend gradually around obstructions in joist voids and ceiling zones without kinking, while the smooth interior keeps air resistance low. Both diameters use push-fit sealing rings at manifold and plenum connections, and the pipe itself can be cut to length on site with a dedicated cutting tool. AIZO's semi-rigid range is anti-microbial and anti-static, actively resisting microbial growth on the duct wall and preventing electrostatic build-up that would otherwise pull dust from the airstream onto the pipe interior.
Key Benefits of Radial Ducting for MVHR
No Cross-Room Sound Transfer
Because each room's duct terminates independently at the manifold, sound cannot travel duct-to-duct. Voices from the kitchen won't carry to the master bedroom through shared ductwork — eliminating the need for acoustic attenuators at every valve.
Faster First-Fix Installation
Push-fit sealing rings replace tape and mastic. Two installers can drop a semi-rigid run from manifold to plenum box in minutes — no cutting tees to length, no worrying about airtight joints on the fly.
Fits Where Rigid Ductwork Can't
75mm and 90mm semi-rigid duct threads through 145mm or larger joist voids easily. The gradual bend radius means no fittings for changes of direction — critical for retrofit installations where you're working around cables, plumbing, and existing services.
Simpler Commissioning & Balancing
Each room's airflow is adjusted at the manifold — one central location — not at each individual valve. A commissioning engineer can balance a whole house in a fraction of the time it would take with branch ducting.
Lower Air Resistance = Lower Running Cost
Smooth-bore semi-rigid duct with gradual bends drops static pressure losses dramatically compared with corrugated flexi or branched rigid steel. The MVHR fan sees a lower system resistance, runs at a lower speed, and consumes less electricity year-round.
Hygienic and Long-Life
The anti-microbial inner coating actively suppresses bacterial and fungal growth on the duct wall, and the anti-static surface prevents electrostatic dust attraction. The result is a system that stays cleaner for longer, with fewer service interventions over its design life.
EPP Ducting: The Thermal-Insulated Trunk System
While the 75mm and 90mm semi-rigid ducts distribute air to individual rooms, the trunk runs between the MVHR unit and the outdoor terminals — where warm, moisture-laden extract air is exhausted, and where cold, fresh supply air enters — carry the full system airflow through what is often an unheated loft space. This is where EPP (Expanded Polypropylene) ducting comes in.
EPP is a closed-cell foam moulded into rigid duct sections, bends, and tees in 125mm, 160mm, and 200mm diameters. It has three properties that make it ideal for MVHR trunk runs:
Integral Thermal Insulation
EPP has a low thermal conductivity — the duct wall itself is the insulator. There's no separate insulation wrap to specify, cut, and tape. This eliminates condensation on cold intake and exhaust ducts running through unheated lofts, protecting both the fabric of the building and the efficiency of the heat exchanger.
Acoustic Dampening
The foam wall absorbs sound in a way that steel and PVC ducting simply cannot. In-duct fan noise from the MVHR unit is attenuated before it reaches the room terminals, keeping the system whisper-quiet even at boost speeds.
Lightweight and Cuttable
EPP sections can be cut with a fine-tooth handsaw or bread knife on site, and are light enough for a single installer to handle at height. PVC couplers (125mm, 160mm, 200mm) join sections and adapt EPP to standard duct spigots on MVHR units, cowls, and grilles.
Airtight by Design
The moulded connections seat tightly with EPDM gaskets — no tape, no silicone. Airtightness of the trunk is one of the biggest drivers of whole-system SFP (specific fan power), and EPP delivers it without installer variability.
Building Regulations: Part F, Part L, and SAP
Every whole-house MVHR system in the UK is subject to Building Regulations Part F (ventilation rates) and Part L (energy efficiency). The MVHR unit's SAP PCDB listing captures its heat recovery efficiency and specific fan power under standardised test conditions — but the ductwork installation determines whether the deployed system actually performs to those figures.
- Radial ducting reduces installation variability. Push-fit joints, factory-sealed manifolds, and gradual bends mean the system-as-built matches the system-as-designed.
- Fire stopping is mandatory at every compartment penetration. Ducts crossing a party wall or floor between dwellings, or between an attached garage and habitable space, need intumescent collars or fire-stop sleeves to maintain Part B compliance.
- SAP PCDB verification. If your installation needs to feed into an SAP calculation for Building Regulations, the MVHR unit must be PCDB-listed and the ducting configuration should follow the tabulated layouts published by Vectaire — this is why the entire Vectaire range at AIZO is SAP-listed as standard.
- Airtightness targets at commissioning: a well-installed radial system typically leaks less than 3% of the design airflow, well within any Part F assessment.
Radial vs Traditional Branch Ducting
For anyone considering a first MVHR installation — or replacing failed branch ductwork in an older system — the practical differences between the two distribution methods are substantial:
| Consideration | Radial (Semi-Rigid + EPP) | Branch (Rigid Steel or PVC) |
|---|---|---|
| Cross-room sound transfer | ✓ None — each duct is independent | ✗ High — needs attenuators at every valve |
| Typical duct diameter | 75mm or 90mm to rooms; 125–200mm EPP trunk | 150mm or 204×60mm flat rectangular |
| Fits through standard joists | ✓ Yes, even in existing 145mm voids | ✗ Often requires cutting or dropping ceilings |
| Joint sealing method | Push-fit EPDM rings — factory-sealed | Foil tape and mastic — installer-dependent |
| Air resistance | Low (smooth bore, gradual bends) | Higher (tees, elbows, transitions) |
| Fan energy consumption | Lower — SFP typically 0.40–0.50 W/l/s | Higher — SFP often above 0.60 W/l/s |
| Commissioning & balancing | Central adjustment at manifold | Adjustment at each room valve |
| Thermal loss on trunk runs | Very low (EPP is self-insulating) | Requires separate insulation wrap |
| Installation time (typical 4-bed) | 1–2 days for first-fix | 3–4 days for first-fix |
| Suitable for retrofit | ✓ Excellent — flexible and compact | ✗ Difficult — requires structural allowances |
Step-by-Step: Installing a Radial + EPP MVHR Duct System
Every installation should follow the manufacturer's specific instructions and be commissioned by a competent person in accordance with Building Regulations Part F. The steps below outline the workflow for a typical whole-house MVHR retrofit using Casavent Radial Ducting with a Vectaire MVHR unit — from planning through to commissioning.
Design & Airflow Calculation
Calculate the total supply and extract airflow required for the property under Building Regulations Part F Table 1.1. Confirm the MVHR unit's capacity (177 l/s for the Vectaire Maxi handles up to 250 m²; 238 l/s for the Maxi Plus BY handles up to 400 m²). Mark supply valves in habitable rooms and extract valves in wet rooms.
MVHR Unit Positioning
Mount the Vectaire unit vertically in a loft, cupboard, or utility space. Allow the required 200mm clearance on each side for motor access. Fix using the supplied wall bracket. Ensure the condensate drain point is accessible and correctly falls to waste.
Plan the Manifold Location
Position the 75mm EPP manifold within 1–2 metres of the MVHR unit — ideally directly above or adjacent. Two manifolds are typical: one for supply, one for extract. Manifold port count (8, 10, 12, or 13 spigots) should match the total valve count plus one spare per side for future expansion.
Trunk Run Between Unit and Manifold
Connect the MVHR unit's 149mm (Maxi) or 125mm (Midi) spigots to the manifold inlets using EPP duct sections. Use 125mm, 160mm, or 200mm EPP as appropriate — sized to match manifold inlet. PVC couplers join sections; 90° EPP bends route around obstructions. Keep runs as short and straight as possible.
Outdoor Intake and Exhaust
Extend EPP ducting from the MVHR unit's remaining two spigots to the external wall grilles or roof cowls. Maintain at least 1.5m separation between intake and exhaust terminals to prevent air recirculation. Fire-stopping sleeves are required at any compartment penetration.
Room-by-Room Semi-Rigid Runs
Push one end of a 75mm or 90mm semi-rigid duct into a manifold spigot with sealing ring. Route the duct through joist voids or above ceilings to the target room. Cut to length on-site using the Radial Duct Cutting Tool, ensuring a clean square end. Connect the other end into a wall or ceiling plenum box with sealing ring.
Fit Plenum Boxes and Air Valves
Mount 125mm air valve plenum boxes (1, 2, or 3 spigot variants available) at each room's ceiling or wall position. The plenum accepts the semi-rigid duct via push-fit sealing ring and presents a 125mm circular valve aperture to the room. Fit adjustable disc valves or grilles at the room face for final flow trimming.
Electrical Connection & Commissioning
Wire the MVHR unit to a 230V fused spur. Connect boost triggers (bathroom light switch, remote humidistat, or PIR) via switched live. Power up and use the integral or remote LCD commissioning unit to set trickle, boost, and purge speeds. Balance airflow at the manifold to match the design values calculated in Step 1.
The Vectaire MVHR Range
A radial ducting system is only as good as the unit driving it. The Vectaire range at AIZO Quality Heating covers residential properties from 170 m² to 400 m², all with counter-flow heat exchangers, automatic summer bypass, integral frost protection, and low-energy EC motors. Every unit is SAP PCDB listed for Building Regulations compliance and manufactured in the UK to ISO 9001:2015.
Vectaire Midi MVHR — Up to 170 m²
Compact whole-house MVHR unit delivering up to 94% heat recovery efficiency — the highest in the Vectaire range. 101 l/s peak airflow, EC brushless motors drawing just 2.2 W at trickle speed. Ideal for two- and three-bedroom homes, flats, and airtight new builds.
£1,995.00 Inc. VAT
Vectaire Maxi MVHR — Up to 250 m²
The workhorse of the Vectaire range for four- and five-bedroom family homes. 92% counter-flow heat recovery, 177 l/s peak airflow, SFP as low as 0.40 W/l/s. Compatible with the Vectaire Cool Breeze active cooling module for Approved Document O overheating mitigation.
£2,370.00 Inc. VAT
Vectaire Maxi Plus BY MVHR — Up to 400 m²
The flagship unit for large detached homes and light commercial. 238 l/s peak airflow, 92% heat recovery efficiency, integral automatic summer bypass, and Passivhaus-certified option available. Pairs with the Cool Breeze Plus cooling module for larger properties.
£3,100.00 Inc. VAT
MVHR Unit Specifications at a Glance
| Specification | Vectaire Midi | Vectaire Maxi | Vectaire Maxi Plus BY |
|---|---|---|---|
| Max property size | 170 m² | 250 m² | 400 m² |
| Peak airflow | 101 l/s (364 m³/hr) | 177 l/s (637 m³/hr) | 238 l/s |
| Heat recovery efficiency (max) | 94% | 92% | 92% |
| Specific fan power (min) | 0.50 W/l/s | 0.40 W/l/s | 0.40 W/l/s |
| Duct connection diameter | Ø125 mm (4 spigots) | Ø149 mm (4 spigots) | Ø149 mm (4 spigots) |
| Weight | 24 kg | 37 kg | ~45 kg |
| Dimensions (W×H×D) | 600 × 555 × 382 mm | 758 × 727 × 649 mm | Larger case |
| Automatic summer bypass | Yes | Yes | Yes |
| Integral frost-stat | Yes (+3°C to −8°C) | Yes (+3°C to −8°C) | Yes (+3°C to −8°C) |
| Sound @ 3m (100% / 20%) | 33.9 / 19.7 dBA | 38.1 / 20.2 dBA | Independently tested |
| Warranty | 5 years | 5 years | 5 years |
| SAP PCDB listed | Yes | Yes | Yes |
| Cool Breeze compatible | — | Yes (Cool Breeze) | Yes (Cool Breeze Plus) |
Radial & EPP Ducting Components
A full radial installation combines semi-rigid distribution ducting, EPP trunk sections, manifolds, plenum boxes, and sealing accessories. The full Casavent range is available in the Radial Ducting collection; the key components below are the ones every installation needs:
75mm Semi-Rigid Duct (50m coil)
From £199.00
90mm Semi-Rigid Duct (50m coil)
From £270.00
EPP Duct 1.5m (125/160/200mm)
From £75.00
EPP 90° Bend
From £37.00
EPP Equal Tee (160/200mm)
From £53.00
Round PVC Coupler (125/160/200mm)
From £11.30
Manifold 125mm × 8 spigot
£177.00
Manifold 160mm × 12 spigot
£190.00
Manifold 200mm × 13 spigot
£230.00
Air Valve Plenum (1/2/3 spigot)
From £33.00
Sealing Rings & End Caps
£3.00 each
Radial Duct Cutting Tool
£23.00
For alternative distribution methods depending on installation constraints, see also the Rigid Thermal Ducting 160mm collection or the Flexible Duct range.
Frequently Asked Questions
Which MVHR unit do I need for my house?
The Vectaire Midi handles properties up to 170 m² (typical two- to three-bedroom homes and flats). The Vectaire Maxi is suitable up to 250 m² — the sweet spot for four- and five-bedroom family homes. The Maxi Plus BY covers larger detached properties and light commercial installations up to 400 m². If you are close to a size threshold, always size up: an MVHR unit running at 60–70% of its capacity is quieter and more efficient than one running flat out.
Can I use radial ducting in a retrofit installation?
Yes — radial ducting is particularly well-suited to retrofit work. The 75mm and 90mm semi-rigid ducts fit through standard joist voids without structural modifications, and the flexible corrugated construction allows gradual bends around existing services (cables, pipework, downlighters). Most 3- and 4-bedroom UK homes can be fitted with a full MVHR system without lifting the finished flooring.
How often do I need to service the ducting?
The semi-rigid duct itself needs no periodic servicing — the anti-microbial and anti-static properties prevent build-up in normal use. The MVHR unit's G3 disposable filters should be replaced every 6–12 months depending on local air quality. An annual professional service is recommended to inspect the heat exchanger, verify airflow balance, and confirm all valves are unobstructed.
What is the difference between the MVHR unit's spigot size and the radial duct diameter?
The MVHR unit connects to the trunk via 125mm (Midi) or 149mm (Maxi/Maxi Plus BY) spigots. These trunk runs — usually EPP ducting — carry the full system airflow between the MVHR unit and either the outdoor terminals or the manifold. From the manifold, the smaller 75mm or 90mm semi-rigid ducts distribute to individual rooms. The step-down from trunk to radial happens at the manifold, not at the MVHR unit itself.
Is DIY installation possible, or do I need a professional?
The mechanical installation of the ductwork is well within the capability of a competent DIY installer familiar with routing pipework through joists. However, Building Regulations Part F requires the system to be commissioned by a competent person — someone who can verify airflow rates match the design and issue commissioning certificates. For new-build and major renovation projects, this is a Building Control requirement.
Can I add rooms to the system later?
Yes — this is one of the strengths of a manifold-based radial system. When specifying the manifold, add one or two spare spigots on each of the supply and extract manifolds. Sealing rings and end caps close the unused ports for now; when you want to add a room, you simply remove the cap, install a new semi-rigid run to a new plenum and valve, and re-balance the manifold. No re-plumbing of the trunk is needed.
Certifications & Standards
Ready to specify your radial MVHR system?
Our technical team can help size the right Vectaire MVHR unit for your property, quote the full radial ducting kit, and provide CAD drawings and airflow calculations for Building Control submissions.