Line vs Performance Line vs Circle: Which Ventmann Diffuser Should You Specify?
Three Ventmann ranges solve the same problem — moving air into a room without putting a visible grille on the ceiling. They are not interchangeable. This is the straight comparison: what each one is, what it costs, where it wins, and where it is the wrong call.
The short answer
Line — the architectural default. A clean frameless slot, lowest entry price, ideal for residential and light commercial supply and extract.
Performance Line (PLINE) — the same slot language with a curved internal face that works the Coanda effect harder. Specify it where throw, comfort and duty matter: AC supply, higher flow rates, bigger rooms.
Performance Circle (PCIRCLE) — a round frameless diffuser for point supply. Lowest unit price, simplest install, the honest choice when you do not need a linear aesthetic.
Price and spec at a glance
Prices below are the published starting prices on ventmann.co.uk, excluding VAT, correct at the time of writing. Trade accounts take a further 10% off every order.
| Line | Performance Line | Performance Circle | |
|---|---|---|---|
| From (ex VAT) | £125.00 | £148.11 | £86.65 |
| Form | Linear slot | Linear slot, curved inner face | Circular |
| Lengths / sizes | 625, 1000, 1250 mm | 625, 1000, 1250 mm | Ø125, 160, 200 mm spigot |
| Slot options | 1, 2 or 3 slots · 12 or 18 mm gap | 1, 2 or 3 slots · 20 mm gap | 1 or 2 slot |
| Air pattern | Coanda, adjustable deflector | Enhanced Coanda — wider dispersal, lower perceptible velocity | Radial Coanda, 360° |
| Deflector options | Standard | Standard, perforated, or none | Integral |
| Indicative duty | Up to ~Ø160 spigot | Up to ~Ø200 spigot | ~200 m³/h at Ø160; ~300 m³/h at Ø200 |
| Plenum | GTG (top) or VTBOX (side) | GTG (top) or VTBOX (side) | Direct spigot connection |
| Material | Natural gypsum composite throughout — sanded, jointed and painted flush with the plasterboard | ||
| Warranty | 5-year manufacturer's warranty on all three ranges | ||
Line: the architectural default
The Line diffuser is the range most Ventmann projects start with. A gypsum body with one, two or three slots, set into the ceiling or wall, jointed with plasterboard compound and painted. Once decorated there is no frame, no fixing, no sightline — just a shadow gap in the plane.
You choose two things: length (625, 1000 or 1250 mm) and slot gap (12 or 18 mm). The 12 mm gap is the quieter architectural detail and reads almost as a reveal; the 18 mm gap gives you more free area for the same length, which matters when you are tight on ceiling run.
Specify Line when: the duty is residential or light commercial, the ceiling has room for a continuous run, and the aesthetic requirement is "make it disappear". It is also the sensible choice for extract, where throw performance is irrelevant and you are paying for the look.
Don't specify Line when: you are pushing cooled air at a high rate into a large volume. The slot will do it, but you are asking a standard profile to do a Performance Line's job, and the first thing the client notices is a draught.
Performance Line: the same look, more engineering
The Performance Line (PLINE) looks near-identical in the ceiling. The difference is inside: a curved inner face that attaches the air jet to the surrounding surface far more effectively, spreading supply air wide and low-velocity rather than throwing it in a concentrated stream. That geometry is what earned the range a Red Dot Award, and it is the whole reason for the £23 premium over a Line of the same length.
It also gives you deflector choice. The standard deflector suits most supply applications; the perforated deflector softens the discharge further where occupants sit directly beneath; removing it entirely maximises free area for extract.
The rule of thumb: if the diffuser is on a cooling circuit, or sits above a desk, bed, dining table or any seat someone occupies for an hour at a time, the Performance Line pays for itself in complaints you never receive. Read more on how the Coanda effect works.
Performance Circle: point supply, lowest cost
The Performance Circle (PCIRCLE) is the one people overlook, and it is the cheapest frameless diffuser Ventmann makes — from £86.65 against £125.00 for a Line. It connects straight to a Ø125, Ø160 or Ø200 spigot with a click system, so there is no plenum box to buy, no plenum to coordinate and no plenum to hang.
That changes the real cost comparison significantly. A Line at £125.00 needs a GTG acoustic box from £85.41 behind it — call it £210 installed-ready. A Performance Circle at £86.65 is the complete item. For a plant room corridor, a utility, a WC, a plant-adjacent extract point or any location where nobody is going to stand underneath admiring the linearity, it is a straightforward saving of more than half.
Duty is respectable too: up to around 200 m³/h on the Ø160 and up to around 300 m³/h on the Ø200 — enough for most single-room residential supplies.
Don't specify Circle when: the ceiling design is built on lines — a reflected ceiling plan with linear lighting, a shadow-gap perimeter, a coffered rhythm. A round diffuser in a linear ceiling is the one element that reads as services.
Cost in context: the real line item
Comparing diffuser unit prices alone is misleading. Here is what a single supply point actually costs to buy, ready to install.
| Supply point | Diffuser | Plenum | Indicative total |
|---|---|---|---|
| Performance Circle Ø160 | £86.65 | none required | £86.65 |
| Line 625 mm + GTG | £125.00 | £85.41 | £210.41 |
| Performance Line 625 mm + GTG | £148.11 | £85.41 | £233.52 |
| Performance Line 625 mm + VTBOX (side entry) | £148.11 | £94.58 | £242.69 |
Starting prices ex VAT, correct at the time of writing. Longer lengths, additional slots and larger width codes increase both figures.
How to choose in thirty seconds
Work down this list and stop at the first yes.
- Is the ceiling design linear? If the RCP is built on lines, you are choosing between Line and Performance Line. Skip to 3.
- Is the location utilitarian or hidden? Specify Performance Circle and bank the saving.
- Is it on a cooling circuit, or above an occupied seat? Specify Performance Line.
- Is it extract, or supply-only heating at modest rates? Specify Line.
- Still unsure? Run the duty through the product selector, which filters by flow rate, pressure drop, dB(A) and installation length.
What all three share
Whichever you pick, the fundamentals do not change: a natural gypsum composite body that takes jointing compound and paint like the board around it, a frameless finish with no visible fixings, a 5-year manufacturer's warranty, BIM objects for Revit, and next business day despatch on orders placed before 11:00 am.
Next step
Send us the schedule and we will size the diffusers, match the plenums and price it — no charge, usually same day.
Request a free quotation · Open a trade account (10% off) · Download the technical catalogue
Browse the ranges: Line · Performance Line · Performance Circle · Square · Plenum boxes
Where to go next
- Sizing a diffuser from your design rates — the spigot and slot-count arithmetic behind the choice
- Picking the plenum box: GTG vs VTBOX — what goes behind a Line or Performance Line
- Hotel guest room ventilation — specifying for noise first
- Office diffusers and draughts — why the jet drops, and how to stop it
- MVHR diffusers for self-build — the residential version of the same decision
