Acoustic Box vs Thermo Box: Which Plenum Does Your Slot Diffuser Need?

Every Ventmann linear slot diffuser breathes through a plenum box hidden above the ceiling — and Ventmann makes two: the Acoustic Box (GTG series) and the Thermo Box (VTBOX series). They do the same job — evening out air pressure so the slot delivers a clean, uniform stream — but they are built for different situations.

The short answer

Let the duct decide. If your ducting drops onto the plenum from above, use the Acoustic Box — its foam insert soaks up airflow noise on the way down. If the duct comes in from the side, use the Thermo Box — its insulated V-shape body is made for lateral entry and keeps the supply air at temperature.

What they have in common

• The same job — balancing air pressure behind the diffuser so airflow and velocity are even across the whole slot.
• The same lengths — 625, 1000 and 1250 mm, matched to Ventmann linear slot diffusers.
• Width codes — each box carries a width code that must match the code on your slot diffuser.
• Pre-scored knock-outs — remove the one you need and fit the spigot from outside with tech screws and sealant.
• The same installation sequence — temporary batons, pre-assembled diffuser and plenum, fix, board, fill, sand, paint. See the plenum box installation guide.
• A 5-year warranty and next-day UK delivery on orders before 11:00 AM.

The real difference: where the duct comes in

Acoustic Box (GTG) — duct enters from above

Thermo Box (VTBOX) — duct enters from the side

The Acoustic Box takes a top-entry connection. Air dropping vertically into a plenum is where turbulence — and therefore noise — is generated, so the GTG puts a high-quality foam insert right where it matters. The result is a quieter slot in bedrooms, offices, hotels and classrooms.

The Thermo Box takes a lateral (side) entry. Its insulated, V-shaped body carries the air across to the slot while limiting heat exchange with the ceiling void — better energy efficiency when the supply air is heated or cooled. It arrives flat-packed, folds into shape and screws together with the supplied SQ2 screws, which makes it easy to move around site.

Side-by-side comparison

	Acoustic Box (GTG)	Thermo Box (VTBOX)
Duct entry	Top — from above	Lateral — from the side
Headline benefit	Foam insert reduces airflow noise	Insulated body for thermal regulation
Body shape	Box plenum	Insulated V-shape
Delivery format	Ready to fit	Flat-packed — fold and screw (SQ2 bit)
Spigot knock-outs	75, 125, 160 mm	125, 160, 200 mm
Lengths	625, 1000, 1250 mm	625, 1000, 1250 mm
BIM objects	GTG series (.rfa)	V Shape Insulated A, B, C (.rfa)
Price from (excl. VAT)	£85.41	£94.58
Best for	Quiet rooms with duct above	Shallow voids and heated or cooled supply

See them on their product pages: Acoustic Box and Thermo Box.

Where each one belongs

Choose the Acoustic Box for

• Bedrooms, offices, hospitality and educational spaces where airflow noise would be noticed.
• Risers and voids where the ducting arrives vertically, straight onto the top of the plenum.
• MVHR supply terminals — low-pressure systems stay whisper-quiet when the plenum absorbs the turbulence. New to heat recovery? Start with What is MVHR?

Choose the Thermo Box for

• Ceiling voids where the duct must run horizontally into the side of the plenum.
• Heated or cooled supply air — the insulation limits energy loss into the void.
• Projects that want flat-packed logistics — easy to transport, store and assemble on site.

Specifying tips

• Match the width code. The code on the box (A, B or C) must match the code required by your LINE or PLINE slot diffuser.
• Size from airflow, not looks. Enter your flow rate in the Product Selector and tick “Matching Plenum Boxes”.
• Pick the knock-out to suit your ducting — spigots are supplied separately and fixed from outside.
• Balance the branch — an inline foam damper in the spigot trims an overblowing run without adding noise.

Quick answers

Can I use a Thermo Box with a top-entry duct?

Ventmann’s guidance is simple: top entry takes the Acoustic Box (GTG series), side entry takes the Thermo Box (VTBOX series). Choosing by entry direction keeps the airflow path — and the performance — as designed.

Does the Acoustic Box insulate like the Thermo Box?

No — its job is noise. The foam insert reduces airflow noise; if limiting heat exchange in the void is the priority, the insulated Thermo Box is the right pick.

What do the width codes A, B and C mean?

They match the plenum opening to the width of your slot diffuser. Always order the box with the same width code your diffuser requires.

Do they install differently?

The sequence is the same — batons, pre-assembled diffuser and plenum, fix, board, fill, paint — except the Thermo Box is folded and screwed together first. Both are covered in the installation guide, each with a short video.