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How to fit an automatic air vent

Updated: 23 October 2026

The towel rail or the loft pipe has gone cold at the top again, and you have a new float vent on the bench. Fitting it is straightforward on an accessible pipe you can drain, and it is the wrong job if the old vent is inside a gas boiler. This is how to fit an automatic air vent on the pipe: upright, where you can see a drip, with the cap set the way that maker describes. Product choice stays on the [automatic air vent comparison](/hub/diy-supplies/automatic-air-vent). Products Compared used maker sheets and amazon.co.uk reviews, not a workshop test. If you buy through our links, we may earn a commission at no extra cost to you.

The towel rail or the loft pipe has gone cold at the top again, and you have a new float vent on the bench.

Our top picks

Flamco Flamco Flexvent 3/8 inch

Best overall

Flamco Flamco Flexvent 3/8 inch
8.9/10about £16.79
View offer
Altecnic Altecnic Robocal 15 mm

Best value

Altecnic Altecnic Robocal 15 mm
7.5/10about £10.69
View offer
Wärmer System Wärmer System 15 mm

Alternative

Wärmer System Wärmer System 15 mm
5.0/10about £7.99
View offer
On this page
  1. 1.Before you undo anything
  2. 2.Make the joint
  3. 3.Cap, refill, and radiators
  4. 4.When to stop and call someone

Before you undo anything

Let the system go cool. Hot water at a high point will scald, and a vent opened on a hot circuit dumps water as well as air. Find the filling loop before you drain. If a shut-off sits under the old head, you may only need to isolate that branch. On the Flamco Flexvent, reviewers lift the head later without a full drain. If there is no isolator, drain to the level of the vent. Stop if the part is inside the boiler case. Gas Safe Register treats work on a gas boiler as a job for a registered engineer. Match the new vent to the old joint first, using the automatic air vent buying guide, so you are not standing with a 15 mm nut in a 3/8 inch port.

Make the joint

A 15 mm compression vent, such as the Altecnic Robocal, tightens with a nut and olive on clean copper or barrier pipe. You do not wrap PTFE around the olive. If the old olive will not come off, one reviewer cut about 15 mm of pipe back and made a fresh joint. A male screwed vent, such as the Flamco 3/8 inch, wants sealant on the thread. Reviewers commonly use PTFE tape. An O-ring port may not want tape, because the ring is the seal. The body has to stand upright at a high point. Flamco's manual says vertical, and not above something that cannot get wet. A float on its side does not close. Leave room above the cap. Tighten the joint so it is firm, then stop.
Flamco Flamco Flexvent 3/8 inch

Testsieger

Flamco Flamco Flexvent 3/8 inch
8.9/10about £16.79
View offer

Cap, refill, and radiators

The sheets do not agree on the cap. Flamco's manual says loosen it about half a turn while you fill, then nip it hand-tight. Caleffi's Robocal instruction is one full turn open, left open. Reviewers of the Altecnic and the Wärmer found that a tight red or plastic top traps the air. Follow the sheet for the valve in your hand. Refill slowly through the filling loop. You should hear air at the vent, then the noise should stop. Bleed each radiator with a key on this first fill. Set the cold pressure to the figure in the boiler manual. A domestic gauge often sits around 1 to 1.5 bar cold, which is a rule of thumb. After a drain, dose inhibitor in line with BS 7593.

When to stop and call someone

Stop if the pressure will not hold after the refill and the cap is set the way that maker asked. A falling gauge with a dry cap can be a radiator union, the filling loop, or the expansion vessel. Stop if you cannot tell 3/8 inch from 1/2 inch. The unbranded cylinder prints both. Stop if water keeps coming from a correctly set cap. That is a failed float, or a vent that arrived unassembled. Call an engineer for the boiler case, for a system you cannot refill, and for a landlord's heating you are not entitled to drain. A float vent does not retire the radiator key.

Common questions

Does the vent have to be vertical?
Yes. The float has to rise and close the needle. Flamco and Caleffi both want it upright at a high point you can see. A vent laid sideways on a radiator plug will not do that job. Leave access to the cap, and do not put it where a drip lands on electrics or where an uninsulated loft can freeze it.
Should the cap be tight or loose?
It depends on the maker. Flamco says about half a turn open while filling, then hand-tight. Caleffi's Robocal sheet says one full turn open, left open. Reviewers of the Altecnic and the Wärmer found a tight cap simply trapped the air. Read the sheet in the box you bought, or the maker's PDF for that code, and do not treat one brand's cap as universal.
Do I drain the whole system?
You drain down to the vent if there is no isolator under it. With a shut-off, which the Flamco includes and the Altecnic listing does not, reviewers change the head without a full drain. Either way the system should be cool, and you need a way to refill and dose inhibitor afterwards. If you cannot do that, do not open the joint.
Do I use PTFE on a compression nut?
Not on the olive. The olive seals against the pipe. PTFE belongs on a male thread when that joint is a screwed port, which is how Flamco reviewers sealed the 3/8 inch. An O-ring joint may seal worse with tape. One reviewer cut the pipe back when an old olive was stuck, then made a new compression joint.
Can I fit the vent inside the boiler myself?
No. A gas boiler case is for a Gas Safe registered engineer. The HTS listing sold against Vaillant 104521 is that sort of part, not a 15 mm fitting you add in the airing cupboard. Water from inside the case is an engineer visit, not a tighter cap.
Do I still bleed the radiators after fitting one?
Yes, on the first fill, and any time a single radiator is cold at the top. The pipe vent only clears the high point where it stands. Panels further along the run can still hold air. Keep the bleed key. A vent that is doing its job should then stay dry. If it weeps with the cap set correctly, replace it rather than leaving a bucket under it for the winter.

Related pages

Automatic air vent leaking

Automatic air vent vs bleed key

Automatic air vent

Best automatic air vent: top pick and best value 2026

Automatic air vent buying guide: match the joint first

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 19 October 2026.

  1. Altecnic, how an automatic air vent works (retrieved 19.10.2026)
  2. Altecnic 502 Robocal data sheet, October 2024 (retrieved 19.10.2026)
  3. Flamco Flexvent installation manual (retrieved 19.10.2026)
  4. Flamco catalogue chapter, order code 27750 (retrieved 19.10.2026)
  5. Caleffi Robocal 01033 (retrieved 19.10.2026)
  6. Gas Safe Register boiler appliance guide (retrieved 19.10.2026)
  7. Vaillant 104521 superseded, Heating Spare Parts (retrieved 19.10.2026)
  8. BES Robocal, WRAS 2012359 (retrieved 19.10.2026)