Automatic air vent buying guide
Updated: 23 October 2026
You are looking at a loft pipe, a manifold or the high point above a towel rail, and the old vent is in your hand. The amazon.co.uk results for an automatic air vent mix 15 mm compression nuts, 3/8 inch screws, 1/2 inch screws and a boiler spare with no thread. This automatic air vent buying guide is the order we use before the rows on the [automatic air vent comparison](/hub/diy-supplies/automatic-air-vent): match the joint you can see, then the pressure line, then whether you can take the head off later. Prices are amazon.co.uk shelf prices from 19 October 2026. If you buy through our links, we may earn a commission at no extra cost to you.
You are looking at a loft pipe, a manifold or the high point above a towel rail, and the old vent is in your hand.
You are looking at a loft pipe, a manifold or the high point above a towel rail, and the old vent is in your hand. The amazon.co.uk results for an automatic air vent mix 15 mm compression nuts, 3/8 inch screws, 1/2 inch screws and a boiler spare with no thread. This automatic air vent buying guide is the order we use before the rows on the automatic air vent comparison: match the joint you can see, then the pressure line, then whether you can take the head off later. Prices are amazon.co.uk shelf prices from 19 October 2026. If you buy through our links, we may earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Connection | Shut-off | Max pressure | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Flamco Flamco Flexvent 3/8 inch | 3/8 inch | Yes | 6 bar (10 bar peak) | 8.9 | about £16.79 | View offer |
| Best value |  Altecnic Altecnic Robocal 15 mm | 15 mm compression | n/a | 6 bar venting, 10 bar working | 7.5 | about £10.69 | View offer |
| Alternative |  Wärmer System Wärmer System 15 mm | 15 mm compression | n/a | n/a | 5.0 | about £7.99 | View offer |
On this page
Start with the connection you can see
A float vent only helps at the high point where you fit it, and only if the joint matches the pipe or the port already there. On a typical UK heating run that joint is 15 mm compression. The Altecnic Robocal 502649, at about £10.69, is that nut. The Wärmer System 7313B, at about £7.99, is the same family of fitting, with no pressure figure on the listing.
If the old part is a male thread, measure it. A 3/8 inch port is smaller than a 1/2 inch BSP port. The Flamco Flexvent 27750 is the 3/8 inch male vent in this set, at about £16.79. The unbranded cylinder at about £8.84 prints 1/2 inch in the title and 3/8 x 1/2 in the bullets.
Shut-off or a plain body
The shut-off is a small valve under the float so you can lift the head later without draining to that point. In this amazon.co.uk set, only the Flamco listing states that it is included. The Altecnic page does not state a shut-off. One reviewer added an isolating valve before the Robocal, which is an extra joint.
Pay the gap from about £10.69 to about £16.79 when a drain is messy: a loft, a boxed-in riser, or a manifold. Skip the shut-off when you can drain to that tee easily. If you already know the size, the best automatic air vent page names who each listing is for.
Pressure, temperature, and glycol
A domestic sealed system is often filled cold to around 1 to 1.5 bar. That is a rule of thumb, and your boiler manual is the number you restore. The Flamco bullets say below 90 °C with a 120 °C tip, and 6 bar with a 10 bar peak. The Altecnic listing says 6 bar maximum venting and 10 bar maximum working. The maker's sheet also gives 0-115 °C and glycol to 30 percent.
The ERGAOBOY listings claim 1.6 MPa (16 bar) and 150 °C. The only UK review on the 1/2 inch pair says the valve does not vent by itself. WRAS 2012359 appears on a merchant page for the Robocal. It is not printed on the amazon.co.uk listing, and it does not cover a generic brass vent.
What this set is not
The HTS listing at about £11.50 is sold against Vaillant reference 104521. The page names HTS, prints no thread and no GC number, and the reviews talk about inserting a part. Opening a gas boiler case is work for a Gas Safe registered engineer. A hygroscopic radiator bleeder is a different object and is not in this set. The mizikuu three-pack is sold as 1/2 inch radiator bleeders, closer to a radiator tail than to a 15 mm loft bottle.
Entry prices sit around £8 to £10, the mid band around £10 to £13, and the Flamco at about £16.79. If the port is still ambiguous, photograph the old thread before you order.
Common questions
Do I need 15 mm compression or a screwed port?
Look at the fitting you are removing. A nut and olive on 15 mm copper or barrier pipe is compression, which in this set means the Altecnic 502649 or the Wärmer 7313B. A male thread into a brass boss is a screwed port. Measure it. The Flamco 27750 is 3/8 inch. A 1/2 inch port is a different screw, and the cylinder listing that prints both sizes does not decide it for you.
Is 3/8 inch the same as 1/2 inch BSP?
No. They are different threads, and a 3/8 inch vent will not seal in a 1/2 inch port. One Flamco reviewer thought they had received 1/2 inch while the title says 3/8 inch, so check the port and the delivered thread before you fill. The Regun and Fdit pages say G 1/2, which you should match to the port you have rather than treating every half-inch line as identical.
Is a shut-off worth the extra pounds?
It is worth it when the vent is somewhere you do not want to drain twice. The Flamco at about £16.79 includes one, and reviewers use it to lift the head later. The Altecnic at about £10.69 does not state one. You can add an isolator, though that is another joint. If a drain to that tee is easy, the plain 15 mm body is the simpler buy.
What pressure should a domestic vent be rated for?
Your cold fill is often around 1 to 1.5 bar, as a rule of thumb, and the boiler manual sets the figure you restore. A listing at 6 bar working and 10 bar peak, which is what Flamco and Altecnic print, covers that with room. A claimed 16 bar on a vent with a single bad review does not make it the safer choice.
Can the same vent go on a solar loop?
Only if that maker's sheet allows the temperature and the glycol strength, and the reviews support it. The Altecnic sheet allows glycol to 30 percent and up to 115 °C. The ERGAOBOY 1 inch page claims solar use, and the only written review, which is not from the UK, says it would not seal on a solar mat and that water followed the air. Flamco's manual rules the Flexvent out of drinking water.
Does WRAS on one data sheet cover every brass vent?
No. Merchant pages cite WRAS 2012359 for the Robocal. That number is not on the amazon.co.uk listing in this capture, and it does not transfer to the Wärmer, the Regun, the Fdit or the unbranded cylinder. If a water-regulations approval matters for your job, you need it on the document for the exact code you are fitting, not on a neighbour's brass body.
Related pages
Sources
Public UK sources and the amazon.co.uk listings compared on this page. Listings captured 19 October 2026.
- Altecnic, how an automatic air vent works (retrieved 19.10.2026)
- Altecnic 502 Robocal data sheet, October 2024 (retrieved 19.10.2026)
- Flamco Flexvent installation manual (retrieved 19.10.2026)
- Flamco catalogue chapter, order code 27750 (retrieved 19.10.2026)
- Caleffi Robocal 01033 (retrieved 19.10.2026)
- Gas Safe Register boiler appliance guide (retrieved 19.10.2026)
- Vaillant 104521 superseded, Heating Spare Parts (retrieved 19.10.2026)
- BES Robocal, WRAS 2012359 (retrieved 19.10.2026)