Automatic air vent leaking
Updated: 23 October 2026
You have water at the automatic air vent: a wet cap in the cylinder cupboard, a stain under the loft pipe, or a boiler gauge lower than last week. An automatic air vent leaking is a cap left too far open for that model, or a float that no longer seals. This page separates those two faults, and it stops if the water is inside a gas boiler. Which part to order stays on the [automatic air vent comparison](/hub/diy-supplies/automatic-air-vent). Products Compared used maker sheets and amazon.co.uk reviews from 19 October 2026. A purchase through our links may earn a commission at no extra cost to you.
You have water at the automatic air vent: a wet cap in the cylinder cupboard, a stain under the loft pipe, or a boiler gauge lower than last week.
You have water at the automatic air vent: a wet cap in the cylinder cupboard, a stain under the loft pipe, or a boiler gauge lower than last week. An automatic air vent leaking is a cap left too far open for that model, or a float that no longer seals. This page separates those two faults, and it stops if the water is inside a gas boiler. Which part to order stays on the automatic air vent comparison. Products Compared used maker sheets and amazon.co.uk reviews from 19 October 2026. A purchase through our links may earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Connection | Shut-off | Mechanism | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Flamco Flamco Flexvent 3/8 inch | 3/8 inch | Yes | Float | 8.9 | about £16.79 | View offer |
| Best value |  Altecnic Altecnic Robocal 15 mm | 15 mm compression | n/a | n/a | 7.5 | about £10.69 | View offer |
| Alternative |  PlumbEZ PlumbEZ 15 mm slimline | 15 mm compression | n/a | Float | 3.2 | about £9.99 | View offer |
On this page
Stop the water reaching electrics or a ceiling
Let the heating go cold. A vent on a hot circuit spits scalding water. Stand a bowl under the cap, switch the heating off, and read the gauge cold. A sealed system often sits near 1 to 1.5 bar when cold. That is a rule of thumb. If the needle is near zero, stop topping up while water is still leaving the vent.
Closing the cap stops the weep and also stops the venting. Use a shut cap only as a plug until you know whether the setting was wrong. Flamco's manual says the expanding rings close the airway when wet. If the vent is inside the boiler case, leave the case on.
Cap setting versus a real leak
There is no single cap position. Caleffi's Robocal note says one full turn open, left there. Flamco's Flexvent is about half a turn open while you fill, then hand-tight. Using the Caleffi setting on a Flamco lets water follow the air. A Wärmer review describes water from a slightly open red top.
A working vent should stay dry in the maker's running position. A green stain on the brass is a reason to replace the valve. If the cap already matches the sheet and water still comes, replace the vent. Fitting is on how to fit an automatic air vent.
A float that never sealed
Air escapes while the float is down. Water should lift the float and shut the needle. A loose float leaves the needle off the seat. These bodies are not sold as a seal kit. Replace the whole vent.
A PlumbEZ review says the float was not assembled. On 19 October 2026 that listing was about £9.99, at 3.4 from four ratings, so this is one report. One Flamco review mentions a leak in a pack of six. After any drain, dose inhibitor again. BS 7593 is the code of practice for that refill.
Pipe cupboard or boiler case
You can change the vent when it is on pipe you can reach, isolate or drain, and refill. A 15 mm compression nut does not fit a 3/8 inch port.
Only the Flamco Flexvent 27750 in this comparison states a shut-off. On 19 October 2026 it was about £16.79, at 4.5 from 1,255 ratings. The Altecnic Robocal 502649 was about £10.69 and does not include a shut-off. Without an isolator, drain to the level of the vent.
Water inside the boiler case is not this repair. Gas Safe Register treats that as work for a registered engineer. Leave the case on. If the pipe vent is dry and the gauge still falls, look at a union, the filling loop, or the expansion vessel.
Common questions
Why is water coming out of the top?
The needle is off its seat while the airway is open. A Caleffi Robocal runs one full turn open. A Flamco Flexvent runs hand-tight after filling, not that open turn. Water with the cap already in the maker's position means a failed float, a float that was never assembled, or dirt on the seat. A working vent stays dry. Cleaning the outside of the cap does not restore the seal.
Should I screw the cap tight and leave it?
A closed cap usually stops the drip and will protect a ceiling until you replace the valve. It also stops automatic venting, so air collects and radiators can go cold at the top. National Pumps and Boilers call that a temporary stop. Leave the cap shut only until the system is cool and a new vent is ready, or until an engineer attends a vent you cannot reach.
Is a slow weep normal?
No. With the cap in the maker's running position, the vent should stay dry. A weep from a cap that is too far open is a setting fault, which is what the Wärmer review describes when a slightly open red top passed water. A weep from a correctly set cap is a failed seal. A green stain on the brass is a reason to replace the valve.
Can a new vent leak because the float was loose?
Yes. A PlumbEZ buyer on amazon.co.uk reported a float that was not assembled. The listing had four ratings, at 3.4, on 19 October 2026, so treat that as one report. Check the float moves in its guide before you drain. A loose float will pass water as soon as the system is full.
Will this drop the boiler pressure?
Yes, if water is leaving the cap, because that water is system water. A cold sealed system often reads about 1 to 1.5 bar, as a rule of thumb, and a needle near zero beside a wet cap fits this fault. A dry vent with a falling gauge can instead be a radiator union, the filling loop, or the expansion vessel. Do not keep adding water while the cap is still dripping.
Can I swap a loft vent without draining if it has a shut-off?
You can if you can see the shut-off and you close it first. In this set only the Flamco Flexvent 27750 states one. Close it, catch the water left in the head, and fit the new vent upright. With no isolator, drain to the vent, refill, and dose inhibitor under BS 7593. A bowl in the loft does not replace a shut-off that is not there.
What if the drip is inside the boiler?
Leave the case on and book a Gas Safe registered engineer. Water inside a gas boiler is appliance work. Homecure Plumbers treat a leak in the case the same way. Note the cold pressure. Do not fit a generic vent in place of the boiler maker's part, and do not remove the cover to look.
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)