A slow water weep from the boiler, with the system pressure dropping a little each week. Often the cause is a failing automatic air vent, or AAV. It's a known wear part on most modern boilers and one of the more common slow-leak diagnoses we see.
What an AAV does
An automatic air vent releases trapped air from the heating system without anyone having to bleed it manually. Air gets into a sealed system most commonly during maintenance: when you flush the system, drain it down to replace a radiator, or top up the pressure significantly. The AAV picks up that trapped air and lets it out as the system runs.
Inside is a small float chamber with a needle valve. As air collects, the float drops and opens the valve to let the air out. When water rises behind the air, the float lifts and closes the valve again. No manual bleeding required.
It's a useful component when it's working. When it fails, it tends to fail by leaking water rather than air.
Where to find it
The location depends on the boiler and the system:
- Most modern combi boilers: the AAV is inside the boiler casing rather than visibly mounted on top. You won't see it without taking the front cover off, which should only be done by a Gas Safe engineer
- Some specific models: notably the Baxi Duo-Tec range, the AAV is mounted externally on top of the boiler and visible without opening the casing
- Older gravity-fed (open-vented) systems: AAVs are often fitted to the pipework rather than to the boiler. Common locations include the loft near the cold-water tank, or where a vertical pipe rises to the highest point of the system
If you suspect a failing AAV, where to look depends on which type of system you have. An engineer's diagnostic visit identifies it quickly.
The symptoms
A failing AAV usually shows up as one or more of:
- A slow water leak in the area around the AAV. Visible from outside the boiler on Baxi Duo-Tec models and older system types. On most modern boilers the leak shows up as moisture or staining inside the casing, or sometimes as water collecting at the base of the unit
- White or greenish mineral staining around the AAV body or below it. Dried residue from water that's evaporated leaves a clear trail back to the source
- Slow loss of system pressure over weeks. Topping up the filling loop fixes it temporarily, but the pressure drops again within days
- Damp patches on the boiler internal trim, on a wall or ceiling near a loft-mounted AAV, or on the floor below a leaking unit
Why it happens
The AAV has a rubber seal inside that closes against the needle valve. Like all rubber components in a heating system, that seal perishes over time. Heat cycles, mineral deposits, and the slight pressure differences across the seal during normal operation all gradually wear it out.
When the seal stops sealing properly, water seeps past the closed valve and out of the AAV. The system loses a tiny amount of pressure each day, which adds up over weeks to a noticeable drop on the pressure gauge.
This isn't a fault as such. It's normal wear. AAVs are designed to be replaced periodically, the same way you'd expect to replace a tap washer at home.
Why this is worth catching early: A slow leak from the AAV is a small problem on its own. Left long enough, the constantly damp surrounding components start to corrode. Electrical connectors close to the leak can be compromised. Catching the AAV failure early keeps the repair to a part-only job. Catching it late can mean the boiler needs more extensive work.
What it costs to fix
An AAV is a part-only replacement on most boilers. The replacement itself is around £150 inclusive of parts and labour, with the work taking 30 to 60 minutes once it's underway. Diagnosis happens first under the £99 call-out, after which you get a clear quote for the fix.
If you've been topping up the filling loop every few weeks because the pressure keeps dropping, replacing the AAV usually solves it for years.
Is it definitely the AAV?
A slow water leak with pressure loss isn't always an AAV. Other possibilities worth ruling out:
- The pressure relief valve (PRV), which dumps water externally if the system pressure goes too high. A failed expansion vessel can cause the PRV to weep even when the pressure looks normal
- The heat exchanger, which can develop pinhole leaks over many years. This is a more serious diagnosis that may make the boiler beyond economic repair
- A leaking joint anywhere on the system. Solder, compression, and press-fit joints can all weep over time if they weren't installed perfectly. Joints inside the boiler are usually O-ring and clip designs rather than compression, but the connection from the boiler out to your household pipework is often a compression joint that can fail
- A radiator valve or towel rail joint, often a slow weep that's been going on long enough to leave staining on the wall or floor below
Diagnosis is a job for an engineer who knows what to look for. Once the source is confirmed, the fix is straightforward.
When to call
If you're seeing any combination of damp around the boiler, mineral trails on internal components, and pressure dropping faster than you'd expect, give us a call. The £99 call-out covers the diagnosis. If the cause is an AAV, the part is usually on the van so the fix can often go ahead at a separately quoted cost without a return visit.
Related advice
If your boiler is also showing low pressure faults and you're topping up the filling loop more often than usual, our article on how to use the filling loop covers what to do in the short term and what's likely behind the recurring loss.
