The Great Automatic Air Vent Debate.

The Great Air Vent Debate: Should Your Boiler's Automatic Air Vent Be Open or Closed? 

It's a question that puzzles many homeowners and even some seasoned gas engineers: what exactly is the deal with those little caps on your boiler's automatic air vents (AAVs)? 

Should they be tightly screwed down, or left loose and open? 

The answer, as with many things in heating, isn't quite as simple as a yes or no – it depends on the type of vent and its purpose in your system. 

Let's clear the air, once and for all! 

Understanding Automatic Air Vents (AAVs) First, a quick refresher: AAVs are vital components in your central heating system, designed to automatically release air that gets trapped inside. 

Why is this important? 

 * Noise: Trapped air causes gurgling, banging, and whistling noises. 

 * Inefficiency: Air pockets prevent hot water from circulating properly, leading to cold spots in radiators and reduced heating efficiency. 

 * Corrosion: Oxygen in the air can cause internal corrosion, shortening the lifespan of your boiler and pipework. 

AAVs work using a simple float mechanism. When air collects, the float drops, opening a small valve to release the air. As water fills the chamber, the float rises, closing the valve and preventing water from escaping. 

Now, let's tackle the two main types: 

1. Integral Automatic Air Vents (Found Inside Combi Boilers) Many modern combi boilers, like those from Worcester Bosch, Vaillant, Ideal, and Baxi, feature a small, often brass or plastic-capped AAV located internally, usually near the heat exchanger or pump. 

The Confusion: People often assume this cap should be screwed down after commissioning. The Truth: For integral automatic air vents, the cap should almost universally be left loose or unscrewed during normal operation. 

Why? 

 * Continuous Function: These vents are designed to provide ongoing protection against air build-up within the boiler's delicate components. If the cap is screwed down tightly, it prevents the vent from doing its job, trapping air inside the boiler. 

 * Self-Closing Valve: The actual valve mechanism inside the AAV is what closes when water pressure is present. The cap is more of a secondary seal or for maintenance, not for shutting off the automatic function. As one Vaillant manual explicitly states: "This cap must be left slackened during boiler operation to ensure any residual air or system gases are released." 

So, for your integral boiler AAV, ensure the cap is loosened a couple of turns to allow it to function. 

2. External Automatic Air Vents (Found on System/Heat-Only Boilers or High Points) Not all boilers have an integral AAV, especially older models, or "heat-only" and "system" boilers that rely on external components. In these cases, or for optimal venting, AAVs are often installed externally at the highest points of the heating circuit, such as: 

 * On a manifold. 

 * Near the top of the hot water cylinder's coil. 

 * At the highest point of a radiator circuit (e.g., in an upstairs cupboard). 

The Confusion: Again, the question arises: open or closed? And are they just for commissioning? 

The Truth: External AAVs also need to be left loose or unscrewed during normal operation. They are crucial for both commissioning and ongoing maintenance of a healthy, air-free system. 

Why? 

 * System-Wide Air Removal: Unlike integral boiler vents which handle air within the boiler itself, external AAVs tackle air that accumulates throughout the entire heating circuit. 

Since air naturally rises, these vents are strategically placed to catch it. 

 * Continuous Protection: Just like integral vents, external AAVs don't just work during the initial fill. Air can slowly build up over time due to various factors (e.g., micro-leaks, water top-ups, or even chemical reactions within the system). 

Leaving the cap loose allows the AAV to continuously purge this air, preventing the issues mentioned above. 

The "Commissioning-Only" Misconception: While external AAVs are absolutely essential during the initial filling and purging of a new or drained system, their job doesn't end there. They are a permanent, active component designed to keep your system efficient and quiet long-term. 

The Bottom Line For virtually all automatic air vents, whether integral to your boiler or installed externally, the rule of thumb is this: 

 * During commissioning: Ensure the cap is loose/unscrewed to allow the vent to expel air. 

 * During normal operation: Leave the cap loose/unscrewed. 

The vent's internal mechanism will close the valve when water is present, but the loose cap allows it to automatically open and release any new air that might accumulate. 

When to screw the cap down? 

Only if you need to perform maintenance on the system and want to create a vacuum within the system when draining, or vent itself  is faulty and leaking water (in which case, it needs servicing or replacing!). 

Understanding this distinction can save you from unnecessary noise, cold radiators, and potentially more serious system problems down the line. 

If you're ever unsure about your specific boiler or heating system, always consult your boiler's manual or a Gas Safe registered engineer in Horsham or Shoreham (or wherever you are!) for expert advice.

Andrew Tyler 07958248040