Dripping in the airing cupboard - are we about to blow up?

[grendel]

Just went to put towels away in the airing cupboard and found everything on the top shelf absolutely dripping wet. Pulled everything out and discovered a vertical pipe with some sort of valve thing on the top, rapidly dripping water. I turned the little red screw cap on top to the right and it stopped dripping.
Problem solved? Maybe...
But now I'm worrying that this is some sort of pressure relief valve that I've turned off (NOT that I know anything about plumbing) and we are now in danger of exploding if the hot water tank overheats.
We've lived here for 6 years and never had a problem with drips/leaks in the airing cupboard. House is 30 years old and all the plumbing looks like it hasn't been touched since then.

Can any plumbing-savvy people out there tell whether I should spend ££££s to get a real plumber in to look at it to be on the safe side, or is it probably OK?

[hellhasnofurylikeahungrywoman]

It needs looking at. I suspect you've turned off the inlet valve, once the tank drains dry the element will boil dry so turn off the electric to the tank too.

[grendel]

I don't think it's the inlet valve. Wasn't one of those big stop-cock type valves - there are several of those in the airing cupboard so I know what they look like.
This is a vertical pipe which just ends in a chunky black plastic stopper type thing with a tiny little red cap on the top. The water was dripping/running out from underneath the red cap until I turned it 'off'.

[grendel]

But thanks for the amazingly quick reply Hell.

[hellhasnofurylikeahungrywoman]

I am assuming it's an immersion cylinder you've got?

[grendel]

It looks like this:

www.silver-magpie.com/images/valvething.jpg

[grendel]

Well, it does have an immersion heater but we hardly ever use that. Normally it is heated by the central heating boiler.

[hellhasnofurylikeahungrywoman]

My other half thinks it may be an <a class="break-all" href="http://www.google.co.uk/imgres?start=210&hl=en&cr=countryUK%7CcountryGB&biw=1454&bih=697&tbs=ctr:countryUK%7CcountryGB,itp:photo&tbm=isch&tbnid=vaju7-XZfLzLIM:&imgrefurl=www.ukhps.co.uk/products/Honeywell-Auto-Air-Vent-EA122.html&docid=MNH5GYRJ9wq7iM&imgurl=www.ukhps.co.uk/product_images/m/254/honeywell-ea122-aav__73842_std.jpg&w=220&h=220&ei=Yb-NT5rcJoSX0QWf27j6DA&zoom=1&iact=hc&vpx=962&vpy=247&dur=480&hovh=163&hovw=157&tx=107&ty=72&sig=109486547969439837800&page=9&tbnh=152&tbnw=140&ndsp=28&ved=1t:429,r:18,s:210,i:132" rel="nofollow noindex" target="_blank"> air vent but isn't absolutely sure

[grendel]

Thanks Hell - it does look rather like one of those. So the next question is, does it matter that I've screwed the cap tightly closed, or should it be left open in order to function correctly/safely?

[grendel]

Your help is very much appreciated, by the way.

[hellhasnofurylikeahungrywoman]

He's not 100% sure what the consequences would be, he could ask at work in the morning if it helps but he says he wouldn't leave it unchecked for long if it was him.

[nocake]

It's an air vent in the central heating system. It automatically allows trapped air out of the system and yours is broken. It needs to be replaced... although most systems don't have them and you bleed air out from the radiators instead.

[nocake]

And there's no problem leaving it closed for a while until it can be fixed.

[grendel]

Thank you so much Hell and nocake. I can sleep easy tonight without fretting that we are all about to be blown to kingdom come!
Now just to the minor task of re-washing and re-drying half the contents of my airing cupboard. [sigh]

[PigletJohn]

yes, it is an automatic air vent (AAV) and it needs changing, and yes, it is OK to screw the cap down if it is leaking water. By the look of all that PTFE tape on the threads it is not the original, or was fitted by someone who is either rather clumsy, or not a pro. You will need some new tape when you fit the new one, it is very cheap.

It looks rather as if it is made of plastic. It is not a pressure-relief valve so is not a safety-critical device. here's a brass one

Does your system have a pressurised boiler, with a pressure gauge on it, or does it have a feed and expansion tank in the loft? It makes a difference to how much water will squirt out when you undo it. There might be a service valve on the 15mm pipe underneath. You can get better quality ones, and you can get manual ones that you bleed, like a radiator, when you remember. The manual ones tend not to go wrong, and are simpler and cheaper. You might find when you take yours off that it is clogged with black sludge. That's due to a different problem which we can talk about another day.

[grendel]

Thanks for all the extra info PigletJohn. That's very helpful. I will investigate getting a new one fitted.