failed the gas pressure-drop test

We are about to let our house and the landlord's gas safety test has detected a fault. When the system is shut off from the main the pressure after at meter drops by about 0.01 mB per second. I think the pressure was around 20 mB. It was a digital meter.
Now, the first puzzle is that BG and another plumber both passed the pressure test a year ago when the meter was moved and some pipwork rerouted. Nothing has been done to the gas system since then.
The second puzzle is that he said that when he measured it, the pressur "held OK for a bit and then started to drop".
The house has a mix of copper and old iron gas pipes, all are capped except at the boiler. It's a 3 bed end terrace.
Any thoughts?
. Robert
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On Fri, 13 Nov 2009 02:56:45 -0800 (PST), RobertL

Don't strike a match :-)
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Frank Erskine

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I had exactly the same failure when my tame plumber came to fit a new pipe for the gas cooker in my revamped kitchen. After some head-scratching, we arrived at the open-flame gas fire in the lounge (one of those things with ceramic 'coals' in it.) At some point during the summer season, the pilot had gone out, so the auto cut-off had dropped the gas supply. However, the valve hadn't *quite* closed solidly, allowing a tiny amount of gas to continuously seep past it and out of the pilot jet. The plumber said this was quite common, and happened because the valve was not a regularly 'exercised' device. He gave it a sharp rap with the butt end of a spanner, and you could actually hear it click. The pressure test then held ok.
Arfa
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On Fri, 13 Nov 2009 02:56:45 -0800, RobertL wrote:

So a mechanical system that's faulty now was OK a year ago. Why is that a puzzle? If your car's OK now does that mean it'll still be OK a year from now?!

0.01mBar per second (if that's correct) equates to a drop of 1.2mB over 120s which is within the allowed tolerance for a standard meter and standard domestic pipework, if appliances are connected.
What's the drop with the boiler isolated?
If it's within tolerance (no discernable drop) with the boiler isolated then the drop is within the boiler which is OK, though you'd want to check the gas valve (I'm assuming it's an old boiler) and internal boiler gas pipework to ensure there's nothing actually leaking in there.
If it drops with the boiler isolated then you need to find and fix the leak. First port of call is always the hose connecting the test instrument to the gas meter test nipple :-)
--
John Stumbles -- http://yaph.co.uk

Many hands make light work. Too many cooks spoil the broth.
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POint taken, but I wouldn't normally expect pipework to suddenly start leaking unless it had been worked on or hit. The leak is not in the boiler (the only applicance).

Yes, it was 0.1 mB/sec I watched the display of his meter.

exactly the same.

That's what I suspect also, especially as it gave an OK result at first and then started to drop. However, his first port of call is to disconnect the parts of the pipework that lead to terminations that don't have appliances on them.
We are getting the original plumber aback who did the previous test (and some pipe works).
Thanks for your guidance people.
Robert
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RobertL wrote:

If you bought your own manometer (B&Q sell them) you could rule out leaks at the nipple. If it turned out to be that then you could probably ask for a retest.
Tim
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Remember there are 2 ways of doing gas pipework:
A) Cheap copper - high labour Can involve labour hours of cutting, jointing, soldering. Leaves you wish bad soldering risk be error or deliberate.
B) Pricey tracpipe - low labour Uncoil, few minutes to terminate, factory y/o plastic coating. No defect risk except at joints - which must be accessible :-) The only downside is that Tracpipe is large, 22mm is actually 28mm o.d.
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THat's an interesting idea. In fact I'm havoing another trusted plumber in to retest. I've cancelled the proposed repiping of the firt guy.
I worked out that to drop 0.01mB per second it must vent many litres of gas per day. I woudl expect to be able smell that (we live in the house at presnt and have done for many years). There is no smell inside the house or under the floor where the pipes are routed.
Roert
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On Fri, 13 Nov 2009 08:55:40 -0800, RobertL wrote:

I saw your follow-ups but for the record, reasons pipework could start leaking include:
1. some corrodible pipework has corroded through
2. a fluxed but un-soldered joint has dried out
3. something else neither you nor I have thought of ;-)
--
John Stumbles -- http://yaph.co.uk

I've got nothing against racists - I just wouldn't want my daughter to marry one
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Thank you, there are all intersting possibilities. For good measure the second plumbver also wandered all over the place with a sniffer but found nothing. Tyhree's no smell of gas anywhere, including under the floor.
Robert
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If it held up for a while then started to drop I would immediately wonder if the bloke knew what governor lock up is.
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On 13 Nov,

Possibly due to the volume of gas at higher pressure before the regulator keeping the pressure up until exausted. Pilot light somewhere?
--
B Thumbs
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Yes, that's a thought. Given tha tthere is no smell of gas anywhere I am suspecting his manomoeter hose connection.
We'll see when the second plumber gets here next week. He's the one who tested it previously. I post further so people hear the whole story.
R
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UPdate:
second plumber came today. Tested (several times to be sure), found all acceptable and issued certificate.
Robert
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