Gas pipes too narrow - safety issue?

[Lobster]

I was talking to somebody today who was bemoaning the fact that, as a residential property landlord, he had this year changed the CORGI who performed the annual gas safety inspection on a flat he lets out, and as a result, found himself with a whole lot of extra work needing doing.

Apparently the installation got condemned because the gas feed pipe was of too narrow bore for the demand; and to gain the certificate, he had to have it replaced with bigger-bore stuff. The theory being that inadequate gas supply could lead to CO formation I suppose? I don't know what gas appliances are in the property.

Does this sound plausible? I thought that use of too narrow a supply pipe just meant your appliances worked at less efficiency, not that it was a safety issue?

So - just wondering which CORGI was at fault - the original one who apparently failed to spot a potentially lethal error for several years, or the new CORGI for his fraudulently insisting on unecessary work?!

David

[Guy King]

The message from Lobster contains these words:

It's hard to see how. Too little gas would lead to too lean a mixture - an excess of oxygen.

[dom]

The FAQ says otherwise:

low a pressure may lead to an inadequate air mixture being drawn into the burner.

[John Stumbles]

Not at all likely to happen.

The real danger with inadequately sized pipe is that when the boiler fires up it may cause a ring on the hob to go out, which then spews out unburnt gas.

Regardless of the potential interaction between appliances I've described above, if the working pressure at the inlet to the boiler is less than its burner's working pressure it would be a cause for concern and should possibly be designated At Risk.

It's a grey area. As far as I can recall (without traipsing out to the van to consult my guide) the CORGGI guide to assessing unsafe situations such as this doesn't explicitly mention this scenario, therefore it's a matter for the fitter's judgement. In the trend to arse-covering prevalent in more than just this industry (cf surveyors' reports) it's safer for the installer to fail than to pass the installation.

[John Rumm]

It is certainly plausible. An undersized pipe will result in too much pressure drop between the meter and the appliance. This will affect different appliances in different ways. It could certainly affect the gas / air mixture and hence the production of combustion by products. It could for example starve an appliance of gas - perhaps causing a hob to extinguish when a boiler fires up.

Hard to say without more detail (i.e. list of appliances and their power ratings, plus a description of the pipework (i.e. length, size, number of bends etc)

[Aidan]

for his >fraudulently insisting on unnecessary work?!

It is not hard to size gas pipework. A quick measurement and calculation for a few installations would show whether the pipework is undersized and whether CORGI 1 or 2 cocked it up.

Presumably CORGI 2 found the burner pressure dropped below the minimum specified value with all the appliances going?

[Doctor Drivel]

Rail burners are much more forgiving. Pre-mix burners expects a certain pressure at the appliance and a lot less forgiving. Hence problems with poor quality Transco meter regulators.

[Doctor Drivel]

The reason why a dedicated supply for the boiler be fitted back to the meter.

[Ed Sirett]

The correct pipes are those that only drop 1 mbar between the meter and all the appliances when all the appliances are full on.

In practice a substantial majority of UK domestic gas installations would fail the absolute "by-the-book" standard as laid down in BS 6891 (link below). There are a number of helpful factors.

1) The meter almost certainly gives 21-22 mbar for most people most of the while. 2) The meter standard has been relaxed in the last few years to 19-23mbar which means that appliances are expected to have gas delivered at over 18mbar. This means that in practice there is (informally/ de facto) 3mbar for the pipes.

Against this is (as Drivel correctly states) premix burners are less tolerant of under gassing. Also in extreme conditions (e.g. a JCB+gas service pipe incident) the pressure at the meter might fall to 15mbar giving only 14mbar at the appliances or less with poor pipework.

For the installer the 'risk' for not making the user/owner aware of the problem can be large, in the worst case. It could result in the fitter having to replace a lot of the installation pipe work with 28 or even 35mm pipe. This would be at the fitter own expense, on pain of losing his/her registration.

OTOH any fitter who applied every jot and tittle of the regs would have about one customer a week (either because others would be more popular with the landlords or because it would take a week to get everything absolutely right).

Over gas leaks, flueing, ventilation, safety controls and other more direct concerns, a less pragmatic approach will be taken.

[Doctor Drivel]

Drivel? You have a cheek. As I recently pointed out your FAQ is a shambles of misinformation, which looks like it was written in 1986, and half the time you clearly haven't much of a clue for a so-called professional. It is bad enough dealing with the amateur uk-d-i-y Lunatic Association on here, who give appalling advice on matter they know sweet FA about. I am not on here as a buddies club to speak to the idiots. It is to give solid advise top people who request it.

I feel sorry for the poor bastards who read your sadly lacking FAQ and believe it.

[John Rumm]

So you prefer Dribble these days then?

Let us see you do better then... (as if you could!)

Oh the irony!

This one could have been written just about you dribble:

I feel sorry for the poor bastards who read your sadly lacking FAQ and > believe it.

You should feel sorry for the rest of us having to put up with the prattle you spout.

Get back to your flags boy!

[Doctor Drivel]

He is from Essex you know.