Gas pressure and boiler problem

[Pete Zahut]

We have an Alpha CD32C condensing boiler up in the loft that has worked faultlessly for the last 3 and a half years (in fact, it's been bloody brilliant :o)) but, about 2 months ago, it started playing up and locking out intermittently. Sometimes it'll reset first time, sometimes not. When it does reset, you can hear a slight "whumph" as the gas ignites.

We have the Alpha Aftercare contract so for just £12/month all parts and labour are covered, so it's just a matter of a phone call and someone comes out (usually) the same day to sort it. Over the last two months they've changed the spark generator, the gas valve and the gas valve lead (which apparently is a resistive lead and can be damaged if the spark genny fails - or so they said anyway).

They've just been out again today and the Alpha (or I should more correctly say the Alpha sub-contracted agency) guy measured a standing gas pressure of

23mbar at the boiler but when the boiler was trying to light he said that it dropped down to 15 or 16mbar, which was too low. He said that I should contact British Gas to report low pressure and they would sort it (incidentally, the whole estate got new gas meter, governor and main out to the street 12 months ago).

Anyway, National Grid/BG guy turned up within 20 minutes and measured working pressure (all 4 rings on the cooker lit and boiler providing hot water) at the meter as 21mbar (their range being 19 - 23mbar) so that's OK.

Assuming we haven't got a gas leak (no smells at all) they reckon it could be crud in the pipework. My question (at last, I hear you cry) is this: (a) how does crud get into a gas pipe in the first place and (b) how the hell do you get it out? Is it likely, or is it that they don't know what to do next so are blaming anything?

Pete

[Pete Zahut]

Doh, sorry - that should read 3 rings lit on the cooker, boiler providing hot water and 19mbar at the meter. Standing pressure with nothing on was

21mbar.

[Andrew Gabriel]

Could be that the pipe is too small a bore for its length. There are tables for working out pressure drop given length, bore, and numbers of bends. However, the fact that it has worked for 3+ years, presumably with the same pipe run, would cast doubt on that being the issue.

Your Alpha sub-contractor should have measured the pressure at the meter outlet when the boiler was firing up before suggesting calling out Transco. To be honest, it sounds like the subcontractor doesn't know what he's doing, given the number of failed repair attempts. I suggest you go back to Alpha Aftercare and ask for a different person.

[Pete Zahut]

No, I can't see it being a pipe-sizing issue either because it really has been marvellous for more than 3 years.

Hmm... tend to agree with you there Andrew. Cheers mate.

Pete

[Toby]

Unless, I suppose, the new gas meter (and presumably regulator) was outputting a higher pressure before, it was then replaced with a new one, that was also on the high side, and this has now reduced slightly to become just out of tolerance for the boiler?

Toby...

[John Rumm]

That sounds a bit suspect. The static pressure tells you nothing in particular in this case - however the pipe sizing should be such that there is not that much fall in pressure on load. The transco bod seems to have confirmed that the meter and governor are doing their stuff (although normally they arrange it such that the system will provide

21mBar on load at the meter. That allows for a 1mBar drop in your pipework, and the dynamic pressure at the boiler test point to be near enough 20mBar).

You would need a substantial leak to drop that much pressure!

Probably the latter.

It could be the supply pipe is a tad undersized and some other change in the supply pressure has made this significant.

[cynic]

If work was done on the gas mains there is every chance that "some" debris may have been dislodged and ended up in your system. I have come across clogged inlet filters on domestic gas valves. I cannot say if your boiler model has a gas valve incorporating an inlet filter but Ed may be able to help here. I have also come across jobs where the gas main pressure varies depending on the time of day (all the other houses in the street cooking tea with boilers running etc) so Transco toddle along at some other time and find no fault. You really need an engineer with time to dig into the problem and sort it out fully. I suspect the answer will lie downstream of the meter

[YAPH]

So you've got 21mbar at the meter under load (including the boiler providing hot water, which is likely to be its maximum load) but only 15 or 16mbar at the boiler itself. How much, if any, 15mm pipework is there in the supply line, and how much of that is elbows (sharp right angle bends)? And what is the length of 22mm pipework, in metres? How many elbows or tees where the gas flow makes a sharp 90 degree bend?

And what's the rating of the boiler?

As a rule of thumb for a 24kW boiler I reckon about 22 metres of 22mm pipework, where you count 0.5m for each 90 degree bend, is the limit. Even

1 metre of 15mm pipework (with the sae allowance for elbows) is too much.

[John Rumm]

It was a 32kW boiler was it not? So about 15m effective length of 22mm tops.

[YAPH]

Ah! "Alpha CD32C" - yes looks like it.

[Pete Zahut]

Just been trying to guesstimate the pipe run - didn't do the job myself for various reasons. Anyway, I _think_ that there's about 22m (including elbows and 90deg bends) of 22mm pipe - no 15mm in the run at all. I'm grateful for all help given by you folks but I really don't see how the pipe run/pipe size could be the problem because it's been working brilliantly for 3+ years - although, obviously, you're the experts and I'm willing to learn :o)

Pete

[John Rumm]

It sounds like it has always been wrong - but something else was hiding the fault.

The difficulty you now have is that the pressure at the meter looks ok (perhaps a little low - but not far out), but the pipe is demonstrably undersized. So you may find the only way back to a working system (or at least one where it can't be used as an excuse for another fault) is to upgrade the pipe.

[YAPH]

But you said:

...

So it's been playing up *since* your new meter was installed. Some connection maybe?

If you can replace about 6 metres-worth (preferably a bit more) of 22mm with 28mm you should be OK. It doesn't have to be the run right from the meter: any convenient length(s) that you can get at will do.

(28mm has as near as dammit zero resistance at the flow rates we're talking about: it's something like a power-of-4 relationship so a few inches of

15mm, a few tens of metres of 22mm or probably some hundreds or so metres of 28mm have the same flow resistance. For exact figures I think the CDA website has some tables or graphs.)

[Doctor Drivel]

Size it up from here:

OK, which on the surface it appears not to be as 22m of 22mm pipe is over

60 foot, but check, then you need a shock pump. The gas pipe is disconnected at the boiler and meter. The pump is charged by hand, and the pressure let loose in one blast. It usually shifts any blockage, or may move it if you are unlucky.

If installing 28mm install it from the meter only, with a 28mm meter union. available from

pressure has to be tested at the meter and the boiler. If OK at the meter when the boiler is on, then is it OK at the boiler. Before the new district governor was fitted, the gas rate pressure may have been too high, hiding the undersized pipe problem.

[Doctor Drivel]

> If OK, which on the surface it appears not to be as 22m of 22mm pipe is

If you install a length of 28mm pipe at the meter, it is best to ensure the cooker pipe is a dedicated supply back to the meter teeing into the 28mm. Then one does not rob the other.

[baz]

just seen your posting ia ma experienceing same problem with a worcester bosch boiler.worked ok for severl years,played up last year so pcb and a couple of other parts replaced such as gas valve. despite last years probllems boiler has always deliverd good hw and ch but since christmas hw and ch temps have dropped dramatically. service engineer came 2 weeks ago out reckoned tested and said i didnt have enough gas pressure at the boiler. ansd need to put in bigger pipe size

[Jim]

Is it a combi?

Depending on the distance and the number of bends 22mm may not be enough.

Have a look at this PDF from Worcester Bosch which explains a rule-of-thumb method for calculating what size of pipe you need. The boiler requirement in metres cubed/hour should be somewhere in your boiler manual.

[Doctor Drivel]

there is to be no more than 1 mb drop in pressure from the meter to the boiler. 22mb at meter, 21mb at boiler.

[Usenet Nutter]

Dunno where this post originated as my newsreader only shows the follow up from Jim to Baz . Is it likely that at no time in 7 years the boiler,gas fire and 3 rings have been on together as he says it has worked fine in that time .

[Graham.]

Because the OP replied to a thread started in June last year via a dodgy leeching web service.

Here is the entire thread courtesy of another dodgy leeching webservice ;-)