Gas pressure at boiler

(apologies if you see 3 copies of this, after waiting 9 hours, my original post didn't arrive, and then after waiting over a day, the repost hadn't arrived either!).
I wanted to try and find the pressure of gas getting to my boiler so I know its suitable for my planned new boiler. However, I found out after connecting the u-gauge that its just the burner pressure - not the main pressure. Seems like I've no way of checking the mains pressure as there's no test point on the boiler, or anything extra fitted.
The manual for my (old) boiler says the burner should be set to 8.5mb - it was actually set to 10mb. When I came to adjust it (whilst burning), I couldn't get it any higher than 16mb. Is it likely that the boiler governor drops it a fixed minimum amount, or was this minimum amount likely to be the true pressure coming out of the pipe.
New boiler claims to want 20mb into the boiler (but only 13.5mb at the burner). I've heard that some/most boilers will work lower than designed. How low is 16mb? I'm hoping that the governor on this boiler does drop it a fixed amount before the adjustor screw makes any changes - which might make the pressure slightly more - like 18 or maybe (I wish!) 20mb.
The boiler in question is an old Baxi WM 531 RS boiler on its last legs.
Thanks
David
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In an earlier contribution to this discussion,

I don't know for sure, but it seems extremely likely that, with the boiler governor on its max setting, there is still *some* restriction so that it's not just feeding mains pressure to the burner. If you wanted to be sure, you'd have to build another test point into the supply pipe just before the boiler - but that's probably a bit OTT!
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Set Square
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On Sun, 1 Feb 2004 21:07:52 -0000, "David Hearn"

This is a really bad idea because you are increasing it to double the intended value. Whether this doubles the burn rate I don't know (probably it isn't linear) but it is way over the rating of the appliance and you could be creating a dangerous situation.

It's likely that the pressure on the input side is a bit higher (it should be no less than 20mB) but you can only tell by measurement.

The 20mB figure is based on the British Standard, which (to extract a small piece) says that there should be no more than 1mB of pressure drop from the meter to the appliance. This should be measured with any other appliances that would affect it also running.
The regulator will need to have some amount of pressure drop across it in order to work. The manufacturer will have selected an appropriate one that will give the required burner pressure assuming an input of 20mB. It may be that the regulator will work at lower pressures, but this should not be assumed.
You can do a calculation of whether the pipe sizing is likely to be adequate
http://www.cda.org.uk/megab2/build/pub124/default.htm
I had exactly the same issue as you when I changed my boiler - no pressure test point on the input side of the regulator on the old boiler.
I did the sums for the existing pipework and on full firing, it looked as though the pressure would have been out of spec - about 17mB. However, the pipework is steel for most of the run, and the paper above is for copper so I was working on approx. figures.
I therefore put in a gas cock with a test point immediately in front of the old boiler and measured it. There was less than 1mB drop from the meter with the boiler at its 15kW max. burn rate.
The new boiler has a max burn rate of 28kW when doing a hot water cycle, so the pressure was checked after installation (this one does have a test point on the input gas cock) and fortunately it was in spec.
I would recommend that you do a proper pressure test before you take out the old boiler, because the corrective action would be to put in a new pipe run of larger size. Don't assume that the old boiler was installed with correct pipework sizing because there was no way to measure the pressure. It's entirely possible that even a professional fitter could have done (gasp) a bodge.
At least then, you know what you are dealing with and can get a new length run in if you need to do so.
All of this assumes that you have the competence to carry out the work of course......

.andy
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Andy Hall wrote:

How would the difference between steel/copper affect it? Are steel pipes larger diameter or have less friction? I assume threaded pipes are steel? If that's the case, I estimate that I have 5.5m of steel pipe (22mm size - or similar) and then 6.8m of 15mm copper. If I've worked out (see below) a 1.6mb drop from meter (21mb I think) assuming copper pipes - would steel pipes bump it up slightly - enough to make it closer to 20mb?
Is there a likelyhood that a 1.6mb drop is going to be seriously out-of-spec? How accurate is the estimating on that site? If I worked out 1.6mb - how accurate do you think that would be?
I guess that if I can replace some 90 deg joints with bends I can reduce it further.

Are test points easy enough to buy? Would they just be like a standard in-line T or something like that? Just cut the current pipe, add the test point and then re-connect?
I've not seen a test point that isn't integrated onto the unit (ie. boiler/gas fire) so I want to make sure I can get one.

Yep - I'd like to do a proper check on the pressure.
Sadly due to the way the pipes are plumbed in, I have quite a bit of 15mm pipe going from the kitchen floor, up a small boxed in section for the pipe into the ceiling, and then across a joist and down again into the boiler. From what I can see I have threaded 22mm pipe (or similar size) from the meter and I have threaded 22mm pipe going to the bayonet connector on the cooker. As for what then happens under the floor to the 15mm going up - I've no idea.
Estimating, I think I may have 6.8m of 15mm pipe from the floor to the boiler, and 5x 90 degree fittings, adding a further 2.5m of effective length. Using the link you gave, I estimate a 1.6mb drop from meter to boiler.
Without taking up the kitchen floor (vinyl on vinyl on old wooden floorboards) I can't find where the piping goes. Also, I cannot get to where the 15mm pipe comes out the floor because its behind some cupboards with a worktop on... then there's the problem that the cupboards are fitted on top of the floorboards, so possibly need to cut them up to lift them...
I do, in theory, have a possible other route I could take the pipe - which would be about 1.5m up the wall (rather than 2.5m to the ceiling), open up the boxed in part and run the pipe across to the boiler. This would result in 4.8m of 15mm pipe, and 3x 90 degree fittings giving a total length of 6.3m of 15mm and an estimated total drop of 0.86mb. The only problem with this is the boxed in part is probably 2" square, and cutting a pipe in that space would be difficult, and then would probably require a compression fitting (which can't be boxed in - right?).
I think my best bet is to put in a test point and then see what the pressure actually is at the boiler.
Thanks
David
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In an earlier contribution to this discussion,

Yes. They will be either 1/2" or 3/4" nominal bore size. 1/2" pipes have roughly the capacity of 15mm copper but have an outer diameter of around 21mm. 3/4" pipes have roughly the capacity of 22mm copper but have an outer diameter of around 26mm.

The test point on my Baxi Solo boiler is about 9mm in diameter, and is screwed into a boss welded onto the burner pipe. I don't know what thread the test point has - it is approx. 1/8" BSP in size, but is probably a gas thread of some sort. If you could get one of those, you'd need to get a tee-piece with a female threaded spur to screw it onto. If all the connections on the tee are threaded, you'll need male iron to copper solder connectors to join it to your pipework. [I'm sure you're aware that you cannot use compression fittings designed for water pipes on gas pipes].
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Set Square wrote:

Are you sure about that? Is is that they are completely imcompatible, or that some are designed for both? The reason I ask is that when I look on various plumbing supplies websites, in the Gas Fittings section they don't have any normal compression fittings (T's, 90's, inline connectors) - only gas valves with compression ends. Either the compression connectors they supply are suitable for both, or they just don't list them on their website.
Thanks
David
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In an earlier contribution to this discussion,

Well I thought I was! But I've just had a look at the Gas Fitting FAQ at http://www.makewrite.demon.co.uk/GasFitting.html and that seems to be saying that you *can* use compression joints providing they are accessible - like not under floorboards.
So now, I'm not so sure! I don't know, for instance, whether gas compression fittings are the same as those used for water. I have a feeling that they may use softer olives. I'm sure others (Ed etc.) will be able to elaborate!
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Much of the materials in the FAQ is drawn from a couple of gas fittings text books. These books mostly point to the relevant normative documents (Building regs, statutory instrument, British Standard, Euronorm).
As far as I know a compression joint is acceptable if accessible. Wrapping PTFE on the reads or the olive would be unacceptable for water let alone gas.
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Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Ed Sirett wrote:

Intrigued now [1]. What is the purpose of gas grade ptfe tape? Is it only supposed to be used for tapered BSP joints? What situations would merit it's use over jointing compound or a naked joint
[1] In a kind of I don't need to know but it's probably wise to know way.
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Toby.

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On Mon, 02 Feb 2004 18:44:07 +0000, Set Square wrote:

Test points are available and can be made into fittings that would T into 15 or 22 pipe.
The thread on a gas meter test nipple IIRC is 1/8"BSP however that is effectively trivia since they only thing that goes into the hole is the plug.
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On Mon, 2 Feb 2004 16:39:52 -0000, "David Hearn"

For the size I had, IIRC, the bore worked out somewhere between 15 and 22mm. Also, because the pipework was buried in the building, it wasn't possible to know for certain how many elbows there were so an assumption was made. From that, I knew that it was fairly close.

Yes.
I think that that is too close to call for the calculations to be that accurate.

You might be able to improve things if you had to by replacing the 15mm copper with 22mm.

If you go to
www.bes.ltd.uk and put in 'gas test point' as keywords in the search, you will get a page of them, both as separate fittings and integrated into valves.
Before you do anything, do look at Ed Sirett's gas fitting FAQ and be sure that you know how to do the various tests, especially for soundness. If you are not 110% certain of what you are doing, then don't :-)

I think so too. Also, I would measure the gas rate of the existing appliance and calculate its actual input burn rate. Compare that with the spec. of the new boiler.
As long as pressure drop is OK and the input rate are comparable, then the pipework should be OK. It is worth doing this before ripping out the old boiler, because you can at least deal with the problem beforehand if needed. It would be tearful to discover that you need upgraded pipes once you have fitted the new boiler and can't use it.
.andy
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On Sun, 1 Feb 2004 21:07:52 -0000, "David Hearn"

I wouldnt worry about it. Assuming you arent fitting an appliance whos gas rate is higher than can be supplied by the meter/incoming service then the worst you might have to do is run some larger diameter pipe if the existing one doesnt come up to scratch.
joe
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On Sun, 01 Feb 2004 21:07:52 +0000, David Hearn wrote:

That must have caused the boiler to wake up. Not sure that such a course of action counts as competant gas fitting. 8-( ?!?
Most of the gas valves on typical low tech pilot+themrocouple boilers have a test point on the valve for the inlet pressure. This is often in the form of a steep metal cone the centre of whicvh contains a small screw deeply recessed. The screw is slackened and the tube place on the cone.

If then makers say 20 then it's not good, 20 is what's needed. However bear in mind that the meter is 20-22 on its outlet. Which means that no appliance ought to be unhappy with only 19 nor should any maker require 20 as a _minimum_ although 20 is the nominal inlet pressure.
If the burner is not a forced premix type the realistically 16-17 should work but most makers say 18 minimum.
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Ed Sirett wrote:

Well, the current boiler is a range rated boiler adjustable via burner pressure. The 8.5mb setting is the middle range with it going as high as 13mb for the upper range. 16mb is a tad high for the top range I agree though it was probably only for a few seconds...

So the screw is kept in place? When I tested it on our boiler, I removed the screw completely (with the gas off) and then connected the tube up before turning the gas back on and testing.
Sadly there doesn't appear to be any inlet test point on this boiler, or on the inlet pipework. Gas pipe goes straight into the gas cock and then into the regulator (via about 3 or 4 screw-on 90 degree bends for some reason! These won't be in the new pipework for the new boiler so removing these might up the pressure a little. The only test point is on the regulator output.

I'm guessing that the manual may say one thing, but the boiler may accept something different. From what I've read in the manual, the pressure is likely to affect the pilot light (which from what I understand, is re-lit every time the burner fires). Seeing as I can get 16mb out the current regulator, then 13.5 (what the burner needs) shouldn't be a problem - its just whether the pilot is okay.

Forced pre-mix being one which has a fan? This new one is a plain and simple, basic cast-iron boiler with balanced flue - no fans at all.
Thanks for the advice. I think that it'll all be okay - thanks to everyone's help on here, and assistance from my Dad (who's done his similar boiler recently) I'm sure we can do a competent job. It might take a couple of days and working slowly, but I think we can do a good, safe (non-bodged) job.
David
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On Tue, 03 Feb 2004 10:20:18 +0000, David Hearn wrote:

...but with electronic ignition rather that a permanent pilot light. Oh well by the book you supply the boiler with at least 19 mbar of gas.
The forced premix type are usually only found on state of the art condensing units. Even having a fan does not make the boiler this sort in all likelihood, let alone not having a fan.
There is likely a pilot adjustment on the new boiler.
Read the FAQ (which you likely have done).
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