A bit of a dilemma with existing boiler installation!!!

I don't agree, if you bought a new car and the back wings went rusty after only six months due to a bad batch of steel, and the maker said because the oil in the engine was changed at 5000miles instead of 3000miles, so your back wings ain't covered, thats just a crock of ****e, the engine has naff all to do with the bodywork, just like the crap pump has no bearing (pun haha) on the gas pipe.
Guess what, you don't need to be RGI to change the pump, unless of course you need to dismantle the gas valve or combustion cover to remove pump.

The boiler maker should sort out the pump and not fudge the issue over gas pipe sizing, which by the sounds of it isn't really a problem at all.

Few years back I contacted CORGI regarding a really bad undersized installation on a private house, guess what, they didn't give a toss, with whats described in the OP, it cant possibly be that much of a problem.

I also queried a rented properties undersized pipework, the landlord contacted CORGI themselves to be told, it's only at risk, well unless Gassafe is any different, I cant see them creaming their pants over it.
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Using the sizes I mentioned (which aren't exact, but thereabouts) I found a simple calculation online from http://www.arca53.dsl.pipex.com/index_files/gas7.htm and I calculated as follows:

12.5m + 1.8 (for fittings and bends)
1.4 + 1.2 (for 90 deg fittings)

= 16.9m in total

38KW gives required flow of 0.0009819m3/s

The table I used only went between 15m and 20m, so I extrapolated inbetween to get the figure for 16.9m and it calculated as 0.0009822.
This is amazingly close to my required flow!

Just right if was all 22mm, but part of mine is 15mm for short distance as it drops from ceiling down the back of the boiler. But would that make that much of a difference to warrant a loss of pressure big enough to fall below 17mbar ? What should be standard mains pressure be?

So if vaillant not honouring their warranty is the only issue...then luckily I have B Gas contract on the boiler.
Alternatively, even if I could demand the full 38KW from the boiler, is it really at that much risk due to being a couple of Mbar below what the manufacturer states that it warrants being switched off?
There seems to be a large difference of opinion?
I am not a plumber and have no expertise in that area but I have a few observations.

You state this house was a new build when you took occupancy, so was the boiler part of the property when you moved in, or did you have it installed at your cost? I am asking this question because in my layman’s opinion this sounds like an installation problem so does it have anything to do with the manufacturer of the boiler?

If you paid for the system to be installed then it would be down to you to seek recompense\remedial work from the installer, but if I am reading your post correctly the boiler was in situ when you moved in. If this assumption is correct then in my opinion it is down to the builder\developer to arrange to carry out any remedial work.

Hind sight is a talent that can sometimes be annoying but the first time you were aware that the piping was undersized was the right time to start getting this issue sorted.

It is highly possible that the developer never spotted the incorrect piping and unless you have told them, I guess they are still not aware.

If the undersized piping is illegal then there must surely be a responsibility of care by the inspecting authority? As soon as this installation has been found to contravene the current regulations then if the correct action is to condemn it, then it should be condemned?? (I have no idea if this is the correct action and only have your post to guide me)

the gas pressure feed is a couple of mBar lower than the legal requirement

He then informed me that his ‘Vaillant mate’ said he would condemn our system as unsafe due to the low pressure and isolate it. BG engineer pointed out that there was a 15mm gas pipe going into the boiler and that this was the problem.

In this modern age of everyone blaming everyone else, I would still like to think that as a house owner if someone told me the pressure was
below the legal requirement
I would have acted there and then to rectify this issue’

A number of professional and no doubt highly skilled folks have debated that this may be a piping or a pump issue to me that is not your problem and to me this is academic, the issue is you have a system that is not performing to a standard it was designed for and the reasons for that is allegedly a poor installation, be that pumping or piping and if this was a new build, with the heating system ordered by and paid for by the developer, then that is where your letters should be aimed.

I am not a plumber and I have no idea whether this piping contravenes any regulations, or your system should be condemned or merely labelled ‘At Risk’ but it does sound like this issue has been caused by the installer and not the manufacturer of the boiler.

I have submitted this post to stimulate the discussion onto a possible different course and am NOT suggesting I am right, but I do believe you should be seeking recompense from the developer\previous owner of the property.

Good luck,
Get someone to check the working pressure - not standing pressure - at the gas valve inlet inside the boiler. As long as that satisfies the manufacturer's requirements, all is well.
Certainly, there should not be more than 1mbar drop through the pipework, but the boiler manufacturer specifies the pressure required at the boiler.

That may well be the attitude of someone who works for a manufacturer.

However, an installer has to make the installation according to the relevant standards and if not risks a defects notice from Gas Safe! And we added to their "risky" list and get inspected more regularly.

Furthermore, most premix boilers will become difficult to set up if fitted to a gas supply with widely varying working pressures as the load varies. Thats one of the problems we have in the field.

To the OP, any engineer seeing a length of 15mm supplying a high power boiler will immediately know from experience that its going to be undersized.

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Just reconfirmed the pressures.
Standing pressure at both the meter and the boiler is 21mBar.

Working pressure when it is switched to full power mode drops to 15mBar....indicating it seems that the 15mm feed at least may be restrictive enough to drop 6mBar under full load.
I have emailed the installer for the second time to give him time to think about a response...i will call him in a day or two...then it will have to be a matter of contacting Gas safe for advice.
Am I correct insaying that the installer is liable for such problems if reported within 6 years? This is how I interpreted the Corgi certificate.

On the pump issue, I'm sure I know what the issue is...it is because they replaced the original modulating pump, which had metal impeller, with current ones that have a plastic impeller. This is what Vaillant engineer said....and I have found the same explanation by googling the issue. They will replace it, but I've got to make sure they don't leave me with no boiler as threatened!

Original installer came back and upped to 28mm until it enters the room where the boiler is...the last few metres were increased to 22mm.
Just managed to get it up to 17mbar working pressure though!
It was just enough to call out Vaillant who have now fixed the problem with a modified pump (he told me he fixed the same problem on the same boiler 3 doors up a couple of years back...which was built by the same developer). It actually was the friend of the B Gas engineer...but he didn't even stick on a manometer to check the pressure...after all that!!!!

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