Boiler gas feed pipe pressure...

[JMA]

Hi, I'm a newbie here (and not a plumber!) so please forgive my lack of knowledge but I could really do with some information and help!

I had a new boiler fitted last week - worcester Bosch Greenstar combi - and the fitting engineer told us that the pressure in the gas-feed pipe to the boiler is not high enough for the boiler to operate at maximum efficiency or be certified. They also recommended that I get our gas supplier (BG) to replace the meter.

The pipe that feeds the boiler is a 22mm copper pipe and they want to replace it with a 28mm pipe from the meter to the boiler. Assuming the pressure of the gas at the meter is a constant, then increasing the bore of the pipe coming from the meter will reduce the pressure in the pipe and therefore at the boiler won't it? If so, then surely just replacing the meter is the correct way forward?

The engineers and representatives from the company are very decent guys and, I think, very honest but I'm just not sure they've suggested the correct solution for the problem we have.

Any advice/insight gratefully received.

Jules

[anz]

A smaller pipe means that the gas will hit the sides of the pipe more, increasing resistance, therefore reducing pressure. You can't increase the pressure at the meter, 21mb is the usual standard pressure.

[justlead1]

Hi. As Anz have posted, when the boiler is using gas, the gas is flowing through the pipe. In doing so frictional resistance is caused by the walls of the pipe, length and elbow and bends all add to a drop in pressure at the appliance(the energy used to drive the gas from one end of the pipe to the other.) The longer the pipe the more pressure loss. To overcome this increasing the pipe size allows more gas but less friction. Gas engineers with the use of data/tables calculate pipe sizes to achieve the required standard as your engineers seem to have done, all be it in a more practical way. Good Luck

[markfxy]

youve got no knowledge of plumbing, youre not a plumber but you think you know better than your plumbers, customers like you really **** me off

[JMA]

Originally Posted by markfxy

youve got no knowledge of plumbing, youre not a plumber but you think you know better than your plumbers, customers like you really **** me off

In which case you're not a very good plumber. You should listen to your customers, consult with them and explain things they are unclear on. With an attitide like this, your business will last less than the blink of an eye.

I don't think I know better than my plumber but Boyle's law (basic Physics) governing the bahaviour of gasses in confined spaces clearly states that increasing the volume of a container in which a gas is kept will decrease the pressure of the gas (there are other considerations such as whether the other variables in the equation are constants or not etc but for our purposes we'll ignore them) so to the untrained eye increasing the bore of a gas pipe to increase the pressure of the gas in it flies in the face of logic.

Your colleague (and I doubt he or she would thank me for using that term) has provided a helpful and detailed enough explaination as to why this is not the case when gas flows through a pipe under pressure. I'll wager that they are a successful plumber with a loyal customer base.

If you were a good plumber, you'd look forward to having customers like me who are interested in what you do and ask questions when they don't understand, rather than looking for little old ladies who won't question your greedy and cynical advice to rip out and replace their entire heating and hot water system at a vastly inflated price because their garden tap doesn't stop dripping.

Plumbers like you really **** me off and you give your whole industry a bad name.

[Alanka]

JMA, can you tell us the approximate length of the 22mm pipe leading from the gas meter to your boiler? This would enable us to calculate approximately what pressure drop is likely.

[grahamep]

Not only do you need to provide the pipe length but also the size of boiler as the pressure drop will depend on the demand. As regards the comments about Boyle's law, this applies to standing gas in a sealed vessel. In this case the pipework is connected to a pressurised main. Increasing the gas supply pipe (as already stated elsewhere) reduces the friction losses.
If you already have a 22mm feed then it seems unlikely that the pressure drop is too great, unless the pipe is very long or takes a tortuous route with lots of bends. Although the general rule is that gas pressure should be 20mb at the appliance, manufacturers instructions take precedence and for most Worcesters minimum inlet pressure is 18.5mb so you have a fair bit more latitude.

[dts]

General rule of thumb not written in stone
0-9 metres 22mm
9-15 metres 28mm
15 metres on 35mm

[grahamep]

Had more thoughts on this. Actually the installer should have identified that the gas feed was inadequate at the time of assessing the job. Or at the very least suspected that there might be a problem involving additional cost and advised accordingly. Suggest that if it really does need upgrading it is the responsibility of the boiler installer.

[JMA]

Thanks all for your replies, they are very informative and helpful.

As it turns out, at tyhe time of installation there were gas works in the road 50 yards from my house so the installers have returned to double-check the pressures at the supply and at the (new) boiler and they are satisfied that they are fine - the pressure at the supply was 21mb and at the boiler it was 19mb under normal load - under boost operation the pressure did drop to 18mb at the boiler - 0.5mb below Worcester's recommended minimum operating pressure but as we only feed one kitchen one bathroom and 7 radiators and we use the hot water on a combi function rather than storage we'll rarely, if ever, use boost.

Anyway, it would've been a huge job to replace the whole pipe from the meter to the boiler as it takes a rather circuitous route under floors and up inaccessible risers and there is only about 20cm of accessible pipe at either end so replacing this would've made no difference either.

The installers were happy sign off the work and give us a written guarantee so I'm happy with that - I didn't want my house ripped apart for the sake of 0.5mb of pressure and I don't think they were too keen on a difficult and time-consuming job done at cost (which they'd already agreed to)

I think everyone's a winner - thanks again for all your information and help.

Jules