Why the need for larger gas pipes for Condensing Boilers?

I've got some experience of life cost of Combi boilers over the past 10 years as I started to get rid of all my old back boilers thinking this was the way to go-No leaks from tanks in lofts any more. No more copper tanks & pipes going to it rusty any more leaking to downstairs-Great!
But then, I have found nearly all my Bosch Worcestor Combi 24i's, new fans £130ish needed after 4-5 years, circuit boards £200+ after 3-4 years, plus other bits.
Roughly, after about 8-9 years of the boiler being in, repairs of £500ish. When the back boiler major tank repair may have been cheaper. Don't tell u this when they buy the combi. And I do mean EVERY combi-I've had approx 30 Worcestors been fitted has needed these repairs.}
And last 4 years, Baxi Duo-Tech 24 HE I believe, done about 20 of them, AND EVERY ONE HAS LEAKED WITHIN 2 YEARS.

I have carried out a lot of training on gas regs and also electrical installations in the past however admittedly more in the mobile and leisure industry (caravans etc). I have always been told and trained that manufacturers instructions of an appliance will always supercede gas safe regs as the manufacturer has to put their product through stringent inspection and testing to gain CE approval which is specific to their appliance. Why is there a need to also change pipe work for a pre-existing installation? If valiant as an example require 16mB at the regulator then that should be the end of the argument?

you ought to try Atmos Inter HE - dutch boiler developed for the social housing sector in Holland based on whole life costs. more expensive to buy, beautiful design, only 4 moving parts, fitted these for some landlords who were prepared to pay a bit more, so far have been ok. watch to video of how they make them! BMW plant in Germany use their engine casting plant. very impressive bit of kit.

but agree with you on the old boilers - I have customers with boilers over 30 years old, they come to us after they have been told to scrap them, we keep them going where we can. a bit of tlc and clean the system, I bet the efficiency is not far off - most losses are with poor insulation and muck and lime scale in the system. another landlord where we did change the boiler, has spent over £800 this year, neglected leaks had corroded much of the water section, parts alone were over £500. should have changed the boiler but he was reluctant after only being in 6 years.

have fun!

Re boilers, most plumbers/gas engineers I know all say Worcesters are really good but my experience is NO, in fact anything else has been better for me at least.
I have 5 Worcester combis and everyone has broken down over the last 5-7 years costing hundreds each time. Yet I have others like Ferroli, Vaillant, and Sime all of which need minimal repair if any ! Recently had my gas engineer fir a couple of Vokeras he rates them and they have five year parts and labour which helps....

I'm not sure where you got the figures you quoted for gas pressure delivery, but since 1970s, manufacturers of gas appliances have designed gas appliances to work at an inlet pressure of 20millibar. This has applied to all appliances: cookers, fires and boilers. Combis weren't commonplace then but simple boilers still demanded a 20mb gas pressure at the inlet. Gas pipes have a consistent pressure loss and engineers would design the pipework to deliver a pressure drop of only 1mb at the appliance. ie, 20mb if the pressure at meter was 21mb. This rule is even more important now when all combi boilers raise the temperature from about 10oC to 45oC immediately. Registered gas installer.

I think it's really a meticulous matter that should be observed by an highly experienced and qualified engineer before training anyone for plumbing and other related work. I called an engineer but he couldn't make up the work.

Reply to the comment left by " " at "29/03/2013 - 20:05":

Hello Joseph,
We have just had a Vaillant ecotec plus 831 boiler installed, replacing our Vaillant Turbomax. We have a pipe run in 22mm of approx 80 feet with four 90 degree elbows. The last 5 feet is in 15mm with a 90 degree elbow. The pipework continues in 22 mm for about 25 feet to the gas hob. We have approx 20 mbars at the meter but only 13.54 mbars at the boiler. Our installer says that converting to 28mm will put things right. My worry is that spending £425 pounds on the new pipework may not provide the correct pressure. Is it possible to calculate the correct diameter of pipework required?
Martin D'Arcy

Yes, very simple to calculate, this page will give you the figures, you can do it yourself for peace of mind!
http://www.ukcopperboard.co.uk/literature/pdfs/Installation-Tips/Domestic-gas-planning-and-sizing.pdf

Remember it is not just pressure, you have to make sure the flow is sufficient for the larger combis. check what the manufacturers instructions say the flow needs to be, then you use the table to see what the maximum length pipework would be for a particular diameter to give the flow rate at the pressure needed.

there is of course an app that does the calculation for you, you put in boiler size, length, number of elbows, and gives you the answer! costs about £1.50, well worth it.

The safety concerns with undersized gas supply pipes, is that they provide low working gas pressures to ALL appliances in a property. This occurs most, if powerful combi boiler, gas fire and gas cooker are all working at the same time.

Modern boilers can work safely with very low working gas pressures as they all have safety systems that will detect if the flame has gone out. The cases,flues/chimneys are also sealed from the room, so if any carbon monoxide is produced through poor combustion, it should go outside.

This is not however the case with cookers and fires. If you imagine that a gas hob is on, simmering some spuds for dinner, set to its lowest setting. You decide to run the bath for the kids, while thats cooking. Hot tap on, 35KW combi boiler fires up. Due to the way most work now, they can almost suck the gas from the pipework for themselves. This can give a situation where the gas pressure at the hob burner, can be so low that the flame can go out. When the bath tap is turned off, the boiler stops, the gas pressure goes back up and the cooker now gets gas, but it is not lit !!!

So you now have a gas burner with gas coming out that is not lit. If this were left for some time, you can imagine the possible scenarios.
A lot of cooker burners do have flame supervision devices now, which would turn off the gas if the flame goes out, but a lot do not. All cookers in flats or HMO should have these safety devices fitted.

The same situation can happen with older gas fires.

I'm a registered gas installer.

Boiler installation, repair and service should only be handled by a gas safe registered engineer. If you were required to change to bigger pipes, your engineer should be able to explain to you why. I have Vaillant boilers in East London and I've needed repairs a few times. I only call my trusted engineers to do the repair.