Boilers and 22mm gas pipes

[Martin Pentreath]

Hi all,

I have a combi boiler around 12 years old which is supplied by a 15mm gas p ipe. In fact that pipe was probably run originally in the early 80s to a pr evious boiler. When I had a man in to inspect it recently he mentioned that when this one goes t*ts-up a new boiler will need 22mm back to the meter. This will be a significant pain in the arse. Is this true for all cases? Th e boiler is in a very small flat, so I could get away with something lower powered, although of course with a combi a minimum is needed to get a reas onable flow rate for domestic hot water.

(By the way, the current 27kw combi runs very happily on its 15mm supply, b ut I guess regs have changed since it was installed?)

Cheers!

Martin

[Fredxxx]

There are some pressure drop calculators. If the pipe lengths are short enough with few elbows it may still be possible to route most of the pipe in 15mm.

You might still find you fall foul of some new rules such as not allowing pipe runs within internal stud walls.

[John Rumm]

No not true for all cases - depends on the gas rate required.

The regs have not changed, as far as I am aware. The same principles apply - that the pipework should drop no more than 1mBar of dynamic pressure at full gas rate.

[YAPH]

If you have a 27kW boiler then about 300mm of 15mm pipe will drop the permissible 1mb of pressure between the meter and the appliance! Your boiler may be running "happily" but it will have seriously low pressure at its inlet and will probably be running below its rated power.[1]

Since pressure drop as a function of flow rate is very non-linear increasing the pipe diameter will allow much greater flow: you could run almost 20m in 22mm pipe with an allowable drop[2], and with 28mm you could run it down your street. And if the original boiler was rated at around 12kW just for heating the house with a little extra for the HW cylinder then it would probably have been fine on yards and yards of 15mm.

[1] you can measure it by timing the rate your gas meter runs with only the boiler drawing gas. If you have a meter with a clock dial showing cubic feet then time it for one revolution and divide 1100 by the time in seconds to get kW. If it's digital take a couple of readings of the last 3 digits a minute or so apart, subtract the readings (to get litres of gas) and multiply 38.6 by this volume and divide by the time (in seconds) to get kW. [2] counting every sharp 90 degree elbow or Tee as half a metre

[YAPH]

And running at full power: on a combi run the hot water at full flow

[Martin Pentreath]

Thanks for all replies, especially YAPH.

The original boiler was a system boiler with a cylinder for DHW. With only four radiators in the flat I guess that boiler was pretty low-powered and 1

5mm was fine. I would guess that the distance back to the meter is easily 1 2 metres. Clearly the guy who put in the 27kw combi that replaced it cut so me corners. Anyway, it sounds like if I want a combi to replace the existin g arrangements I will need a new gas pipe, so I'd better bite the bullet.

(BTW I'm assuming that this is not an ongoing safety issue? The boiler is i n a rented flat, and has always been given a certificate on its annual chec kup.)

[Adam Funk]

We just had a combination boiler (30 kW) installed to replace the

32-year-old (IANMTU) slave boiler + cistern + cylinder system. The plumber upgraded the first section of pipe (from the meter to the T) from 22 to 28 mm & the third section (the last couple of meters) from 15 to 22 mm. (The middle section remained at 22. The T supplies the pipe up to the boiler & the pipe to the hob.)

The 32-Y-O boiler was still running, although the new system actually gets the house up to a decent temperature & stops, instead of running flat out whenever it's on. The guy who used to service the boiler had also been warning me that spares would not be available these days.

[Capitol]

That's the problem with these young 32 year young boilers, you can't get the parts, although you can for 45 year old boilers like mine!Bought a new Honeywell gas valve as a spare this year ($50), just happens to fit British boilers. OK, I accept that there are very few parts in this boiler and that's why it's so reliable.

[Adam Funk]

I only know how old the boiler was because, while I was looking for something else a few days ago, I found some papers that came with the house. The old one had no electrical connection, just water in & out, gas in, & a balanced flue through the wall. It had an internal flow valve that turned up the gas when water was being pumped through it, & some kind of non-electrical cut-off (which worked) to detect a pilot light failure. The plumbers found a data plate on the back which said it was 16 kW in & 11 kW out (so 69% efficient --- the new one is supposed to be 92%).