Gas pipe diameter - new regs

[--tom]

chris, thank you for such an excellent explanation.
A few further Q's
Q1. are you saying that all combi's require a 22mm supply only for it to be reduced to 15mm at the appliance? Q2. What is the purpose in increasing at the meter connection from 22mm to 28mm? Given that the meter usually seems to be 22mm.

 

[ollski]

pressure loss is directly proportional to

length
specific gravity
friction

ie. if any of these quantities are doubled the pressure loss would also be doubled

 

[ChrisR]

--Tom
combis are 24-35kW
Theoretically you can go about 4 metres with 15mm for the smallest of those. But thats 4 metres of EFFECTIVE Pipe Length. Every elbow is half a metre, and the isolating valve on the boiler is always smaller than the pipe. Smaller combis all have a 15mm connection, which is daft. So you can almost never use 15mm with a combi.

The resistances of every bit of pipe add up - it isn't a "weakest link" thing. SOmetimes you need to increase the capacity of the supply where half of it is buried in concrete. So you increase the part which is practical (which almost removes its resistance, the difference is so great) to bring the overall resistance to an acceptable figure. It doesn't matter which part of the pipe - larger or smaller, comes first, if there are to tees off to other appliances.

I recently ran 2 combis off one meter, (which was the maximum capacity for the meter). It had a 3/4" outlet connection. I split the pipe to two 28mm's and ran one to each boiler. All the pipe ends were deburred, I used slow radius bends, and changed the isolating tap on the boiler to a larger diameter one. I STILL had a bigger pressure drop than the book said, but I'd chosen boilers which burn at 12mbar, so had plenty in hand.

Anything above 28mm is "Industrial". The gas behaves differently in the pipe, so a different set of procedures has to be used, not covered by "domestic" corgis.

 

[ConcreteCow]

It's quite common to have only the built-in oven removable, so you have to cut a lot of 2ft long bits of 28mm and solder a bit on, slide it along, solder another bit on... etc, until you come to a hole in the wall you drilled from under the stairs..

I thought I was bad enough doing that for 15mm heating pipe under my bedroom floor. Glad to know that I was doing the "right thing"

regards

 

[ConcreteCow]

--Tom
combis are 24-35kW
Theoretically you can go about 4 metres with 15mm for the smallest of those. .

Looks like I will have to get the feed to my Ravenheat condensing CSI Primary boiler rerun in new 22mm then, as it is fed with ~3M of 15mm pipe, with 4 elbows and 2 bends. Nominally range rated ~12 - 24KW.

Or do I?

regards,

 

[ChrisR]

Measure the gas:
Put cold taps on full to bring boiler on at full rate.

If it's an imperial meter. Measure how long it takes for 1 cubic foot of gas to pass (one rev of the fastest moving dial on the meter). Call that T.

(3600/T) x 1034 gives Btu/hr. Dvide by 3412 converts Btu/hr to kW.

If it's metric, Measure how much gas passes in one minute (first you have to work out what your dials mean!). Multiply by 666 gives kW. Mult by 3412 converts to Btu/hr.