On 03/10/2020 10:46, John Smith wrote:
On 2020-10-02 22:20:46 +0000, said:

In most cases a 24kW boiler is not permitted to run on 15mm now,
current regs are more demanding re pressure drop. Worcester Bosch do a
pdf:
Technical Training Bulletin: Natural Gas (G20) Supply Pipe Sizing.
that explains how to calculate what's required.

We had a new Vaillant condensing system boiler installed yesterday and
the installer played it safe and put in a 18kw unit as he said the 15mm
gas pipe run was probably quite long and it is about 4 metres in a
straight line from the meter without taking into account the true path
under the hallway/kitchen.. The boiler so far is working fine and we
have hot water and heating after 10 days without and it's comfortably
driving 15 rads in about the same time as the old 24kw unit.

24kW equates to around 2.16 m^3/hour [1]

So if you work within the 1mBar pressure drop permitted, your maximum
pipe length will be somewhere 3 and 6m [2]

So if you take the 6m discharge rate, and scale it for the slightly
higher actual demand you get :

1 mBar pressure drop = ( 1 / 6 x 1.9 ) x actual length x 2.19

1/actual length = 0.192

actual length = 5.2m

So yup with 4m of straight pipe, potentially another 1m or so of
equivalent length in elbows etc, plus the rises to the boiler and the
meter, you would be pushing it at 24kW

(in reality it would probably still work since most boilers will be a
little more flexible than the pipe sizing regulations would imply)

Only hitch was a bleed hole rounded off in one old rad so that's been
taken out of the system for now.


[1] http://wiki.diyfaq.org.uk/index.php/...its#Converting

[2] http://wiki.diyfaq.org.uk/index.php/...ng#Design_Data

--
Cheers,

John.

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