On 07/10/2020 13:19, Fredxx wrote:
On 07/10/2020 01:10:11, John Rumm wrote:
On 06/10/2020 20:53, Fredxx wrote:
On 02/10/2020 23:42:17, Fredxx wrote:
On 02/10/2020 23:20:46, wrote:
On Wednesday, 30 September 2020 20:35:29 UTC+1, John SmithÂ* wrote:
On 2020-09-30 18:58:53 +0000, John Rumm said:

With modern modulating boilers that is less of a problem in the
past. A
boiler with a wide modulation range is also desirable. The Vaillant
24kW models often go do to around 6kW.

John, thanks for your suggesting about the PCBs but at 20 years old I
think it's time to replace the whole thing and also it will be
quicker.

maybe you could find a similar used one & swap it, but probably you
won't


One possible problem regarding rating and the way new boilers work -
our gas supply pipe to the current boiler is 15mm - is this likely to
be a problem for a condensing system boiler - I know it would be
for a
combi boiler. Maybe there is a ceiling on rating or some makers only
specify 22mm.

Digging up the kitchen floor is not a great option...

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.

Do you have a link?

I didn't think so.

I did find this:

http://www.worcester-bosch.co.uk/sup...ownload/TB0053

Your google must be better than mine. I did look for a short while but
gave up.

That was somewhat tricky to find and then disentangle from the google
redirect to it, since could not find an actual landing page that links
to it. However it just dawned on me that I can now do a link search on
that one. So we get to:

https://contracts.worcester-bosch.co.../boiler?page=3

(there are a few other interesting docs there)

I find it annoying when someone makes a claim and then doesn't back it
up, the request for a link was genuine.

Well par for the course...

A stove was fitted where I've worked on a branch going to a boiler. It's
a long length of pipe to both boiler and stove and I can can hear the
gas ring change note momentarily as the boiler fires up.

If it dips and then recovers, it could just be a reflection of the main
gas governor taking a moment to adjust in response to a large step
change in demand. (which arguable a long pipe run might actually smooth
slightly, rather than make worse)

Although their note that a 24kW boiler's max gas rate is 2.6 m^3/h is
slightly surprising... it would suggest an efficiency of only 80% if
its giving a true 24kW output, and hence drawing gas at a rate of
nearly 30kW

That's a little worrying?

Well they say that's peak rate 10 mins after ignition. If the boiler
attempts to modulate on actual output power rather than gas rate[1],
then when the system is warming up, but not yet enough to modulate the
output down, condensation efficiency would likely be tailing off a bit.

[1] and that's a big if, since it seem more complicated to do.

(my previous calc was assuming that the input rate is 24kW, which is
likely an underestimate, since most boiler specs now quote output
power rather than input).

Oh, and I missed the brackets round the 6 x 1.9

If I redo my sum with their flow rate, that brings the maximum down to
~4.4m total effective length. (although they also indicate that the
pressure at the boiler inlet only need be 18mBar rather than the 20
one designs for)

I think boilers are designed to cater for low pressures because of poor
regulation rather than any assumptions about pressure drop in pipes.

Quite possibly - but it also explains why a number of installations seem
to work in practice, even when they their supply pipework is well out of
spec.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/