On 30/09/2020 20:35, John Smith 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.

At that age it owes you nothing, so probably worth going for a new
system since it will cost lest to run as well.

(whether it will also last 20 years is another matter!)

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.

A new 24kW boiler will have the same maximum gas rate (or possibly
slightly lower[1]) than an old 24kW boiler. So in that sense - no it
does not need upgrading if its already adequate.

(also worth noting that even very high consumption boilers often sitill
have 15mm inlets - the anticipation that the end user will likely want
the "show work" pipes in an out to be as unobtrusive as possible).

However to complication matters there are lots of "rule of thunb"
fitters out there that don't actually do the sums and just insist that
it needs a 22mm feed whether it does or not. Since that way they are
covered, and very few end users would question it.

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

Do a quick set of sums to check the actual state of play:

http://wiki.diyfaq.org.uk/index.php/Gas_pipe_sizing

Then at least of the fitter wants to do otherwise you can argue with
some data to back it up.

[1] If the 24kW on the old boiler was its specified output power, then
it will have needed to consume gas a bit quicker than a modern 24kW
output power boiler. (many would quote gas input power though, and just
deliver a bit less)

You can convert between units like this:

http://wiki.diyfaq.org.uk/index.php/Gas_units


--
Cheers,

John.

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