On 05/03/2021 13:40, Kevin H wrote:
On Friday, 5 March 2021 at 12:02:00 UTC, John Rumm wrote:

Also there was a trend in the past to specify the heat input into
the boiler rather than output. So 14kW might only mean 10kW of
output. Whereas 18kW these days, probably means that as an actual
output)

14kW is the output (18kW input). As you have said with modulation and
similar pricing for 18/21/24kW boilers I can't see any reason not to
go large (worst case having to upgrade to 22mm). Minimum output of
the Greenstar 24Ri is 8kW vs 6kW for the 18Ri model which should not
present any issues.

IIRC, the minimum output on my Vaillant 624 (24kW) system boiler is
about 6kW

The 3kW for DHW is a bit suspect these days... it was probably
about right for an old C-plan system with a slow recovery cylinder,
but for a more modern setup with S or W plan and a fast recovery
cylinder, the cylinder will usually be able to absorb the full
output of the boiler for a sizeable part of the reheat (and hence
do it in minutes rather than hours)

We have a 250l Megaflow pressurised HW tank with a fast recovery
coil. The extra output from the larger boiler should as you say help
with quick re-heating. Still on a Y plan system which means all the
output going through one 3 way valve. Wonder if moving to S plan
would improve flow to the tank (or rads) to take advantage of the
additional 10kW output that would be available?

The 3 way is ok, but you probably want the timing set so that you don't
run CH and DHW at the same time - the the full output (or close to it)
of the boiler can go to the cylinder. I installed a 210L Unistor
unvented cylinder, and that can consume heat at up to about 22kW - so
the boiler will run close to flat out into that appart from the last 10
mins or so.



--
Cheers,

John.

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