On 20/05/2017 20:37, Andrew wrote:
On 20/05/2017 19:44, Phil L wrote:

If all four showers are used concurrently, while someone is also
using hot water in the kitchen then there will be a problem.

It's called the laws of physics. The faster you move cold water
through the combi, the less heat is transferred into it from the
heat exchanger. So choose between showers running like a poor
old sod with a prostate problem or lukewarm water.

I found a 35kW combi will do a pair of showers concurrently. Not going
to do 4 obviously - but its unlikely in a domestic setting that all 4
would be in use at exactly the same time.

And while this is happening all the heating stops.
This can be a problem with an old leaky house.

That is also a problem with modern unvented cylinders as well.

Old cylinders had low heat transfer rate coils that would only absorb
say 5kW at best. That meant there was logic to using a Y plan
arrangement which allowed the boiler to run more efficiently feeding the
rads and the cylinder, rather than short cycling with just the cylinder
that could not take its full output.

A modern unvented cylinder will have a much larger coil that can take
heat at 20kW plus. That means you can direct the full output of the
boiler at it. Hence its better suited to W or S plan style control
systems (unless you have a very powerful boiler) but no heating while
its recharging.

(there is another reason for directing the full output of the boiler at
the cylinder, in that it then allows split temperature operation -
running much higher flow temperatures into the cylinder than you use for
the rads, gaining extra condensing efficiency on your heating)


--
Cheers,

John.

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