John Orrett wrote:

At present we need both the immersion and the boiler to give us a
decent bathful of hot water.


OK. Is that because the boiler on its own heats the water too
slowly or not to a high enough temperature or some other reason?


Basically because the immersion tank itself does not give enough hot water
due to its volume. The boiler is the 'top up'

OK Reading between the lines he

I am getting the impression that your boiler must already be connected
to your hot water cylinder.

(If your cylinder was heated only by the immersion as it first sounded,
then the boiler and central heating settings would have no effect at all
on the amount of hot water available).

That complicates matters somewhat! Your plumber will be able to discuss
options for improving the performance of your current setup if its not
up to its current job.

Just the one bathroom. If we need a top up of water then as said previously,
I turn the central heating down for an hour so the boiler has enough hot
water to finish off the bath.

Apologies if I am barking up the wrong tree, but I think you might have
a slight misunderstanding about how your current boiler operates. The
boiler itself will not directly heat the water that fills your bath.
Instead it has a loop of pipework that leaves the boiler and the returns
to it. There will be a pump somewhere that just circulates this same
water round and round, each time it goes through the boiler it warms it
up a bit. The radiators are connected between these feed and return
pipes, each time the water goes through these they cool it down again
before it goes back to the boiler.

Your hot water cylinder can also be connected in a variety of different
ways. Most cylinders that are heated from a boiler will have a coil of
pipe inside them, through which the water heated by the boiler can be
pumped. This enables the heat from the boilers water (which will be
contaminated and stagnant) to pass to the water you get in your bath
without actually getting mixed into it. If you look at this type of
cylinder you would expect to see four piped connections to it. One will
the cold water in from you tank in the loft (typically fed into the
bottom of the cylinder), one will be the hot water out (top), and the
other two will be the feed and return for the hot water from the boiler
(typically middle and bottom).

It could be you have a very simple setup at the moment with either the
hot water cylinder relying on unpumped convection circulation of water
from the boiler, or it has actually been installed "just like a radiator".

In the first case you will get slower and less efficent heating of the
cylinder, in the latter you will have the cylinder and the radiators
contesting for the available heat from the boiler.

You may well be able to get better performance with the setup you have
by having your plumber improve the way your boiler heats your current
cylinder. Also raising the temperature of the water stored in the
cylinder will allow you to mix more cold with it when running a bath,
giving you more "bath temperature" water.


--
Cheers,

John.

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