On 10/03/17 20:29, David wrote:
On Fri, 10 Mar 2017 16:26:02 +0000, AnthonyL wrote:

On 10 Mar 2017 12:46:02 GMT, David wrote:

On Fri, 10 Mar 2017 11:56:14 +0000, AnthonyL wrote:

Since 1992 my friend has had an impossible to control shower which
drives his family (daughter especially) and visitors mad.

It is difficult to believe that given the installation no improvements
were suggested when the Greenstar 28i Junior was installed 7 years
ago.

The shower is on the first floor. The tap is one of those old
fashioned two-into-one with a plunger to change the destination from
down into the bath to up to the shower head.

The hot water is fed at mains pressure, the cold is from a header tank
in the attic directly above, which by my calculations means that cold
water will never make it to the shower head.

On my visit last week I suggested that we simply turn the hot water
temperature down - but no, unlike my (older) boiler there is no
separate control, despite no mention of this in the (on-line) manual.

The guy who posted this:
http://forums.moneysavingexpert.com/....php?t=3584437

seems to have the exact same issue.

My immediate thought is to take a feed from the inlet of the header
tank and connect to the bath's cold water, giving it mains pressure.

Any thoughts?

As far as I am aware you should never mix tank and mains pressure at a
shower. Unless there is a non-return valve on the cold this looks a very
dodgy set up because you are connecting mains pressure to tank pressure
through the shower head.

I assume that there is still a hot water tank, otherwise why the header
tank in the loft? That is, the boiler is working as both a combi
(directly heating mains hot) and a system boiler (heating some kind of
hot water store).
snip

It is unlikely that there would have been a hot water tank in the loft.

Usually a system is all mains pressure (as with a combi boiler) or
(almost) all gravity.

With a gravity system you have a cold water header tank in the loft and
then a hot water copper cylinder in an airing cupboard, often close to or
in the bathroom. Hot water is pre heated and then when the hot tap is
turned on cold water from the loft flows into the bottom of the hot water
cylinder and forces hot out of the top and through the hot tap.

One important thing is to have a mains cold water tap for drinking water
as cold water in the loft tank can sit there for a while and can also get
contaminated by vermin.

What you describe is not a normal system and sounds suspiciously like a
cowboy installation. Usually if a hot water cylinder is removed and a combi
boiler fitted then all the cold water is converted to mains pressure and
the header tank is removed.

The only usual reason for a header tank in the loft is to provide a head
of water for the hot water tank. In that case the cold feed to a mixer tap
should also be from the header tank.

Your original suggestion that mains pressure should be connected to the
cold side of the shower was sensible; however we are asking further
questions to try and establish if this is an unusual but generally logical
set up.

For example there may still be a hot water cylinder which supplies all the
hot taps apart from the shower, and the combi boiler may have a circuit on
the central heating side which heats both the radiators and the hot water
cylinder.

The second point being made is that if the shower mixer is a hang over
from a low pressure gravity feed system then it may be unsuitable for a
mains pressure system even when both hot and cold feeds are at mains
pressure.

If you have a mains pressure combi boiler there is a limit to the amount
of hot water it can supply by instantaneously heating the incoming cold
water and the shower has to be designed to produce a good high pressure
spray at this flow rate. It has to work at a low flow rate and a high
pressure.

Low pressure tank fed showers tend to pass a lot more water through at a
much lower pressure to provide a satisfying shower. If you use a low
pressure shower mixer with a mains pressure supply you may never get a
decent shower.

From online specifications the boiler should provide a hot water flow of
between 10 and 13 litres per minute (assuming enough pressure and flow
from the incoming main). If the shower is not designed to operate at that
flow rate then you are wasting your time. Connect the cold mains to the
input to the shower mixer to match the hot mains, but be prepared to also
fit a new mixer designed to work with a combi boiler.

I do have a (possibly unworthy) suspicion that if this shower has been
endured for so long there may be a reason why it hasn't been fixed.
Perhaps a reluctance to spend money?

TL;DR - connect mains cold to the bath, and fit a new mixer.

Cheers


Dave R

Cowboy installation? Maybe.

It took me quite a while to get my head round the water supply in our
bungalow. It has a standard (non-combi) gas boiler, and the usual tanks
in the loft - cold water and expansion. There was also a double-sized
hot water cylinder.

There are four showers! One is an instant electric one in one bedroom.
The master bedroom has an en-suite, and the main bathroom has a shower
cubicle, and shower attachment on the bath taps (of the sort shown in
the previous post link). The en-suite and bathroom cubicle showers have
pumps (hence the need for a double-sized HW cylinder). The HW and CW
supply to the shower pumps is taken from the CW tank and HW cylinder. So
far so good. It is when I started looking at the CW supply to the rest
of the bathroom and en-suite fittings that it became interesting.

I found the bath shower attachment very sensitive to the CW tap setting.
With the HW tap fully on, slightly opening the CW tap led to a cold
shower only. And if left for a few minutes, the HW didn't immediately
reappear when the CW was turned off. It was obvious that CW was being
forced back into the HW system. It then became clear why I had had to
turn off the isolator valve almost completely for the CW supply to the
bath and en-suite basins. It was very easy to open the quarter-turn taps
fully and get drenched with splashback from the basin!

Effectively, only the showers were fed (via the pumps) from the CW tank.
All other CW supply, to the bath, basins, and cisterns, was at mains
pressure. When I mentioned this to our plumber one time, he said that it
was unusual, but he had seen it occasional.

--

Jeff