wrote in message
oups.com...
During this problem the pressure at the shower head never drops the
water just goes hot & cold. I just tested the hot water tap in the
kitchen where the boiler is situated,so I can view the control panel.
Once hot tap is turned on the green light comes on followed by the
amber light & the water temperature increases to approx 65c this
continues for approx 3-4mins. Then the amber light goes out the green
light remains on & the water temp starts dropping to below 38c the hot
water from the tap starts to go luke warm but pressure seems the same.

Once the water temp drops to approx 38c the amber light engages (turns
on) again & the water temp display shows the water temp increasing
again & the water at the tap starts to get hot again.


Interesting. Unfortunately, I am not an expert in comi fault diagnosis!
However, I have one of the Vaillant Turbomax combis, and have had
a bit of a poke around it when I first got it to see how it was laid out.

I believe yours will have an lcd diagnostic display on the front panel :
what
are the fault codes that it is showing? You should have an installation and
operation manual with your boiler. If not, you can download a pdf version
from the Vaillant website. I believe they may also have a technical helpline
( not absolutely sure about that ).

If I were you, and wanted to see if I could diagnose it myself, I
would
start by looking at the faultcodes and their meanings. There should be a
list
of fault codes accesible on the lcd display, in the historical order that
they
occurred. Be aware that ust because a faultcode says something like
'no water in heating loop' doesn't actually mean you've nailed the fault. It
may
be a duff sensor or just a random error. However yours sounds repeatable,
so the diags will give you clues. If you delve into it you can also see
whether
the combi is modulating ( throttling the flame up or down according to
load )
by looking at the modulator current ( I haven't got the destructions in
front of me, so I can't
remember the exact info code you need to punch in ) which is available via
the lcd display.

I'm not saying that'll tell you the answer, but it might be a clue as to
what the combi is up to
whilst it goes through this hot cold routine. The rest is up to you, get the
instructions,
familiarise yourself with the operation and layout of the combi, and do some
detective work,
or get a combi expert in!

NB, I can provide a comparison with a working 828 Vaillant Turbomax if you
want to figure
out what a working vaillant should be doing, as long as it doesn't involve
taking mine apart
I'm happy to tell you what the lcd display says in various circumstances
etc.

Andy.