"David W.E. Roberts" wrote in message
...

snip

Taps:
------

Sounds (from a logical view, not an expert on Combi boilers) that whatever
detects that water is flowing and therefore needs heating (pressure
valve?)
is sticking a bit.

Open tap - no action.
Open tap, close (pressure raised above norm briefly) open again (larger
pressure drop) and the bolier kicks in. Effectively you are 'kicking' the
pressure sensor.
[Could low mains pressure make this happen also?? i.e. not enough
difference
between taps closed and open to trigger the pressure sensor.]

Once the boiler is going, presumably (?) it is turned off once max
temperature is reached, so hot water continues to flow.

So your problem is getting the boiler to turn on not turn off.

This is supported by the fact that once the boiler is running (for the
central heating) you get hot water straight away.
[Although presumably this would work even if the pressure sensor was
totally
non-functional?]

Central Heating:
----------------

O.K. at first, but now whatever senses that the water is under temperature
in the central heating circuit (or just the boiler) is not functioning
(boiler thermostat?). So although water is being pumped round the boiler
is
not kicking in because it cannot detect that the water temperature is low
(switch on).

However when you start the boiler (hot water tap pressure switch) the
boiler
starts, and then heats the water in the radiator circuit also. This
suggests
that the pump is running but the 'on' thermostat is not working.

One thing is not clear from your account.

Does the central heating only work whilst the hot tap is on?
If so, the thermostat in the central heating circuit is completely
thrashed.

Does the central heating warm up fully once a hot tap has been run for a
little?
If so it is only the 'on' action of the thermostat which is not working.
[And you are going to spend a lot of time turning hot taps on for a bit to
keep the house warm]

As I stated at the start, this is a (hopefully) logical analysis of the
symptoms - I don't know the bits of a Combi but I am assuming:

Hot water boiler on - pressure sensor
Hot water boiler off - h/w thermostat
CH boiler on - c/h thermostat
CH boiler off - c/h thermostat

This assumes that the valves and pump are working correctly and the
timer/programmer is supplying the correct voltages.

Reading through this there are inconsistencies (you talk about the boiler
'cycling' so at some stage the 'on' and 'off' functions of the thermostat
are working) but hey - logic is at best a weak and damaged tool :-)

There are also more complex interactions e.g. the hot water flow to the
taps
is getting too hot (so turn the boiler off) but the CH water is cool (so
turn the boiler on). I presume the HW would over-ride the CH in the
interest
of safety but this shows that a simple analysis could be a load of tosh
:-))
Also that you could just have a thrashed logic circuit in the controller.

Summary:

Sound like you have more than one problem
You may have problems with your pressure sensor and your boiler
thermostat.
Or something else entirely.

Cheers
Dave R

P.S. we seem to have a rash of 'my combi system doesn't work after the
summer layoff' threads at the moment.
So:

(1) As I was advised with car aircon over winter "just because you don't
need it don't ignore it; switch it on at least once a month to keep it
happy" so I would suggest that the CH on combi systems should perhaps be
turned on at least once a month over the summer just to check it is still
working and exercise the controls. At least you will know before winter if
you have problems.

(2) My old conventional boiler is still running well, and we have been in
the house nearly 20 years. Lord only knows how old it is, but it passed
the
last Corgi inspection with flying colours. It is less efficient than a
modern combi but all the bits are obvious and easy to replace. Projected
savings with a new combi are all very well, but if bits keep dropping off
then the savings may not materialise. My mum-in-law is on her second Combi
in 10 years (mind you the first one was cheap crap).
I am drawn to a comparison with cars - my old Moggie Minor was slow and
inefficient, but I could fix everything on it. My more recent Volvo Turbo
Estate (now quite elderly) has so much electronic gadgetry under the
bonnet
and fancy doodads and chips and analysis sockets that it frightens me
every
time I open the bonnet. Faster, more powerful, etc. but a bugger if it
goes
wrong.
So is simplicity best at times?



An interesting analysis - but it doesn't quite take account of the fact that
a combi boiler EITHER heats the hot water OR the central heating - but not
both at the same time. It achieves this by means of an internal diverter
valve which directs the heated water either through its internal heat
exchanger (for hot water) or through the external circuit (for central
heating).

The symptoms described suggest to me that the diverter valve is not working
properly - either as a result of a problem with the valve itself or (perhaps
more likely) as a result of a problem with the logic and sensors which tell
the valve what to do.

Roger