In an earlier contribution to this discussion,
wrote:

I'm trying to sort out a "no hot water" problem when boiler is set to
heat "hot water only, central heating off". Just get normal flows of
cold water from all the hot water taps.
The hot water flow pipe going into cylinder from boiler is very hot,
the return pipe coming out of the cylinder to the boiler is luke warm
where it exits cylinder.
The cylinder itself (including top) feels cold everywhere.
The boiler fires up but goes off after few minutes (say every 5
minutes).
I can hear water swishing through the pump which is running.

Its a conventional indirect system, Potterton Fuelsaver 65F boiler,
cylinder on first floor, expansion tank in loft (which is full).

I thought this was an airlock in the cylinder in the non-coil part of
cylinder, so attached hose & forced water out through cylinder out of
vent pipe, left it coming out for say 5 minutes & thought this would
be enough time to clear any air.
As this didnt fix problem,my question is how much water might need to
be forced through hose,cylinder & out vent pipe to get rid of air
lock. Might I need to replace the entire cylinder contents. I went
onto think the problem is now in the coil boiler fed part of the
cylinder but its hot going into the cylinder coil.
Help...
Steve

One thing is certain - you *don't* have an airlock in the DHW system
itself - otherwise you's get *nothing* rather than cold water out of the hot
taps.

For whatever reason, you have insufficient flow through the *primary*
circuit (boiler, pump, coil inside HW cylinder) to heat the water in the
tank. So the boiler fires for a little while, and then turns off on its
(internal) stat because the heat produced isn't being transferred to the
cylinder.

Could you give us a bit more detail about your system - particularly what is
supposed to control it. If the pump is running on HW-only, that suggestes
that it's a fully pumped system rather than gravity HW and pumped CH. Is
that your understanding? If this is so, there are likely to be one ore more
motorised valves in the system - probably (though not necessarily) in the
airing cupboard near the hot cylinder. How many are there - and what type?
[Likely to be one 3-port valve or two 2-port valves].

Is there a cylinder thermostat (as part of the controls for the
boiler/valve(s), *not* the one inside any immersion heater which may be
present)? Does it click when you turn it back and forth? What temperature is
it currently set at?

Is there a gate valve - or somesuch - on the output pipe from the cylinder
coil (i.e. the boiler return pipe)? If so, is it open?

You say that the expansion tank is full (that's the *little* tank which acts
as a header for the primary circuit, right?) Is there a tap or gate valve on
the pipe coming out of the bottom of that, which feeds the primary circuit?
If so, is it open?

You started by saying that this problem occurs when on HW-only with CH off.
What happens if you turn the CH on - do you then get get both CH *and* HW?
[I know you don't *want* the CH on this tgime of the year, but a quick test
may help to isolate the problem].
--
Cheers,
Roger
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