Simon wrote:

I did the test with the settings as suggested by Terry: ie, bypass
valve a quarter-turn open, heating valve completely closed, gas boiler
thermostat set on 4 (max is 5), TRV-like capillary valve fully open).
After half an hour the tank was slightly warmer (you could feel it was
a bit warmer but not much and certainly not near "hot"). The pipe
going from the tank to the TRV capillary valve was somewhere between
"no difference" to maybe "slightly warm"), but the pipe going from the
TRV capillary valve off to the top tank and the rest of the system was
very hot (too hot to touch). Is this the way it should be?

After 30 mins I ran the bathwater and the water was warm but not
enough for a bath. After 90 mins I ran the bath and it was running too
hot to touch and there was enough for a bath.

I'm extremely pleased you have got this far, and hopefully (someone)
enjoyed a bath!

I'm not sure from the above description exactly what's heppening, but
that's my fault.

You may have the bypass gate valve too far shut - try opening it to
half a turn. It lets water through the bypass, but you need some to
keep the pump happy.

So... yippee!!! Many many thanks Terry! This is the first time we have
ever been able to have a hot bath without having the electric heater
on for more than an hour (usually 90mins min). Just this on its own
will save me a fortune and make life so much easier. All i need to do
now is try the gas boiler thermostat on diff settings for diff lengths
of time to find the optimum setting for water temperature, time to
heat, and units used etc.

No, no, no...

Leave the boiler thermostat setting where it is for now. The boiler is
designed to run to its own settings - that's why you have control
valves such as the TRV-like capillary valve and the TRVs on the
radiator. Trying to run the whole system from the boiler is simply not
the way to go - let each part it do its own job.

You control the temperature of the water delivered to the hot water
taps (domestic hot water or DHW) via the TRV-like capillary valve
only.

I suggested setting it on 'max' as an initial setting. Clearly it's
too hot - possibly dangerously so. On the one in the system I used to
have, the difference between setting no. 4 and setting no. 5 (out of
8) was the difference between hot water and very hot water.

I suggest you now try a setting about half-way as a start, and then
experiment over the next few days to give you the DHW at the
temperature you want. This will be a major milestone in getting your
system running properly.

But... from the above, is this HW system now working well or should it
be better?

Difficult to say, but you do need to get the DHW temperature down to
something more reasonable (hot but not very hot....you'll need to
decide for yourself).

btw - When I say "gas boiler thermostat" set on 4" I presumed you
meant the actual dial-setting on the boiler itself, and not something
on the water tank or associated pipework. Is this right?

Yes, it was the setting on the boiler thermostat.

I couldn't
see anything else on the tank that could be adjusted and so I presumed
that the dial setting on the boiler was what you meant.

Yes, that's right.

Because of the success of the above test I didn't remove the top of
the TRV-like capillary valve and see if the pin was stuck in a
partially closed position, from the above do you think this is
necessary or a still a good idea?

It's not necessary. What I suggested was an experiment in case the DHW
wasn't heating up at all. Clearly this isn't the case and your
TRV-like capillary valve appears to be working fine - except that it
just could be stuck open. However, if you can control the temperature
of the DHW by adjusting it as mentioned above, that rules out another
problem.

Because the water can reach boiling in the tank I guess it means there
is a failed thermostat that needs replacing as you say, and this is
something I should look at (even though i presume limiting the heat is
purely a safety measure, and is not something that is done for the
efficiency or otherwise of the system).

I take it you mean when the immersion is on. Hopefully you won't need
it now 'til Spring.

Replacing an immersion heater thermostat is a straightforward job if
one knows what one is doing. You could ask the question in a separate
thread to get more knowledgeable people than myself to offer more
informed comment.

@B Trends - I live in a hard water area (I can tell from the build-up
in the kettle, the taps etc, and how hard it is to get up decent suds
for washing). I had thought about this possibly being part of the
problem and could see how limescale could serious affect the
efficiency of a CH/ DHW system. One solution I looked at was
Scalewatcher (http://www.selfbuilder.net/products/scalewatcher.html)
which looked good but expensive (£150ish). What do you think about the
cost/benefits of these kind of limescale removal systems? Are they
worth it? Is it applicable to my situation?

I'm sorry, I couldn't advise on that. You might like to ask a seperate
question to get informed comment from people who have installed and
used these. I would suggest not rushing out to spend big money yet as
although the DHW is on its way to working properly things have yet to
be fine-tuned.

When we move into winter and we want to have the radiators on for at
least part of the day, will it be okay to switch between having the
heating valve fully open to take the heat of the gas boiler to the
radiators, and then close it fully to heat the water for a bath, and
then after the bath to open the heating valve again to heat the
radiators, or could this cause problems? Is it okay to simply switch
between fully open and fully closed in this fashion?

When winter strikes, or you just feel cold, put the system on and open
the Heating Valve gate-valve fully. The flow of hot primary water from
the boiler will be shared by both the DHW and CH circuits, so it will
take longer to get a tankful of hot water and longer for the radiators
to heat up fully. However, if you have set the TRV-like capillary
valve to give you the DHW temperature you want, and the TRVs on the
radiators to give the room temperatures you want, then these various
valves will after a time start shutting down and thus passing hot
primary water to the other circuit. When they've all shut down, the
only flow left - necessary to keep the pump happy - is through the
bypass system.

It may take noticeably longer to get a tankful of DHW under these
circumstances, and you may need to anticipate family demands for DHW
and room heating! Starting with last night's DHW still in the tank
goes a fair way to getting everything up to temperature sooner -
starting with a tankful of cold water straight from the mains means
much longer for everything to heat up.

On the sytem I had, heating up a tank of mains-cold water took about
an hour. Starting with last night's DHW in the tank, it took an hour
to heat the water and get the rooms warm. Conserving hot water in this
way can help a lot, as can anticipating family demands. After all the
upheaval of the last few days, take time to ensure the lagging jacket
on the HWT is properly fitted.

Once again, many thanks for all your help. Any further advice that
could help is gratefully received.

I'm glad I was able to help - but you do need to get the DHW
temperature right for you by adjusting the TRV-like caplillary-valve
settings.

ps - i have ordered R.D.Treloar's "Plumbing: Heating & Gas
Installations" to get a better overall understanding of how these
systems should be set-up and should work. Supposedly it's the best one
for the layman. It's on its way and I'm really looking forward to it!

Well, I've just taken over a house with a complex thermal-store system
and more than twice the number of radiators I had - some of the
radiators have TRVs and some don't, and there's a room thermostat as
well. Although the system in seven years old, all the lockshield
valves (LSVs) on the radiators are fully open, meaning the system has
never been balanced. Using the groups' FAQ, I have now set about this
task, you can find it he

http://www.diyfaq.org.uk/

However, I would caution you not to make too many changes to your
system at one time, otherwise you could easily get lost in maze of
changes that don't give you the results you anticipated. You've come a
long way to get the DHW system started, and you need to fine-tune
that. With the DHW system running satisfactorily, there'll be the
whole new world of Central Heating to explore....take it one step at a
time. Good Luck!

TF