The boiler supports valving according to tech help at Potterton so the
easiest option till I replace the boiler is to fit a 2 port to the HW
system.

You might want to stick a pump on there too. You may need a relay or two so
that you can run the pumps independently. Alternatively, don't use a 2 port
valve at all, just use a pump, an antigravity valve and a relay for boiler
interlock.

Please excuse the dimwit questions but is this a major job, and capping
off the gravity circuit means what exactly?

An alternative is to cap off the gravity circuit and run the hot water
cylinder from the pumped central heating side with modern format controls.
The gravity circuit is usually then used solely as the vent and water feed,
with any actual heating appliance connections capped off. The advantage of
doing this is that it simplifies the installation of a new boiler in the
future, as the pipework layout and heating controls are to modern standards.

When making modifications to old boilers, you must consider the vent/feed
paths. The vent path must be unvalved all the way to the exit. Unless your
boiler has a secondary lockout, it must also not be possible to block the
feed path to the boiler. The feed and vent may not share pipework, as the
escaping gas in the vent will block the ingress of quenching water. These
requirements don't apply to boilers that have an overheat cutout, as such
boilers don't rely on the quenching effect of new water introduced into the
system.

Christian.




"Charlie Pie" wrote in message
...

Christian McArdle Wrote:
The cure for this problem is either to convert the system to a fully
pumped-
setup with two zone valves or a single 3-port valve (S-Plan or
Y-Plan), OR
to convert it to a C-Plan system.-

Another solution if the boiler is the type that can't support valving
on the
gravity circuit (only solid fuel or really old boilers might be like
this),
is to:

(a) Put a cylinder thermostat inline with the HW On output from the
programmer. This will limit temperature and improve fuel efficiency
when on
HW only mode. When the CH is actually on, it will still overheat,
though.

(b) To solve that problem, stick a TMV (thermostatic mixing valve) on
the
output. This will mix the very hot water with cold, to bring it down to
a
safe temperature (i.e. 55C). You may need to add a pump to get good
shower
performance.

Obviously, a better solution would be to replace the boiler in this
sort of
situation. If you have a boiler that can support valving, or, better
still,
capping off the gravity circuit, then go with Set Square's suggestion.

Christian.

Thanks all for advice so far.
As suggested both HW and CH are independent and connected to the boiler
(potterton neteheat 16-22 Mk2 F) with 28mm pipes.
The boiler supports valving according to tech help at Potterton so the
easiest option till I replace the boiler is to fit a 2 port to the HW
system.
Please excuse the dimwit questions but is this a major job, and capping
off the gravity circuit means what exactly?


--
Charlie Pie