In an earlier contribution to this discussion,
wrote:

Hi,

HW has been working all summer, but attempting to re-start CH lead to
blown fuse (on timer/control/punp/valve circuit) - not sure if this
was coincidence or not. Now the HW does not work - boiler won't start
(pilot on and running) - no CH either.

After replacing the fuse, I confirmed that the pump is running, by
powering it directly. I also suspected the two-port valve (stuck
shut?) and tried the manual open switch, but can't budge this past
the central position (so I'm not sure if the valve is open or
closed). Boiler won't start in any case.

The system is 20+ years old, and the coil in the HW tank appears to
have a hole, as I am getting water through the overflow (even though
the boiler isn't on...), and is probably full of scale by now. So, I
will replace the cylinder and the valve, but would like to be sure
there isn't a further problem here that will stop the system working
when it is back together (especially as the valve is so conveniently
placed behind the cylinder, making future access very difficult).

Any suggestions what else I can try to confirm that a dead valve is
really the problem?

Alternatively, am I barking up the wrong tree completely, and if I am,
what else do you recommend I check?

Thanks/Paul

I'm not entirely sure from your description what sort of system you have.
You mention a 2-port valve - which suggests that you might have a C-Plan
system (see http://content.honeywell.com/uk/homes/systems.htm) with gravity
hot water and pumped heating. Is that what you have?

Or is it perhaps a Y-Plan (or even W-Plan) fully pumped system with a 3-port
(one inlet and 2 outlets) valve rather than a 2-port valve?

Once we've established this, we can tell you what to look for.

Is it the small fill & expansion tank which is overflowing, and is the level
in this *lower* than that in the main cold water heated tank for the hot
water system? If so, it's possible that the indirect coil inside the hot
cylinder is leaking - assuming you've eliminated the ball valve on the F&E
tank as the culprit.
--
Cheers,
Set Square
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