On 15/08/2016 12:20, Davey wrote:
On Mon, 15 Aug 2016 11:38:28 +0100
"Dan S. MacAbre" wrote:

Muddymike wrote:
On 15/08/2016 10:50, Dan S. MacAbre wrote:
My mum and Dad's boiler wants to stay on all the time. I thought
the best thing to do, besides just switching it off for now, would
be to try and work out how it's all wired together, and then work
out possible causes. I've only so far had time for a quick look,
but something has struck me as odd. They have two motorised zone
valves, but there is no room thermostat anywhere. I'd always
thought that each zone valve responded to a thermostat. Is there
any valid configuration that has two zone valves, but no room
thermostat?

Is there a thermostat on the hot water tank? If so is it set higher
than the boiler thermostat? If it is that is what is causing the
problem.

Mike

I'll check. This has apparently only just started happening, and
neither of them has admitted to changing the tank stat. I don't
think they could even find it. There is a timer, which has, AFAIK,
worked until now. A mechanical clock type, with a red/blue wheel for
manual control in the top right corner. Even when that is switched
off, the boiler still lights, but I thought the boiler got its power
from the timer.

It could be that the tank stat has failed and never says OK any more...
my parents did that once with scalding water from the taps.

It is an odd failure mode for midsummer. More often you get the opposite
that the CH is satisfied so it fails to heat the hot water.

Is there a junction box somewhere near the boiler or 'stat? That might
help provide information by identifying the wires connected inside it.
Otherwise, you will indeed have to do what I had to do earlier this
year, start from scratch with a pad and pencil, and make your own
schematic. Keep careful note of changes of colours where wires from
different components are connected.

The sudden change in operation could possibly indicate that one of the
zone valves has failed Open, thereby telling the boiler to run all the
time. Also, what temp. is the boiler thermostat set at?

Look for as much information as possible on components, ie Manufacturer,
model, etc so that you can look them up online.

But two zone valves and no room 'stat is a real puzzle. What controls
them?

Could be other timers but then you would have more than one system timer
but there should be thermostats somewhere. My parents is like that with
thermostats and zones being switched at night to only heat the bedroom
in use (and with a lower set point).

The usual cause of sudden changes resulting in either mad hot hot water
or insanely hot radiators is that a valve has stopped opening or closing
when commanded to do so by the thermostat (or the thermostat is dead).

Find the thermostats and then you might be able to test it by altering
the settings.

I am intrigued.

--
Regards,
Martin Brown