Pulsacoil 2000 melted wiring

Three years ago I bought a flat fitted with a Pulsacoil 2000 water heater.  It was fitted at the time of build in 2004, so is now 9yrs old.  I've had no issues with it until now, apart from the overheat protection (red light) tripping from time to time.  I have been careful to ensure that the header tank is kept topped up.

Last week I had no hot water in the morning and discovered that the circuit breaker for off-peak had tripped.  I put the breaker back and nothing seemed wrong, but had no hot water the next day either and noticed that the mechanical timer was not working.

I suspected the timer and replaced it, this time with one that had an LED indicator.  This did not work and showed a lack of power going to the timer.

At this point I reached for my trusty multimeter and discovered that neither the boost nor the off peak circuits were giving voltage (though interestingly, the on-peak live and off-peak neutral connections were live).

Confused, I turned off all the power and started removing switches from the wall.  The power to the unit comes down three pieces of white mains flex, running from switched spurs on the wall (one for on-peak, one for off-peak and one for the boiler circuit mains supply).  These flexes look like the sort of thing you'd get on a 4-way mains extension lead.  The builders had also used the same cable to connect the DPST switch for the off-peak to the timer switch.  Behind both the off-peak and on-peak DPST switches I discovered that this cable had melted, damaging the switches to the point that the circuit was broken. In the case off off-peak there had been a short to earth, which is what had tripped the breaker.

I have replaced the melted wiring and both switches and now have hot water again.

My question is, what has caused this?  From looking on this forum and other web searches it would appear that this can happen when either a relay or thermostat fails and the heater is permanently on.  Looking at the main circuit board I have a slow blink (one flash per second) indicating a sensor failure.  Haven't checked the elements - is it possible that these have also been damaged?

One thing I also found odd is that the boost switch has both off-peak and on-peak inputs paralleled together.  

For safety I've turned off all circuits at the mains while I am out of the house as the  melted switches looked close to having caught fire.

Appreciate any tips.

Have you the manual on the PC 2000?
If not down load one for free from Gledhill.
Some PC 2000's did have a relay.
But later No relay.
Some had an overheat reset button at the top to the left.
And a probe went in a brass pocket again near the top on the left.
The over heat can be removed and new stats fitted and the over heat wires are joined.
This comes as a kit.
You really need to check the wiring.
The hard wire circuits need to be 2.5 twin and earth.
The flexes need to be;
PC 2000 supply 0.75
PC 2000 OFF Peak 1.5 3 core
PC 2000 On peak 1.5 3 core.
The best flex is butyl
The 2nd best is heat resistant
Nothing else can be used.
It is normal to have
A switch for the Supply
A switch for the OFF peak.
A switch for the ON peak followed by a Boost timer 30 / 60 / 120 minute. Which if pressed for any of those times stops after that period.
The off peak whether E 7 or E 10 comes on for that period.
Once the unit has heated with in that period it will stop via the stat.
To re cap.
If there is a separate OFF peak and a ON peak supply.
No timer is required.
If still in trouble you will have to post some pictures.
Mike

Thanks Mike.

I have the manual for mine, it was in a pocket on the back of the front cover.

Mine has a relay exactly as marked in the service manual - large grey thing that the live wires from the various mains inputs are taken to. It buzzes quite loudly - is this normal?

I have the overheat sensor button and could be interested in the upgrade - does it make a significant improvement? (Is this the part MIP80?)

I will check the flexes - they are probably 1.5mm, but no more. Will also check the on/off peak wiring again. This is all Twin & Earth, solid core, heavy duty and it was only the flexes that had melted at all.

Agreed on the timer, but that's how it's been installed. Also, my Boost is just an on-off switch, not something more sophisticated as I've seen in e.g. the TLC Direct catalogue. I can see the merit of a boost timer - as I need a new switch I might replace with one of these. Slightly surprised that most of these only seem to be 13A rated vs 16A for mechanical timeswitches.

It was working perfectly until recently, so will restore wiring to new-build condition (as per manual) then have Gledhill Response / MK Group do a fixed price repair on the boiler itself and 12 month guarantee job as I'll be renting the flat out soon. Suspect relay/stat/sensor problems are causing it to overheat. My electricity usage is up in the past few months, which could be explained by this - I've been spending less time at home, so doesn't make sense otherwise.

In case you're missing one of the two manuals, you can request it from here - > Pulsacoil 2000 Manual