Overheat error

Hello,

I recently moved into a rented flat with a Pulsacoil A-Class. A few weeks ago the airing cupboard filled with steam and the red light was flashing on the front panel. Having switched the system off and left it to cool, I noticed the water level in the top-up cistern was very low so I topped that up.

The system worked fine for another week but then the water in the top-up cistern started boiling off again; having read a few threads on this forum (many thanks Mike - very useful for people learning the basics!) and downloaded the service manual from the MK Group's website (thanks also for the intro video - very useful), I took off the front panel and interrogated the control board.

The sensor control setpoints are all about right. The 5 most recent lockout error codes are no. 10 - overheat error, then previous to that no. 11 (which is not listed in the service manual I downloaded). The 8 most recent block-out error codes are no. 37 - S1 overheat 1 open. One instance further back of no. 29 (again not listed in the document I have).

Does this seem like a case of simply needing to replace the OHT sensor? (metal cartridge with prong pressing against the insert copper tube)   I'm not sure how to check if it's failed or not - any advice much appreciated!

Also, all contactors look fine - nothing burnt

Kind regards,
Simon in Cambridge

Change the top left hand sensor.
And might be wise to change bottom & middle too.
Also check off peak relay
And On peak relay
And interface relay.
There are 2 more relays on the ACB too.
If you have an OFF peak supply which only comes on at night (11 or 12)
Then I always connect a lead to the unit (off peak out) Lead goes in.
Then apply power to the unit (Off peak)
Unplug the supply & put boost on. Make sure unit is not to hot if it is boost will not work.
While the boost is ON
Plug in the temporary OFF peak supply.
The BOOST should switch off instantly. If it does Interface relay is OK.
Watch the 2 relays on the ACB you will see them working as see thru.
Mike

Mike – many thanks for the prompt response last week. Upon reading your message I checked the relays as you outlined and all seemed ok.

I then ordered a replacement overheat sensor for the top left position from Supplied Direct (came to just under £25 inc. delivery). This stopped the water in the top-up cistern boiling off in the event of an overheat lockout error (fault code 10), although it seemed there was still a fault causing the system to try to overheat in the first place. Having then logged the temperatures reported by the various sensors on the ACB over a couple of evenings when hot water was being used, I suspected the control thermistor for the off peak heater in the bottom left position was under-reading. Hence I replaced that (again sourced from Supplied Direct, same price as above) and in the few days since the system has been stable - sensors for both the off-peak and on-peak heaters reading steady, mostly between 75 & 80C.

Ah - arrived home from work today to see the no.10 lockout overheat error had returned. It's probably time I arranged for an expert to look at the relays.

The only other thing that springs to mind is that the previous tenants had apparently left the digital timer switch on the wall on "override", meaning supplies to both the bottom (off-peak) and middle (on-peak) store heaters were available all the time (confirmed by the ACB display).

I re-programmed the off-peak supply to cut in at 0000 - 0700 hours, but wondered if the scenario of always having power to the bottom store heater could have been a factor in previous overheating, or is it something that simply increases the bills?

Anyway Mike - many thanks again for your time & advice.

Kind regards, Simon