No Hot Water

The Electramate 2000 has been working fine for several years, other than leaking when we first moved in 6 years ago which was fixed.  Both hot water and heating has worked when needed.

This week however the hot water has ceased to work effectively.  Drawing hot water gives a few seconds of warmth, then runs cold.  I switched the heating on and that seems to work fine, radiators heat up and stay hot until thermostat switches them off.

I removed the cover on the EM2000 to investigate.  All looks OK, with no leaks and feels hot.  When the heating comes on the 'HEATING PUMP ON' LED illuminates as it should.  When water is drawn from the hot tap no LEDs illuminate.  The LED which should be permanently on is not.  I did not see any other green LEDs illuminate in the few minutes I was watching.

Can you advise me on what my next steps for diagnosis are?

I looked at the temperature sensor readings via the switches on the ACB.  The store was reading 80°C but the DHW was at 85°C!  as I cannot see how the DHW can be hotter than the store, especially as it felt cold to the touch.  I am tending to think that the DHW sensor has developed a fault.  Therefore that is the first thing I will try changing.  One is on order and should turn up in the next couple of days.

How difficult is it to replace?  It looks fairly easy, as long as I can work out where it connects to the ACB.  Am I correct in supposing that the system will not require draining down to fit?

I replaced the DHW sensor in about 5 minutes.  All is now working within normal parameters.