Systemate lll Part Deux Request for advice/help.

Hello again,

I posted here sometime ago about my systemate 3 (see previous post) and after some fantastic advice on here and some so so advice but expensive advice from Gledhill, me and the engineer got my systemate up and running again after installing a replacement boiler.   Well everything has been fine and dandy until the last week,  it seems my systemate has once again hit an issue (cries). Whats the problem i hear you ask?  Well when showering or using any tap including running a bath, the HW pulses from extremely hot to warm to cold back to extremely hot in a never ending circle.  The HW water is at times scaldingly hot?   

Having read your wonderful site, I'm thinking it could be a heat sensor failure? but I could really do with some help trying fault diagnose the issue and fashion a repair before i call out an engineer.

As always, many thanks in advance for any help offered.

Geths

I just wanted to thank Mike for the advice he gave me with the above fault.   I ended up draining the systemate, replacing the DHW pump and related valves & sensors and the old girl was saved from the knackers yard.