Greenstar Heatslave Oil Boiler Overheating and Pump Running 24/7

[Johntheo5]

Very good, you are on the right track, check now that the flow switch is permanently made by checking for 230V between the end of the wire and N or E that you disconnected at the PCB, if you are then flow switch faulty, you may be able to clean it, @Exedon will advise.
It would seem though that when the flow switch is made that it overrides the DHW thermostat and demands constant boiler firing, a bit strange in one way, if true, in a boiler that fires flat out with no modulation, the flow switch setting is probably set to a flowrate that will ensure that the DHW temperature doesn't rise above say maybe 65C, a 18kW boiler may have the switch at a minimum flowrate of ~ 5.0LPM or so.

 

[scottwhittaker]

Sorry I'm not great with a multimeter! Attached are photos of the flow meter and PCB board.
I've inserted the multimeter prongs into the two terminals on the end of the flow switch.
Should it read zero on ohms/continuity in either open or closed state?

I get 265 with the tap off and 145 with the tap open

Now Ive seen the flow switch, I did change the little mesh filter near the flow switch a year or so ago as it had a seep.
The washer was completely gone and I noticed the little filter gauze was completely corroded as well.

Ive ordered a flow switch as Im not against a little proactive upkeep. I would also say we've sometimes turned the shower or bath on and not got hot water until we've turned it off and back on again. Its quite rare that it happens but would point to a flow switch hiccup albeit this issue seems the opposite.

 

[Johntheo5]

Put the m.meter to its its lowest ohms resistance scale, 200 below. With the probes held apart, the reading should be like in "Open" below. With the probes held (or clamped) together, the reading should be like in "Closed", below.

With the flow switch in place and no flow the reading should be as in "Open", with a Hot Tap open the reading should be as in "Closed".
With the flow switch taken out, the reading should be as in "Open", if you (can) press the paddle the reading should be as in "Closed".
Ensure the probes are touching the bare ends of the wires.