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.

 

[Johntheo5]

Have you tested the old Flow Switch?, when/if you do, it may be no harm to check out the DHW thermostat similarly, ensure the DHW temperature higher than the stat minimum turn down setting, remove the two wires, attach the M.meter (set to resistance) probes and turn the stat setting up/down to prove that the contacts are opening/closing.

 

[scottwhittaker]

Sorry I got back too late last night. With the flow switch Im always getting a high reading. The boiler has been off for over 12 hours and the hot water stat reads 00.4 like your closed photo.

I will test it later when I know the water is up to/above temp.

Basically we can get hot water by using the shower or any hot outlet as that stops the boiler overheating but then I turn it back off at the spur.

When the new flow switch arrives today or tomorrow I'll test it again against the new one.

Genuinely, thank you to both of you for all your help so far

 

[scottwhittaker]

New flow switch arrived today and the m/meter readings read like you said above, so I switched out the old one and turned the boiler back on. It fired up and then switched off after it had heated it's heatslave water store so it looks like that was the main issue

Checking the DHW thermostat now. The boiler has been on and heated the water store with the dial set to 4 (dial has 1-6) however when I turn the dial UP (or down) it doesn't click or fire the boiler, but I'm sure it used to when increasing the temp.

Iv done the m/meter test on the two wires when it's warm and it read 1, given it was 00.4 when it was cold does that mean its working?

I may just have a dial issue!?

 

[Johntheo5]

Yes, when the water is cold then the the thermostat contacts should be closed, 00.4, and when the water is heated to the thermostat setting, the contacts should open, 1. I would have thought it should click when you turn the setting up/down, see how it goes in actual service.

 

[scottwhittaker]

Iv left it an hour and half roughly gone back in and the dial ignited the boiler when I turned it up. Oddly the CH dial also wasn't doing anything before and is also clicking now (CH is off though) so not sure what that's about. Presumably it was above the max temp or something and now it's cooled slightly. Maybe the thermostat is working albeit not accurately.