Main combi 24 he, heating always on!

[noviced1y]

Hi All,
I am hope someone can help with this.
I have a Main Combi 24 he. Problem is heating is always on even with selector switch on water only.
I have replaced the diverter valve and no change. Then replaced the pcb, still no change. I have also disconnected the thermostat as a precaution. still no change.
As soon as you turn the dial from off to hot water only, boiler fires up instantly and rads heat up. This occurs even without hot tap demand.

Has anyone come across this before?
Any advice would be apreciated.

Thanks!

 

[picasso]

Main thermistor acts as a frost stat so brings on the heating when the temp drops or it goes out of spec, might be your problem.

 

[ianmcd]

When you replaced the pcb it is possible that you have connected the selector control knob 180 degrees out, so what you think is switching off is actually switching on

 

[cross thread]

If early model it can be hot water micro switch not moving back to off,( on the end of the left hand front brass housing )

 

[gas112]

Would be looking at micro switch

 

[CBW]

When you replaced the pcb it is possible that you have connected the selector control knob 180 degrees out, so what you think is switching off is actually switching on

PCB was replaced because of the fault existing beforehand, if I read it right?

 

[DP]

Is this the same boiler where the heating did not work but hot water was ok

 

[noviced1y]

Microswitches seem to be working fine.

 

[noviced1y]

Thanks for the suggestions, but m

If early model it can be hot water micro switch not moving back to off,( on the end of the left hand front brass housing )

Would be looking at micro switch

icroswitches seem to be working fine.

 

[noviced1y]

PCB was replaced because of the fault existing beforehand, if I read it right?

Correct.

 

[noviced1y]

Main thermistor acts as a frost stat so brings on the heating when the temp drops or it goes out of spec, might be your problem.

Sounds intresting.......
Ill give it a go. Will keep you posted.

 

[gas112]

Thanks for the suggestions, but m



icroswitches seem to be working fine.

You have changed divertor so nothing up in there pins returning etc. So If boiler is firing minute you put it to hot water with no demand then the microswitch is stuck inside 99% of time, disconnect the leads from board /or switch ifitdont firethats your problem if it continues then its main sensor

 

[noviced1y]

You have changed divertor so nothing up in there pins returning etc. So If boiler is firing minute you put it to hot water with no demand then the microswitch is stuck inside 99% of time, disconnect the leads from board /or switch ifitdont firethats your problem if it continues then its main sensor

Good way to be sure. Ill let you know.

 

[DP]

Good way to be sure. Ill let you know.

Gas is the king when it comes to these boilers and lot more. He does not need to know but perhaps feedback to say sorted

 

[noviced1y]

Thermistor replaced, but still the same.
Micro switch is activated by the vertical pin on the diverter which extends when boiler is turned on. So I assume micro switch is working fine.
With microswitch disconnected, and boiler switched to water only. Fan and pump seem to run without any demand.....
Anything else I can try?