BM II Green Light

Hi,

I have a BMII system and recently noticed that the green light seems to be on a lot and the pump seems to be running all the time the light is on even if the boiler is not firing. I have tried to calibrate it today but the green light stays on even when the thermostat is turned right into the blue summer zone. The hot water and heating are generally ok although a couple of times the rads have been a bit cooler to the touch. Do you think the themostat on the BMII needs replacing and is it a straightforward job?

Cheers

Mr F

The green light is just neon, which is illuminated by the switched live circuit. As you correctly say the boiler doesn’t need to be firing for the boiler pump to be running. This is because the boiler pump is controlled by the delay timer, which will keep power to the boiler pump after the store temperature has been satisfied. The delay timer will keep the boiler pump energised for 5mins±1min nominally after the store temperature has been satisfied. This is to clear hot water from the heat exchanger.
If your boiler pump is running all the time, I would first check the pump delay timer PCB. Instructions for checking the timer PCB are given in the BM2 Installation and Servicing booklet. This booklet can be downloaded from the Gledhill website.
BM2 PCB replacement is a straight forward job as is changing the thermostat.
You could also check what your actual store temperature is. I state this as you said your radiators are cooler than normal.
Also check your F&E tank has water in it although this doesn’t mean that your BM2 is full of water as you could have a blocked feed pipe!
I hope all this helps? I am not a qualified Heating Engineer, but a competent DIYer and have successfully changed various parts on my BM2 to keep it operational.
Mike the Boilerman is the thermal store expert, but from my understanding of how a BM2 works then all the above is correct. Please make sure you understand what I have written and the implications of testing/repairing your BM2
Robin

Thanks for the reply.
I think the pump delay board is ok - i only replaced it not too long ago and the pump does stop after the green neon light is extinguised for about 5 mins.
I just wanted to clarify that it may be the thermostat that is causing the issue as the green light never seems to goout after the boiler has fired even though all rads are hot and hot water is very hot! Should i just bite the bullet and buy one and fit it?
cheers

Mr F

If you have correctly diagnosed that the store thermostat is faulty, you can just buy another and fit it. However, you could disconnect the wires to the thermostat first and check for continuity/continuity interruption when you turn the thermostat dial. I have never tired this, but it should work….
You may be able to hear the store thermostat click on/off when you turn the dial?
If the thermostat wires always create a closed circuit then this would explain why the green neon is always illuminated on your BM2.
Be careful to isolate the BM2 from the supply AC before wire disconnection. Not trying to teach you to suck eggs here, but don’t know what you have done before to keep your BM2 operational and therefore believe I need to state some H&S advice!
Hope this helps?
Please post your findings to help others on the forum.
Robin