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Michael
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I have a Potterton Promax FSB 30 HE boiler. It was installed

Resolved Question:

I have a Potterton Promax FSB 30 HE boiler. It was installed in December 2011.
It provides central heating and hot water. The pump is a Grundfos. The system is controlled by a Honeywell ST9400C Programmer. The hot water is controlled by a radio Drayton receiver allied to a Drayton thermostat on the exterior or the immersion tank. This works well.The central heating is worked by a HC60NG relay module with a HCW 80 room thermostat. The two separate systems (hot water and central heating) by two Honeywell V4034H motorised valves.
The system worked well until about 3 months ago
The problem is that the central heating comes on OK when requested in the morning, and happily switches itself on and off according to demand from the HCW 80 until about 10 am. After that the central heating does not switch off when the HCW 80 tells it to. It keeps running although the green light on the HC60NG is not lit. EVEN if I switch the ST9400C to off for heating, the heating keeps running. The only way I can stop it, and then not always, is to press the button on the HC60NG to get the green light, wait 3 seconds, switch everything off at the mains, then switch it all on again after about 30 seconds.. This works about 1/3 of the time.
The ST9400C programme was replaced by Honeywell in June this year because the original was faulty (they had a dodgy batch). I have replaced the HC60NG relay: the results are the same. I have checked the signal strength as instructed. I have teached-in the relay to the HCW 80 thermostat. All to no avail. Any suggestions?
Submitted: 3 years ago.
Category: Appliance
Expert:  Michael replied 3 years ago.
Hi , yes they did have a bad batch. but this can be only one of two things here. Either the main receiver or the the controller is still bad.
I would bet that you simply got another bad control:-(
They had a lot of them.
But really if one more doesn't work, I would have the installer replace the main control as there are relay circuits that could cause the same problem.
Customer: replied 3 years ago.

The main controller was replaced by Honeywell. This was before the present problem. I have replaced the main controller myself (another 70 quid) but the same result. So this is the second replacement controller.

Where are these relay circuits? are they in the HC60NG or in the ST9400C controller?

Having replaced the ST9400C twice and the HC60NG once, I am quite puzzled.

Could the PC wireless or the Panasonic cordless phone (both may yards away) be doing it? altho they are all on very different frequencies....

Expert:  Michael replied 3 years ago.
Hi Steve they are on the main board HC60NG .
But if the replacement didn't help,then its simply an issue with the controls. The phone could very well be playing havoc as they are close frequency wise.But I doubt that.
I would have the tech come back because this sounds as if the new board may also be faulty for one reason or another. I see this all the time lol;-)
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Expert:  Michael replied 3 years ago.
Hi steve