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JamesI
JamesI, Handyman
Category: Appliance
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Experience:  I have repaired and maintained numerous items over the years, from fridges to freezers, radios to TV's.
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We have a Honeywell ST772 sundial system, it has developed a

Resolved Question:

Hi there,
We have a Honeywell ST772 sundial system, it has developed a curious issue
When flipping the switch over to continuous the controller turns itself off after the boiler fires for 1 or 2 seconds , and will only come back on after pressing the reset button
Any suggestions?
Thanks
Submitted: 1 year ago.
Category: Appliance
Expert:  JamesI replied 1 year ago.

Good morning my name is ***** ***** I will assist with your query today.

Please give me a few minutes to look up your specific model

Customer: replied 1 year ago.
Cheers James
Expert:  JamesI replied 1 year ago.

So the controller display just goes blank when the boiler starts?

Customer: replied 1 year ago.
Correct, as soon as you switch to continuous the whole controller goes blank, never happened in the 10 years we have been her
Customer: replied 1 year ago.
It does work in timer mode after resetting , we are getting hot water in the morning?
Expert:  JamesI replied 1 year ago.

This sounds like a short on the circuit board, the switch is most likely faulty.

Are you able to induce it by pushing the switch half way over?

Expert:  JamesI replied 1 year ago.

Can you also try the same with moving the HW to continuous?

Customer: replied 1 year ago.
Would appear it's the heating switch causing the issue ,h/w seems to be ok , as soon as the heating switch clicks to continuous the unit goes blank
Customer: replied 1 year ago.
Yep just tried it again , heating switch seems ok on , once , twice or off but it doesn't like continuous....
Expert:  JamesI replied 1 year ago.

Ok that proves the fault, its nothing to do with the relay or boiler as the fault would occur each time the boiler started (on auto or continuous).

The cheapest option is to replace the timer, the Honeywell model which replaces your unit is the ST9400C, which costs around £70.

You could try and replace the switch but unless you have an electrical engineer background and handy with a soldering iron its beyond most peoples DIY skills.

Customer: replied 1 year ago.
Ok thanks James
Expert:  JamesI replied 1 year ago.

Since the fault only occurs on continuous, I might be inclined to leave it until it completely fails.

Customer: replied 1 year ago.
might be a good idea, it's not that cold yet..... Is there anyway I could get the heating on for longer without the continuous working?
Expert:  JamesI replied 1 year ago.

Give me a minute while I find the manual again.

Expert:  JamesI replied 1 year ago.

You can set the switch to "Once" - The heating will turn ON at the first ON time set and stay on until the last OFF time set each day.

Expert:  JamesI replied 1 year ago.

If you just want to override it, you can press the "change" button to the right of the switch, which will turn the CH on and stay on until either the timer tells it to switch off or you press the button again to manually turn it off.

Customer: replied 1 year ago.
That's interesting, just changed the times for the heating on and off to see if It would accept it, unit went blank again, looks like we have no heating unless I get it changed now...I think I need a new unit
Expert:  JamesI replied 1 year ago.

Its not a big job changing it, just make sure you switch off the power to the boiler (as that is what serves the timer.

I know Screwfix are selling this unit for about £80 if you want to pick it up today.

Customer: replied 1 year ago.
Thanks again James , it is the heating switch so I will probably go to screwing today
Customer: replied 1 year ago.
Damn spell checker, screwfix I meant to say!Apologies
Expert:  JamesI replied 1 year ago.

I wont blame you when you look at their prices!

Appreciate if you can rate the service received today.

Customer: replied 1 year ago.
Will do, I think Amazon have them for about £65 so I might just wait and tell the kids to put jumpers on!
Expert:  JamesI replied 1 year ago.

A quick google showed you can pick it up for £61.50 from here, including delivery or on ebay here for £58.99

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