Grant Vortex Condensing Combi Internal 26 - won't fire CH unless Store Thermistor disconnected

13 Jan 2023
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United Kingdom
Hi All,
Just hoping someone might have an idea on this. The boiler is running fine in terms of hot water but stopped running the Central Heating when called for from the Hive (It's sending a Live feed to the boiler from the Hive so thats all good.) We thought it might have been a PCB problem so we put in a new one and new thermistors too. It still won't work unless we disconnect the Store Thermistor from the PCB and then the pump kicks in (and the Red LED for CH Pump) and once it drops temperature from the system as it circulates the burner will also kick in. As soon as we reconnect the (brand new) Store Thermistor the shuts down the CH and goes back to only running the Hot Water. The pressure is good on the system... bit lost and wondering if anyone had any ideas?

Thanks in Advance
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Have you a multimeter and are competent to use it?
Check the selection switches are operating correctly they are known to fail. Have you checked flow switch is closed?
When it's running in hot water mode does it ever stop?
If you start it from cold and it's running on hot water, any heat going up either heating flow or return?
Thanks guys, the selection switches were our (my dad is the 'doer' I'm the fetcher and coffee maker) next point of attack. I had questioned why the Flow / DHW Pump never shut down (or at least the LED's didn't) Is that a symptom of a stuck flow switch? or perhaps the Switches. Either way might replace both of them and see what happens.
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