Baxi HE133 Boiler Fault Finding Help Please

[hodder_fisher]

Hi,

I have a fault on my boiler which is causing the Divert valve to continuously motor open/close with consequential impact on the CH and cycling hot/cold water when DHW demanded.

I have found 240v is permanently on the CLOSE terminal on the divert valve, when CH is demanded 240v is applied to the OPEN terminal but as power is still on the CLOSE terminal the actuator just continues to run OPEN - CLOSE as power is permanently applied to both sides.

I suspected the PCB so acquired another board (second hand but supposedly from a working boiler !) so either the 2nd PCB features the same problem (unlikely ??? but possible !!) or something else is preventing the CLOSE demand being switched off when CH is demanded.

DHW functions fine no CH demand and CH works fine if I isolate the divert valve actuator in the OPEN position by removing the moles connector.

Any ideas ?

 

[picasso]

dhw flow switch stuck on ?

 

[hodder_fisher]

Yeah could be, I need to have a deeper look as I disconnected the wiring harness to the switch and the fault didn't change but I will go back and look I was just google the circuit for the switch when you replied.

Thanks

 

[hodder_fisher]

Ok, double checked the DHW flow switch: open circuit - no water demand, close circuit - hot water demand.

So looks fine

I suspect the DHW Diverter valve relay on the PCB is stuck in the closed position, its probably the mechanical part on the PCB that gets worked the most (Hot water demand on & off.....)

But any other suggestions welcomed !

 

[AGAS]

what readings are you getting from the pcb terminals when in each demand ,
I've seen plenty of times with baxi PCB'S to have problems with just ch demand with hw and service mode working correctly

 

[DP]

post deleted

 

[AGAS]

I think will me the diverter motor. End of travel switch faulty so it keeps going round

i think it could that because i think he was testing from diverter end , if he's getting right readings from pcb it should be the diverter motor

edit ; or the wiring of course

 

[DP]

Agas, I posted that and then looked at boiler model. 133 has honeywell. Not seen Honeywell do that but ones fitted to current combis, seen all sorts of failures. Often possible to get a working diverter from collection of duds

 

[AGAS]

TBH dp i didn't look

 

[picasso]

You should have permanent 240vac on what you call closed and 240vac on open when central heating is called for, the diverter changes over to hot water when you open the contacts on "open" wire, there is a pdf on the Honeywell vc4012 head which explains it better, so looks like you need a new diverter valve head.

 

[hodder_fisher]

Thank you for the guidance.

A little more thinking / investigation as follows:

I suspected the Honeywell actuator 1st, so swopped out for a new (secondhand) one, same symptoms.

The diagram above is from the Honeywell spec.

I agree 240vac should be permanently on the NC terminal (Default Normally Closed position), until CH demanded which should switch the relay contact over to NO (the SPDT controlled in diagram above) this removes the supply from the NC contact and allows the motor to move to the open position, once open SW2 'breaks' and SW1 'makes' thus ready to be motored back to open once a supply is made on the NC (terminal 3 pin).

The problem I have is 240v is on both NC & NO pins at the same time hence it cycles !

The original PCB I swopped out I have traced the connections from the 7 pin connector back to a JQ1P-12V-F relay.

The relay is wired like:

The NC pin is connected to br wire (with a 270 ohm resistance) on the Diverter valve & the NO pin is bk wire (Nb wiring harness colours match the Honeywell diagram above)

So I am certain the relay is the cause & the PCB I have checked (original one removed) is not normally showing COM to NC continuity other than occasionally when the PCB has been moved about and the contacts make, but must have been failing in a condition that shorted the NC & NO terminals.

Views ?

 

[picasso]

I think the molex connector denotes the 2 wire and common so you have the wrong diagram, its likely that the spst controller (on the pcb?) is stuck giving 240 on both wires at the valve head.

 

[hodder_fisher]

Which diagram do you think is wrong the relay ? I agree the actuator is 2 wire & common (3 wires) but I am sure having belled out the PCB the relay is the one I have inserted the diagram for above.

thanks

 

[picasso]

Figure 3. the first one you posted , cant see how you can have 240 on both brown and black with a spdt relay.

 

[hodder_fisher]

ok, Fig 3 is from the Honeywell manual for VC series valves and aligns with the wiring diagram shown on the actuator.

There is the chance the PCB I currently have fitted to the boiler and have measured the voltages off is a different PCB config than the one I have inspected !

I have ordered new relay for the PCB, so will update when I have changed and fitted back into the boiler.

Thanks