VAILLANT 20/1 T3 W WON’T SWITCH OFF

[gorselands]

Vaillant 20/1 T3 W Combi, Randall 103 timer, no room thermostat.

C/H and H/W all work fine. Noticed that controls on timer were not working so switched timer to another Randall 103 I knew to be working. (2 identical boilers and timers side by side – big old house). Same problem, so not the timer.

Bit stuck now the obvious is out of the way. I am switching the boiler on and off using fused mains switch.

Tried to look after these for the past 22 years and was massively helped by forum members 2 years ago when when wax thermostat problem was diagnosed and I replaced the divertor and did a general strip and overhaul,.

Very much a novice. I can see the wires from the timer go into a black terminal box but what lurks in and beyond that I haven't a clue. I'm assuming it's an electrical problem or could it be mechanical?

All help and advice much appreciated

Mike

 

[Bunnyman]

the water valve will be stuck,turn the left hand switch off with rad symbol on it,if its still running the boiler thinks u are calling for hot water which ur obviously not.it will then dump its heat to the rads,change the water valve 011156 u still get em if u hunt hard

 

[gas112]

Bunny would you not just strip and clean and fit new diaphragm at same time tenner as opposed to a ton for the valve.

Do you still fit many flow switches(diverters) on these got a couple that i may sell on ebay if there is still a demand

 

[gorselands]

Hi again Bunnyman and thanks gas112.

Turned off the LH rad switch and boiler immediately shut down, switched it on again and boiler fires up. Do you still think it's the water valve? Not quite sure as you say 'if it's still running.'

Where is the water valve? gas 112 refers to flow switch (diverter) that I replaced less than 2 years ago. So bit confused.

Thanks

Mike

 

[gorselands]

Just a thought, tried turning rad switch off and turning hot tap on. Boiler fires and I get hot water. Turn tap off and boiler goes off ok, So diverter seems to be working.

 

[gorselands]

Hi Bunnyman, looked up the part number you gave me and looks like the unit under the switch box. Has a 70mm diaphram in it.

 

[Agile]

I am now a little confused by what you think is wrong?

Maybe the others follow you but I dont. Its not helping using confusing names.

I use DHW flow switch at bottom, diverter for rear left and pump flow switch for bit in the middle with small tubes.

Tony

 

[The831Bunny]

From whats been said it sounds like the water section is fine if the boiler is going off after dhw tap closed and heating switch off.

The heating switch sends 230V out of T3 to your external controls, so if the boiler goes off when this switch is turned off, it has to be a problem with external wiring/controls still.

Remove the 103 completely from the back plate, the boiler should be off for heating then, even with the heating switch on (unless it's wired incorrectly). If not the only other possibility is you have a frost stat wired in parallel across T3 and T4. See if you can find one.

Also check you haven't got the hot water pre heat switch (switch on rhs of mains switch) turned to on. It should be pressed in at the bottom (red/white rectangle)

 

[The831Bunny]

I am now a little confused by what you think is wrong?

Maybe the others follow you but I dont. Its not helping using confusing names.

I use DHW flow switch at bottom, diverter for rear left and pump flow switch for bit in the middle with small tubes.

Tony

For you Tony: (Vaillant terminology 1st)

Flow Switch = Diverter Valve
Water Section = DHW Flow Swtch
Pressure Differential Valve = Pump Flow Switch

 

[gorselands]

Now you're losing me. Where is '103' T3, presumably the terminal box. As the boiler has functioned for as long as it has I don't think it's wired incorrectly. I did check the fuses. The HW pre heat switch is always pressed in at bottom, R/W triangle.

All I can say is that it has worked fine for the last two years then last Thursday failed to switch off on timer.

So, CH and HW work fine. HW works when CH switch is off. CH switch off switches off boiler, on and boiler fires up in-spite timer being off.

 

[Agile]

All that points to your timer!

What have you done to test that?

All faults are found following testing! That usually involves electrical tests.

Tony

 

[The831Bunny]

You have to eliminate the timer to move on I'm afraid. T3 and T4 are the terminals in the boiler terminal box that connect to your external controls. Remove the timer from the back plate (slide it up like when you changed it) and then see if the boiler fires for heating. It shouldn't.

If it does, you have a live to T4 telling the boiler to fire. i.e. a frost stat or incorrect wiring. Thats it.

 

[gorselands]

First of all thanks for staying with me. Tony, I have 2 timers for 2 boilers next to each other. The timer on boiler 1 works fine so I slid it off and swapped it with timer 2. Boiler 2 still did not work so ruled out timer.

The831Bunny I am about to remove the timer as you suggest and see if the boiler fires for heating and will let you know the result. I wasn't aware of a frost stat but have found it on the wiring diagram. Is it a separate box and where is it? I can see it's a switch, is it a relay that could be stuck?

Mike

 

[gorselands]

Removed timer from back plate and boiler still fires up - CH and HW.

Got continuity from T4 on boiler to T3 on timer so no break in wire. The run from the timer to T4 does not appear to be interrupted so I am wondering if there is a fault on the timer back plate.

Mike

 

[Agile]

The way to test the timer is to measure its connections at the boiler!

That includes in the test the timer, backplate and cable.

Tony