Main Boiler circuit fuse blowing.

[Stevex]

This morning woke up to no central heating, but the hot water had been on earlier in the morning.
I checked the programmer and it was indicating CH was on.
Went to the room thermostat and it didnt click when rotated, but then started clicking without any input.
I had a spare room thermostat, so replaced it. However when I rotated the new stat to call for heat the main boiler/CH/HW switch fuse blew.
I checked the new thermostat with a continuity check and the live and neutral continuity made and broke when the stat was rotated, so that seems to work normally.
Now I'm at a lose and would appreciate advice on what to check next. My CH/HW is a vented Y plan.

 

[Stevex]

Just to confirm, live and neutral are obvious and I've wired the call to number 4.

 

[Deleted member 120897]

I think your call is 2 or 3, not 4, this is why it’s blowing, because you’re creating a short/dead short circuit.

 

[Stevex]

Thinking the spare I had was faulty as it's been sat in my garage for about 15 years.
Just bought a Honeywell T6360. The original stat had 3 wires red, blue and yellow with the wiring labelled L, N and call.
I traced these wires to the junction box and the red is live, the blue goes to the CH pump and the yellow to the 3 port valve, so which terminals do the blue and yellow go to on the Honeywell?

 

[ShaunCorbs]

that doesn’t make sense you sure it’s the same blue wire from the stat ?

 

[Deleted member 120897]

that doesn’t make sense you sure it’s the same blue wire from the stat ?

Unless OP is getting confused and it’s a bank of neutral wires, which one happens to go to the pump? @ShaunCorbs - do know if I answered correctly in post #3? My wiring skills/diagram learning isn’t the best.

 

[Stevex]

I went into the HW/CH junction box, only 1 cable with the same yellow wire was present but too far apart to con check. I traced the wires out to locations as stated. I've returned the Honeywell and am getting what is hopefully a direct replacement Drayton 3 wire stat to fit. I presume when the stat makes it turns the pump on and selects the 3 port valve to the CH setting using their neutrals as returns.

 

[ShaunCorbs]

Yes logically

Red / brown live
Blue neutral
Yellow switch
Yellow and green earth
that’s how it should of been wired

 

[Deleted member 120897]

I presume when the stat makes it turns the pump on and selects the 3 port valve to the CH setting using their neutrals as returns.

No, it’s not the stat that brings the pump on, it’s a series of switch lives. Have a look here

 

[Stevex]

Just fitted a replacement Drayton RST1, same as the one that was fitted and all working normally now.
Wish I'd taken that option first thing, would have saved a **** load of hassle.
Thanks for your inputs.
The one thing I've learnt today is that all room stats are not equal.

 

[ShaunCorbs]

Picture of backplate please

 

[Stevex]

This is the original backplate, new Drayton is exactly the same; see previous post, all sorted.

 

[ShaunCorbs]

Yes blue is neutral 1000%

 

[John.g]

Thinking the spare I had was faulty as it's been sat in my garage for about 15 years.
Just bought a Honeywell T6360. The original stat had 3 wires red, blue and yellow with the wiring labelled L, N and call.
I traced these wires to the junction box and the red is live, the blue goes to the CH pump and the yellow to the 3 port valve, so which terminals do the blue and yellow go to on the Honeywell?

Academic now but I installed one of these as a swap for a RTS1 for someone a while ago and connected Live to 1, Call (yellow) to 3 and N to 2 as far as I remember.