Boiler Fault Finder

Y Plan (3 port mid-position valve)

Y Plan (3 port mid-position valve) Suspect the mid-position valve as being faulty if the valve does not operate after the following checks (in order): Heating mode: Electrically isolate mains supply and remove fuse – always check a 3 amp fuse is fitted Remove junction box cover Ensure there is no 240V present at L and N terminals in junction box Disconnect the grey wire from the valve (this connects to HW off on the time controller and may also connect to terminal 2 on cylinder stat which may be the satisfied terminal) Disconnect also, the white wire from the valve (this may connect to terminal 3 on the room stat which may be the calling terminal) Connect both the grey and white wires to the permanent live terminal in junction box (this should force the valve to a HW off but CH on position, once connected to mains supply) Switch on mains supply – valve motor should now move to fully open the heating (port A) Once fully open, it should stay in this position as long as power is applied to the white and grey wires When (port A) is fully open, the orange wire becomes live to start the pump and boiler Always check (port A) outlet has a temperature rise and is progressively getting warmer Hot water mode: Electrically isolate mains supply and remove fuse – always check a 3 amp fuse is fitted Once the valve has been de-energised, the HW (port B) will rest in open position and close the HTG (port A) Remove the grey and white wires from the permanent live connection (from before) and temporarily make safe (not connected to terminal block at all or touching one another) Remove cylinder stat wire only from terminal 6 (common) – This should connect to ‘HW ON’ on the time controller Then connect the wire to permanent live (this should force the valve to a HW on but CH off position once connected to mains supply) Switch on fused spur, cylinder stat must be calling for heat – pump and boiler should start Always check (port B) outlet has a temperature rise and is progressively getting warmer Hot water and heating mode together: Electrically isolate mains supply and remove fuse – always check a 3 amp fuse is fitted Replace cylinder stat wire back to terminal 6 (common) Keep grey wire made safe but connect white wire temporarily back to permanent live Switch on fused spur, motor should now move to mid-position and stop Cylinder stat must be calling for heat and both ports (A and B) are now open, boiler and pump should start Always check (port A and B) outlets have a temperature rise and are progressively getting warmer Electrically isolate mains supply, reconnect white and grey wires back to their respective terminals in the junction box. In some cases, there may not appear to be any numbers on the terminals within the junction box. In these situations, trace back the respective wire from the cylinder stat. If this functional check proves to be correct, the problem is not the motorised valve, but elsewhere in the circuit – possibly cylinder stat or room stat. A typical Y plan schematic:…

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