Intermittent fault on Y plan

Emkew · May 3, 2023

Customer complains of Intermittent heating on Y plan with Stat satisfied.
New boiler fitted last year (no permanent live required or fitted previously) on an existing Y plan system. Existing timer was a Randall 103 (no hot water off position as its a timer not a programmer). The customer wanted a digital replacement so I fitted a digital timer (only one switch live out feeding cyl stat and room stat and no H/W off connection) so I couldn't fit a programmer without significant disruption and additional wiring which the customer didn't want. All seemed well, then a few months later he asked me to fit a digital stat Honeywell (DT90E) in place of the old mechanical stat. I think their was an additional wire connected to the stat which didn't appear to be needed on the new one, so I isolated it just in case. Is it possible that the original wiring took the heating satisfied connection as a hot water off live feed?
I have been studying wiring diagrams I don't see how this fault is occurring. If the room stat is satisfied there is no power to the white and the 3 port valve should be in the H/W only position requiring power from the cyl stat to power the pump and boiler. If there is power to the grey and white. It should close off the H/W, but once the room stat is satisfied, the power on the white would stop and the valve should spring return shouldn't it?
Any help or advice would be much appreciated.
Thanks in advance.

John.g · May 4, 2023

Very strange, with both wires still disconnected to term5 suggest disconnecting 2 wires at term7 and see if valve still goes to mid position?

Are you sure its a midposition replacement top and not a diverter valve top.

If you have still got the old head you can do a few bench tests with it?.

Emkew · May 4, 2023

John.g said:
Very strange, with both wires still disconnected to term5 suggest disconnecting 2 wires at term7 and see if valve still goes to mid position?

Are you sure its a midposition replacement top and not a diverter valve top.

It says mid position on it so I assume it is. I plan to revisit next week and do the following checking for the correct voltages at each step. Room stat turned down for tests A-C
A. disconnect and leave disconnected the wire from Cyl stat to No.8 and check nothing works. (no rogue voltages apparent)
B. as above but turn the cylinder stat satisfied and check 240 on grey only
C. as above but turn cylinder stat back up and provide an external 240 supply to No8 and check 240 only on Orange (no rogue voltages apparent).
D. wire in an different new valve head and check it has the correct voltages when the stats are turned up and down

If all this fails I (or an electrician) will be rewiring the whole system.

I will update you when I have done these tests.

John.g · May 4, 2023

Have you still got the old head?.

Emkew · May 4, 2023

John.g said:
Have you still got the old head?.

Unfortunately not.

Emkew · May 9, 2023

Emkew said:
Unfortunately not.

I have traced the issue to the new valve head being faulty. Which manifested the exact same fault as the original faulty valve head, This is why it was tricky to diagnose, (that's what you get for trying to save your customer money and buying a cheaper version from Amazon!). The orange wire which is live when only hot water is required (as it is connected to the pump and boiler which are getting the feed from the cylinder stat) is back-feeding up the white wire and driving the valve to the mid position when it should be in the H/W position. It shouldn't be able to do if the diode inside is working.
Anyway my £38.34 will be returned to me from the seller within 5-7 days of them receiving the faulty part.
Thanks to all who helped, you are much appreciated.