Y Plan - Heating only problem

[DIY-Si]

Recently move house and experiencing problems with the heating and hot water system.

Heating only works when hot water is on.

Suspected it was a problem with the 3 port valve and called in my usual heating engineer, who did a couple of tests and replaced the power head. Didn’t solve the issue and boiler will still not fire independently of the hot water. He couldn’t find the problem and suggested I called an electrician.

Decided to try and trace all the wiring myself referring to a Y plan diagram and identified there was no connection from Hot water off terminal on programmer to grey wire on 3 port valve. Found a spare wire to make the connection and felt confident this would fix the issue…. But nope!

With heating only on there should be 240v on white, orange and grey, but orange and grey are only showing 62v

Anyone got any ideas?

 

[ianmcd]

better to post pics of all controls an wiring

 

[DIY-Si]

Unfortunately it’s not easy to get a photograph that would show all connections clearly. The wiring centre is upstairs in the airing cupboard and the programmer downstairs by the boiler. Took me ages to trace where everything goes.

 

[DIY-Si]

Not the tidiest

 

[Deleted member 267285]

Where did you take the hot water off connection from?

 

[DIY-Si]

From terminal 1 (HW off) on programmer to the connector block in wiring centre (7 for the left) which is also connected to grey wire from 3 port valve and cylinder stat terminal 3 (Sat).

 

[DIY-Si]

Correction- terminal 2 on cylinder stat

 

[jeff the gasman]

Hot water off, or cylinder stat satisfied not feeding the grey.

 

[DIY-Si]

I’ll test both independently tomorrow, but can’t understand why it’s reading 62v unless programmer is faulty.

 

[RandomGrinch]

I’ll test both independently tomorrow, but can’t understand why it’s reading 62v unless programmer is faulty.

I may be wrong, but this is my interpretation...

Compared to a standard Y plan, when heating only is selected (courtesy of flameport)...

As your H/W off is disconnected, your system is left in an invalid state...

In this state, white is receiving power, driving the valve to the mid-position, and here it stalls. The switched live to the boiler is also missing.

Here is a schematic of a typical 3 port valve - it may not match yours - and this is the state it may have been driven to...

In this case, the white is supplied with power, and this is half wave rectified to about 210V DC by the diode, and the motor is stalled.
Because the grey wire is effectively disconnected, both the grey and orange wires are left floating.
Your measurement of 63V, may just be the voltage drop across the stalled motor.

I hope some of that makes sense!

 

[DIY-Si]

Yes that would make sense if the connection between HW off and the grey wire in not good. I will try and check the continuity.