Central heating stays on when hot water is on

[rufusfrog]

System is S-plan wired with 2 Honeywell motorised valves.

Boiler is Vaillant Ecotec condensing.

Central heating is pressurised system but hot water is open vent with conventional hot water tank.

Programmer and room stat are standard Honeywell digital models.

The boiler, programmer & room stat were installed 4 years ago. The only recent change is the installation of an additional towel rail type rad with no local stat.

If the hot water is 'off' at the programmer then the central heating responds to the room thermostat as it should.

If the hot water is 'on' at the programmer then the central heating stays on and the radiators with no local stats keep getting hotter and hotter.

I searched the internet and it seemed that the most likely source of the problem was a failed microswitch in the central heating valve (or less likely a 'sticky' valve).

So bought a new valve, swapped over the head, which didn't solve the problem. Then drained down the system, swapped over the valve, which also didn't solve the problem.

So then I turned my attention to the wiring in the Honeywell junction box, which is completely in a different order but I eventually mapped all the positions onto those in the Honeywell wiring guide, version 16.

I have a labelled photo that I would like to upload (but apparently cannot as a newbie).

The only thing 'wrong' with the way it is wired is that the grey wires from the two motorised valves are not wired into permanent live but into switched live along with the brown wires - this is presumably bad practice but could it cause the problem? I checked the various connections with a voltmeter with every combination of HW and CH off/on etc. to confirm that what appeared to be happening really is happening.

I am now at a loss as to what is wrong with the system. The only thing I can think of is that the new valve also has a duff microswitch which is stuck in the always connected position, which seems a bit of a coincidence.

Can anyone suggest any other possible causes or how I should proceed to diagnose the problem?

 

[Andy™]

has this problem always existing or did it just start happening?

you would probably be best getting someone to look at it who knows heating wiring, at least you will find out what the problem is rather than randomly replacing bits and hoping for the best, which so far hasnt really worked

 

[Murdoch]

Have you considered that the controller could be the problem? 

If the system is set to water and the radiators are getting hotter something sounds wrong. 

As above, when did it start doing this? 

Have you had a new boiler? new controller? changed the room stat? 

 

[Blue Duck]

Could be worse I suppose....

 

[rufusfrog]

Thought I said: boiler, programmer, room stat were installed 4 years ago. I haven't noticed the problem before but that doesn't mean it wasn't there.

We used to have two radiators without stats - a towel one in the bathroom and a normal one in the hallway and it never felt ridiculously hot. This summer we put another towel rail in the kitchen, it is very close to the one in the hall and thus the 2 of them together could be putting enough heat out to make the temperature insufferable.

 

[rufusfrog]

In the next of my attempts at

randomly replacing bits and hoping for the best

I have moved the grey wire from the central heating valve from the switched live to the permanent live (where it should have been all along according to the diagram). Now the system appears to be working correctly...

 

[Lurch]

In the next of my attempts at

I have moved the grey wire from the central heating valve from the switched live to the permanent live (where it should have been all along according to the diagram). Now the system appears to be working correctly...

That alone wouldn't have made a difference.

It is more likely that the valve is not closing properly, might be intermittently failing to close or could be some debris in the valve.

 

[user 29611]

In the next of my attempts at

I have moved the grey wire from the central heating valve from the switched live to the permanent live (where it should have been all along according to the diagram). Now the system appears to be working correctly...

This alone would make the difference when the heating has been on.  Once the valve is open the micro switch in the valve is closed.  Whilst the hot water is on and calling the boiler for heat you will get a back feed from the boiler down the orange to the heating valve, through the micro switch and to the grey which is in the same terminal as the brown, therefore giving the valve motor power to remain open, even though the heating timer and thermostat is off.  This would then revert back to normal operation once there was no demand for the boiler from the hot water.

If the radiators became hot when the hot water was on, without the heating thermostat being on then yes there would most likely be something else causing this.  But either way the grey wires should always be permanent live and be separated from the brown's by a time or thermostat switch somewhere.

 

[Murdoch]

If the hot water is 'off' at the programmer then the central heating responds to the room thermostat as it should.

If the hot water is 'on' at the programmer then the central heating stays on and the radiators with no local stats keep getting hotter and hotter.

Are all these radiators on the hot water "circuit"?

 

[Ruth French]

That alone wouldn't have made a difference.

It is more likely that the valve is not closing properly, might be intermittently failing to close or could be some debris in the valve.

My multimeter says otherwise. Also I thought I'd ruled out the valve when I put a new one in.

Are all these radiators on the hot water "circuit"?

Lol but no.

 

[Ruth French]

This alone would make the difference when the heating has been on.  Once the valve is open the micro switch in the valve is closed.  Whilst the hot water is on and calling the boiler for heat you will get a back feed from the boiler down the orange to the heating valve, through the micro switch and to the grey which is in the same terminal as the brown, therefore giving the valve motor power to remain open, even though the heating timer and thermostat is off.  This would then revert back to normal operation once there was no demand for the boiler from the hot water.

Thank you Rolf,  finally a useful reply that makes sense of the situation!

We've had no problems since moving that grey wire.

 

[rufusfrog]

Sorry I accidentally used Facebook account to log on earlier. Can finally show the awful wiring (th numbers I added to map the pins used to the right ones!)

 

[Rob_the_rich]

Bonjour Ruth, as an aside, I noticed in your photo that the second from right twin and earth cable (at bottom with red to 6 and black to 4) does not seem to have its earth wire connected. It looks like the cylinder stat cable. 

There is also an unconnected earth from one of the flexible cables.

Both these earths should be connected to either terminal 3. I think you should just about be able to get a connector onto the t&e earth, put the flex earth in and put a new piece of earth wire from that connector to term 3

It might be worth checking the cylinder stat, and whatever the flex goes to, to see if the earths are safely terminated there (not touching live components, preferably in a connector.

Switch everything off first, everything, main switch at fuseboard, mcb and local fused spur