honeywell cm927 to valliant 937

[Jag-1978]

Hi There - I've been in my house a few years and recently noticed issues with my heating constantly going off and on, could be fine for a week but then for the next few days it clicks on and off randomly, sometimes giving me heat for 30 minutes sometimes 5 minutes, nothing consistent. When its not working its reporting an s.30 error

I get the feeling the thermostat is playing up so may consider changing it.. Anyway I've looked at the wiring and just wanted someone to roll their eyes over the pics and look at the way it's connected.

Seems there is a connection from the fused spur going to the honeywell receiver then another wire from the receiver to the boiler.. I thought it would be the other way round.. fuse to boiler and boiler to stat receiver..

Any thoughts..

thanks

Jag

 

[stem]

The thermostat is simply an on/off switch. When heating is required terminals A and B are electrically connected and the heating comes on, when not required, they are disconnected and the heating goes off.

The switching on/off time period will change. This is because it is being controlled by the actual air temperature in the room where the thermostat is located, so it will be on longer when the weather is cooler and when the timer first switches the heating on in the morning, and less when the weather is warmer or the heating has been on for some time. The test of a thermostat is: Is it controlling the heating to the set temperature, such that the heating goes 'off' when the set temperature in the room the thermostat is located has been reached, and then cycles on and off to maintain it? If it's switching off when it shouldn't be, then it's possible that there is a fault, or a loose connection. [Some stats do turn off a degree earlier to prevent overshoot]

Regarding the supply from the fused spur: It matters not if it goes to the thermostat first and then the boiler or the other way around. it's simply providing a permanent 230V mains supply for both of them.

The code on the boiler would appear to mean that there isn't an 'on' signal from the thermostat to the boiler, I'm not a Vailiant expert, but I seem to remember that it shows every time the thermostat has switched off, and may not necessarily indicate a fault. Hopefully a Vailiant pro will be along later and will be able to assist further.

 

[Jag-1978]

fair enough thanks for your input definitely something dodgy going on as it shouldn't be clicking on and off constantly every second for a few minutes then go off completely then back on randomly..

however from the looks of things the thermostat and receiver seem to be lighting the usual lights and showing things are okay.. but thats just visually..

 

[Harry Bloomfield]

Where is the stat mounted?

Is the stat actually clicking in time with the boiler starting and stopping?

Could the boiler be short cycling, itself cutting off early, when the stat is still demanding heat? If so, it might be short cycling due to the boiler over- heating, because it cannot get rid of the heat it is generating quick enough.

Stats should be mounted on an internal wall, well away from radiators or other heat sources and not where sun from a window will be able to reach them. The radiator nearest the stat, should not be fitted with a TRV.

 

[Jag-1978]

The stat is on a stand mostly sits in the bedroom away from radiators. its a portable stat so can take it to any room.. but mostly in the bedroom where it has sat for the last 5 years and has been working fine up until now.. I can force the boiler in to chimney sweep mode and the heating does work that way round for 10 minutes. The clicking in and out has no baring on what the stat is doing. I'm using it on manual mode as the heating is on constantly. if i switch it off the receiver does click on and off like it should do.. but doesn't seem to be mirroring on the boiler.. The boiler just has a mind of its own..

 

[stem]

it shouldn't be clicking on and off constantly every second for a few minutes then go off completely then back on randomly..

You're right; it should be switching 'on' and 'off' based on the actual room temperature compared to the set temperature, and not randomly. So for example if it's switching off when the room is at 19 degrees and the thermostat is set to 22 degrees then something is wrong.

If you are using it in a room where a radiator has a TRV fitted, that can interfere with the operation of the room thermostat. If there is a TRV in the same room as the thermostat the TRV should be set to maximum.

However, if the operation of the thermostat seems different from previous years and nothing else has changed, it does sound as if it may be malfunctioning.

 

[Jag-1978]

maybe time to upgrade to a Nest.. just a shame I can't pinpoint if it is the stat that is causing the issue.. I might have to see if i can disconnect it completely and see what the boiler does

 

[CBW]

It’s difficult to tell, but there appears to be no wire in the RT side of the boiler? Just what appears to be a link from Live to the non functional Side.

@stem is correct, S.30 means that the room thermostat is blocking, basically meaning the thermostat isn’t requesting heat currently, it’s not an error code it’s a status code, hence the ‘S’.

Also opening that particular boiler forms part of the combustion circuit and should be tested by a competent person afterwards.

 

[stem]

.. I might have to see if i can disconnect it completely and see what the boiler does

If you disconnect it the boiler will do nothing from a central heating aspect, the heating just won't work at all. If you were to move the wire from thermostat B and put it in A with the wire already there, then the heating will be 'on' permanently. If then the boiler runs continually and there's no S.30 message, all is well with the boiler.....you might find it gets a bit warm though.

 

[Jag-1978]

If you disconnect it the boiler will do nothing from a central heating aspect, the heating just won't work at all. If you were to move the wire from thermostat B and put it in A with the wire already there, then the heating will be 'on' permanently. If then the boiler runs continually and there's no S.30 message, all is well with the boiler.....you might find it gets a bit warm though.

Might be a good test just to get the boiler working and seeing if i get the status message or not. I'll move the B wire on the stat receiver to A and see if that kicks the heating in

 

[ianmcd]

S codes are status codes, all fault codes begin with F

 

[ianmcd]

It’s difficult to tell, but there appears to be no wire in the RT side of the boiler? Just what appears to be a link from Live to the non functional Side

Agree, can you post a better pic of the wires going into the RT 240v plug at the boiler

 

[SpecialK]

Im not a boiler engineer so I'm probably wrong - why is the input live bridged to RT?
Does RT not output a voltage to go through a volt free contact and back to switched live?

 

[CBW]

Afaik, that side doesn’t do anything, so I don’t know why it’s bridged, which is what I queried in post #8 and @ianmcd in post #12. Can you post a better photo as requested?

 

[lloyda]

Seems there is a connection from the fused spur going to the honeywell receiver then another wire from the receiver to the boiler.. I thought it would be the other way round.. fuse to boiler and boiler to stat receiver..

The connection from the fused spur is powering the receiver. The second connection from the receiver to the boiler is not providing power to the boiler, but making/breaking a connection that enables the boiler. Hence:

If you were to move the wire from thermostat B and put it in A with the wire already there, then the heating will be 'on' permanently.

There will be another cable (perhaps from the same fused spur) powering the boiler.