Wiring for CMT927 - Diverter Valve getting hot

[infrequentuser]

My house had a Danfoss "RMT230 (with parallel accelerator)" fitted as a room stat to a metal back box.

The stat is supplied by a 3-core-and-earth cable with old colours : Red, Yellow, Blue and "Bare".

By inspecting the wiring arrangement of the RMT230 and by checking the Red and Yellow conductors I believe that the wires are as follows :

Yellow = Live (from timer/programmer)
Red = Switched Live (to wiring centre – CH On)
Blue = Neutral
Bare = Earth

I wanted to upgrade the CH control to a programmable wireless stat and I chose a Honeywell CMT927(BDR91).

I fitted the CMT927/BDR91 in place of the RMT230 and wired the BDR91 wireless receiver as below :

Blue → 'Electronics' Terminal 'N'
Yellow → 'Electronics' Terminal 'L'
Red → 'Heating' Terminal 'B'
Earth → Back Box only
+ Short length of conductor connecting Terminals 'L' and 'A'

The new stat appears to function perfectly correctly and I can find no 'problems' except in one regard : I recently discovered that after switching on the whole system from scratch, the Diverter Valve gradually heats up to a temperature which seems excessive (very hot to touch) and does not appear to cool down even when there is no 'active' CH in progress, eg even overnight when the stat temp is set so low as to be 'off'.

It's as if the motor in the valve is 'working' even though the call for heat is absent.

Can anyone suggest a cause for this ? Have I wired up the CMT927 correctly ? Can the Diverter Valve remain 'very hot to touch' even when there is no call on CH and no call on HW ?

Thanks

 

[D_Hailsham]

Is this a new problem or have you only just noticed that the valve gets hot?

 

[infrequentuser]

Is this a new problem or have you only just noticed that the valve gets hot?

Apologies for the delayed response - I've been tied up with other matters.

I'm afraid I don't know whether the diverter valve has been getting very hot only recently or for a longer time. By way of more background, the CMT927 was fitted about a year ago and seemed to function perfectly since then.

However, very recently I experienced 'typical' diverter valve issues - room temperature going much higher than room stat setting because of valve sticking in mid position etc. As part of attemting to understand what was happening I observed the system, including 'touching' components at different times of day, and with different combinations of HW/CH activation. It was then that I discovered that the diverter valve head was getting hotter than seemed reasonable and particularly that it was very hot even when the system had been fully inactive for a number of hours.

I had someone look at it and the diverter head was found to be sticking and a new one was fitted (the valve body itself was found to be OK). I mentioned that I thought the old valve was getting very hot but this didn't seem to cause surprise.

The new diverter valve head solved the 'functional' problems which had developed - that is to say the system now seems to function perfectly once again. But I decided to keep an eye on the diverter head temperature mainly because it had puzzled me etc etc. I found that the new diverter valve head also gets very hot just as the old one did.

I thought the valve defaulted, on a spring, to HW only. So what puzzled me particularly was that the valve head was getting/staying hot when I considered the system to be effectively 'fully' off - hence no 'work' would be being done by the valve head. Hence, why is it getting/staying hot and not cooling down ?

Subsequently I've discovered your sticky FAQ post on the workings of a diverter valve which indicates that the valve may not really default to 'HW only' often - rather it stays in the last position used (eg CH only) until it gets a further call for HW or the power is switched off. This makes things possibly more understandable to me : the valve head can indeed get warm/hot even when its effectively not being used. I suppose the question is 'how hot can/should it get ?'

I could revert the room stat to the original RMT230 to test whether the CMT927 is to blame ? However, with the RMT230 fitted, the CH will be called less often and this could presumably lead to more chance that an 'HW call' will cause the valve to default to 'HW off' and so cool down ?

Any ideas ?

 

[D_Hailsham]

Subsequently I've discovered your sticky FAQ post on the workings of a diverter valve which indicates that the valve may not really default to 'HW only' often - rather it stays in the last position used (eg CH only) until it gets a further call for HW or the power is switched off. This makes things possibly more understandable to me : the valve head can indeed get warm/hot even when its effectively not being used. I suppose the question is 'how hot can/should it get ?'

The valve will get fairly warm when it is stalled in the mid or CH only positions as there is a current going through the motor (it's rated at 6W) which is in a fairly confined space with no ventilation.

I could revert the room stat to the original RMT230 to test whether the CMT927 is to blame ? However, with the RMT230 fitted, the CH will be called less often and this could presumably lead to more chance that an 'HW call' will cause the valve to default to 'HW off' and so cool down?

I doubt if changing the stat will have any effect on how warm the valve gets as the valve will still stay in the CH position when CH is satisfied and there is no call for HW.

 

[infrequentuser]

Thanks D_Hailsham

Your FAQ is good at explaining how the valve head can be energised when there is no call for HW and no call for CH. You suggest this can lead to the valve becoming fairly warm.

The reason I posted this query was that I was surprised just how warm/hot it often seems to feel, even at the end of a 'call-free' night. (Note that I have confirmed that the valve head does indeed de-energise and cool down overnight if the last 'call' of the evening is HW on; CH off.)

I have a cheap infra-red thermometer, so I don't know how accurate it is, but it suggests a temperature of around 55 -60 C degrees at the hottest point of the head. By touch, I found the previous (different brand) valve head to be getting equally as warm/hot.

So the valve head has been replaced by a new (different branded) valve head and still reaches the same temperature - this tends to make me think there's unlikely to be anything wrong with the valve head. I've setout details of the only change I've made to the system - swapping the CMT927 for the RMT230 - in my first post and I can't see any problems with how I've done this. Finally, the system seems to function perfectly - all different combinations of HW on/off & CH on/off seem to work precisely as I'd expect.

Any final ideas/comments ?

 

[D_Hailsham]

Any final ideas/comments ?

You have found out that this is a "feature" of this type of valve; so now you have to learn to live with it!!