Heatslave 15/19, Honeywell CM927 & Honeywell ST699

8 Feb 2005
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United Kingdom
Hi Everyone,

Looking for some help please. I have a Heatslave 15/19 Oil boiler. When we came here it had just a Honeywell ST699 controller (no room thermostat).

We wanted to add some more control over the system, so I purchased a Honeywell CM927. I wired it with Live & Neutral from the boiler feed. I removed the jumper from Pin 2 & 3 of X2 in the Heatslave. Then connected A to pin 3 of X2 and then connected B to pin 2 of X2.

I left the ST699 in place, but set both CH & DHW to continuous. Obviously now the jumper has been removed, the CM927 should control the CH.

So, this morning my wife gets up and says.. damn it's hot in here.. and sure enough, the radiators are on. I go downstairs and the CM927 is set to off. The boiler also appears to be idle.

So what's gone on? Why did the CH run without control from the CM927? I have tried putting the ST699 CH control to off, but then the CM927 can't turn it on either.

Confused. Any help much appreciated.

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Well it looks like you have wired it up correctly so the only thing I can suggest is have a good read of the user instructions, it may be not off when you think it is ;)
You state that the boiler appeared to be idle. Does this mean that it wasn't on at the time? Was the pump running?
Check your times and temperatures on the 927; had the boiler been on and gone off? You need to check outputs on the PCB to be sure exactly what is happening.
Did you buy the CM927 brand new in a sealed box?

Does the relay box's green light go on/off when the temperature is turned up/down on the control unit?

Have you changed the CM927 settings to those for an oil boiler? Cycles/hour (set2:Cr) =3 and Min On time (set1:Ot) =4 .
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Sorry for the delay in getting bacl.. I have been 'observing' :D

OK, so the 927 was and is set to off, so it's not running a program at all. So that would rule out a program turning it on. The 927 works fine when used normally, it does indeed turn the indicator light on on the relay when it wants the burner to fire. It was brand new in a sealed box... at least it appeared to be.

Since it doesn't do it all the time, I have to run to the boiler to see what it's doing when I realise the radiators are hot (or ticking which indicates heat). B?y the time I get downstairs, the boiler is always idle (no pump runnnig, no burner, nothing).

The amount of heat seems to vary. This morning when it did it, only one radiator got hot, and only at the top. A couple of days ago, several radiators got hot.

Could it be dumping excess dhw into the CH circuit?

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