Honeywell CM67 Problem

[DerekB1]

Our oil fired central heating system is controlled by a Honeywell CM67 programmer. Most of the time it works OK but occasionally we have the following problem: The temperature is set at (for example) 19.5 deg, the temperature is reached and the boiler switches off. All radiators cool down except the lead one on the system. This one get very hot even though the boiler is not running. We live in France and we cannot get our plumber to understand what the problem is. It has been suggested that we 'turn off' the optimal start on our thermostat as this may be causing the problem. I therefor have two questions

1. Has anyone else suffered the same problem and how did they overcome it.

2. Does anyone know if all CM 67 thermostats have optimal start. We know how to set the controls but there is no indication on the system of being able to 'control' the optimal start.

 

[D_Hailsham]

Won't be anything to do with Optimum Start, which only operates on the first time/temperature setting of the day.

Which make/model boiler do you have?

Do you have any motorised valves on the heating or HW pipes?

After the boiler cuts out, does the problem radiator get hot and then start to cool down? If so it could be acting as a heat sink for the pump overrun.

Optimum Start is an "installer" setting; the default it is ON.

CM67 Installation Instructions