I have a GCH system that has worked fine for many years. It has a basic boiler, Hot water tank, Grundfoss pump, Honeywell contol and a Honeywell Digital Programmable Room Thermostat. No radiator thermostatic valves.
Everything worked fine for ten years with no changes to the system. The Honewell digistat controlled everything beautifully, until about a year ago when the temperature used to get really high and uncontrollable.
Checked the roomstat and even when it was not calling for heat the boiler was on and the pump circulating heated water around the rads. I found that lowering the temperature on the hot water stat seemed to stabilise the temperature, although it was still quite erratic. I recently changed the roomstat after calling Honeywell, who told me that the existing model was too old and likely to cause problems. The replacement was the latest version (Honeywell CM67).
On testing, the heating is doing the same thing, so (I assume) it can't be the roomstat that is causing the problem. After changing the roomstat, I tested by just raising the required temperature. The heating came on and didn't go off even though the roostat had apparently switched it off.
I'm wondering whether (my strongest suspicion) the motorised valve is sticking and letting water through while the hot water stat is calling for heat or whether the hot water stat is causing the problem (the dial seems stiff).
Any suggestions?
Everything worked fine for ten years with no changes to the system. The Honewell digistat controlled everything beautifully, until about a year ago when the temperature used to get really high and uncontrollable.
Checked the roomstat and even when it was not calling for heat the boiler was on and the pump circulating heated water around the rads. I found that lowering the temperature on the hot water stat seemed to stabilise the temperature, although it was still quite erratic. I recently changed the roomstat after calling Honeywell, who told me that the existing model was too old and likely to cause problems. The replacement was the latest version (Honeywell CM67).
On testing, the heating is doing the same thing, so (I assume) it can't be the roomstat that is causing the problem. After changing the roomstat, I tested by just raising the required temperature. The heating came on and didn't go off even though the roostat had apparently switched it off.
I'm wondering whether (my strongest suspicion) the motorised valve is sticking and letting water through while the hot water stat is calling for heat or whether the hot water stat is causing the problem (the dial seems stiff).
Any suggestions?
