Sigma 60 - 100. There is an intermittent fault. I think it is the CH Flow Temperature Sensor or its controlling circuit. Please advise what further diagnostic tests I should do and, depending on the test results, what repairs are indicated? (I would prefer not simply to replace expensive components on the basis of trial and error!)
Symptoms of Fault. The symptom of the fault is that, with everything having been working correctly (sometimes for several hours), about 3 or 4 times a day I find cold water being pumped around the system (for over an hour at a time) and the flame won't light until I turn the CH Temperature Setting Knob (on the front panel) down to min and back up to max again. The flame always re-lights when I do that and everything seems to work correctly again for up to several hours.
I'v ruled out the Safety Overheat Thermostat because I do not need to press the re-set button to get the boiler to go again after the fault has occured.
Description of System. It is a closed system pressurised to 1 bar with 3 separate CH circuits and a separate HW circuit, each controlled by a motorised gate valve (Gate). Each CH circuit has a room thermostat controlling its Gate Valve. These all work correctly, (although the air temp rarely gets hot enough to trigger the room thermostat high limit to close its Gate because it is a large house). All rads except one, have thermostatic valves fitted. It has a Santon Premier Plus unvented HW system which, I believe, has its own thermostat controlling its Gate Valve. There is a short circuit of pipe from the flow out of the boiler straight back to the return, via an automatic valve, so that the pump can continue to push water through the boiler when all 4 Gate Valves are shut. The boiler is the high point of the system and there is an automatic air vent just above it, out of the Flow pipe.
Action Already Taken. We have already replaced the CH Flow Temperature Sensor but the fault continues. We've checked the gas pressures, which are correct. We've also carried out most of the other checks in the Installation and Servicing Instructions except that we have not checked "that the flow and return differential does not exceed 20C" as stated in Section 3.04 at the bottom of Page 20. How do we do that and is it relevant/necessary?
Conclusions. My observations suggest that, the CH Flow Temperature Sensor and its controlling circuit work correctly some of the time but that it occasionally locks out and will not trigger the flame to re-light even when the temperature of the water in the boiler get cold. Could the new Sensor be faulty as well as the original one? Could there be a fault with its controlling circuit? How can we test and distinguish between these two possibilities? What is the significance of the fact that, after the fault has occured, the flame re-lights only if the Temp Control Knob is turned to min then max?
Many thanks.
Symptoms of Fault. The symptom of the fault is that, with everything having been working correctly (sometimes for several hours), about 3 or 4 times a day I find cold water being pumped around the system (for over an hour at a time) and the flame won't light until I turn the CH Temperature Setting Knob (on the front panel) down to min and back up to max again. The flame always re-lights when I do that and everything seems to work correctly again for up to several hours.
I'v ruled out the Safety Overheat Thermostat because I do not need to press the re-set button to get the boiler to go again after the fault has occured.
Description of System. It is a closed system pressurised to 1 bar with 3 separate CH circuits and a separate HW circuit, each controlled by a motorised gate valve (Gate). Each CH circuit has a room thermostat controlling its Gate Valve. These all work correctly, (although the air temp rarely gets hot enough to trigger the room thermostat high limit to close its Gate because it is a large house). All rads except one, have thermostatic valves fitted. It has a Santon Premier Plus unvented HW system which, I believe, has its own thermostat controlling its Gate Valve. There is a short circuit of pipe from the flow out of the boiler straight back to the return, via an automatic valve, so that the pump can continue to push water through the boiler when all 4 Gate Valves are shut. The boiler is the high point of the system and there is an automatic air vent just above it, out of the Flow pipe.
Action Already Taken. We have already replaced the CH Flow Temperature Sensor but the fault continues. We've checked the gas pressures, which are correct. We've also carried out most of the other checks in the Installation and Servicing Instructions except that we have not checked "that the flow and return differential does not exceed 20C" as stated in Section 3.04 at the bottom of Page 20. How do we do that and is it relevant/necessary?
Conclusions. My observations suggest that, the CH Flow Temperature Sensor and its controlling circuit work correctly some of the time but that it occasionally locks out and will not trigger the flame to re-light even when the temperature of the water in the boiler get cold. Could the new Sensor be faulty as well as the original one? Could there be a fault with its controlling circuit? How can we test and distinguish between these two possibilities? What is the significance of the fact that, after the fault has occured, the flame re-lights only if the Temp Control Knob is turned to min then max?
Many thanks.
