My boss's house is large and heated by a 60kW boiler in an outhouse, the system is antiquated but the boiler ( gas, non condensing, recently replaced a free standing oil boiler). It seems to be traditional S plan but with large commercial valves on the ch circuit. I noticed a strangeness some while back in that if the single thermostat ( one zone only) called for the ch valve to be open the dhw solenoid was inhibited.
The boiler is constantly on and controlled only by its internal water temperature. It has a manual valve controlled bypass between hot out and return, so there is constant blending of return and hot into the boiler. I understand this was to keep the oil boiler from condensing but why would the gas boiler have retained this backend protection? On the wood boilers I deal with this blending is done with a thermostatic valve.
I would have expected the thermostat to have controlled the boiler and the ch solenoid valve. As it is this valve has failed and of course the system now doesn't react to the room stat at all.
AJH