I have a system that relies on the one thermostat (in the hallway of my bungalow)--set to 19c. ---If I close the valves on the radiators to halfway,what effect on gas used would this have( and what actully happens regarding flow of water through them when this happens?
The system would still be trying to achieve 19C at the stat in the hallway.
The restricted flow of the system (as a result of partially closing the radiator valves) will result in the boiler firing up for a shorter period of time, but more frequently to achieve 19C. The net result would be the same gas usage (albeit slightly less due to the system taking longer to initially reach 19C).
You also run the risk of upsetting the balance of the system, resulting in uneven temperatures throughout the property. Much better to reduce the stat if you want to use less gas – or reduce the overall time that the system is operating.
Just one thing to add, kenny. Quite often, a system will have one or more "heat leak" or "heat sink" rads.
This is a rad fitted with manual (not thermostatic) valves that should not be turned off. It's there to dissipate any excess heat created when, maybe, the boiler stat is a bit high, and no rads are "calling".
If any excess heat can't be lost, then boiler sensors may upset the system in other ways.