That is apparently the theory on which the heating system in my house was put together (long before I bought it). There's a single boiler, a single primary water circuit, and a single timeswitch, but instead of a central thermostat switching the radiator pump on and off, the pump runs all the time the heating is on, and each radiator has its own thermostatic valve.

It's an appealing theory, but the problem is that all the pipes to and from the radiators remain hot even in rooms where the radiator is off; in other words the problem isn't with the low conductivity of the water, it's with the high conductivity of the (no) insulation on the copper pipes. If the electricity flowing round my house were equally poorly-insulated, it'd be a deathtrap!

Peter