....in that case, if the pump is not running when CH is on, then it is either borked, or the switching circuit is non-functional.
Probably the only reliable way of testing is to measure for the presence of current at the pump terminals (or a nearby junction box) when the CH function is switched on and the (any) thermostat is turned up high enough to fire things up. Lack of volts means it isn't the pump.
If you have hot water, I'm assuming (with a gravity system) it's relying on the boiler thermostat to trip the boiler out out once the hot water is to temperature. As you say the boiler won't fire up when CH is selected, is the boiler still hot from (gravity) water heating (which would keep the boiler tripped-out)? If you're selecting CH with a cold boiler and the boiler isn't firing up, the switching arrangements are much more suspect than the pump.
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