taco 700 constantly running....
It is not normal for the circulating pump to be running unless the thermostat is calling for heat. Not sure what type of zone valves you have, but if the motorized type, such a s Honeywell, the thermostat simply opens and closes the zone valve. In turn, there is an end switch on the zone valve that opens and closes the circuit that powers the circulating pump. If you have this type of zone valve, check for a faulty end switch that keeps the pump running even if the zone valve is closed. Disconnect one of the end switch wires from one zone valve and see if the pump quits. If not reconnect, and try the other one.
If none of the zones are calling for heat, the furnace should not fire when the temp drops below low temp set point. There is a relay in the control that tells the furnace to fire when the water temp drops ONLY if the circulating pump is running. Since yours is running all the time, that is probably the reason the furnace fires when it shouldn't.
In one case however, I saw a system that did not have the relay to perform the function discussed above. It was simple project to add the relay.