Remedy
System is not filled with water or low system pressure.
Open vent system - Check feed and expansion tank.
Sealed system - Check system pressure.
No water flow, because CH system is closed.
No water flow, because pump is not operating correctly.
Check pump is operating freely check voltage to pump.
Check the isolation valves.
Check for air locks in the system, purge if required.
Check flow through appliance and system pressure.