Is there a way of fitting a water control valve to the heater on a Dduratec engined Plus 4?
As far as I can make out the heater core does not have its own water circuit but has been included in the engine recirculating flow circuit, which should not be shut off.
Most car heaters receive their heat from the coolant in a user controlled manner via the control valve which may be set anywhere between full on or off. As the flow to/from heater effectively sits piggy back (in parallel) with the thermostat bypass circuit there is no restriction to engine cooling or warm up with heater valve completely closed.
This doesn't preclude the likelihood that the heater core may have its own dedicated feed from the cylinder head.
Hopefully this helps