I too am having this problem on my mk5 fiesta , I have changed the heater control valve and the control switch panel and it is still blowing cold air
There is power up to the hcv
Do you need water runnig through the hcv to make the valve work ? i would not have thought so with it being controlled electrically if i hold the hcv off the car you can blow through the unit ,as soon as it it connected to the plug the valve closes and you cannot blow through it ,but does not open again when you turn the heater switch to hot in the car , should this be able to open while the unit is not connected to the water pipes ?
What else could the problem be if it is not the hcv and the switch panel