Hi.....the Festa heater has an electrically-operated valve. [It is located, under the bonnet, right under the windscreen area..central....with two rubber [coolant] hoses in & out.

The heater motor/tap 'defaults' to open..ie if the motor, or the electrics, fail, then the tap is open, ie hot.

I have a similar issue.

I suspect either a dodgy electrical contact somewhere, or a failing motor.

It is also entirely possible the heater system is acquiring a vapour lock [or, air lock]....which results in little or no coolant actually remaining in the heater, if commencing with the heater in the 'off' position.

Again, I sometimes get this myself.

It can be identified by watching the engine temperature gauge when stressing the engine out......on mine, I suspect, an airlock has formed if the temp. gauge shoots right up to the hot zone [all the way to the right]

This occurs when I have the heater on 'cold'.

To sort it, I turn the heater 'on' [hot], and the gauge returns to normal [central] posiiton.

That seems to expel the airlock.

So, I suggest, when it blows 'hot' with the dial on 'cold'..check the engine temp gauge....turn the dial to 'hot' for a while, then turn it to 'cold' again..see if that makes a difference?

Possibly best to 'start' the engine with the dial on 'hot'....allow the engine temp to settle, then turn the heater to 'cold'...see what happens...hopefully, airlocks will be dispelled?

My car also has IBS..after a run, the engine turned off, the car continues to burp and f*** in a most incontinent fashion.

I have traced this intemperance to the small hose connected to the coolant bottle....air is slowly bleeding back into the header tank bottle, from the heater area.

I suspect Ford [Fiesta] heaters aren't particularly well sorted with regards to air locks?

For the journey with the youngster..if it 'blows' hot, I recommend directing the 'heat' to the dash vents [face vents]..with the vents shut tight.

This keeps most of the heat away from the footwells, directed to an area that can be better controlled.

Then, open the windows.....at least an inch or so, to 'suck' any hotter air out of the car...plenty of breeze is good [except for the hair]...enjoy the fresh air?

If the blast of outside air is intolerable to the passenger, then shut their window, and open yours a few inches?

[I usually drive with my window [almost ] shut, but the passenger window open a few inches, when on my own.....nice fresh breeze, without dislodging the barnet?]

Edited by alastairq on 21/08/2013 at 19:06

Is the radiator fan cutting in correctly?

In my experience if the temperature is low/normal most of the time, but shoots right up under various odd circumstances, then I suspect there is an airlock forming within the heater matrix, or associated pipes.
To prove this [or not]...next time [in traffic perhaps] the temp gauge shoots right up [if the heater is on 'cold']...try turning the heater to 'hot'..and see if the temp gauge quickly returns to 'normal?'