Well here is a strange happening.Car Heater
 

The heater in the car (75 Diesel), has all of a sudden started to play silly luggers,, From cold with the temp set at 20 both sides the hot air starts to increase as the engine temp rises. All good, but after 20/30 miles at a sustained speed cold air starts to blow thru the vents, turn the temp up and you get warm air. There are no sign of over heating. The pipes to the heater matrix are both hot when the cold air starts to come thru the vents:shrug:

Any one have any suggestions??

Check coolant level.
Have you done the top thermostat mod ?

I have done the top thermostat Mod.

But this was done some time ago, the level of water never changes And this has only recently started to do this.

Oh and it has been bled correctly

switch off the auto mode, and switch to recirc (there is a combination of settings similar to this, but I can never recall it).

Mine does this, but it is limited to the upper drivers side air vents. Feet, and passenger sides remain warm. I have read this is a normal safety function to keep the driver alert.

If all of the vents turn to cold, then I would suggest an issue with the vent mixer flaps, or an issue with the air sampling inlet vent.

Underneath the clock is a vent control.

I believe this selects between (cold(er)) fresh external air, and (warmer) air to the driver side air vents.
( I expect somebody can clarify this).

If somebody has fiddled with it, it could explain the recent issue.

The environment controller is generally very very reliable. Unfortunately the cooling thermostats are known to be far less unreliable, with age, and may need replacing.

That's basically correct Mark but it affects all the face level vents, not just those on the driver's side.
John; that sounds like thermostat trouble to me, i.e. it is not compensating for the cooling effect of the increased air flow through the radiator when road speed is increased and sustained.

Simon

When the vents blow cold what is the outside temperature as if warm outside then this is quite normal as the sunlight sensor on top of dash will see and put vents on slightly, is engine temperature gauge reading normal?

Thanks for the reply, The outside temp is anything between 1c and 10c when this is happening so not hot out side.

The temp gauge sits at half way and never moves, Always been there.

Well it had a new stat fitted a few years ago, and I left the old Renault 5 stat in situ.. but was told it would do no harm to leave it in place.:shrug: