(Home) Heater problems

I have a 1990 2.0TD townie and although the heater in the front works great when your driving along,it starts to blow cold when the engine is ticking over,only to start blowing hot again when the revs increase.Only have to increase revs slightly to achieve this.Is this normal or do I have a problem.
With the rear heater system can you only get it to blow hot air through the bottom vents and air-con through the top?
Any help on these queries greatly appreciated. - (#222) peter bennett, 27 May 01

Heater going cold at idle is probably one of two things. Is the system FULL of water? run the engine with the heater at hot, then switch off and let the engine cool. Check the level in the expansion bottle, and that coolant comes the whole way to the "radiator cap" neck.
Other than that, is the idle speed correct?- too slow will mean low coolant flow- thus the idle up switch.
You are correct about the rear heater, BTW
HTH - (#225) david miller, 27 May 01

Have checked water levels and all are correct.
Tick-over is at about 700 rpm,is this too low?, as when i switch the idle-up switch on the heater begins to work and this only lifts the revs by a couple of hundred.
Will try turning tick-over up and see if it cures my probs.
THX. - (#235) peter bennett, 28 May 01

Idle speed should be 750- 850rpm. - (#236) david miller, 29 May 01