E36 heater problem

Hi,
I've got a 1994 e36 318i, and am having a problem with the heater.
It normally just blows cold air, occasionally warm or hot, this is with leaving the temp setting at MAX.
I think there's a v slight leak in the cooling system, whilst hot I can hear a fait hissing noise near the thermostat. I'm going to have a new thermostat and cooling flush done this week, if the heater problem continues, what shall I try next? Could the heater valve be stuck or blocked so that no coolant's going to the heat exchanger?
Please don't tell me its a head gasket, I've had enough of that with a Rover K series engine in the past!
Cheers, Nick
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Nick wrote:

Yes, the heater valve could be stuck, or if you're lucky, your small leak has put an air block in the system that will be corrected when you fix the thermostat and re-bleed the system.
Good Luck
Frank
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