Changed the thermostat on my car yesterday, took me about 45minutes. I'd read somewhere about getting airlocks etc. in the heater matrix and these were a nightmare to remove. Well after changing the part and refilling with antifreeze, the heater worked. I switched off the engine to recheck the coolant level (it was fine), started up again and the heater was blowing cold air - Aaaaargh !!!!
I panicked, then revved the engine once to get the water pump to turn faster and it cured the problem.
The car feels better now the thermostat is working. My engine temp before depended on what speed I was doing and if it was sunny or not. The car heats to just under 90 in about 10 minutes at idle, or about 5 mins on the road. Great !!!