No heat in cab

[johnymingle]

 A4 2.0 tdi b7 2008.... I have noticed when putting my heaters on, I get hot/Warm air for a few minutes then it turns cool, my temp is normal at 90. My scan pointed to heater circulation flap.......it was indeed knackered so I replaced that. Still the same so I replaced the thermostat thinking it might be stuck open and made no difference.....the top hose gets hot and the bottom is cold..... hoses on the bulkhead to the matrix, bottom is hot and top is half hot and when pried back there's a trickle of coolant. ...... expansion vessel cool to mildly warm with not a trickle in the return.....replaced timing belt 4 years ago and new SKF pump.....always used fresh coolant and its very clean....My gut feeling is the water pump, as the water doesn't circulate, but then i think is it the heater matrix!! Im stumped really, as I would expect a good pump like skf to last longer than 4 years. And if it's the matrix, then why isn't the rest of the engine circulating, (or does it have to circulate through the matrix to make a circuit) I would appreciate some experienced help thanks.

[alpha omega]

can we rule out an air lock as they can be stubborn

[johnymingle]

Wel  I took off the top bulkead hose (the one with a little hole for bleeding) and expecting bubbles but there was nothing. Pulled it right off and coolant was flowing back towards the engine, but a very slow rate. The engine was normal temperature. I held the expansion vessel as high as I could......its like all coolant anywhere isn't moving but its not overheating. I replaced the radiator about 15 months ago.