So let me start this thread with the issue at hand I have a 2015 Audi a3 2.0t my heat stopped working (fan blows great) so I flushed the heater core and pulled the dash (to replace a motor for the hvac flaps) then I found out the bag of silica beads had busted and thus clogged my system so after flushing the system multiple ways and multiple times the heat started to work great but only if you revved the car in the slightest bit if it just stays idling the car will get to operating temp but no heat and both hoses going to the heater core are cold so would it be the heater control valve? Or would it just not work at all if that was it it’s cost me a lot of money and time and can’t seem to find any answers  

I had similar issues on mine and it obviously only bothered me in the colder months. Do you go through engine coolant fairly quickly? I had mine flushed and it improved for a bit, but ultimately, it needed a new heater matrix @ £370.00 fitted. 

@ op did you sort this ? 
 

@ Ryan, that’s a pretty good price take it that was not main dealer?