I bought Liana nearly half an year ago. It had the same problem, heater was blowing cold to very very mild hot air. I detached both, inlet and outlet pipe, going into heater core (as shown in your picture), Attached water hose inside one them and turned-on the water tap. To my amazement mud like water started coming out of the core. The problem was the mud stuck inside of it. After the flush, I attached the hoses back on and filled the coolant to level. The heater afterwards was blowing very hot air.