I have a Vauxhall ZAFIRA ELITE CDTI 1686cc, reg. 2011. Since I bought it second hand, it had a heater issue. The heater always blows cold (or at best lukewarm) air. Two mechanics failed to properly diagnose the problem, so I am investigating it myself. I have checked the heater fan and it is working properly at all speeds, and the pollen filter is new, and I have checked the flaps, and they are all working fine. I removed the heating matrix bottom cover and touched the heating core and it was dead cold although the engine temp was around 88c. I then flushed the heating core with plenty of water from a hot tap, and there were some dirt and white flakes but not much. All the above did not help. So I disconnected the heater hoses from the fire wall while the engine is running (expecting water to gush out from one of them), and to my surprise no water came out of any, just a trickle from the top one. Is there a valve in the engine compartment that controls the flow of water to the heating core? If no, could it be the water pump? For information, there is a faint whining noise that sometimes comes from the engine bay, but I don't remember having an issue with overheating in the summer. It once reached 98-99 on a hot summer day but did not go above that. In fact, with the mild weather in the UK, it is rarely hot enough to experience overheating anyway. Is there a way to check the functioning of the water pump?