Hope you don't mind me chipping in.
I would have thought that the Evaporator (the part of the A/C system which provides the cooling) is sited at the firt point of air flow entering the car. In other words, the air hits this first upon entering the car. From here, it flows over the heater matrix for heating if required. The reason for this is to dry the air which in turn prevents (or removes) the misting as mentioned above. In the winter, the air has to be heated somehow for this facility (which is a side effect of A/C) to be employed. In any case, it does not matter which way around the Evaporator and Matrix sit, the effect is the same.
Going back to the HBV issue, if this is leaking, please get it sorted quickly or you will be looking at a new DIS module as well!
Just my thoughs on the matter and hope I have not offended.