[stuart bowes]

Not to overstate the obvious but the heater matrix is by very definition a way of taking heat from the water and distributing it into the air so yes, to some degree hot water will go in and slightly cooler water will come back out afterwards, I think you already know that.. the question is *how* cool is that other pipe.. one man's too hot to touch is another man's quite warm

Anyway you've proven water is flowing through fine, so hot water must be getting in there surely

Personally my next step would be to try a recalibration on the Hvac, or checking the thermistor as you already mentioned (depending which type you have), do you have VIDA or know anyone that does