Could be the distribution flap sticking in the heater box,when you start up from cold the flaps to the limit of travel in each direction.If the Hevac Ecu does not see full movement on all three blend motors it flags a fault.
The distribution flaps seem to be tight in the box,when they warm up a bit they free off enough to work.Can be a bit of a bu--er to sort out.
You really need to stick it on testbook to find out exactly what is going on.Hevac on LP RR's is the fussiest system on the car and can be a real pain to sort out,but at least the fault codes usually relate fairly closely to whats going on.(Unlike EAS)