Just trying to sort a mates classic wrx. He replaced the heater matrix after the old one started leaking but now has no hot air coming through the vents.

I've run the engine with the header tank cap off and there's no air locks. Both pipes going in and out of the matrix are hot.

Anyone got any ideas what could be causing this ?

 

Sorted it now

It was the motor that moves the flap to blow hot/cold air down in the passenger footwell that wasn't plugged in.