Heaters blow luke warm one side, cold the other

[alexthecheese]

Hi guys. In van at moment, down at Durdle Door, and am fiddling with the van and trying to get both heaters blowing hot when we're on the move. The drivers side blows luke warm air, but the passenger and central blow cold. Any idea what this is? I want to have a look under the dash but don't evenknow where to start with that. Any ideas?

[bazw]

take the filler cap of and run the engine , look to see if theres water pumping into bottle , this will let you know if water pump is ok
then assuming you have water in the system face the van up hill and give the throttle a blip or two , this will move any air lock in the hater matrix and see if that helps

if not then its time to see if the controls are moving the levers that open the water valve to the heater