There's no heat control valve on the water, the heater matrix is always the same temperature as the engine, it's all done by mixing hot & cold air with flaps on the heater itself & they're unlikely to change with speed.

 

That sounds like a circulation issue. Is the coolant topped up to the max? And when you changed the pump and stat, did nice clean pink coolant come out when you drained down, or was it rusty brown?
A little test to see where the problem is, is to feel the flow and return pipes to the heater, they should both be hot, with the return being slightly cooler. If the flow is hot, and the return cold, it's a circulation issue. If they are both hot, there is a mixing flap / controls issue in the heater box.
Also did this problem exist before changing the pump and stat, or after changing them, or sometime since changing them?

 

Hi, It's been like this since I've had the T5, I've changed the water pump and thermostat twice and still no joy, water is returning into the expansion tank from the radiator. The heater in the back is blowing cold too so I wonder if its worth me taking the three pipes off in the engine bay that go under the van and clearing them out with a hose pipe

 

Hi, Thanks for the reply, at least that's one thing less to check. I just can't understand why at normal engine temperature and being stationary my heater is barely warm whereas when driving the heating is hot is it a flow problem?

 

We had another 'over-cooler' the other day but I don't think he came back. It's probably worth checking that the auxillary coolant pump under the van isn't stuck on all the time speeding circulation and make sure the radiator fan is off most of the time.

If you have access to VCDS it does know about the auxillary coolant pump so you could look for codes in the engine ECU. If you do have acess to VCDS you can log the two temp sensors from cold and see if anything strange is happening. This is what my 2.5 did. This is one 20 mile drive split in the middle. You can see from cold that the outlet (bottom hose) stays the same temp until the thermostat opens and allows full circulation to start. This is how it should be. After that it depends on load (hills/high speed/traffic etc.)

 

That's great thank you for the info, I will look into that

 

could be an airlock. worst case scenario and air lock from combustion gasses = HG.