Hello, I am the onsite mechanic at a retirement community. We have a bus that is a 2011 Ford F-550 with the V-10 gas engine Chassis. There is a problem I am having with the blending door for the heater. It doesn't get as hot as it should, and only hot at idle speed. When you increase the RPM's it goes cold. The higher you go the colder it gets. When you bring it back to idle it is warm again, however not hot like it should be. Do this year Ford use vacuum to control the blending door? Does anyone have any idea what it might be? I have only seen that in Chevy's back in the early 2000 that used vacuum to control the blending door.