Squealing belt when heater on high and/or at low RPM

[ErnestoR02]

Hello, I have a Ford E-150 2003 (4.2L V6 w/ AC). The belt squeals terribly mainly when I put the heater on high, at low RPM and when starting the van. The squealing noise goes away for a bit when I spray belt dressing on the belt.
I replaced the alternator last winter. I've also put a new belt tensioner and both idler pulleys with a new belt about a month ago. The squealing noise stopped for a few days and came back. The only thing I can think of now would be either the AC compressor or the power steering pump? But are any of these affected by the heater? Please I need help it's driving me insane! Thank you so much!

[fredjacksonsan]

Double check to make sure you have the right length of the belt… Even an inch or two can make a difference between squealing and not squealing.

While you have the belt off, check all of the pulleys and accessories to make sure they turn easily.

You said you install the new belt tensioner; was it remanufactured? I have heard that sometimes those can be substandard.