Musty smell when heat first turned on.

[Bluevitz-rs]

It's because you're using the A/C when the defrost setting is used. So it takes moisture out of the air but after years of use there can be an accumulation of dust on the evaporation coil that never dries out and gets moldy. The only way to avoid the smell is to avoid using any defrost setting. Even the mixed mode uses A/C.

I know on my Echo the setting for floor only still put air up on the windshield so I never use any defrost settings.