How often do the trains have their HVAC off?

I was just in an R179 train with its HVAC off (I don't know why), but the interior sounded very unique and relaxing, prior to this, I haven't been in a train with an HVAC off since an R40 back in the 2000s and it brought me back to this experience. Obviously during the summer months the HVAC is usually on, but is there any time where the HVAC could be off on certain lines?