I've been using a p10 mask for a year and until recently decided to try a full face mask. (I'm tired of trying different methods to solve mouth leak/chipmunk chin).
So I order a ResMed airfit F20 mask (LARGE size) and try out.
The good news is that firstly I don't have claustrophobia and secondly the mask seal perfectly (mask leak is even lower than P10, amazing) and I felt comfortable wearing it.

The bad news is that as soon as I put on F20 for the first time, I could clearly feel that it does not blow enough air into my lung compared with the nasal mask-- it felt like the air from the tube firstly blow into the space btw the mask and my face, which "diluting" the pressure. I heard before that a Full face mask typically requires a higher pressure setting than a nasal mask -- so I dial up the pressure setting from APAP 11-14 to APAP 12-20.
3 nights in, I could see some patterns by reviewing my oscar data.
1. There are tons of FLOW LIMITS in my oscar chart but the pressure never went up beyond 15 mmh2o. (I never have any flow limits when using nasal mask)
2. There is 1-2 RERA per night which rarely occur when I'm using a nasal mask. I could remember I woke up several times a night but could remember why.
3. I feel ok for one night and suboptimal for the other nights. The overall sleep quality correlates well with if and how many times I woke up during the night.
I tried to figure out what the heck is Flow limits and RERA and into the rabbit hole of Upper airway resistance syndrome. I'm still confused that if the machine sense there is resistance from "flow limits' why dont it just increase the flow pressure to overcome it? And is it bc my mask is too big that there is too large a space under it that the air is diluted before reaching my nose?
first night 2nd night 3rd night