I can't believe they even sell so called bivvy heaters !! A guy in Angling ~direct tried selling me one , even after me telling him my 7 year son was doing the night with me , I think the look on my face told him all he needed to know .
PLEASE PLEASE just use proper thermal clothing , any heat source is always going to be restricted due to the thermal properties (lack of) of the the bivvy any way.
Carbon monoxide kills with NO warning .
As for pointing towards a youtube video to try and prove a gas heater is safe REALLY , are all tents / bivvies the same size , material , ventilation .
Just imagine your next of kin telling the coroner - well he watched a youtube video and the guy in it said they were safe to use .
Buy proper clothing .