and possibly ac with no cabin air heat exchangers on the exhaust pipes.
CO from the exhaust can be sucked into the fuselage at any underside hatch which is not airtight, and build up over time. The most sensitive place for the CO monitor in our Jodel was the rear seat. When moved to the heater air entry, it stopped bleeping. The underfuselage access hatch was not properly sealed. It took over 90 minutes to trigger the alarm.