Stu Gotts wrote:

Agreed, but you are assuming that the exhaust system is sealed well.

Actually, I think I'm assuming that the heater system is very slightly
pressurized (versus the engine cowl space). If the input fresh air SCAT tube for
the heater is kaput (obstructed, disconnected, missing), then the heater system
may not be so pressurized, and exhaust gases may enter the cabin if the under
cowl pressure is higher than the cabin pressure.

Sorry, but I can't and won't make that assumption, even on my own plane.

I wouldn't dream of trying to convince you to be less conservative, *BUT* IMHO
the risk of an otherwise intact heating system (as described by Ben Jackson)
being a source of CO poisoning is no worse a risk than CO poisoning via exhaust
fumes entering the cabin via other routes (leaky firewall throughways, leaky door
seals, ...). Were I in Ben's shoes, I'd patch the tear in the SCAT tube (Home
Depot HVAC repair tape), verify the integrity of the remainder of the under cowl
portion of the heating system, order a replacement SCAT, and schedule replacement
at the next convenient downtime (oil change, snowstorm, whatever).

Russell Kent