Natural gas detectors and Co detectors

["paul@xxxxxxxxxxxxx" <paul@xxxxxxxxxxxxx>]

I use Ademco Vista 20p panels.  I need to add 2 Co detectors and 2
Natural gas detectors.
Ademco has a Co type for fault reporting, but I do not see a Natural
gas faul code.   What do you guys use?

 Also can I put all 4 detectors on a zone expander?  The manual says
don't use zone doubling and Ademco Tech said don't use expansion
module but he seemed a bit confused when I asked him why.  Anybody
know why?   Thanks Paul.