At a meeting last night it was mentioned that when the manufacturer of a CAT IV appliance allows drawing the combustion from the interior of the structure instead of directly from the exterior of via a dedicated intake pipe directly connected to the appliance, that if you elect this option a 90+ efficient appliance can drop into the 80+ category.

I've tried looking through the installation instructions for several manufacturers, but I've found no reference to this efficiency drop.

Can anyone point me to a source confirming or refuting this?

Thanks