But yes, it will make the home slightly negative.
Those rules can be found here.
The installation company said they could do it for me, but gave me this warning -
Furnaces with fresh air intake from outside sometimes get junk (leaves and yard wastes) sucked from the outside and this problem is not covered by the warranty...
That's true but not an issue if you dont let leaves build up. The intake and exhaust should be at least 12" above the ground,or above normal snow levels.
I think cleaner burning might be in reference to either the fuel air ratio changing due to colder denser air. IT also might be expressing how much longer the HE will take to warm up if cold outside air is able to cool it off. I could see this causing some sooting as well as possibly causing some initial condensation in the primary heat exchanger.
No trap on the indoor coil drain either, around my neck of the woods that's a redtag
the drain gentleman will possibly spew water out where the humidifier is tied in due to the fact that it is tee-d in at the same height as the coil without a trap, Air pushes past the water at the humidifier.. The humidifier drain shoud be tied in at the floor. Trust me . Ive come back to fix this from my installers a bunch of times. Water all over the floor.