Furnace Plug/Knockout Required?

[greyboy39]

Saw an open condensate line plug and open knockout (correct me if my terminology is lacking) on a forced air Cat IV furnace this morning. Are there any requirements from most manufacturers as to plugging an unused condensate plug? Furthermore, any requirements from manufacturers in covering unused knockouts on the furnace cabinet?  If so on either count what adverse effects can they cause. And yes, i do see the manufacturers installation instructions sitting on top of the furnace and did consult them, I wasn't able to determine if either was required and or recommended. Thanks in advance!

[Jim Katen]

Was it located in a conditioned space? If it's in an unconditioned space, I'd seal the open unused plug (and the condensate line cleanout opening).  Otherwise, I wouldn't care. 

Was it set up as direct vent or non-direct vent? If it's a direct vent setup, I'd seal the unused openings at the burner compartment because only outdoor air is supposed to be there. In a non-direct vent setup, I wouldn't bother.