Clean air intake

[Brian C. Foreman]

I see most furnaces have the clean air intake vented outside. This one is not. Is it safe?

[David A. Andersen]

Safe yes (unless you have a Radon problem).
Efficient, no.

You must have sufficient room size to supply combustion air to the appliance.

Down draft furnace?
No filter rack cover.
Where it it located in the house?

[Gary Reecher]

Depends.
Is there a natural draft water heater in the same room? If so the draft should be checked on the water heater with the furnace operating to ensure no back drafting.

[David A. Andersen]

It always "Depends"!