How large should the fresh air intake be?

[Musclemass]

I'm not sure if reading in this forum leads me to having more answers or more questions.

I have an 80% 120K furnace that has what seems to be about a 5 inch diameter sheet metal exhaust pipe. There's about a 2 inch diameter fresh air intake in the room to supply the furnace. Will that supply enough air or should I try to enlarge it? Thanks.

[Musclemass]

No one has a suggestion? This isn't a DIY question (I'm not a DIY-type) and it's also not to criticize anyone's work, it was how the house (that I bought 10 years ago) was built 40 years ago.

Thanks in advance for any ideas.

[Kevin Weaver]

Where is the furnace located, basement, garage, attic, closet? That pipe you are referring to is a combustion air pipe, lets fresh air into the area for the burners. Its probably a 3 inch pipe. If its in a closet which I suspect, the closet should have metal bi-fold doors. We need more information to answer the question.

[Musclemass]

Yes, it's in a closet, behind a standard one piece wood door.

[TwincamDave]

Is there a gas water heater in the closet also?

[Musclemass]

No, the water heater is in the garage

[Kevin Weaver]

When you close the closet door is there a 6 inch clearance from the closet door to the face of the furnace? You should have at least 6 inches. When the door is close the air in that room can burn up and then the furnace that needs air to mix with the gas for complete combustion goes away. the 3 inch pipe is a start but not code. You can check but here you need a 6 inch pipe from attic high, and a 6 inch pipe low for combustion air, or replace doors with complete louvered doors.

[Musclemass]

Yes, it's a bit more than 6 inches back from the door. Louvered doors wouldn't bet a good fit appearance (or sound) wise, so I'll need the air supplies increased.

It's time for me to set up an HVAC annual service call, so I'll point that out to my contractor. Thanks again.