In the basement, which the framing is about 90% done, I have to build a wall to accomidate the bathroom vanity. The top plate of that wall covers the sole cold air return in the basement.

I was planning on just moving the vent to another location, actually add an additional trunk to the end of the run (which ends in the middle of the basement) and run a line through the soffit which will be perpindicular to the bathroom wall, facing the largest room (the rec room).

I probably plan to call an HVAC specialist to see if I meet requirements to pull combustional air back into the furnace/water heaters with that, but can you pull air in from the outside? I can install a trunk easier to the cold air return easier from the furnace end to grab air from the outside, but is that ok? legal? Just wondering.

 

Yes you can take in outside air. Dont forget its cold air . There are units that you can use to take in outside air and let inside air out. If you close the room to the furnace and water heater. You need a open grill to the room/ 1 sq " for ever 1000 btu of heat.

 

If I remember right, the furnace is 100,000 BTU and the water heater is 36000. We plan on adding a second water heater. So figure the same number for that in BTUs . Total 172,000 BTUs.

The mechanical room will be closed off, but in my 'revised' layout (damn wife, lol), pushing the vanity wall under the return, shortens up the opening to the mechanical room and I can only install a single (louvered) door which will be 36'' wide. I think before the plans got revised.. we would have had atleast 60'' of room for a couple of doors.

My biggest worry is to have enough combustable air moving back to feed the furnace/water heaters.

I've read (somewhere) that you take the total BTUs from the furnace/water heaters and divide that total by 20; which gives you the number of cubic feet you need (minimum) to feed them.

So I need 8600 cubic feet of available air? (the 172,000 total from above). The total size of the basement doesn't even come close to that. Off hand, I think its 42 x 22 x 8 (7744 cubic feet). Does that afore mentioned "way to figure out air" take into consideration the cold air returns in the main level of the house? Because I don't have the room in a bare basement to feed that.. needless to say, when I start to close stuff off.

The louvered door should satisfy the 1 sq in. per 1000 BTU's? Eh, now I'm confussed, lol.

Do HVAC specialists act like used car salesmen? Cause I'll probably call one out to my specific needs but I don't want him/her to say I need this or that when I actually dont, lol.

 

As I said here code calls for 1sq" per 1000 btu. So you need 172sq" of air into that room.

Do HVAC specialists act like used car salesmen?

Yes some do.