We are located in San Francisco Bay area. There's a huge fire almost every year. We recently moved to a house where central AC brings in smokey outside air. We have 2 young children and I'm worried about air being too dirty. Does anyone know how to fix this? Much appreciate your help!
There are two ways I could think of that you could be drawing in outside air:
The AC system should be sucking return air, heating or cooling it, and then supplying that same volume (approximately) back to the house. In the first case above, the AC intake is sucking from the return duct but also from the outside, because the return duct is at a lower pressure than the outside.
In the second case, a hole in a supply duct means the conditioned air is shooting out of a high(er) pressure duct to the outside. You might not think this is such a big deal, but that volume of air being lost means the AC intake is now lowering the pressure of the entire house trying to find makeup air to keep the total air volume the same. This means air will be sucked through any crack in your house to try to equalize the pressure - door seals, dryer vents, window sills, bathroom fans, etc.
Sometimes a window leaks bad and you can see the curtains rustling, especially when the wind blows. If the leaks are bad enough, the air will permeate through the house regardless of what the AC is doing.
Since you're saying the smells only happen while the AC is on, I would guess it's a leaking duct, again in an unconditioned space. It's not immediately clear to me how your return is setup, but I would guess there's a large intake grill on the other side of one of those walls that just dumps into the air handler intake.
That would narrow it down to an air leak in a supply duct in an unconditioned space. I would check for it by starting in the attic or crawl space and trying to listen for it. You might also be able to feel a temperature difference in the insulation.
Finally, the spots for leaks would probably be at the joints where sections meet, like elbows. If I had to bet, I guess something like a sagging duct under a crawlspace ripped open an elbow flange and the air is shooting out.
Make up air ducts are normally on the intake side of your furnace. I understand your concern about the smoke you may want to block the intake partially but not having any fresh air can also be not so healthy. The houses I built in that area the main trunk was in the attic for all of them and I usually put a small damper in close to the intake so it could be adjusted , if the furnace is in the garage it may be close to the furnace. I know some guys that just punch a hole in that case a piece of duct tape could be used to block it.
I would recommend leaving a small amount of opening, possibly filtering the intake. these intakes are usually under 4” about the size of a dryer vent (if your furnace is in a closet and there is a flex duct on it that may be the intake). These are the areas I have put them and seen them.
