I have a Rheem Classic 90 Plus that's 9 years old. When I run the AC and my neighbor is burning leaves the smoke is pulled into my house via the PVC pipes that run outside my house. I noticed that if I cover one of the two PVC pipes that HVAC system uses I don't get the smell in the house but I also don't get any air pressure from the AC. Is this normal for HVAC systems to draw in outside air or is my Rheem crappy or malfunctioning? I've had a local company (Bel-o) service it twice a year. Thanks in advance for your help!