Hi, I am renting a 25 yr old townhome that has a small HVAC unit-serves all levels. Top floor is very warm and basement, where unit is, is coolest. Thermostat is on main floor. Question: I have a renter in the basement level who wants to keep the sliding door open for "fresh air"(95 degrees and orange level air quality danger!)and thinks it doesn't affect the a/c efficieny, cost or cooling of the place. I have tried covering the intake vent down there and closing his a/c air vents, but still feel it's a huge drain on the a/c and increases my bill and is wasteful. Any proof I can offer him form someone who knows?
Thanks!

Fact is, heat rises. So unless the floors, doors, stairways, etc are extremely well sealed and insulated, the heat from the downstairs will come up (while all your cool air is falling down and out his door) and cuase your a/c unit to run much much more than it would if the basement were cool. Good luck, I hope you can talk the tenant into keeping windows and door closed during high temps, otherwise, you will see increased bill and wear and tear on your a/c unit.

Tell your tenant to close all incoming airducts and then tape plastic sheeting over any return in takes, if there are any. Close half of the supply reisters on the main floor and be sure all registers are up up stairs. Remember to reverse all this in the fall. Old home were just no designed well for AC on 3 levels.

Bob, wouldn't you agree that even if all supplys and returns were closed on the lowest floor, unless the ceilling between the low and main floor were well sealed and insulated, the heat would rise and still cause an issue?

Yes, I'd agree to a certain extent. I advised closing the basement returns so as to not suck in the hot outside air he was letting in with the sliding door open. If there is a door between the basement and the first fl;oor the OP should feel more comfortable. Me personally, I find a new tenant.

Thanks so much!!