How many stories do you have total? How many ac units do you have? If you have one unit yes all the cold air is just rolling down the stairs to that level. You would need to close some of the supply ducts down there and close the door. You would also need to make sure you have return air ducts on each level.

We have return air ducts on each level, and a newer central AC. Main floor and basement.

But only one AC unit not zoned correct?

They are not on separate thermostats, if that is what you mean. We have returns and vents on both floors. Thanks!

Yes it would help but we have a ceiling fan over the top of the stairs pulling the cold air up and a standing fan at the bottom blowing the cold air up. We have an open staircase so can't block it off. I am surprised leaving the furnace fan on all day isn't helping. Ours always brings the house down at least two degrees when we run it.

I have the same issue and also we don't have a door down to the basement. I would think having a door would keep the cold air down there. I think as suggested, try turning some of the vents to cool the rooms off, but keep the "return" ones on. Do you have a dehumidifier running?

Cold air drops and warm air rises. You need to keep the air circulating.

We do use the house fan, and it helps, just not enough - AC vents and returns on both floors, so the air circulates. Since a door is possible, it seems smart. Hubby and I disagree. I want a door (precisely cuz of the physics of cold air), he thinks the air won't mix if we have one. Two pros have said to add a door. We live in an arid climate, so don't need a dehumidifier. We do have some of the vents in the basement blocked. We also have a black roof (to be replaced as soon as we have a hail storm) which just makes the whole main floor hot. Thanks for the responses!

Do you have access to the attic- if so I would spray foam long before I replaced the roof. As long as you have returns on both floors you can add the door, though at some point you should look at having the system zoned so only one floor comes on at a time (sometimes it’s tricky to retrofit into an existing house). My last house had one AC unit for three stories connected by a staircase and I could control each floor independently even though there was only one hvac unit.

My finished basement has one supply vent which I keep closed and a door at the top of the stairs leading to the first floor. The basement is usually 10 degrees colder than the first floor. Your house may be different, but I am skeptical adding a door will have much of an affect on the basement temperature.

Would it be possible to hang a blanket in the doorway to see if it has any affect?

Mike, It's not the basement temp she is trying to change as such, it's the upstairs.

Correct, Angel. Mike, I do have a curtain up, and it helps, but I've had a couple of non-professionals say that a door wouldn't help or would interfere with airflow. Just wanted to confirm before I spend on the door. I've now had three professionals say yes, install the door, so I will!

By house fan do you mean an Attic Fan?

If so, that depressurizes the house, and makes the HVAC work harder. Its good to have them & ceiling fans if the HVAC dies, but if you run it while the HVAC is running your venting the conditioned air into the attic which then depending on the gables and vents in the attic sends the air outside.

You can most likely achieve what your looking for temporarily if you have the right amount of returns and supplies until you get your system zoned or replace it with ducted or ductless mini splits

Try closing the supply vents in the basement floor 3/4ths of the way and leave the ones upstairs open. Its a stopgap if it works and will allow the main floor to cool down quicker while maintaining airflow to the basement. This adjusts the static pressure in the conditioned space envelope. Zoning solves this mostly.

in the winter you may need to re-adjust it.

What temp is the basement?

We do have some of the vents in the basement blocked.

How many vents are there in the basement compared to the rest of the house? The basement needs no cooling so all vents can be closed. The only reason to not close all the vents is it could cause too much static air pressure in the rest of the duct work. With the vents closed the return in the basement may pull some of the warm air from the first floor. You should experiment and see if there is an change in air flow.

We do use the house fan

Is this a whole house fan? If so it should not be operated when the AC is on and the windows are closed. You can use it at night when the temperature drops and the windows are opened. This would work well in your dry climate.

Not an attic fan. The fan that runs when the furnace or AC is running, it can be set to auto or on, is currently on so it pulls up basement air even when AC isn't active. Hubby says all the vents in the basement are closed (there are several vents on each floor). It is still about 20 degrees cooler down there. The stairwell is in center of house and the main floor AC'd air just sinks. Zoning is next. Already foamed the attic. Windows are very poorly oriented so there is no cross flow in the house for overnight cooling. Neither the north or south side have any windows at all, and no space for window fans because screen is on inside of louvred windows. Adding windows is on the to-do list. Black shingles will be switched out for more reflective ones.

As I mentioned, I have had three professionals (who could try to sell me a new AC instead) say that I should add a door. So that is the immediate step with the others to follow.

Thanks everyone!

Instead of the door get some magnetic covers and cover the returns about 3/4 to 7/8th the way close the supplies 7/8th the way in the basement. You can use some packing or duct tape at first and put those on the returns to test.

That will give you some ventilation in the basement and circulate the air but not too much and should allow the main to cool off quicker.

Right now the basement returns are pulling the air from the main floor via the stairs.

Adding a door would solve that, but give you a static pressure difference between the 2 floors because the returns would still be pulling air.

There is also a return at the top of the stairs on the main floor, so don't think the pressure difference should be an issue. Thanks for all your input!

Had an HVAC pro come look and they say the best option short of zoning is to install a door. Zoning will come soon, but costs a lot more and they can't get to it immediately due to the heat wave. Prioritizing those without working AC above us, rightly. The door will save electicity and keep us cooler in the meantime.