Circulate basement air upstairs
 

I've got a 2-level house that has a single air handler in the basement level, as well as an air return on each level. The temperature in the basement level is 70 degrees while upstairs it's 80. Would it be a bad idea to block the upstairs return, forcing hot air down the stairwell into the basement return to get the air to mix? I'd also want to close the downstairs registers.

I’ve done a lot of messing around with similar circumstances. My current goto is a quiet blower fan / squirrel cage type. Set on top of a floor register in our bedroom in the upper level. Took a cardboard box, opened the bottom, ducted the output through the top of the box. Leave it running on low all the time once the room starts getting too hot. When the ac runs a lot, seems to draw extra to the room, and it will be the coolest room in the house. When the ac doesn’t run a lot, it circulates enough that it’s the same temp as down stairs. Trying to keep 2 sets of blackout curtains closed as much as we can too. Other setups I’ve tried don’t ever pan out for the milder days

Imagine how effective this monstrosity has to be, to be wife approved
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I closed all but 1 register downstairs and ended up turning the AC on to hopefully get the girl to sleep. I'll try blocking the upstairs air return tomorrow to see if I can get the cool downstairs air to mix upstairs. Maintaining temperature parity will be difficult in this house... I can already hear freebeard typing a link to a dome home :P

I'm just not sure if blocking a return will put too much strain on the air handler or if closing half my registers will put too much pressure on the ducting.

I had a window AC unit when we lived in a different house. The accordion rubber seal would let in a lot of light, so I cut a Rainier beer box and inserted it to block the light. Of course, I kept the printed side toward the inside instead of boring cardboard.

I've easily got to be the most hillbilly in my neighborhood.

Yeah did it on my friends house.
It eliminated them running 2 dehumidifiers in the summer and lowered their power bill by at least a third in the summer, this is in mountainous Virginia where it gets hot and humid.
There was a 12x24 duct that connected the up stairs air intake to the air handler in the basement.
All we did was disconnect a sheet metal elbow from the circuit to test it out.
Later my friend removed another elbow further away from the air handler to really open it up.
The air filter is located at the air handler.

Wouldn't it work better to pull the hot air from the upper levels and expel cool air in the lower level to force air flow from bottom up?

I have a somewhat similar problem in my single story house, all the returns and vents are in the floor. It's great in the winter, but in the summer the floor will get 50° while the thermostat still struggles to get the rest of the house down to a comfortable temp.

I just want the air to mix between the levels, which should be comfortable.

I ran the AC for the first time last night for 2hrs (we just moved in over the weekend).

At the moment the lower level registers are closed, but I didn't block any returns. That means all air is going out upstairs, which should force airflow downstairs.

If both are finished out with ducting and all you may need to really open up that air handler to draw mostly air from the lower level. Do something like pull off the return ducting or cut a hole in the return duct next to the air handler.

The downstairs return actually isn't sealed in correctly and pulls air from the wall, mostly bypassing the filter. That's on my list of things to fix.

I wonder how difficult it will be to heat the downstairs in the winter.

My father did this at the house I grew up in, the basement was usually 60F while the upstairs could be 80-90F.

All he did was shut off the pilot on the furnace and bypass the thermostat turning the blower on during the day,
all of the air into the furnace blower came from the floor of the basement and the returns just dumped into random spots down below (aka most intake air was from the basement not circulated directly from upstairs)

Worked quite well and in even hotter weather there was a window directly in front of the furnace, my father would put a medium sized window air conditioner there and tarp it’s output over to the blower intake and it actually kept the house just as cool as central air.