I have a 2-story condo, 1240 sf, almost 30 yrs old, with a newer heat pump that was serviced last year. The blower is downstairs and has a return intake of 18x18. Upstairs, directly above the unit there is another 12x24 intake that has no duct, just the space between the studs. I keep the filters clean. When I open the door to the little blower space, there is quite a bit of vacuum, which leads me to believe I need to add more return. Someone told me once that most all townhouse condos have inadequate return.
My question is how much should I add (I'm thinking pick a common size filter-maybe 16x20) and better to put it upstairs or does it matter? The same ducts service both up and downstairs and we have already shut those vents off at the registers.

That vacuum in the blower box is supposed to be that way. When the door is closed that vac is what pulls the air through the returns. If your up stairs is too warm try partially blocking the lower return. It probably leads directly to the plenum and the upstairs return is common to use the stud space. Condos do frequently have poor circulation due to architects who try to get more space and less mechanical room.

I have a 2-story condo, 1240 sf, almost 30 yrs old, with a newer heat pump that was serviced last year. The blower is downstairs and has a return intake of 18x18. Upstairs, directly above the unit there is another 12x24 intake that has no duct, just the space between the studs. I keep the filters clean. When I open the door to the little blower space, there is quite a bit of vacuum, which leads me to believe I need to add more return. Someone told me once that most all townhouse condos have inadequate return.
My question is how much should I add (I'm thinking pick a common size filter-maybe 16x20) and better to put it upstairs or does it matter? The same ducts service both up and downstairs and we have already shut those vents off at the registers.
Thanks. I understand, but when I open that door even a little, I get more airflow and it's much better/cooler upstairs. So it seems apparent to me that I should increase the filtered return airflow, and by your response, it should be upstairs rather than down. I had thought about putting one in the door to the blower compartment.

You could get a HVAC guy to measure the pressure in your supply and return and he could balance it for you. If expansion of any return is warranted it should be upstairs to pull more hot air out. If you cut open that door more air will be pulled from your lower level and even less from upstairs. Since hot air rises your problem will get worse.

Well, I decided to add a return in the door to the downstairs 'closet' where my blower sits. I bought a 16x20 grill and installed it in July with no difficulty. The difference has been just short of amazing. We quickly began experiencing much cooler temps upstairs and all I did was give my blower more air to work with. I am one happy camper.