Supply and return both on ceiling
This may be a case in which I'm looking for trouble where there isn't any, but I'd feel irresponsible if I didn't ask.
My mother-in-law is thinking about buying a condo. When I asked her a couple basic questions about the HVAC setup, she told me that all the vents--both supply and return--were on the ceiling. Apparently, people who live there think it's fine, and perhaps that's ultimately what I should go by. But to this layperson, it just seems inefficient for heating.
I probably can't provide great detail about the place. I know it's a single story, first-floor (with a "ground floor" beneath it) apartment. She likes "bright and airy", and it has a balcony, so I'm guessing there's a fair amount of glass. This is in the D.C. metro area.
Thanks in advance. It's likely not a big issue, but I'd prefer that she be as informed as possible before spending her money.
Fine for cooling but not the greatest for heating. Things can get a bit cold down at the lower levels of the space. Pretty common though even in cold areas
It will depend a lot on air mixing and if the supply registers have good throw. The common round supply registers often used with flex duct tech to have more lateral throw that a good quality rectangular supply.
It will also depend on wherether the unit is properly sized or oversized as well.
Overall, I think it can get a little overblown. Heat will rise regardless. Only a high velocity system or one with a limited number of ducts with commercail grills and long throws and good face velocity will really mix the air. I have pretty high velocity in my system and upstairs or downstairs, floor or ceiling supplies and floor or ceiling returns, it's always warmer at the ceiling and cooler near the floor.
Depends on the airtightness of the condo and how good the windows are. What's that got do do with supply and return placement? Comfort within a room is more than about air exchange, as important as air exchange is. Air exchange that is controlled is good. Air exchange that occurs because the enclosure leaks air in from and out to the exterior at an excessive rate is not.
Duct leakage is also a concern. It can create a host of comfort problems that should not be there if more care was put into installing and sealing the ducts.
Additionally, if the windows are metal frame single pane, the air can be adequately warm (for winter heating) within the room but occupants will feel uncomfortable due to the radiant heat loss from the body to the cold window.
Bottom line: if building (and duct) air leakage and heat exchange through the enclosure are well controlled, the supply and return location both being on the ceiling is less problematic. Now, if the supply outlet is blowing directly at the return inlet, that's a problem no matter how good the house is sealed or insulated!
Building Physics Rule #1: Hot flows to cold.
Building Physics Rule #2: Higher air pressure moves toward lower air pressure
Building Physics Rule #3: Higher moisture concentration moves toward lower moisture concentration.