Sizing a supplemental window unit

[Commendatore]

Here's my situation. My house is a typical suburban box arrangement with a finished third floor attic. A 2 ton AC unit serves the first story, and a 3.5 ton (single zone) unit serves the second-story (including a bonus room above the garage) and 3rd story.

I'm now working from home full-time and moved my office up to the 3rd floor. its still April, but its already hot up here. With the thermostat on the 2nd floor, the 3rd floor hardly sees any cooling in moderate weather.

The 3rd floor is 300 square feet (clipped attic ceiling) plus about 60 square feet of stairwell. There is one window facing northeast.

My first thought was to size down, since this window unit is going to supplement the central air, BUT there is more to the story.

In a moderate summer, the central air is barely adequate on the second-story. During a scorcher, it can't keep up. In a perfect world it would be time for a new dual zone system, but that is not in the budget. I'm hoping that by reducing the heat load with a 3rd floor window unit the second floor will also be a lot more comfortable.

I see lots of affordable window units out there that are 6,000 or 8,000 BTUs. Quick rule of thumb numbers say I need 7200 BTUs, so should I bump that up or down?

 

[Commendatore]

Crickets, huh?

Here's a sample of the options I'm considering:

6000 BTU, claimed 41-47 dB. I guess it it is undersized and runs non-stop at least it will be quieter.
https://www.lowes.com/pd/Frigidaire...oner-115-Volt-6000-BTU-ENERGY-STAR/1000195131


8000 BTU, claimed 55-59 dB. Larger, louder, etc. At least I won't ever say if only I had gone up a size.
https://www.lowes.com/pd/Frigidaire...ioner-115-Volt-8000-BTU-ENERGY-STAR/999990680

Cost is similar at $269 vs $299. While there are cheaper 6000 BTU units out there, less noise is pretty appealing.

 

[JerryC]

I'm not an AC expert, just a guy with a window unit in his home office.

I have a 5000 btu unit mounted in a ~170 sqft room on the second floor with southern exposure. With the door closed it can keep the room really cool without a problem. Open the door and as a supplement to the central unit it is "ok". I also have the problem of cold air going down the stairs so a lot of the cooling will just go out the door if left open and down the short hall and then down the stairs. I close the central vent in my office and use just the window unit to cool the office and that gives more air to the rest of the second floor.

In your case you'll have to guess how much of the cold air from the window unit will go down the stairs and how much you want to "help" the central unit. Do you want/need the upstairs vents open?

Anyway, my guess would be to go with the bigger unit.

 

[Falcon67]

My experience with the bitty window units is that the 8000 BTU LG is the better of the pack. The 5000s are hit or miss, have tossed more than a couple. I have an 8K LG in the shop cooling the 288 sq/ft work room and it does well, throws air a good 20'. It's not loud and it's going on it's 3rd year in service.

The Frigidaire units are the ones I have had the worst luck with, just FYI.