6000 or 8000 BTUs for a top floor apartment in Boston?

Go with the 8000 BTU units. Being on the top floor in an old Boston building means the roof is basically a giant radiator sitting right above your head. Energy Star guidelines are notoriously conservative and assume decent insulation, wich most 3rd-floor walkups in New England definitely don't have. If you go with 6000, those compressors will run 24/7 during a July heatwave and you'll still be sweating while your electric bill skyrockets.

The humidity concern is valid, but it's usally overblown unless you're putting a 12,000 BTU beast in a tiny closet. At 8000 BTUs for 170 square feet with high heat gain from the roof, the unit will stay on long enought to pull moisture out of the air before it hits the target temp. If you're really worried about that clammy feeling, look for units that have a dedicated **dry mode** or a dehumidify setting. This keeps the fan on a lower speed so the coils stay colder longer, pulling out more water without dropping the temperture too fast.

Also, keep in mind that 8k units are often built on the same chassis as 6ks, so they aren't necessarily much louder. The real noise jump usually happens when you move to 10k or 12k models. Since you're doing four of them, check your electrical situation first. Four 8k units pulling power at the same time might trip an older 15-amp circuit if they're all on the same line, which is common in those older triple-deckers. You might need to be careful about what else is plugged into those circuits.

For a top floor, **oversizing** slightly is almost always the better move. You can always turn an 8000 BTU unit down or set the thermostat higher, but you can't make a 6000 BTU unit work harder than its max capacity. In a Boston swamp summer, you're going to want that extra headroom.

The circuit thing has me a bit nervous because my place definitly has that old-school wiring. If I've got two 8000 BTU units in the bedrooms and maybe a TV going, am I gonna be constantly resetting breakers? I’m also seeing alot of talk about those 'Inverter' ACs lately. They’re supposed to be way more efficient and better at not cycling on and off so much, right? Since I’m worried about the humidity and the noise, woud spending the extra cash on an inverter model actually make a difference for a top-floor setup like mine, or is that just overkill for a small room?

Inverters are the way to go for those old Boston circuits. A standard 8k unit draws a huge surge of power every time the compressor kicks in, which is ussually what trips the breaker. Inverters have a 'soft start'—they ramp up gradually, so you won't see your lights flicker. Since they modulate their speed, an 8k inverter can basically act like a 4k unit once the room gets to temp. It stays running at a low hum instead of cycling on and off, which handles the humidity way better than a standard oversized unit. It's the best fix for your specific setup.

If you end up going with the 8000s, look into those "U-shaped" window units. They let you actually close the window almost all the way down into the unit. Since you’re on the 3rd floor, the street noise in Boston can be brutal, and those things block way more sound than a standard box. Plus, it makes the whole setup feel more secure so you don't worry about it falling out during a storm. Just make sure your window frames are sturdy enough beacuse they're a bit heavier than the cheap 6k models. Getting four of them installed is a workout, so maybe grab a friend to help.

One thing people overlook is where the thermostat acutally sits. Most of those 8k units have the thermistor right at the intake, wich means it cycles off based on the air right next to the window. In an attic-level room with high ceilings, you’ll end up with a 'cold pocket' while the rest of the room stays swampy. Look for a unit with a 'Follow Me' feature or a remote-based thermostat. You keep the remote on your nightstand, and the AC runs untill *that* spot hits the target temp. It stops the short-cycling issues people blame on oversizing and actually handles the Boston humidity way better.