Is central Air conditioning more efficient then individual room air units. If I’m only using 1/3 of the house 99% of the time, why keep it cool? I can’t close the vents because the thermostat is in the largest room which I never use. And sometimes to cool the rooms I want to 74 degrees, I have to set the thermostat to 70. Seems a total waste of money. Plus if the central air breaks, the whole house becomes a furnace (I live in Florida). With individual units, if one breaks its cheaper to replace and while waiting you can at least hide in some other room.
Central a/c units today are getting SEER ratings over 20, while the best window unit is only about 12, so the central a/c can theoretically be twice as energy efficient. With an older central a/c and a brand new window unit the difference won’t be as big, but it’s still non-trivial. Portable units are generally a bust from an efficiency standpoint.
If you’re talking about only cooling 1/3 of the house, then I think you’re in the realm of it being a worthwhile discussion, but it’s not as simple as it may seem. For one, it’s notoriously difficult to seal air leaks around a window unit. Even if you use weather stripping, foam pads, and tape on all the cracks and joints around the terrible window kits, there’s still gaps inside the unit around the case and where the motor spindle comes inside, and also where the wires and refrigerant lines go between the inside and outside. The styrofoam and plastic separating the cold side of the unit from the hot side is also pretty thin, so even if you take the unit apart to stuff more foam inside it and do the best job you can, it’s still a pretty significant break in the room’s thermal envelope compared to any half-decent window. Plus don’t forget to put some foam in between the top and bottom sash of the window (or the left and right sash if it’s a slider), that can be a bigger gap than everything else combined.
On top of all this, remember that the walls between rooms aren’t insulated either. So if you want to condition just one room and let the rest of the house float temperature-wise, your one room is going to act as if it’s a little freestanding building, though well shaded on the sides against the other rooms. So your window a/c is likely going to need to work harder than you might expect, and you may lose some more efficiency over the central a/c which builds up a core of tempered air in the middle of the house that sort of helps mitigate the heat gain. If you just set back the thermostat in the rest of the house, like to 80 degrees or something, then using window units to supplement one or two rooms might work better from a comfort standpoint, but also remember that window a/c units can be pretty darn loud too, so factor that into the comfort equation too.
In addition to all of that, central AC systems can be zoned fairly easily, so you don’t have to cool rooms that you use only occasionally, like a guest room, when nobody’s there.
yes, most modern systems allow you to adjust the temperature per room or area throughout the building.
^ If they have variable speed blowers and compressors, which isn’t all that common yet. Simply closing vents or adding dampers to a system that’s not meant to be zoned will lead to the vents that are closed whistling like crazy, the vents that are open blowing a hurricane, poor dehumidification, and perhaps freezing up of the evaporator coil.
Individual room units doesn’t necessarily mean window units. Mini split systems are one solution, and they work very well for zoning. Worth consideration if you are installing AC.
Another vote for mini split systems - a small compressor outside with conventional line set tubing going to a small evap coil and blower, usually installed above door or window.
There are units with 2 or more heads, allowing you to a/c 2 or more rooms/areas.
What I would suggest is a serious look at the “portable” a/c’s now found in big box stores. I hope there are units built to better specs than most stuff found in big box.
These have a single unit with an exhaust hose (think clothes dryer vent) typically attached to a panel which sits in a window.
The (tiny) wheels on these allow them to be moved from room to room.
I’d give on a try - if it works for you, get another so you can cool the room into which you plan to move next.
My recollection of FL says: DEHUMIDIFY. I’d be inclined to get unit(s) with pails to empty, and not mess with units which try to avoid producing water - while they are cooling very humid air.
The water needs to come out somewhere - I’d prefer a machine which does not try to hide that bit of reality.
I’m a big fan of mini-split systems.
I am not a big fan of portable ACs with exhaust hoses. They are terribly inefficient, you have to manually empty the condensate bucket, and they just don’t work very well.
Splits are what I normally see in homes outside the US, and in hotels anywhere. The only maintenance my mother’s systems have needed is remembering to clean the filters, which is recommended once a year (remove lid, remove filter, wash filter in washer without any soap, let it air-dry indoors, replace it, replace lid). She’s got one in each living room and just closing the doors of the rooms you don’t want to cool makes an evident difference.
If the OP is looking to save some money, then mini-splits aren’t the way to go unless we’re talking about replacing the entire HVAC system. While a window a/c can be had for a few hundred dollars, mini-splits are a few thousand. Portables are also a train wreck as far as efficiency goes, especially single-hose units because they pull the conditioned air out of the room to blow over the condenser coil and outside. Thus they’re causing hot air to be sucked in under the door, through cracks in the windows, etc. to make up for the negative pressure. If you get a portable unit you really must get a dual-hose one, which brings outdoor air in through one and exhausts through the other. Portables are also about double the cost of a window unit of the same capacity.
The real issue is also - how good is your insulation? I live in Canada, where winters are noticeable and poor insulation can cost serious money in heating costs. We have triple-pane sealed PVC windows, weather-sealed doors, a foot of insulation in the attic, and (a newer house) 2x6 exterior walls with fibreglass insulation. I’m guessing that south of the more northern states, the USA does not take R20 house insulating as seriously, especially older homes.
That’s the key. Cooling the whole house is not so bad if the cool stays in. If you are basically cooling your yard too, why bother? Especially, if you live further south where cooling costs dominate over heating cost…
Things to check - how much insulation in the attic? Is it intact, or disturbed and bunched up by humans or animal visitors? Is the attic properly ventilated? Vents in the eaves, not blocked, and vents up top - so on those bake-in-the-sun days, the hot air in the attic rises and escapes and draws cooler air in. No point trying to cool a house or a room if the ceiling is basically the outside of an oven. Do you have newer, sealed windows and doors? Etc.
I agree - if you have a modern, efficient H-Vac system, the next step is insulation.Cheapest is the roof space where there should be at least a foot of insulation; walls will depend on how they are constructed, but if there is a cavity, that should be filled suitably. Don’t forget floors too. If you have a space underneath, it must be ventilated. Windows are a big heat transfer area, but are the most expensive to replace, with a long payback time.
I had a house with lots of window units, and would mostly run the one in the living room, sometimes the dining room, and the master bedroom. When I finally had central A/C installed, my electricity bill in the summer dropped by probably 30% or more, plus the whole house was much more comfortable (such as no longer walking through sweltering hallways to get to a cooler room).
YMMV, of course, but I was really surprised how more comfort came with smaller electricity bills.
What’s interesting is that, at least in the most current energy codes, the far south of the US (Florida, Gulf coast, etc.) actually bumps the insulation up similar to what’s common in the more northern latitudes near the Canadian border due to the high cooling loads. The most permissive (relatively speaking) insulating standards are more around the middle latitudes of the country, like along the Ohio River where while it may get plenty hot in summer and quite cold in winter, you don’t get the relentless heat and humidity of the south for 6+ months of the year, nor do you get the biting below-zero cold for days or weeks on end like you can farther north.
Of course as you said, with older homes all bets are off. More insulation is never a bad thing, but you can’t really do much with just a couple hundred dollars other than some do-it-yourself weather stripping and foaming of building penetrations, which isn’t a bad idea anyway.
Check out Mitsubishi mini splits which can have one outside unit handling 4 or more individual room units inside.
In addition to being able to just tun on the units for the rooms you want cooled/heated, these units (inside and out) make their own highly efficient electricity to run the motors.
They can run varying speeds from slow to fast - giving you just the amount of cooling needed. And when running, run VERY efficiently due to their design (inverter technology).
I’d suggest looking into moving the thermostat as part of the central air project.