Greatest Efficiency - Cooling Problem

Say you live in an apartment w/ two main rooms - one’s the bedroom w/ a regular 8’ ceiling and the other an open plan kitchen/dining area 1.5 times the size of the bedroom w/ an 18’ ceiling (no ceiling fans in either room). There’s an egress-sized window in each room and a central air unit of the sort one sees in a hotel room is installed next to the window in each room. There are double doors btwn the two rooms.

To cool BOTH rooms most efficiently, should one run only the cooler in the bedroom w/ the doors open; only the cooler in the larger, high-ceilinged room w/ the doors open; or the cooler for each room w/ the doors closed? Or is there a 4th, most efficient option I haven’t mentioned?
Signed, one of the hottest Utah summers I’ve ever lived through.

Are both units the same BTU rating? That makes a difference to the answer. So does time of day.

If the same, you’ll want to run both units with a strong portable fan blowing air from the bedroom into the main room.

If they’re different ratings, then there’s math I’m not qualified to do.

If you’re mostly concerned with keeping the bedroom cool, set the living area for a higher temp (78? 80? even 82?) and set the bedroom for a temp slightly above where you want it when you sleep, and close the door. Dial the bedroom AC down to preferred temp when you come home.

I don’t know how hot it is at night, but you can also cool by turning off the AC in the main room at night and using fans to create a cross-breeze. Much cheaper than AC.
Works best if you have windows at opposite ends of the main space, less well if the windows are at a right angle, not at all if there are windows only in one wall. This is not a great solution if you wake up late and the fans are moving hotter air through the place.

Hope this is useful.

Can you define what you mean by “efficiently” in this scenario?

Most efficient would be to keep the doors closed, run the bedroom, keep the kitchen turned off, and just retreat to the bedroom whenever you need a break from the heat.

Next after that would be to keep the doors closed, run the bedroom but not kitchen overnight when you’re mostly in the bedroom, and the kitchen but not the bedroom during the day when you’re mostly in the kitchen.

The basic idea is to only cool the spaces you’re actually using.

Most efficient would be to disconnect the apartment’s power and sit in the dark and sweat.

The OP asked how to cool BOTH rooms most efficiently… To not fight the hypothetical, I’ll assume that means the least power consumption overall.

Is it not true that the most efficient AC unit is one which runs continuously at fixed output to maintain the desired temperature, rather than an overpowered unit that pulses on and off? So theroretically, if a single wall unit in either room could keep both rooms adequately cooled, perhaps with the use of a mixing fan in a doorway, but it had to run 100% of the time to do so, that would be more efficient than both units running 50% of the time.

So first, see if one unit can cool both rooms to your satisfaction; ideally without a mixing fan since that will add heat, but with the fan if the temperature gradient is unacceptable. If that doesn’t work, then it sort of depends on how controllable the units are: single stage compressor? Dual stage? Variable speed? If variable, and still assuming that one unit alone is underpowered, it would be most efficient to run them both continuously at the lowest power level possible. For single stage compressors, it’s a balancing act: if one unit alone is not *quite *enough, then both together are too much. You could turn the thermostat on one unit way down so it ran continuously, and set the other one at your goal temperature and let it pulse five or ten or twenty or a hundred times a day (more is worse for efficiency, but the precise number is out of your control at this point). That would be more efficient than both units being set to the same temperature and both turning on and off more often. You could even move from room to room and tweak the thermostats manually to alternate the ‘colder’ unit to even out the wear and tear; you get the same number of pulses per day but divided evenly between units, at the expense of you monitoring things a lot more closely.

And I have a related question, one I’ve been meaning to ask here for a while.

My AC setup is similar to that of the OP, except all ceilings are 8 foot. And the main room uses a larger 208v in wall AC unit while the bedroom has a smaller 120v unit.

But my question has to do with the “energy efficient” mode and the fact that I’m suspicious of it as being more efficient. When I am running the AC in the regular mode the fan runs continuously and the compressor cycles on and off as needed. In efficient mode the fan cycles as well. I’m not sure what the other differences are.

But here is where I think the scam lies. If I set the AC in energy-efficient mode at 72, the room maintains a temperature of 72 degrees. But if I set it to 72 in normal mode, the room gets MUCH colder.
Now I like to sleep in a really cold room. So right before I go to sleep I change the setting from efficient to normal. That way the fan cycling doesn’t disturb my sleep — and the temperature drops about 5 degrees. Then when I wake up in my icy room I change the setting back to efficient to warm the room up.

So it seems to me that it’s dishonest to compare the energy use of 72 degree energy efficient setting to a 72 degree normal setting, because the difference in the ambient temperature of the room is so drastic. It seems it would be more honest to compare the energy efficient 72 degree to a normal 76 or 77 degree setting, because of the comparable room temperature. But I think if I did that the energy efficient setting might not be efficient at all.

What would actually be honest would be for a setting of 72º to mean 72º.

But are you basing that statement on thermometer readings, or just how cool it feels? Because other things affect human perception of temperature: A fan and a dehumidifier will both make you feel cooler, without changing the temperature. And a well-designed air conditioner would use whatever combination of temperature, humidity, and air movement gave a given perception of coolness, for the least energy consumption.

I took the liberty to check to forecast at SLC … the highs are impressive but them overnight lows are MISERABLE … I see your problem …

My sense is that running one unit makes it work harder, and this generally makes it less efficiency … if I’m reading your OP correctly, you have 18 foot ceilings … up there is where all the hottest air will be, maybe a lil’ mixing fan is counter-productive to keeping things cool down in the living area …

If it’s just money you’re trying to save … then all this can be easily tested … find your electric meter and write down the number, run one unit a couple days and write that number down again, run both units a couple of days and write the number down again … whichever number is smaller is the cheapest configuration … as long as the temperatures are roughly the same and you are consistent with all your other electricity usage … this should be a great way to limit your costs … so you’ll have to set aside your arc-welding hobby these few days and turn off the cyclotrons …

I’m sorry, you’re right - that was too vague. In this situation, efficient to me would mean the most cooling for the least energy output.
Both cooling units are the same BTUs but do not have a numeric temperature selector, only Cooler or Warmer and a red to blue range button like those you find in a car’s AC.

Also: What is affecting your wall & ceiling temps?

In a stand alone house with a lot of shade an over size unit set to one temp and left there will cool the walls and ceiling & contents also and save the extra cost of cooling everything in the space everyday when you come in from work or where ever. Same principal as your refrigerator. Don’t keep starting over.

Major sun and no shade or mountain to block the sun + poor insulation = $$$$$ ( just move )

Foil the windows if they get a lot of direct sun or good thermal shade that hug the wall trapping the heat right there and keeping it from the living space.

Then do like advised up thread, do some meter checks to see power consumption.


You don’t want your refrigerator to “keep starting over”, because food will spoil while it’s warm. But for your home, if there are times when it doesn’t matter what the temperature is (because you’re at work, say, or you’re spending 8 hours straight in just one room), it’s absolutely more efficient to turn off the AC (or heat, if appropriate) for those times. If you want it to be a nice temperature as soon as you come home, then install a programmable thermostat that will kick in a half-hour or so before you get home.

Thank you for all the responses, folks! I sent the thread to my apartment-dwelling friend and he’s getting good ideas and confirmation of those he already thought. First step will be blocking up the window that’s 3/4 up the high-ceilinged room’s west-facing wall, b/c it’s a beam of radiation coming through every afternoon.
He can’t make any electric or permanent alterations to the place, unfortunately.