Here in Okinawa, each room has its own wall air conditioner, and each wall unit has its own thermostat. I have never seen central heating or air here. At this time of year, it is hot enough outside that even if you put one AC on in a small closed room at 27 C, it runs constantly to keep the room at that temp. Since the unit is running constantly anyway, does it use more (or much more) energy if I turn the unit’s thermostat down to say, 22 C? Also, is it more efficient in our open floor plan apartment to have one AC on at a low temp, or two ACs on at higher temps?
IME, the only thing a thermostat does on any A/C is keep it running until the temperature drops to whatever you have it set at. If it’s running constantly at 27 C (81 F for my own sanity…that’s pretty warm), you could set it at 0 C and it wouldn’t make any difference. Except that when it gets dark or cloudy and the temperature in the room drops, it won’t shut off.
You probably have an undersized unit for your room, or something’s wrong with the A/C.
ETA: What’s the temperature outside in Okinawa? Here, we have been hitting 95 F (35 C) every day and I have one large added on room with a wall unit that will make the room uncomfortably cold after running for about 30 minutes. I have a hard time keeping the rest of the house under 80 F during the daytime with the Central Unit running.
I think these must be different from American units–they also heat in the winter and have a dehumidify setting. Lowering the temp definitely makes a difference in the air coming out of the units, much like a car thermostat. We have 5 of these in our 1000 sf apt, and even with turning them all off when we’re gone and only cooling the bedroom at night the electric bills in the summer are about $300. It isn’t insanely hot here–about 90 F, but it’s extremely humid.
If the thermostat makes everything turn on and off, and everything is running constantly at some setting, moving to a colder setting won’t use any more electricity.
But, you’d have to be sure there weren’t parts of the system going on and off, to be certain you weren’t using more electricity. Some HVAC equipment can run in several modes, such as only running the fan so it brings in fresh air from elsewhere.
Are these regular stand-alone window or wall-mount units, or are they part of a larger built-in system such as Mitsubishi’s “Mr. Slim” or “City Multi?”
These newer Mistubishi units run at different power levels depending on demand - they have multiple small compressors that get switched in and out for higher efficiency compared to one big compressor that always runs at the same power.
Part of this involves thermostats that are more than the usual binary on or off type - they have a third “Kick it up!” mode that gets invoked when the system comes out of a setback time, or if the room temperature is more than something like five degrees away from the desired temperature.
I don’t know how common these are in Japan, but they’re pretty rare and exotic in the US so far.
You must have a sizeable draft going through the appartment bringing in moist air. Dehumidifying air takes a lot of energy. Much more than just cooling it. Anyway, figure out how to stop the draft (although it will mean stuffier air). Also, turn the A/C’s fan on High. This makes it less of a dehumidifier and more of a cooler.
The dehumidify setting could be key. The more humidity the unit is removing from the air, the cooler it should get (I think).
Most car A/C’s don’t have a thermostat. The temperature knob just opens the heater core to mix warm air in. You’re still running your compressor the whole time. Adding heat in a car doesn’t draw any extra power from the alternator since you’re just using the hot water that’s a natural byproduct of a water-cooled engine to heat the air.
If your wall-unit A/C in your house works like that, then adding heat would mean kicking on an electric coil to warm the air-conditioned air. That would be a really high power draw.
The window/wall A/C’s I’ve seen that had both Heat and Air had them as two completely separate functions. You couldn’t use both at the same time. The Heat function worked basically a space heater with a blower.
If the A/C is running, when you turn the temperature knob higher, does the whole unit shut down, or does the blower continue to run with the compressor off? (you should be able to hear the compressor kick off) If so, then it’s just blowing fan air when the compressor is off, either recirculating the inside air, or introducing hot outside air.
They’re stand alone, built into the wall above windows.
Ok, just did some playing around. When I turn the temp from 23 to 30 (the highest it will go on AC setting) it the fan stops blowing as hard but the air coming out is the same temperature. The compressors are outside, and seem to run as long as the AC is on. So perhaps the temp control only controls fan speed? If so, I would think that the temp setting wouldn’t have all that much of an impact on energy usage, right? Slight increase, but not huge?
Here are some pictures to illustrate what we’re talking about:
AC: http://picasaweb.google.com/raenjapan/OkinawaApartment/photo#5216350515143109730
There are 5 of these in the house.
Living room: http://picasaweb.google.com/raenjapan/OkinawaApartment/photo#5216352723831646898
I post the living room pic to demonstrate what the house is like–tons of sliding glass doors. We have seven total in what is basically a four room apartment. However, they are pretty air-tight. We get a lot of wind off the water (not in the summer, unless it’s a typhoon, unfortunately) and you can tell when a door isn’t sealed properly. I think I would know if we had a major draft.
All of these are run by digital remote. There is a dehumidify button, a heat button, an AC button, and a fan button. As far as I can tell each of these functions is separate from the others–if you push the fan button, it turns off the AC, etc.
When it isn’t so hot out, we can turn on the AC and the blower, at least, cycles on and off to keep the room at the desired temperature, but now they all run constantly (never all at the same time, we only cool the rooms we are actively using) whether they’re set at 20 C or 27 C, but the room feels much cooler when the AC is on a lower setting.
Let me explain the dehumidify and the fan setting. Dehumidifying works by drawing the air slowly (ie, at low fan speed) over the condenser giving it time to really cool down. This squeezes a lot of water from the air. Temperature works in a mostly linear way. Yet the humidity curve is exponential. A five degree temperature drop makes the air a little cooler but a lot drier. When you work out the lower volume of air and all that, you’ll get the result that lower fan speed = drier room.
Now, as for energy usage: Water vapor contains huge amounts of energy because of its heat of vaporization. To heat a gram of water ten degrees C takes just 40 joules. But to turn it into water vapor at the same temperature takes 2300. This is the same if you boil it or if you just let it sit around and vaporize. It’s why sweat is so effective, after all.
At 95F and 90% humidity, a cubic meter of air has 40 grams water and 1.4 kg of nitrogen, oxygen, and other crap. (Damn… air is heavy.) Cooling that much dry air by 10C will require the movement of 10 kJ. But taking even half of the water out will move 50 kJ.
In short: Yeah, fan speed makes a pretty big difference if you’re humidity-limited. And the more humidity the unit is removing, the warmer the room will be.