Continuing from the last post, it was surprisingly how much I was able to reduce the temperature in the room with the various measures. It made enough difference that the AC didn’t need to be replaced. Because we were gone during the day, the windows were all shut and although we closed the blinds, the temperature would really rise in the room, above the outside air temperature. I was able to lower the temperature to close to outside air during the day before the AC was used so it didn’t have as much of a burden.
My apartment is a similar size, about 700 SF and I have two window a/c units, one is 10,000 BTU in the living room and the other is 5,500 BTU in the bedroom. When the temperature gets above 90 they struggle to keep up but can still maintain about 74 as long as I don’t use the oven or cook a huge pot of pasta at 7:00 pm. This is a brick building constructed in 1914 in Cincinnati that gets beat pretty hard by the late afternoon/evening sun. It’s dark red brick and I’m on the top floor of the four story building, so I’m probably in the hottest unit of all. There’s decent attic insulation which helps a lot, but not much can be done about un-insulated 8" brick walls, or the original double-hung windows with fairly rickety triple-track storm windows from the 1960s. All that said I can still turn the a/c units off while I’m away at work. If it’s going to be over 90 they need to turn on by 2:30 or at the latest 3:00. I turn them down at night to around 71 or 72 degrees, so there’s a good pool of cool air and it doesn’t get much hotter than 79 or 80 by the time they turn on in the afternoon. The heat soaking through the walls seems to peak around 8:00 pm so if I wait until I get home to turn them on (at which point it would probably be closer to 82 or 83) they wouldn’t be able to catch up adequately. Even as it is, they are both likely to run continuously throughout the evening from body heat, cooking, computers, etc, but part of that is because the larger unit is a newer GE that has a ridiculous 4-degree split on its thermostat (I think that’s an Energy Star thing, and technically it’s a 3-ish-degree split, where if you set the temperature to 74 it turns on when it hits 76 and turns off when it hits 72, but it’s really turning on at 76.0 and turning off at 72.9 assuming that’s the level of granularity in the sensor, but it’s still too much swing for my comfort).
So anyway, if the temperature outside is going to push 100 then I’d definitely have a problem. I’d need to just leave them running all day, and hope that I could build up some cool air overnight or in the morning. Big windows that get a lot of direct sun are killer, so that could also be a differentiating factor here with the OP. Everyone’s comments about the delta-t (temperature split) is spot on. If the room is 70 degrees the a/c will be blowing out 50 degree air. On the other hand, if the room is 80 degrees then the a/c can only blow out 60 degree air. It’s still removing just as much heat, but it can only remove so much in any one pass.
There is another factor at play too. As the temperature goes up outside, the temperature split inside is likely to go down a bit. The same is true if the outdoor condenser coil is dirty.* Restricted airflow is basically the same as running the unit in a hotter outdoor environment. An air conditioner can maintain the same cooling capacity, even though it’s hotter outside, by pumping the system to a higher condensing pressure. The particulars of that are beyond what we need to go into here, but the result is that the unit will use more electricity, and it will strain the compressor motor more, but it will still maintain basically the same cooling ability. Up to a point. Beyond that point, it does start to lose capacity, which means the 20 degree temperature split is going to be more like 15 degrees, but that’s also when you need the most cooling, so it’s a double-whammy. The same principle applies to a heat pump in the winter, only the reverse. There’s less heat to extract out of the air in very cold temperatures, and the system can only pull the evaporator pressures so low to try to extract that heat before it can’t do any more. In this case there’s so much heat outside, limited condenser area to try to dissipate the heat, and the system can only ramp up its pressure so much to try to expel that heat before it overloads. It’s like trying to cool yourself off on a hot day by jumping in a warm swimming pool. It’ll still work, but not as well as if the pool was cold.
So to mitigate that effect you just need more capacity, more BTUs if you can’t reduce the heat gain coming through the walls, windows, or drafts. Very hot climates do just need much larger a/c units. They’re also more likely to have multiple stages, whether completely separate parallel refrigeration circuits, or low and high speed compressors and fans, so that the occupants aren’t frozen out by short blasts of very cold air during milder days, and so the equipment doesn’t short-cycle, which makes dehumidifying a lot more difficult. If your a/c isn’t able to keep up and the air is getting quite dry inside (like below 40% RH) that’s a good indicator it’s simply undersized for the load. If the humidity is still high (like above 50% RH) then you may want to try tracking down air leaks around windows, doors, and vents. In multi-story apartment buildings, don’t overlook the door to the common hallways and stairs, as well as gaps under baseboards and around radiator pipes or behind cabinets.
Also, I have an Extech infrared thermometer, and it does not like taking readings into a moving air stream. I’m not sure what the mechanism is there, but it’ll fluctuate as much as 20 degrees. If it’s held perfectly still with the a/c blowing cold air at it, you’ll see the temperature reading go down very fast. Similarly if hot air blows at it, the temperature reading will go up. Maybe moisture or heating/cooling of the shroud/sensor/lens is causing it to mis-read, but I’ve found them to be very fickle for this sort of thing. If you can get a reading without air blowing on it, then it should work, just beware. I just keep one of these jammed in my main a/c to see how it’s doing.
*I was able to get a good 4-5 degree increase in temperature split by cleaning the condenser coils on both my a/c units. It had been several years and it they were very clogged up. Nearly all window a/c units collect the water that drips off the evaporator inside and direct it into a pan under the condenser outside. The condenser fan then slings this water around and sprays it on the condenser helping to cool it. That does work, but it means that instead of dust building up on the coil over time, you get sludge and mud, which doesn’t rinse off as easily. Also it means the condenser fan has to blow through the coil from inside to outside, so you can’t easily see the buildup. It does make spraying it with a hose a bit more effective since when you spray from the outside in it pushes the muck back the way it came, but that’ll only get you so far. I had to take mine apart to get in there and properly clean them with scrub brushes soap and hot water.
All I can add is my very comfortable 350 sq ft bedroom with a 15 year old Goldstar 8,000 (maybe 10,000) BTU unit works a lot better then some puny 15 to 20 degree drop. The unit has been running a few hours so far today. Outside is 80 degrees. Inside everything is controlled to 75 by this time - the walls, ceiling and air temp. Humidity outside is 70% and was 80% this morning. Inside humidity is 45% or lower throughout the house .
The window A/C is putting out 42 degree air so that’s a 33 degree drop in temperature from the 75 degree room air. The unit cycles on and off as it should and runs about maybe half the time. The room is well insulated - R30 ceiling and R16 walls.
I’m honestly going to assume temperature reading errors, a 8000 BTU LG AC is not going to cool vented air 33 degrees from what it brought in.
A very well insulated room can be cooled well beyond 20F below outside temperature though, because in a well insulated room over time the AC is bringing in air it has already cooled and cooling it further and further. But that is only going to occur under certain circumstances. In the top floor of a brick building with big windows letting the sun in, that’s unlikely to happen.
On the flipside, you can take a small room and heavily insulate it, and I mean heavily, then get heavy duty freezer type door for it, cut a hole in a wall and plop in a wall AC, and turn that small room into a walk-in fridge. There is a company that makes a device to facilitate this and I’ve heard of people using them for years on various forums. The way it works is the small and heavily insulated room will rapidly cool. A typical AC unit’s sensors won’t really let you cool below 60F, but nothing physically stops it from cooling below that. The third party device tricks the sensors to continually think the room is hot, well above 60F, so it keeps cooling up to the point where the coils actually freeze. But that point is so low that the small, well insulated room can get as cold as 34F, perfect for a walk-in fridge. When the device detects that the coils are about to actually ice, it kicks them off, and keeps recycling as needed to prevent it from warming back up.
But this application requires a very well insulated room with very good air sealing, and a small room with what ends up being a very oversized AC cooling it, and a third party device that fools the AC’s built in sensors.
No reading errors. 42 degrees is what I can read just inside the outlet vents as I just used a non contacting readout. I don’t think I have any clip on thermometers anymore. I use to have my HVAC ticket for auto use and low 40s was what we always tried to achieve on cars. The old R-13 systems could get so cold they sometimes discharged fog.
What he means is that if the room’s 84 degrees, the air coming out of the AC unit should be no warmer than 69 degrees, which should feel pretty cool if the room is 84.
And FYI, AC units don’t have a temperature adjustment- they just run and cool the air as much as they can. What makes various temperatures happen is the mix of cold and warm air in the room, which is adjusted by the length of time the AC runs.
Generally speaking, like @am77494 said, the output air should be roughly 15-20 degrees cooler than the air that’s being cooled.
If your air isn’t that cool, then you probably have low refrigerant and/or the unit is freezing up. If it’s a central unit, you can get someone out to check it and fix it/recharge it. If it’s a window unit, I’m not sure- those may be one-shot devices.
If the air is cool like it should be, then perhaps it’s a sizing or air distribution issue.
That’s the kind of strange reading I get with my infrared thermometer pointed into the air outlet. I’d be very interested to know the reading from a proper probe thermometer. Older a/c units could discharge colder air, but they were generally using smaller coils and blowing less air through them. If you block up the air intake and then take a temperature reading it’ll drop to near freezing pretty quickly. That means there’s less heat transfer going on though, and thus less efficiency. A blocked filter can cause that to happen.
OK. The temperature/humidity readout I use in the bedroom has a wired thermister for the outdoor air temp. I placed it in the air outlet and ran the A/C on low fan speed for lowest discharge temperature. The room is once again at 75 degrees F. The outlet air temp is 42 degrees F.
I suspect the A/C manufacturers have adopted a new spec, like the furnace companies did. When I installed a new furnace in my home around 1990 I used the old ASHRAE standard for design conditions. They listed the 50 year low temperature in your area, which is -20 degrees F around here. I sized the furnace to allow for future expansion which I have since completed. The temp has only fallen to -20 once since then but -10 several times. I have no problem keeping the house at my normal 70 degrees. It does not run full time during those cold spells - it cycles on and off as usual.
We had several days of below zero weather a few years ago and the weathermen were advising people on how to cope by closing off rooms, etc. Now I see the recommended design temp for this area is +5 degrees F. WTF? They are deliberately handicapping output in the name of efficiency as a small furnace heating a home is slightly more efficient then a larger furnace running a shorter time. No thanks. I’ll stick to my calculations. Oh - my heating bill is very reasonable.
DX systems can easily go below 40 degrees. The PTAC unit in my office is currently blowing out air at 35 degrees. Room air temperature (and thus entering air temp at the coil) is 75.
Ah! I asked about that, but nobody knew or remembered. I hadn’t thought to attribute the change to the change in refrigerant.
The a/c in my current car is also just really crap, in a fairly crap cheap GM car. I had it replace it twice last year because of total failure (regassing had no effect), and 6 months later it’s already only barely working.
To answer your questions.
I wonder whether the machine thinks it is doing the job, or even whether it is but there are other factors. It is a machine it does not think. When called for it runs, when the temp is at set point is shuts off.
If it is overworked the discharge temps will increase and efficiency will decrease. And the amount of cooling the unit can provide will also decrease.
If it is 90 degrees out sside and the space temp is 83 either you have a problem or your unit is undersized. What kind of heat load do you have in your spaces. How many servers and conputers and other heat generating equipment are in the rooms?
I do not know how many battles I have had to go through with tenants who thought a little 5000 BTU AC unit should be keeping a server room with over 20,000 BTU of equipment.
YOu need to have someone do a load calculation of your spaces. Then check your equipment. It is all in the numbers.