Lately it's been running in the mid-90s here, and my air conditioner has been running non-stop during the day. I assumed that it was "bad" in some way, but after some further research, I find out that air conditioners in the US are typically set to provide only 20° of cooling, which is why my thermostat never reached its set at 72°.

Who here knows anything about air conditioning? I don't understand where this 20° figure is coming from, for one thing. Certainly it's not like that everywhere. Basically, any information about how I can make my house actually cool down properly would be great.

White blinds?

But seriously, I'd be very curious as well. We have AC issues here too.

I've always been stingy with AC and only run it a few degrees cooler than outside. That was when I lived in the Midwest and the main thing I was after was to reduce the humidity.

I don't have AC in California. It's almost time to put the box fan into the ceiling access hole and get the swamp cooler out of the shed...

Yah, my house is not the greatest.

While I'm not an expert in the field, surely the units power rating dictates its cooling capacity. Chillers I've bought for projects are described as having x-kW's of cooling capacity and are specified by the contactors based on the power consumption of the equipment to be cooled.

EDIT: Or are you talking about the maximum temperature differential between the 'hot' and 'cold' sides of the system?

Yes, differential.

In addition to being a freedom-loving zealot :-), I'm a mechanical engineer. So I know a bit about thermodynamics and air conditioning.

The A/C is not "set" to provide a certain temperature difference, but there is a maximum heat-transfer capacity to the system. The AC works by cooling a coil inside your furnace plenum (the evaporator coil), and then the furnace fan blows air through the evaporator, cooling it. Heat from the air blown across the evaporator is transferred to the refrigerant, which flows outside to the condenser coil (the one outside), where it is exhausted to the outside air.

On really hot days, the air inside the house can be theoretically heated by the ambient environment faster than it can be cooled by the evaporator. As a result, the inside air temperature will reach an equilibrium higher than the thermostatically set temperature (though lower than the outside temperature). In practice, though, you should be able to get it down to 72 degrees as long as you're not repeatedly opening the windows at night to let in a bunch of humidity (you should generally turn on the AC and leave it on for a prolonged period, to avoid repeatedly drying the inside air -- this is referred to as "latent heat of vaporization". Opening with windows at night is the worst thing you can do in a humid climate).

On these days, then, it is really important that you are getting as much capacity from your system as possible. The two major things that decrease capacity are dirty/clogged coils (which reduce airflow across the coils) and low refrigerant level.

What you are describing is usually indicative of reduced performance due to a frozen evaporator. Water vapor condenses out of the air as it passes by the cold evaporator. If the evaporator is clogged or dirty, or if the condensate drain line is clogged, the water will freeze in the evaporator. Once this starts to happen, it reduces the airflow across the evaporator, causing more freezing, until the evaporator is totally frozen and clogged with ice. If you go look at your plenum, you should see two copper lines heading into it and my guess is that one will be completely ice covered. If that's the case, you need to turn off the system (switch the fan to on), and let it thaw out and dry. You can limp through these hot days by thawing out the evaporator every 5-6 hours, but to fix it you'll need to figure out why it's freezing up and resolve that. Cleaning the evaporator is a really difficult task. Unplugging the condensate drain is fairly easy.

Go check your evaporator and if it's frozen up we can talk more about that. If your evaporator is bone dry and cold, then you either have a capacity issue (AC can't keep up), or you may be low on refrigerant due to a system leak.

Yeah, there's something fundamentally flawed about the assumption in that statement.

The A/C unit will simply try to cool things down to the temperature you set it to. The A/C unit doesn't have a "try harder" setting. For instance, turning things down to 60 isn't going to make the unit work any harder than if you turned it down to 72. It's working at its maximum efficiency at all times.

If you turn it down to 72 and your house isn't getting cold enough, you're running into the following possible issues:

- Air conditioning unit is malfunctioning and not cooling at its full potential efficiency.

- Air conditioning unit is not large and powerful enough for your house and/or your geographic location.

- House insulation is extremely poor, so heat leaks in through the walls and especially the attic, negating all the hard work the A/C is doing.

I lived in a house for years which fell into that third camp. It was pretty miserable.

Technically, it's working at it's maximum *capacity* at all times, efficiency is a function of the temperature difference between the high-pressure and low-pressure heat exchangers (more temperature difference, more efficiency of the gas cycle).

Your 3rd issue is basically the same as the 2nd issue -- the AC can't keep up because the insulation is so bad. But most modern houses with a well-maintained AC unit should be able to keep up with mid-90's for ambient temperature, even with a high wet bulb (dew point) temperature.

As you and TigerJimmy said, air conditioners run at full capacity or zero capacity. There is a maximum amount of cooling that it can do. Apparently air conditioners in the US are sized to provide about 20° of cooling, so as to not cycle too much. You don't want your AC to pump 32° air into the house for 30 seconds and then have to do it again five minutes later. You want it to pump 65° air for five minutes and do it again twenty minutes later.

I guess what I want to know is: is there anything I can do to make the unit cool down more – 25° would make sense here – or do I just have to replace it?

Is the problem that the air coming out of the cooler is itself not cold enough, or that there's too little of the cold-enough air to make headway against the ambient heat?

Peter

Did it ever work well? About the only thing you could probably do yourself is to clean the condensor and evaporator. Also check the temperature just after the evap, it should be around 40 degrees F. Any higher than that and you've got some problem going on.

Did the AC ever cool the house down or is this the first time you are trying it?

Check a couple things first, is the temperature difference between the inlet and the outlet right around 20F and does it feel like there is enough air blowing out a vent? Did you replace any windows or anything that may have affected the insulation?

Wow. Somehow I totally missed your extensive post earlier.

The reason I came up with that 20° differential is that that's what the AC service company told me over the phone, and I'm disinclined to believe that they'd lie in order to prevent making money.

I've not been opening windows at night. For one thing, it's not been getting cool enough for that to be helpful, but that's a good point about overworking the AC just to dry out the air.

I know that the AC had the same problem last year. It seems no worse this year, which leads me to believe that it's not a leak. I want to say that for a few years before that, we had relatively mild summers, so I may not have noticed. In addition, I didn't really notice last year. I only really noticed it this year because I happened to be home on a 95°+ day.

75° isn't terrible, but the AC was running all day long. It didn't occur to me that if I'd turned the thermostat up to 75° that it probably would have started cycling normally. I was initially concerned that if it couldn't keep up at 95°, it must have lousy efficiency at 90°, and I was concerned about wasting energy and money.

But when I called the AC service company, they told me that what I was seeing was expected behavior. They said that ACs are usually sized to provide 20° of cooling. But I don't think I've experienced this at other places I've lived.

I'll check to see if the evaporator is frozen. I should also check to see the temperature of the air coming out of the vents.

Actually, that's not true. You want the air conditioner to pump out 32 degree air (it will be more like 50 degrees), which is then mixed in with the rest of the air inside the house through the circulation by the furnace fan. Air conditioning cycles are more efficient the greater the temperature difference between the coils, so they are designed to maximize this. The thermostat doesn't control based on vent temperature, it controls based on the room temperature. It won't cycle every 30 seconds if the evaporator is very cold, because the thermostat isn't on the evaporator, it's in the living room, which cools more slowly.

You want that evaporator as cold as it will get. What you can do to improve that is to check to see if it's frozen and clean both the evaporator and condenser coils to get maximum airflow through the coils.

Report back on the evaporator. It sure sounds like it's frozen.

Efficiency isn't the same thing as power consumption. At 95 degree ambient, the refrigeration cycle is more *efficient*, but it consumes more power. Here's an analogy: a lawn mower engine is very inefficient, but because it's not doing much work, it doesn't consume much fuel. A jet engine is extremely efficient, but because it's developing so much power it consumes a lot of fuel.

On a hot day, the AC runs more *efficiently*, but it also runs more *often*, so it consumes more total power. Efficiency in a refrigeration cycle is measured by the amount of heat transferred to the hot side as a function of the energy put into the cycle.

75 is terrible. You should be able to achieve 68 degrees at 95 degree ambient temperature, even with a high wet bulb (dew point).

One other thing -- if your furnace filter is old and quite dirty, this will reduce airflow across the evaporator which can also lead to a freeze up.

I'm sure that you know a hell of a lot more about this than I do, but I've always understood that it's preferable for an air conditioner to run long cycles because it puts less strain on the mechanical elements the fewer times it has to stop and start. In addition, it's uncomfortable for the air coming out of the vent to be way colder than the ambient air.

So while I totally agree with your assessment from a physics point of view, my understanding of the real world application is different.

In addition, I have found a decent amount of confirmation online about the 20° differential thing. This also seems to make sense to me. The evaporator is going to be as cold as it can be, which seems to me is going to be below 60° at the absolute most, but it cannot transfer all that energy from the passing air instantaneously, and most air conditioners (mine, at least) run air past the heat transfer unit at a constant rate, which all leads to a relatively constant level of cooling.

Again, though, I'm sure that my limited understanding is making me look like an idiot.

Am I correct in remembering that you're in Louisiana? Maybe AC units down there are sized differently than they are here. It's hot and humid in North Carolina, but it's no Gulf Coast.

I am confused about this. Which differential are you talking about? Internal temp to ambient temp, intake vent to output vent or exactly what?

Yeah, now that you say that, I have no idea. Since the outside temp was 95° and the inside wouldn't get below 75°, I'm guessing that differential. Obviously, the evaporator gets much cooler than that, so I guess it's a sizing issue.

I don't know how hot the compressed refrigerant gets, but I'm guessing no more than 175°, so a difference between 85° and 95° in its heat sink (the surrounding air) is probably reasonably significant.

Yeah, you've mentioned our "misuse" of the term "efficiency" a couple of times. I understand your point about the technical meaning, but I was referring to the idea of using more energy (and money) than necessary to do the job.

That is, if the AC is working poorly, due to poor air flow or whatever, even when it is able to cool the room to my set temperature, it is using more energy than necessary to perform that task.

The outside-to-inside differential is entirely dependent on how well-insulated the house is. Which is going to be different for every house, and entirely unpredictable when the company installs the air conditioner. So if there really is some kind of "20 degree rule of thumb", it doesn't sound logical that the 20 degrees should measured from those two particular extremes.

I'm not in Louisna now, but when I, was our upstairs unit would run all day in the summer and keep things at 75-78 depending on whether we had a 95 degree day or a 98 degree one. Another thing you might check is the air temperature at your vents. I heard that
it has its own differential as well although I don't recall what that should be.

The last time I had my AC worked on the guy gave me a range on how much cooler the air on the outlet vent was than the inlet vent. I don't remember what he said, but I want to say when he checked the difference was like 26F after he fixed the AC.

The only way the outside temperature is involved is through the heating of the house. The air that is inside the house should be the air that is getting cooled (obviously there is some mixing of outside air, but it should be negligible). If you have adequate insulation, it should be negligible I guess.

I know my house gets a lot cooler than 20F below ambient. It was 108F this weekend and my house stayed at 78F. Of course our humidity is a lot lower here, but I don't see humidity making that big of a difference in the long term cooling.

Edit: I just realized I have no idea where my thermometer gun is. When I get a new one, I'll check the temp from the inlet vent to the outlet vent to compare them.

The AC has to sink heat to the outside air. I see no reason why the outside air temperature wouldn't affect the amount of heat it was able to absorb.

I imagine the fan sinks on the AC outside get a lot hotter than ambient in most places. If none of the issues mentioned are affecting your installation, you may just have a unit that's undersized for the space. Was the AC installed by the builder or did you have it installed after-market? The first will tend to go as small as possible and the second as large as you can afford.

Yeah, I'm pretty sure you hit the nail on the head. That's my expectation.

That might be where the humidity plays a part. With low humidity you can definitely tell a temperature difference between shade and direct sunlight. With higher humidity the air gets 'thermo-saturated' and you can't tell a difference between the shade or not. My compressor and fan have no problem getting the cooler ambient air (even when it is 110F+) to dump the heat to.

Yeah, that sounds like a much more likely place to measure the temperature differential.

Once you've got that within an expected rule-of-thumb range, then you're down to the quantity of air that is expected to be pushed through the system at that differential, and that's down to the cubic footage of the house versus the capacity of the AC unit (with the house's insulation factor being the wildcard).

I just remember that I had a miserable time in the hot house on the hill. The A/C people looked at it and said that the device was working correctly and that it was the correct size for the house. The problem was that it was the worst-insulated house on the planet, with no shade cover, on a hilltop, so it baked in the sun for the whole summer.

The evaporator is not frozen. Unless it was able to thaw out over a few days. In fact, I can't imagine that it could possibly get cold enough to freeze. It was about as cool as the outside of a glass of iced tea, and produced about as much condensation (per unit of surface area).

The only filters are the ones in the return vents, and we change those regularly. At least I couldn't find any in the furnace/AC unit in the attic.

The ductwork is not exactly well sealed. It looks like there was some sort of dope used to seal the seams, but it's not doing the job, not to mention the sizable holes where the refrigerant pipes go in. But I can't imagine that that would have a huge impact.

There was an odd sound coming from the evaporator. It may well be normal, but it sounded like a leak. It was kind of halfway between a whistle and a hiss. It wasn't constant. It seemed to do it for about 30 seconds, then pause for about a minute, then repeat.

There were two refrigerant tubes running into the ductwork, one insulated and the other bare. I assume that the insulated one should be the cool one. It was moderately cool. Enough to generate some condensation inside the ductwork where it was uninsulated. Inside the unit, it split into two or three smaller tubes that fed the evaporators. On the other end of the evaporators, the return tubes were more and smaller. They were pretty cool. Where they passed out of the ductwork, they were somehow joined into a single return tube, which was sometimes room temperature, and sometimes somewhat warm.

Bitt,

We had problems last year with a split evaporator/condenser unit over the course of several warm months. They kept recharging the refrigerant loop to no avail. Finally a frustrated tech drained/purged the loop (my approximation of his explanation) then recharged it and all was well, relatively-speaking (we replaced that 1.5 ton unit in January with a new 5 ton).

So the tech's take home message was a "bubble" -- a locked up loop that the system was laboring against. It took 4 service visits. The first 3 were "I charged it. Seems fine!"

You're fortunate to have air conditioning. As beefy as our solar electric system is, it can't handle an air conditioner, which sucks in the tropics.

Thanks for mentioning it you insensitive clod