Ah, that makes sense. But the 26 minutes --> 13 minutes is from adversarial training holding the threshold fixed, right?

I think that 3 orders of magnitude is the comparison between "time taken to find a failure by randomly sampling" and "time taken to find a failure if you are deliberately looking using tools."

I still the 25-30% estimate in my original post was basically correct. I think the typical SACC adjustment for single-hose air conditioners ends up being 15%, not 25-30%. I agree this adjustment is based on generous assumptions (5.4 degrees of cooling whereas 10 seems like a more reasonable estimate). If you correct for that, you seem to get to more like 25-30%.  The Goodhart effect is much smaller than this 25-30%, I still think 10% is plausible.

I admit that in total I’ve spent significantly more than 1.5 hours researching air conditioners :) So I’m planning to check out now. If you want to post something else, you are welcome to have the last word.

SACC for 1-hose AC seems to be 15% lower than similar 2-hose models, not 25-30%:

• This site argues for 2-hose ACs being better than 1-hose ACs and cites SACC being 15% lower.
• The top 2-hose AC on amazon has 14,000 BTU that gets adjusted down to 9500 BTU = 68%.  This similarly-sized 1-hose AC is 13,000 BTU and gets adjusted down to 8000 BTU = 61.5%, about 10% lower.
• This site does a comparison of some unspecified pair of ACs and gets 10/11.6 = 14% reduction.

I agree the DOE estimate is too generous to 1-hose AC, though I think it’s <2x:

The SACC adjustment assumes 5.4 degrees of cooling on average, just as you say. I’d guess the real average use case, weighted by importance, is closer to 10 degrees of cooling. I’m skeptical the number is >10—e.g. 95 degree heat is quite rare in the US, and if it’s really hot you will be using real AC not a cheap portable AC (you can’t really cool most rooms from 95->80 with these Acs, so those can't really be very common). Overall the DOE methodology seems basically reasonable up to a few degrees of error.

Still looks similar to my initial estimate:

I’d bet that the simple formula I suggested was close to correct. Apparently 85->80 degrees gives you 15% lower efficiency (11% is the output from my formula). 90->80 would be 20% on my formula but may be more like 30% (e.g. if the gap was explained by me overestimating exhaust temp).

So that seems like it's basically still lining up with the 25-30% I suggested initially, and it's for basically the same reasons. The main thing I think was wrong was me saying "see stats" when it was kind of coincidental that the top rated AC you linked was very inefficient in addition to having a single hose (or something, I don't remember what happened).

The Goodhart effect would be significantly smaller than that:

• I think people primarily estimate AC effectiveness by how cool it makes them and the room, not how cool the air coming out of the AC is.
• The DOE thinks (and I’m inclined to believe) that most of the air that’s pulled in is coming through the window and so heats the room with the AC.
• Other rooms in the house will generally be warmer than the room being air conditioned, so infiltration from them would still warm the room (and to the extent it doesn’t, people do still care more about the AC’d room).

Ok, I think that ~50% estimate is probably wrong. Happy to bet about outcome (though I think someone with working knowledge of air conditioners will also be able to confirm). I'd bet that efficiency and Delta t will be linearly related and will both be reduced by a factor of about (exhaust - outdoor) / (exhaust - indoor) which will be much more than 50%.

I think labeling requirements are based on the expectation of cooling from 95 to 80 (and I expect typical use cases for portable AC are more like that). Actually hot places will usually have central air or window units.

I would have thought that the efficiency lost is roughly (outside temp - inside temp) / (exhaust temp - inside temp). And my guess was that exhaust temp is ~130.

I think the main way the effect could be as big as you are saying is if that model is wrong or if the exhaust is a lot cooler than I think. Those both seem plausible; I don't understand how AC works, so don't trust that calculation too much. I'm curious what your BOTEC was / if you think 130 is too high an estimate for the exhaust temp? 

If that calculation is right, and exhaust is at 130, outside is 100, and house is 70, you'd have 50% loss. But you can't get 50% in your setup this way, since your 2-hose AC definitely isn't going to get the temp below 65 or so. Maybe most plausible 50% scenario would be something like 115 exhaust, 100 outside, 85 inside with single-hose, 70 inside with double-hose.

I doubt you'll see effects that big. I also expect the improvised double hose will have big efficiency losses. I think that 20% is probably the right ballpark (e.g. 130/95/85/82). If it's >50% I think my story above is called into question. (Though note that the efficiency lost from one hose is significantly larger than the bottom line "how much does people's intuitive sense of single-hose AC quality overstate the real efficacy?")

Your AC could also be unusual. My guess is that it just wasn't close to being able to cool your old apartment and that single vs double-hoses was a relatively small part of that, in which case we'd still see small efficiency wins in this experiment. But it's conceivable that it is unreasonably bad in part because it has an unreasonably low exhaust temp, in which case we might see an unreasonably large benefit from a second hose (though I'd discard that concern if it either had similarly good Amazon reviews or a reasonable quoted SACC).

• The top wirecutter recommendation is roughly 3x as expensive as the Amazon AC being reviewed. The top budget pick is a single-hose model.
• People usually want to cool the room they are spending their time in. Those ACs are marketed to cool a 300 sq ft room, not a whole home. That's what reviewers are clearly doing with the unit. 
• I'd guess that in extreme cases (where you care about the room with AC no more than other rooms in the house + rest of house is cool) consumers are overestimating efficiency by ~30%. On average in reality I'd guess they are overestimating value-added by the air conditioner by more like ~10% (since the AC'd room will be cooler and they care less about other rooms).
• I think the OP is misleading if 10% is what's at stake and there are real considerations on the other side.
• I think there is very little chance that the wirecutter reviewers don't understand that infiltration affects heating efficiency. However I agree that your preferences about AC, and the interpretation of their tests, depend on how hot the rest of the building is (and how much you care about keeping it cool). I'm 50-50 on whether someone from the wirecutter would be able to explain that issue if pressed.
• This AC does not report SACC BTU, though many of the top-rated ACs list both (4/6 of the other ones I checked from the "4 stars and above related products"). I agree that some consumers won't see this number.
• The internet will tell you to use a 10,000 BTU portable AC for a 300 sq ft room (in line with the recommendation on Amazon's page) and a 6500 BTU window AC. That is, the "300 sq ft" number and normal internet folklore are mostly taking into account these issues.
• The AC in question does report CEER which I still think includes this issue. It has a quite mediocre CEER of 6.6. It describes this as "super efficient" which is obviously false.
• Note that non-SACC BTU ratings are mostly only a problem when looking at comparisons of single-hose to double-hose AC (since e.g. googling portable AC sizing or looking at recommended sq footage takes this issue into account), and so what mostly matters is whether the Amazon page for a double-hose AC makes this argument in a way that lets it win comparison-shopping customers.

What an argument about air conditioners :)

They measure the temperature in the room, which captures the effect of negative pressure pulling in hot air from the rest of the building. It underestimates the costs if the rest of the building is significantly cooler than the outside (I'd guess by the ballpark of 20-30% in the extreme case where you care equally about all spaces in the building, the rest of your building is kept at the same temp as the room you are cooling, and a negligible fraction of air exchange with the outside is via the room you are cooling).

Which... seems to misunderstand the actual problem of infiltration as it relates to heating efficiency? This is the only mention of the word infiltration in the whole article, and I can't find any other section that discusses infiltration problems in other words. 

I think that paragraph is discussing a second reason that infiltration is bad.

Here is the wirecutter discussion of the distinction for reference:

Starting in 2019, we began comparing dual- and single-hose models according to the same criteria, and we didn’t dismiss any models based on their hose count. Our research, however, ultimately steered us toward single-hose portable models—in part because so many newer models use this design. In fact, we found no compelling new double-hose models from major manufacturers in 2019 or 2020 (although a few new ones cropped up in 2021, including our new top pick). Owner reviews indicate that most people prefer single-hose models, too, since they’re easier to set up and don’t look quite as much like a giant octopus trash sculpture. Although our testing has shown that dual-hose models tend to outperform some single-hose units in extremely hot or muggy weather, the difference is usually minimal, and we don’t think it outweighs the convenience of a single hose.

The one major exception, however, is if you plan on setting up your portable AC in a room with a furnace or hot water heater or anything else that uses combustion. When a single-hose AC model forces air out through its exhaust hose, it can create negative pressure in the room. This produces a slight vacuum effect, which pulls in “infiltration air” from anywhere it can in order to equalize the pressure. In the presence of a gas-powered device such as a furnace, that negative pressure creates a backdraft or downdraft, which can cause the machine to malfunction—or worse, fill the room with gas fumes and carbon monoxide. We don’t think that most people plan to use their portable AC in such a room, but if your home is set up in such a way that you’re concerned about ventilation, skip the rest of our recommendations here and go straight for the Midea Duo MAP12S1TBL or another dual-hose model like the Whynter Elite ARC-122DS or Whynter Elite ARC-122DHP.

Boston summers are hotter than the average summers in the US, and I'd guess are well above the average use case for an AC in the US. I agree having two hoses are more important the larger the temperature difference, and by the time you are cooling from 100 to 70 the difference is fairly large (though there is basically nowhere in the US where that difference is close to typical).

I'd be fine with a summary of "For users who care about temp in the whole house rather than just the room with the AC, one-hose units are maybe 20% less efficient than they feel. Because this factor is harder to measure than price or the convenience of setting up a one-hose unit, consumers don't give it the attention it deserves. As a result, manufacturers don't make as many cheap two-hose units as they should."