Update: I too have now spent like 1.5 hours reading about AC design and statistics, and I can now give a reasonable guess at exactly where the I-claim-obviously-ridiculous 20-30% number came from. Summary: the SACC/CEER standards use a weighted mix of two test conditions, with 80% of the weight on conditions in which outdoor air is only 3°F/1.6°C hotter than indoor air.
The whole backstory of the DOE’s SACC/CEER rating rules is here. Single-hose air conditioners take center stage. The comments on the DOE’s rule proposals can basically be summarized as:
Single-hose AC manufacturers very much did not want infiltration air to be accounted for, and looked for any excuse to ignore it
Electric companies very much did want infiltration air to be accounted for, and in particular wanted SACC to be measured at peak temperatures
The DOE did its best to maintain a straight face in front of all this obvious bullshitting, and respond to it with legibly-reasonable arguments and statistics.
This quote in particular stands out:
De’ Longhi [an AC manufacturer] expressed concern that modifying the AHAM PAC-1-2014 method to account for infiltration air would disproportionately impact single-duct portable AC performance and subsequently cause the removal of such products from the market.
So the manufacturers themselves know perfectly well that these products are shit, and won’t be viable in the market if infiltration is properly accounted for. However, the DOE responds:
However, as discussed further in section III.C.2, section III.C.3, and III.H of this final rule, the rating conditions and SACC calculation proposed in the November 2015 SNOPR mitigate De’ Longhi’s concerns. DOE recognizes that the impact of infiltration on portable AC performance is test-condition dependent and, thus, more extreme outdoor test conditions (i.e., elevated temperature and humidity) emphasize any infiltration related performance differences. The rating conditions and weighting factors proposed in the November 2015 SNOPR, and adopted in this final rule (see section III.C.2.a and section III.C.3 of this final rule), represent more moderate conditions than those proposed in the February 2015 NOPR. Therefore, the performance impact of infiltration air heat transfer on all portable AC configurations is less extreme. In consideration of the changes in test conditions and performance calculations since the February 2015 NOPR 31 and the test procedure established in this final rule, DOE expects that single-duct portable AC performance is significantly less impacted by infiltration air.
Ok, so how are the test conditions somehow making single-hose air conditioners not look blatantly shitty?
The key piece is that the DOE is using a weighted mix of two test conditions: one at outdoor temperature 95°F/35°C (representing “the hottest 750 hours of the year”), the other at outdoor temperature 83°F/28.3°C (representing some kind of average temperature during times when the AC is used at all). For both cases, the DOE assumes that the indoor temperature (i.e. the air which the single-hose air conditioner is taking in) is at 80°F/26.7°C. So for the average-outdoor-temperature case, they’re assuming conditions where there is only a 3°F/1.6°C temperature difference between indoors and outdoors! Obviously infiltration has almost no effect under these conditions… but also do people even bother setting up a portable air conditioner only to bring the temperature from 83°F/28.3°C down to 80°F/26.7°C?
Now for the key question: what weights does the DOE use in the weighted mix of these two conditions? 20% weight on the 95°F/35°C outdoor condition, 80% weight on the 83°F/28.3°C outdoor condition.
Bottom line: the DOE’s SACC/CEER rating puts about 80% of its weight on a test condition in which the indoor and outdoor temperatures are only 3°F/1.6°C apart.
This document has a good dense summary of the relevant formulas. In particular, Qs_95 and Qs_83 on page 74038 are the heats lost to infiltration under the two conditions. Those are incorporated into ACC95 and ACC83 respectively, which are then combined with the weights on page 74039.
One more interesting observation: if we assume that the efficiency loss from infiltration is basically-zero for the 83°F/28.3°C condition, then in order to get an overall efficiency loss of 20-30% for one-hose vs two, the efficiency under the 95°F/35°C condition would have to be somewhere between zero and negative. (Which is possible, given the way the test is setup—it would mean that running the AC in an 80°F/26.7°C house when it’s 95°F/35°C outside makes the house hotter overall. In other words, the equilibrium temperature of the room with that AC cooling it is above 80°F/26.7°C.) I would guess that the efficiency under the 95°F/35°C condition is somewhat higher than this simple estimate implies (since there will be some losses under the the 83°F/28.3°C condition), but I think it is a reasonably-realistic ballpark estimate at the temperatures given.
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.
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).
From the Pro Breeze single-hose AC product description on Amazon:
a mighty 10,000 BTU cooling capacity...
Super Efficient: This portable AC unit for bedroom boasts a 6.6 CEER Energy Efficiency rating to ensure that blowing cold air in the sweltering summer heat is energy efficient and cost effective.
I haven’t looked into the % efficiency loss measurements, but I think it’s interesting that you can still figure out that this is a crap AC if you’re willing to trust this website.
Portable units have to meet a much weaker standard. I actually pushed for a more stringent standard on these products when I was consulting for the Appliance Standards Awareness Project.
Update: I too have now spent like 1.5 hours reading about AC design and statistics, and I can now give a reasonable guess at exactly where the I-claim-obviously-ridiculous 20-30% number came from. Summary: the SACC/CEER standards use a weighted mix of two test conditions, with 80% of the weight on conditions in which outdoor air is only 3°F/1.6°C hotter than indoor air.
The whole backstory of the DOE’s SACC/CEER rating rules is here. Single-hose air conditioners take center stage. The comments on the DOE’s rule proposals can basically be summarized as:
Single-hose AC manufacturers very much did not want infiltration air to be accounted for, and looked for any excuse to ignore it
Electric companies very much did want infiltration air to be accounted for, and in particular wanted SACC to be measured at peak temperatures
The DOE did its best to maintain a straight face in front of all this obvious bullshitting, and respond to it with legibly-reasonable arguments and statistics.
This quote in particular stands out:
So the manufacturers themselves know perfectly well that these products are shit, and won’t be viable in the market if infiltration is properly accounted for. However, the DOE responds:
Ok, so how are the test conditions somehow making single-hose air conditioners not look blatantly shitty?
The key piece is that the DOE is using a weighted mix of two test conditions: one at outdoor temperature 95°F/35°C (representing “the hottest 750 hours of the year”), the other at outdoor temperature 83°F/28.3°C (representing some kind of average temperature during times when the AC is used at all). For both cases, the DOE assumes that the indoor temperature (i.e. the air which the single-hose air conditioner is taking in) is at 80°F/26.7°C. So for the average-outdoor-temperature case, they’re assuming conditions where there is only a 3°F/1.6°C temperature difference between indoors and outdoors! Obviously infiltration has almost no effect under these conditions… but also do people even bother setting up a portable air conditioner only to bring the temperature from 83°F/28.3°C down to 80°F/26.7°C?
Now for the key question: what weights does the DOE use in the weighted mix of these two conditions? 20% weight on the 95°F/35°C outdoor condition, 80% weight on the 83°F/28.3°C outdoor condition.
Bottom line: the DOE’s SACC/CEER rating puts about 80% of its weight on a test condition in which the indoor and outdoor temperatures are only 3°F/1.6°C apart.
This document has a good dense summary of the relevant formulas. In particular, Qs_95 and Qs_83 on page 74038 are the heats lost to infiltration under the two conditions. Those are incorporated into ACC95 and ACC83 respectively, which are then combined with the weights on page 74039.
One more interesting observation: if we assume that the efficiency loss from infiltration is basically-zero for the 83°F/28.3°C condition, then in order to get an overall efficiency loss of 20-30% for one-hose vs two, the efficiency under the 95°F/35°C condition would have to be somewhere between zero and negative. (Which is possible, given the way the test is setup—it would mean that running the AC in an 80°F/26.7°C house when it’s 95°F/35°C outside makes the house hotter overall. In other words, the equilibrium temperature of the room with that AC cooling it is above 80°F/26.7°C.) I would guess that the efficiency under the 95°F/35°C condition is somewhat higher than this simple estimate implies (since there will be some losses under the the 83°F/28.3°C condition), but I think it is a reasonably-realistic ballpark estimate at the temperatures given.
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).
If you wouldn’t mind one last question before checking out: where did that formula you’re using come from?
From PickHVAC.com, “What is a good CEER rating?”:
From the Pro Breeze single-hose AC product description on Amazon:
I haven’t looked into the % efficiency loss measurements, but I think it’s interesting that you can still figure out that this is a crap AC if you’re willing to trust this website.
Portable units have to meet a much weaker standard. I actually pushed for a more stringent standard on these products when I was consulting for the Appliance Standards Awareness Project.