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Now that my partner and I are working from home indefinitely, we decided to install central air conditioning. We got quotes for a unit with a 17 SEER efficiency rating, and ultimately picked this one. Note that 17 SEER is about 30% more efficient than the minimum recommendation of the 2019 energy code.

After the unit was installed, I saw the Energy Guide label, which shows a 14.5 SEER rating:

Energy Guide label

You can see that the model number (YCG18B21S) matches, but the SEER rating does not. What's going on here?


Based on comments and answers, I took a look at the indoor (evaporator) unit, which is made by ADP and has model number HE31124A155A0004AP, which appears to be one of these.

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  • Just for us non-Americans, I a lower SEER is worse than a higher one? Can it be that the better rate is for a newer revision of the unit but you got an older unit?
    – jmk
    Jun 24 at 9:55
  • Related question on diy.SE.
    – LShaver
    Jun 25 at 2:22
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@jmk yes the Seasonal energy efficiency ratio or SEER a high number is better. The system won’t let me comment must be because I am new on this page of the SE .

There are similar questions asked on DIY home improvement.

Basically the outside unit is the compressor and condenser coil. The inside unit could be a very basic coil and orifice and your system would be a low seer value system.

By increasing the coil inside the evaporator to have more fins so the heat transfer is better this brings up the efficiency.

To gain additional efficiency we ad active controls called txv (thermal expansion valves) these valves regulate the amount of liquid that is allowed into the evaporator by adjusting the valves based on the temperature.

By metering the refrigerant slowly or just what is needed the compressor cycles less often increasing the efficiency.

The inside coil size and type with a TXV and possibly a multi speed fan are all that can be done there to increase the efficiency of the system.

To increase the overall system beyond that of txv and evaporator coil the outside unit can have a better condenser coil. (Rare but I did this 1 time when dogs peeing on the outside coil caused it to fail) The other thing that can be done outside is an inverter driven compressor. I have not upgraded to an inverter.

Usually the outside unit is chosen and we match to it or slightly better.

The cost although off topic can be 2x going from a 14 seer system to a 21 seer system.

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Thanks to MonkeyZeus' answer to a related question on DIY.SE, I sorted this out.

The AHRI product search lets you see the SEER rating for different combinations of A/C indoor and outdoor units. (AHRI is the trade organization that maintains efficiency standards and testing procedures.) If I enter just the model number of the outdoor unit (YCG18B21S), there are 3,325 possible combinations, with SEER ratings ranging from 10.05 up to 19.

Sorting out how to search on the indoor unit is a bit more complicated, as they give the results as regular expressions (I think that's what that is?). For example:

  • My indoor unit has part number HE31124A155A0004AP
  • AHRI lists this as H,YE31(1,2)24, which can be read as H or Y, E31, 1 or 2, 24. Everything after that can be ignored.

When I search with this string, there are four matching results (depending on the furnace the unit is paired with), all of which have a SEER rating of 17.00, which is what I expected.

AHRI search results

The interesting thing is that the Energy Guide labels that are linked for each one show a rating of 15 SEER. It seems the Energy Guide labels are just designed to confuse!

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