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A/C Temperature Drops


inspectorwill
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I have always understood that the temperature drop from return to supply during AC operation should be between 14-22 degrees. Any more could result in condensation problems or freezing. However, after reading the Charging&TD Chart I downloaded from this forum, I am confused. AC unit yesterday was 1-2 years old with new ducting and had temperature drops exceeding 30 degrees. Return air temp was 68 and supply temp (closest to coil) was 36 degrees. The filter was clean and the return air seemed correctly sized. Do different systems have different temperature drops? 32 degrees seems excessive. Thanks.

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I have always understood that the temperature drop from return to supply during AC operation should be between 14-22 degrees. Any more could result in condensation problems or freezing. However, after reading the Charging&TD Chart I downloaded from this forum, I am confused. AC unit yesterday was 1-2 years old with new ducting and had temperature drops exceeding 30 degrees. Return air temp was 68 and supply temp (closest to coil) was 36 degrees. The filter was clean and the return air seemed correctly sized. Do different systems have different temperature drops? 32 degrees seems excessive. Thanks.

You bet it's excessive.

Marc

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I have always understood that the temperature drop from return to supply during AC operation should be between 14-22 degrees.

The differential of an AC system depends on both temperature and humidity. If you only look at temperature, you can't tell whether or not the system is working properly. These systems remove water from the air. That takes energy and reduces the temperature differential. If the indoor air is humid, the differential will be lower. If the indoor air is dry, the differential will be higher.

While it's true that most systems on most days will produce a differential between 14 and 22 degrees, it does *not* follow that any differential within that range is correct. An air conditioner producing an 18-degree differential might be improperly charged and working poorly.

Any more could result in condensation problems or freezing.

Not always. These kinds of simplistic statements are not accurate. A system producing a 25-degree differential might be working just right -- if the indoor air is really dry.

However, after reading the Charging&TD Chart I downloaded from this forum, I am confused. AC unit yesterday was 1-2 years old with new ducting and had temperature drops exceeding 30 degrees. Return air temp was 68 and supply temp (closest to coil) was 36 degrees. The filter was clean and the return air seemed correctly sized. Do different systems have different temperature drops? 32 degrees seems excessive. Thanks.

32 degrees is way excessive. There's no combination of temperature and humidity that would allow that much drop in a properly operating system.

The coils might not be properly sized, the blower speed could be too slow, the system could be improperly charged, the metering device screwed up, etc, etc.

- Jim Katen, Oregon

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We had a speaker at a recent ASHI chapter meeting who said higher temp diffs are normal with the newer systems using Puron as refrigerant.

I'm not sure if 30+ diff is in the range but does anybody have input on the differences or wider tolerances one could expect with the newer systems?

Sometimes when an educator speaks, he's actually saying more about himself than the topic being discussed.

My own AC uses R410A which is the same chemical as Puron. Previous system from 3 yrs ago was an R 22. Delta T is the same. The engineers design these systems to accomplish whatever delta T is specified, not the other way around.

Ain't mocking the speaker, just suggesting that he's a little off.

Marc

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Not always. These kinds of simplistic statements are not accurate. A system producing a 25-degree differential might be working just right -- if the indoor air is really dry.

That's an interesting observation. It takes energy to condense vapor into liquid.

We tend to look at how things operate in our neck of the woods. No one here runs A/C unless it is warm/hot outside and with the heat comes humidity. Maybe Arizona not so much. Not having been to a warm dry climate, while testing a A/C system, I'm at a loss to give an authoritative opinion. Have you tested A/C systems in this type of enviroment Jim?

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Not always. These kinds of simplistic statements are not accurate. A system producing a 25-degree differential might be working just right -- if the indoor air is really dry.

That's an interesting observation. It takes energy to condense vapor into liquid.

We tend to look at how things operate in our neck of the woods. No one here runs A/C unless it is warm/hot outside and with the heat comes humidity. Maybe Arizona not so much. Not having been to a warm dry climate, while testing a A/C system, I'm at a loss to give an authoritative opinion. Have you tested A/C systems in this type of enviroment Jim?

Jim's referring to latent heat removal and sensible heat removal. It's a rock solid concept. [;)]

Marc

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Not always. These kinds of simplistic statements are not accurate. A system producing a 25-degree differential might be working just right -- if the indoor air is really dry.

That's an interesting observation. It takes energy to condense vapor into liquid.

We tend to look at how things operate in our neck of the woods. No one here runs A/C unless it is warm/hot outside and with the heat comes humidity. Maybe Arizona not so much. Not having been to a warm dry climate, while testing a A/C system, I'm at a loss to give an authoritative opinion. Have you tested A/C systems in this type of enviroment Jim?

Jim's referring to latent heat removal and sensible heat removal. It's a rock solid concept. [;)]

Marc

Really????

Jim should know where I'm going with this. Given the lack of humidity how many more degrees delta T might we see? (no fair asking your HVAC guru).

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Not always. These kinds of simplistic statements are not accurate. A system producing a 25-degree differential might be working just right -- if the indoor air is really dry.

That's an interesting observation. It takes energy to condense vapor into liquid.

We tend to look at how things operate in our neck of the woods. No one here runs A/C unless it is warm/hot outside and with the heat comes humidity. Maybe Arizona not so much. Not having been to a warm dry climate, while testing a A/C system, I'm at a loss to give an authoritative opinion. Have you tested A/C systems in this type of enviroment Jim?

Sure, that's the environment across much of the west, not just Arizona. Out here, it gets dryer as it gets hotter. (It's one of the reasons why vented crawlspaces aren't a big problem out here, even with uninsulated AC ducts. The humidity is so low that condensation isn't much of a problem.) Most of the time, the systems I test produce 18-22 degree differentials. But sometimes I'll get a 24 or 25 degree differential in a system that I'm convinced is working properly.

- Jim Katen, Oregon

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. . . Jim should know where I'm going with this. Given the lack of humidity how many more degrees delta T might we see? (no fair asking your HVAC guru).

It's a moving target. The attached chart lays it out though.

Download Attachment: icon_adobe.gif Temperature _Differential_Chart_Carrier.pdf

75.9 KB

Have you tested any systems in Arizona Jim?

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. . . Jim should know where I'm going with this. Given the lack of humidity how many more degrees delta T might we see? (no fair asking your HVAC guru).

It's a moving target. The attached chart lays it out though.

Download Attachment: icon_adobe.gif Temperature _Differential_Chart_Carrier.pdf

75.9 KB

Have you tested any systems in Arizona Jim?

I have not.

- Jim Katen, Oregon

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I have found that charts and such maybe good on a brand new, correctly installed system however not so much on a system that has seen a decade worth of dirt.

Any chart is useless if the coils are dirty or if dirty filters are in place. It isn't a substitute for a proper inspection; it's an adjunct to one.

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