Jump to content

65 degrees outside, how much should A/C cool?


esch
 Share

Recommended Posts

How much of a temperature drop should I expect with the return air at or around 65 degrees? 15-20?

Thanks, Matt

Just asking because a Realtor was saying it shouldn't drop it below 60 (my reading)

The Realtor said it shouldn't drop below 60°??? While it's not out of the realm of possibility that the Realtor worked in the trades prior to becoming a Realtor I highly doubt it. I would have asked how they arrived at that theory just to have a little fun. Were they saying that the supply temperature shouldn't drop below 60° if the return air is 65° or that you shouldn't run the system below 60° outdoor temperature? I love talking heads.

There has been a standard that you shouldn't run the ac below 65°. The reason for this being that the refrigerant and oil can blend together and when the compressor starts it will pump out the liquid refrigerant and as we all know you can not compress a liquid - something will break like valves or a crankshaft. While this is very true the standard of 65° outdoor temperature is off. I start ac systems down to 50° outside. Condensing units (the outside unit) have a crankcase heater that keeps the compressor warm thereby keeping the refrigerant in a vapor state and separated from the compressor oil. Most new units have a scroll compressor that is very forgiving for pumping small amounts of liquid. The only time I wouldn't start the unit is if the electrical power has been shut off - no power no crankcase heater.

Now if the Realtor was referring to evaporator delta T (temperature difference across the evap coil) implying that that there will only be a 5° delta T across the coil that's just plain silly.

It is true that the a/c will not run its best with low outdoor and indoor ambient conditions however you will see more than a 5° delta T for sure. When it is cool inside and outside I start the unit to make sure it will run, I feel the suction line to make sure it is getting cold and I rarely take supply air temperatures. I explain the reason for this in my report. If I were to check delta T I would expect anywhere from 10 and up.

To understand why the ac doesn't run as well in cold outdoor/indoor temperatures is a whole separate discussion (needs proper head pressure which correlates to a full column of liquid among other things).

In the end I think it's another case of a talking head just repeating what they heard somewhere before.

Link to comment
Share on other sites

How much of a temperature drop should I expect with the return air at or around 65 degrees? 15-20?

Thanks, Matt

Just asking because a realtor was saying it shouldn't drop it below 60 (my reading)

Even at 65 degrees, the temperature of the outdoor air has little effect on the temperature differential across the evaporator coil. The chilling of the indoor coil is caused by the evaporation of refrigerant from the liquid state to the vapor state. As long as it's receiving liquid refrigerant, the indoor coil doesn't care how cold it is outside. Three factors will influence the temperature differential: the temperature of the return air, the humidity of the return air, and the speed of the air passing over the evaporator coil.

It's not possible to tell if an air conditioner is working properly without knowing the temperature and the humidity of the return air.

-Jim Katen, Oregon

Link to comment
Share on other sites

I was always taught that if the outside temp is about 74 or higher, a normal temp drop (delta) of 14-22 degs is normal, but if less than 74 degs, the temp drop may only be 7-12 degs. Is this wrong?

Yes. Certainly. Where did you hear that?

- Jim in Oregon

I don't recall exactly where I was taught that but it was one of the annual NAHI seminars or a joint ASHI/NAHI seminar from about 6 or 7 few years ago that I went to.

SO, if what I was taught was wrong, should I expect to see the same 14~22 deg delta no matter what the exterior temp is above 60~62 degs? I've run A/C systems in the mid to upper 60s and found delta to be around 10 degs and thought this was normal for that temp.

Link to comment
Share on other sites

. . . I don't recall exactly where I was taught that but it was one of the annual NAHI seminars or a joint ASHI/NAHI seminar from about 6 or 7 few years ago that I went to.

SO, if what I was taught was wrong, should I expect to see the same 14~22 deg delta no matter what the exterior temp is above 60~62 degs? I've run A/C systems in the mid to upper 60s and found delta to be around 10 degs and thought this was normal for that temp.

Go back to Thor's post and download his TD chart. The temperature differential will be governed by the temperature and humidity of the return air (and, of course, the blower speed). I'm confused about Thor's comment about it only being good for fixed orifice systems. I've found that, if they're working properly, both fixed orifice and TXV systems produced differentials within a few degrees of the chart.

I've attached my version of that chart that's a little easier to read.

In my climate, the outdoor temperature is pretty much irrelevant with regard to the TD across the evaporator coil. But I live in a moderate climate. If the outdoor temperatures were really extreme, I suppose that there'd be some effect on TD.

- Jim in Oregon

Download Attachment: icon_adobe.gif Temperature _Differential_Chart_Carrier.pdf

75.9 KB

Link to comment
Share on other sites

I was always taught that if the outside temp is about 74 or higher, a normal temp drop (delta) of 14-22 degs is normal, but if less than 74 degs, the temp drop may only be 7-12 degs. Is this wrong?

Yes. Certainly. Where did you hear that?

- Jim in Oregon

I don't recall exactly where I was taught that but it was one of the annual NAHI seminars or a joint ASHI/NAHI seminar from about 6 or 7 few years ago that I went to.

SO, if what I was taught was wrong, should I expect to see the same 14~22 deg delta no matter what the exterior temp is above 60~62 degs? I've run A/C systems in the mid to upper 60s and found delta to be around 10 degs and thought this was normal for that temp.

Matt,

I want you to know that I've repeated stuff here that I've heard at CE classes or from licensed peeps that's been wrong, as well. That's the beauty of TIJ. They spank you when you're wrong, but are kind enough to explain why so you have a better understanding.

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...