Temp difference between register and return

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  #1  
Old 08-12-10, 10:25 PM
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Temp difference between register and return

What should the temperature difference be between the conditioned air coming through floor register and the return air being drawn back through the return grill?

I had energy survey and my difference was 23 degrees warmed at the return than it was at the register.

I sealed several leaks in my ducts (2 being pretty bad where the supply line had actually come loose fro the trunk. I sealed them with duct sealing paste.

I now have a reading of 12 degrees. 50 degrees at the register and 62 degrees at the return with my thermostat set on 76 degrees.

I also found the return was not even sealed off at all at the top nor calked around the sides. The top was completely open which was pulling hot air from the attic. I sealed it completely with 3 1/2 inch styrofoam and calked around all sides real good. I am sure this helped also.
 
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  #2  
Old 08-13-10, 04:48 AM
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62 at the return grille face WOW? NO WAY unless your indoor temp is around 62 are you hanging meat in there?. Seriously you need to go back and recheck your temp readings that does not sound right at all. Take a reading at the supply closest to the unit and then right at the face of the return grille both should be Dry Bulb readings. On a properly designed and properly operating system if your tstat is set at 76 and the ambient outside temp is say 90 with a indoor temp of around 76 you will normally see a 54 to 58 reading at the floor supply register closest to the unit and a approx reading of 76 at the face of the return grille. All approximations of course.
 
  #3  
Old 08-13-10, 09:20 AM
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Originally Posted by Former Member View Post
62 at the return grille face WOW? NO WAY unless your indoor temp is around 62 are you hanging meat in there?. Seriously you need to go back and recheck your temp readings that does not sound right at all. Take a reading at the supply closest to the unit and then right at the face of the return grille both should be Dry Bulb readings. On a properly designed and properly operating system if your tstat is set at 76 and the ambient outside temp is say 90 with a indoor temp of around 76 you will normally see a 54 to 58 reading at the floor supply register closest to the unit and a approx reading of 76 at the face of the return grille. All approximations of course.
************************************************
I use a air conditioning thermometer (Igloo) which is a dry bulb one.

I have retaken the temperature difference:

Outside temp: 92
Inside temp 76 (Thermo. set on 76)
When compressor cut on, I measured the temp at the register the closest to the unit and stuck the thermo into the grill about 1/3 of the way from the bottom of the grill.

Measured temperature is: 49 at register
Measured temperature is: 64 at return.

I have two returns and both measure the same.

I had an energy survey done by my local power company and they measured it and it was 22.

They said it should read around 15 if it is working properly.

I went under the house and used duct sealing paste to seal up some air leaks that they found during the inspection as well as some they failed to find. Before I did that it was colder under my house than it was in my house.

I did this yesterday before measuring the temperatures.

According to their survey, my unit is now putting ut good since it is near the 15 degree difference.

I have measured it several time with the same results. At no time does the temerature at the return match the thermostat set temperature.

Thanks

Tom
 

Last edited by TGMcCallie; 08-13-10 at 09:23 AM. Reason: correct tape to paste and added more info.
  #4  
Old 08-13-10, 09:41 AM
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Where are your returns ? Aren't they located in your living room ? bed room ? hall way ? the air temp there should be 76, therefore your return air temp should also be 76. they are the same air. I don't understand why you get 64, something is missing.
 
  #5  
Old 08-13-10, 02:43 PM
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Both of them are in the central hall. It is 76 degrees in the hall and the rest of the house. The thermostat is set on 76.

I have taken this now 3 times and this is the true reading.

The energy survey technition from the Power Board took it first before I repaired the air leaks and he got a 22 degree difference so I guess both of us could not be wrong in the way you take the reading.

It would stand to reason that the temperate would be the same, but it is not and don't ask me why as i do not know.

Maybe someone else on this forum may know the answer.

I know the thermometer I am using is correct because if I move it up to next to the thermostat it is 76 degrees. The automic clocks in two locations in my house also support the 76 degrees.

Would anyone know the reasoning behind this.

I have called the energy survey man and he tells me that this is the proper way to take the reading and this supports whether or not your airconditioning system is doing what it shold be doing. He said anything 15 degrees difference or lower is excellent. He said anything above that indicates your system is not performing as it should.

They do hundreds of energy surveys each week.

Tom
 
  #6  
Old 08-13-10, 03:45 PM
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Something wrong here. we are not measureing the same stuff. Usually the temperature split (between cold air and return air) should be at least 15 F degrees. prefer to have 18 or 19 for newer system. below 15 is no good. The energy survey guy is measuring something else.. Let me ask you another question, does your return register blow air out or suck air in ? What is the temperature at about 1 foot out from the return register?
 
  #7  
Old 08-13-10, 06:23 PM
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Smile

I beg your pardon. A air conditioning return will only such air never blow it out unless maybe you have your air conditioning hooked up backwards. HA..Ha...

You don't make sense. If the temperature difference between your return (back to system) and your register (supply cold air from the airconditioner) is higher (say 20+ as you stated) then your system would not be doing as good a job of operating than it would be if your temperature difference was 15 degrees. Think of just a moment. If your temp out of the register was 50 and you temp at the return was 15 you would have a 0 difference so in your theory your system would not be cooling good at all but if the register temp was 50 and your return temp was 80 with your thermostate set at 76 then your system would be working wonderfully. Don't make sense to me.

I suppose you mean for me to measure the temperature 1 foot away from the return while the system is running and give you the temperature. I will do that just as soon as the system comes back on. It is now 81 outside and my system is set on 76 and holding that temperature and the unit is not cycling on very often because it is doing a good job of cooling.

As sooon as it does come back on I will supply you the reading as you asked.

I will also call the man from the power board Monday and tell him that I am participating on a air condition forum and one of the members thinks he is not measuring it correctly. Maybe he needs to be retrained but I don't think so.

Why is it that before I stopped the air leaks in my ducts my difference rearing was 22 and the air conditioner ran constantly and never turned off and now that I have stopped up the air leaks and got the different spread down to 15 the air conditioner is cycling on and off and holding the set temperature which I did not do before. It would be set at 76 and it would be 80 degrees in my house and ;the unit would run all the time.

Riddle me that.

Tom
 

Last edited by TGMcCallie; 08-13-10 at 06:26 PM. Reason: Other thoughts
  #8  
Old 08-13-10, 06:36 PM
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Originally Posted by clocert View Post
Something wrong here. we are not measureing the same stuff. Usually the temperature split (between cold air and return air) should be at least 15 F degrees. prefer to have 18 or 19 for newer system. below 15 is no good. The energy survey guy is measuring something else.. Let me ask you another question, does your return register blow air out or suck air in ? What is the temperature at about 1 foot out from the return register?
************************************************
I measured temp 1 foot out from the return supply register and it was 64:
I measured temp 1 foot out from the return grill and the temp was the same 64.

This is not how the energy man measured it and told me the same. He stuck the thermometer down in the register and also down in the return grill to measure the temperature not 1 foot away.
 
  #9  
Old 08-13-10, 06:52 PM
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I knew you are going to say that. but the reason I asked you if your return register blow the air out(not in) is because all your measuremenrts are completely opposite to mine, so the only way I can explain your theory is your air blow is also backward. Well, Let me say this again, 20 is better than 19, which is better than 18, better than 17,16,15. Below 15 is no good. WHy are you keep saying the other way, I don't know. Mike, Former, Jeg... please help, this one is over my head.
 
  #10  
Old 08-13-10, 07:26 PM
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Where are you located? Are you in a arid climate or humid climate. That make things entirely different.

If you live in a arid climate such as AZ you are correct 20 would be better than 15.
If you live in a humid climate such as North Georgia (where I do reside) this 15 is way better than 20.

Now does that make sense to you?

By the way I still don't follow you on your comments about direction the air is blowing it can only blow one way.

My numbers would be completely opposite to yours if you lived in an arid climate since I live in a extremely humid one. 100 degrees and high humidity is HOT as Hell where 100 degrees in AZ or NV and very dry is not. The humidity makes the difference in his your unit works.

Hope this helps.

Tom
 

Last edited by TGMcCallie; 08-13-10 at 07:29 PM. Reason: After thought
  #11  
Old 08-14-10, 05:21 AM
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take your temps starting at the air handler then work the registers...range should be 18F diff.the reurn to the discharge right off the air handler.if you see anything higher then the last return grill the unit is sucking atti air in and adding load.the discharge registers and duct run out must be supper sealed to deliver the coldest air with the most force.....flex duct turn tooo many kill the velocity leaking as you founf..if your air handler is laying in the attic did you seal the bottom section where the discharge duct connects..big leak point after installers tape dries out and you end up cooling the attic.back on the flex run outs if you can tighten it up minimize bends and pinches air discharge will gain from the effort..
 
  #12  
Old 08-14-10, 05:25 AM
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Airman, Mike, Former, Jeg.. Houston...

This one is over my head, needs your opinion. I knew the humidity level affects the temperatures and the cycle time of the unit, but it should not turn the comparison numbers upside down. TGM, I do not understand that your central hall temp is 76, but your return register air temp is only 64, I thought may be that register blow out cold air (instead of suck in room air), therefore it is 64. By the way, I am in Texas, hot and humid.
 
  #13  
Old 08-14-10, 06:25 AM
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Maybe we can get this thing figured out. If we are doing it the same way, then it should be simular since we are both in humid locations.

Maybe someone can figure out why.

Tom
 
  #14  
Old 08-14-10, 12:04 PM
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Did that survey person tell you why 15 is better than 20 ?
 
  #15  
Old 08-14-10, 02:58 PM
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No he did not, but just think if you have 50 coming out and 65 going back in then you are only dropping 15 degrees where if you have 50 out and 75 in then you are dropping 25 degrees so I guess a lot of things play in this such as not good enough insulation in your house or air leaks somewhere that would cause you to loose effeciency. The air coming out is mixing with hotter air for some reason or the other. This could be caused by a variety of things that have nothing to do with your aircoditioner but your windows, doors, insulation, etc.

I will call him and ask though.


Tom
 
  #16  
Old 08-14-10, 04:13 PM
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If the return temp at the grill is 14 degrees colder than the setpoint then the return grill is in a bad location (inefficient) or the A/C is way over sized for the home (inefficient).
 
  #17  
Old 08-14-10, 06:18 PM
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You are greatly misinformed. The temperature at the return grill is NOT 15 degrees colder than the thermostat set temperature. If you will read all the posts I made, there is 15 degrees difference in the temperature coming out of the register than the temperature going back through the return grill. Thus, there is a drop of 15 degrees between the two NOT 15 degrees colder at the return than the thermometer set temperature.

If that were the case then I would more likely think that the thermostat was not calibrated properly and you would have a very very very high bill.

Tom
 

Last edited by TGMcCallie; 08-14-10 at 06:18 PM. Reason: none
  #18  
Old 08-14-10, 06:25 PM
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Originally Posted by TGMcCallie View Post
You are greatly misinformed. The temperature at the return grill is NOT 15 degrees colder than the thermostat set temperature. If you will read all the posts I made, there is 15 degrees difference in the temperature coming out of the register than the temperature going back through the return grill. Thus, there is a drop of 15 degrees between the two NOT 15 degrees colder at the return than the thermometer set temperature.

If that were the case then I would more likely think that the thermostat was not calibrated properly and you would have a very very very high bill.

Tom
Thermostat setpoint of 76 - return air temp of 64 is 14 degrees.
 
  #19  
Old 08-14-10, 08:22 PM
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TGM, I start understand the way you are thinking. It is the opposite to all of ours. You said 50-65 drop is better than 50-75 drop because the room is leaking, therefore the room is hot, so return 75 is not as good as return 65, Is this the way you are thinking ??
 
  #20  
Old 08-15-10, 06:34 AM
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yes, that makes sense. This is the difference, which in this case is a drop in loss of cooling. Thus if you lost 15 degrees of cooling you are better off than if you lost 20 degrees of cooling.

If you had no difference at all in the two points then your temperature in your house would be the temperature that is coming out of your register, in my case 50 degrees. By the time it leaves your register and travels all through your house in order to get to the return it has lost 15 Degrees in cooling efficiency. If you had lost 20 degrees of cooling efficiency, your system was not as efficient.

I am talking about and figuring out the ability of your house to be save energy. If it has less of a drop in cooling loss you are more efficient in your attempt to save energy. That was what the entire energy survey was about.

I am going to try to get in touch with the man that came out here and see if he can explain it so it will be better understood.

TGM
 

Last edited by TGMcCallie; 08-15-10 at 06:43 AM. Reason: correct spelling
  #21  
Old 08-15-10, 06:48 AM
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Originally Posted by clocert View Post
TGM, I start understand the way you are thinking. It is the opposite to all of ours. You said 50-65 drop is better than 50-75 drop because the room is leaking, therefore the room is hot, so return 75 is not as good as return 65, Is this the way you are thinking ??
************************************************

Check this out: Air Temperature Diagnostic Techniques

In this article he talks about temperature drop measured from many places but one of the places is from register to grill. He also talks about humidity etc.

TGM
 
  #22  
Old 08-15-10, 07:48 AM
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Now I understand. You misunderstood the term 'temperaure split, or temperautre drop' that we used in HVAC world, therefore you use that term in the wrong area. The 'temperature split' is used to measure how good the AC unit is, how much heat the AC unit can remove from the intake to the output. this has nothing to do with the house insulation or how much the heat lost in the house. Often we test the AC unit in the factory in open area, we measure the temp one inch before the air goes in the AC unit, and one inch after the air come out at the other end, the different of the air temperature drop we call it temperature split, there is no house involved at all. So, if you give me 70 degree air, if my unit can give you 60 degree air back, my drop effieicency is 10, If I use a bigger AC unit or a better one, I can give you 50 degree air, because this unit removes more heat from the air. so my drop effieiency is 20. Hope this make sense.
 
  #23  
Old 08-15-10, 10:02 AM
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Then what I am doing, does it make sense. The way I was measuring it made sense to me because before I stopped the leaking air in several places, I went from a 20 degree difference to a 15 degree difference. The energy man said they wanted to get it down to 15 to be better.
 
  #24  
Old 08-15-10, 01:00 PM
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The way you'r doing is right only at that moment, because you seal the leaks, so more cold air comes in, and your room temp drop, therefore your split drop from 20 to 15 at that time(you assume the cold air temp stay the same) . but if you let the AC unit keep running, the cold air will get colder, so your split will go back to 20. House insulation should not affect the temp split, but it will cause the unit to run longer, if the unit is not too bad, it will eventually get to the original split point. This is the way we measure an AC unit, it does not matter where you put it.
 
  #25  
Old 08-15-10, 04:57 PM
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Arrow Energy survey rater is wrong...

Originally Posted by TGMcCallie View Post
Both of them are in the central hall. It is 76 degrees in the hall and the rest of the house. The thermostat is set on 76.

I have taken this now 3 times and this is the true reading.

The energy survey technition from the Power Board took it first before I repaired the air leaks and he got a 22 degree difference so I guess both of us could not be wrong in the way you take the reading.

It would stand to reason that the temperate would be the same, but it is not and don't ask me why as i do not know.

Maybe someone else on this forum may know the answer.

I know the thermometer I am using is correct because if I move it up to next to the thermostat it is 76 degrees. The automic clocks in two locations in my house also support the 76 degrees.

Would anyone know the reasoning behind this.

I have called the energy survey man and he tells me that this is the proper way to take the reading and this supports whether or not your airconditioning system is doing what it shold be doing.

He said anything 15 degrees difference or lower is excellent. He said anything above that indicates your system is not performing as it should.
They do hundreds of energy surveys each week.

Tom
Well, the energy survey rater is wrong.

A 15-F split is only okay if there is a fairly high indoor humidity, however, at 50% RH or lower @ 400-CFM per-ton of cooling, the temp-split should be 18 to 22 or higher depending on how low the humidity is that's going through the indoor coil.
 
  #26  
Old 08-15-10, 06:24 PM
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He is right or wrong, I could not know.
What I do know is this:
The reason I asked for the survey was the fact that my air conditioning was running constantly and not pulling my temperature down to the set temp. This was with 93 degrees etc outside.

He checked the temperature drop between the register and the return grill and said it was 20 degrees and they wanted it around 15.
He then went into my crawl space where my ducts were located. It was colder under my floor then it was in my house.
He found several big air leaks and told me to use duct sealing paste to repair them. He also found that one of my returns was completly open on the top and thus drawing hot air down from the ceiling.

I stopped all the leaks with sealing paste and sealed off the top of my return with 3 1/2 inch solid styrofoam which I also sealed around the edges of the foam.

I then rechecked the temp drop between the register and the return grills and it was 15 degrees difference. That is a 15 degree cooling loss vs 20 degree originally.

After I did that I set my system at 75 and it maintains that temperature. It cycles on and off and it is very comfortable in my house. This is with 93 and 99 etc outside.

And by the way I have a Carrier 5 ton package unit/ gas heat and electric airconditioning. The unit is on the outside of my house. My unit is 5 years old. My house is 2400 square feet of living space.

Right or wrong, now my system is working very good.

Also, I have had 2 different service people out here and they did not even find the air leaks and said my system was working fine even though it ran constantly from morning to late at night.

Tom
 

Last edited by TGMcCallie; 08-15-10 at 06:28 PM. Reason: Added more info
  #27  
Old 08-15-10, 06:31 PM
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Originally Posted by HVAC RETIRED View Post
Well, the energy survey rater is wrong.

A 15-F split is only okay if there is a fairly high indoor humidity, however, at 50% RH or lower @ 400-CFM per-ton of cooling, the temp-split should be 18 to 22 or higher depending on how low the humidity is that's going through the indoor coil.
***********************************************

There is no indoor coil. The coil is outside and it is very very humid outside in North Georgia.
 
  #28  
Old 08-15-10, 06:41 PM
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Originally Posted by TGMcCallie View Post
***********************************************

There is no indoor coil. The coil is outside and it is very very humid outside in North Georgia.
He is referring to the A/C evaporator coil which is indoors in your furnace/air handler. The coil outside is the condensing coil for the A/C.
 
  #29  
Old 08-15-10, 07:03 PM
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Originally Posted by TGMcCallie View Post
He is right or wrong, I could not know.
What I do know is this:
The reason I asked for the survey was the fact that my air conditioning was running constantly and not pulling my temperature down to the set temp. This was with 93 degrees etc outside.

He checked the temperature drop between the register and the return grill and said it was 20 degrees and they wanted it around 15.
He then went into my crawl space where my ducts were located. It was colder under my floor then it was in my house.
He found several big air leaks and told me to use duct sealing paste to repair them. He also found that one of my returns was completly open on the top and thus drawing hot air down from the ceiling.

I stopped all the leaks with sealing paste and sealed off the top of my return with 3 1/2 inch solid styrofoam which I also sealed around the edges of the foam.

I then rechecked the temp drop between the register and the return grills and it was 15 degrees difference. That is a 15 degree cooling loss vs 20 degree originally.(?)

After I did that I set my system at 75 and it maintains that temperature. It cycles on and off and it is very comfortable in my house. This is with 93 and 99 etc outside.

And by the way I have a Carrier 5 ton package unit/ gas heat and electric airconditioning. The unit is on the outside of my house. My unit is 5 years old. My house is 2400 square feet of living space.

Right or wrong, now my system is working very good.

Also, I have had 2 different service people out here and they did not even find the air leaks and said my system was working fine even though it ran constantly from morning to late at night. Tom
That is a 15 degree cooling loss vs 20 degree originally. (?)
A cooling LOSS; I don't understand what you are measuring.

The sensible temperature drop or sensible temp/differential between the Return & Supply should increase NOT decrease as the the duct system, especialy the Return Air leak was sealed.

Where were you measuring the Return Air temp at?

If it was measured just before the return air filter, then the hotter air from the attic would have increased the temp/differential.

I am a long time Tech since the early 1970s, concerning the temp/splits, you can count on it being correct.
Do some Internet searches.

You did excellent work sealing those duct leaks, but normally that should not have reduced the temp/split.
 

Last edited by HVAC RETIRED; 08-15-10 at 07:13 PM. Reason: Added some clarity...
  #30  
Old 08-15-10, 09:14 PM
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TGM, I think the energy survey man lead you to the wrong direction. I don't know what they are trying to do, but many of us here are AC professionals. we fix hundreds of AC systems per year, as mentioned, HVAC RETIRED has more than 40 years experience in this field. Let's forget about the theory for now. I see you said your temp split is 15. this indicates your system is performing at the border line. It is only acceptable (as HVAC RETIRED mentioned), not a good performing system. For a good AC system, we like to see at least 17 or 18 split or even higher..To tune up a system, you can not just measure the temperatures & tape or seal the leaks, you need to check both AC line pressures, make sure the freon level is right, check airflow volumn, check amperage draw,,, etc. these areas can not be done by a DIY home owner, you need to call a licensed AC professional to help.
 
  #31  
Old 08-16-10, 06:54 AM
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Had a professional do a tune up in May. He did not even find the air leaks. I don't think he did anything but spray acid on the coils and hose it off. Did not even check the freon.
I am calling him back today. They are one of the companies that are on the energy survey companies list of professionals to do the job.

I just don't understand. I can open back up the air leaks and get it back to 22 which was where it was before I corrected the air leaks and the faulty return. If I do that according to your figures it should be better and run constantly like it did before.

I just do not understand.
TGM
 
  #32  
Old 08-16-10, 08:20 AM
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Just throwing something out to be gnawed on....no expert of course....

If the ducting after the coil was too small or blocked somehow, but the leaks allowed slightly more airflow across the coil (closer to optimum)...but then the leaks were all sealed, thus restricting airflow.....could that account for the temp change??

It seems slightly counter intuitive....but I was just wondering.
 
  #33  
Old 08-16-10, 09:35 AM
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Arrow That is the way that works...

Originally Posted by TGMcCallie View Post
***********************************************

There is no indoor coil. The coil is outside and it is very very humid outside in North Georgia.
Even if it's an outdoor packaged unit with the evaporator inside the unit, it is insulated against the outdoor air.

A 15-F split is only okay if there is a fairly high indoor humidity, however, at 50% RH or lower @ 400-CFM per-ton of cooling, the temp-split should be 18 to 22 or higher depending on how low the humidity is that's going through the evaporator coil.

That is the way that works & the way it is...
 

Last edited by HVAC RETIRED; 08-16-10 at 09:36 AM. Reason: the evaporator coil...
  #34  
Old 08-16-10, 10:52 AM
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The reason you got 22 is because you did not wait long enough. Youhave to let the system stablelize itself first, wait he get to the T-stat set temp and after a couple of cycle on-off, then go ahead measure the temperature split. Plus once you have leaks, the airflow volumn comes to the room will be different (depends on where the leaks are, intake side or output side, they affect your air temp differently, there are many variables here). So at this point, don't try to verify your theory by un-do your work. Stop leak and clean coils are two major tune up steps. It is good you did already. Next time you call a AC tech in to check your system, ask him to measure the 'subcool' and 'superheat' and other items we mentioned here, may cost you more. some people don't want to spend the money, but it is up to you.
 
  #35  
Old 07-12-13, 08:45 AM
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I know this is an old post but it was kind of comical reading two different perspectives arguing. I was surprised these two camps had the patience to keep this thread going for so long. The energy efficiency man is trying to measure how much heat is being lost in the house (bad) so the lower loss the better from an efficiency of insulation perspective. The other perspective is how efficient the AC unit is. How quickly can it suck heat out of the air provided to it. higher difference is better.

TGM did make things a little confusing with some of the numbers he provided though. he did state at some point that temp at the return was lower than the room temp. which can only be true if these two ducts systems are not completely isolated and or the return grill is too close to the register, sucking the air back before it had a chance to travel through the room. which is I don't believe is the case here either.

AC efficiency is done right next to the AC unit. HVAC professinals don't care where the air is coming from or where it is going. they just want to know how much heat they can extract from the air supplied to them.

Energy folks on the other hand are mostly concerned with the waste of energy through bad insulation / duct work. so the register and return to them are in the same room not by the AC/furnace. They're trying to measure how much heat is added to the cold air coming into the room.
 
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