delta T ?

[bananaboy]

on a Residential A/C

Between me an my fellow coleage we have that discussion.

I say 20-25 F

he says 10-15 F.

My argument is: Temp. after Evap. is approx. 50 F - and in the Return is 75 F.

Help me out - who is right ?

[airman1]

20 to 25 is right ... if you live in the desert then lower delta t has some merit

[HVAC/Stud]

If you subtract 50 from 75, your right.

Now if you are asking what is the correct DT you need a chart and WB temp. Most will be happy with DT of 20. Your fan speed is the biggest factor on how much heat is being removed from the air.


STUD

[umd]

I am a little confused...isn't this a web site for professionals in the field. The very question indicates the author knows nothing about HVAC...

Just my opinion.

[airman1]

hey i know plenty of pros who arent

[bananaboy]

umd,

what is your opinion - if you have one ?

I am sure you are a Super Professional, that is why I humbly ask you for your opinion.

Let's see what are you up to? C'mon be a man, say it ...

[MadeinUSA]

Originally posted by umd
The very question indicates the author knows nothing about HVAC...

Just my opinion.

The man asked a specific question pertaining to Delta T, and showed where he was getting the temperatures to verify this. I think he knows a little more than just nothing, don’t you?

[tinknocker service tech]

this guy is not a pro but reasonable question
depending on blower speed
18 to 25 is good but super heat and subcool is the best way to check

[bananaboy]

madeinusa,

I took the readings right after the Evap. Coil, let's say 3-5 " on the Supply - and 12-24" before filter on the Return.

[tinner73]

20 degree TD

[berad]

Originally posted by umd
I am a little confused...isn't this a web site for professionals in the field. The very question indicates the author knows nothing about HVAC...

Just my opinion.

The guy is in the learning process. Wow, switch to decaf.

[umd]

Ok, I'll take the bait...The dry bulb delta T across the evap coil will be a function of the amount of sensible heat removal from the airstream and the amount of air going across the coil. Now if you were only doing sensible coiling (not removing any moisture) the formula would look like this: BTU/HR(sensible)=CFM X1.08 X DELTA T. Unfortunately in the real world you are usually removing some latent heat which is not measurable by dry bulb temperature. To accurately predict the delta T, you would need to know the wet bulb temperature of the entering and leaving air. You would also need to know the airflow across the coil. Using a psychrometric chart you could predict what your delta T would be by calculating the actual enthalpy per pound of heat in your entering and leaving air and use the formula: BTU/HR (total)= CFM X 4.5 X DELTA H (actual heat content of air).
The bottom line is this: your delta T could be anywhere from 10 to 20 degrees depending on the amount moisture in the air and the amount of air going across your coil. Low humidity equals high delta T; high humidity equals low delta T.

Sorry if you were offended.

[bananaboy]

umd,

I really thank you for your answer.

"Offended" - not.

a bit ticked-off by your first reply - yes !

Not anymore.

Thank you, again.

See you around.

[Andy Schoen]

C'mon boys, let's be more civil here.

Delta T of the air across the evaporator is a function of entering dry and wet bulb temperature, air velocity across the coil, and evaporating temperature. umd provides a good explanation of the psychrometrics involved. Be careful about "rules of thumb". I like tinknocker service tech's response best

[jiminator]

"Ok, I'll take the bait...The dry bulb delta T across the evap coil will be a function of the amount of sensible heat removal from the airstream and the amount of air going across the coil. Now if you were only doing sensible coiling (not removing any moisture) the formula would look like this: BTU/HR(sensible)=CFM X1.08 X DELTA T. Unfortunately in the real world you are usually removing some latent heat which is not measurable by dry bulb temperature. To accurately predict the delta T, you would need to know the wet bulb temperature of the entering and leaving air. You would also need to know the airflow across the coil. Using a psychrometric chart you could predict what your delta T would be by calculating the actual enthalpy per pound of heat in your entering and leaving air and use the formula: BTU/HR (total)= CFM X 4.5 X DELTA H (actual heat content of air).
The bottom line is this: your delta T could be anywhere from 10 to 20 degrees depending on the amount moisture in the air and the amount of air going across your coil. Low humidity equals high delta T; high humidity equals low delta T."

umd,
All that said, I get the feeling that you feel that delta T readings are sorta bogus, kinda like a "beer can cold and sweating" sort of measurement that is often misused/not required at all. Is there any real use for any delta T reading? Since there are so many factors involved in truly measuring this correctly (as you've written), and the results can vary so greatly (depending on so many more things than just the air temp difference), what's the use? Wouldn't proper superheat/subcooling calculations reveal air flow issues? Perhaps this was your point.
I'm just trying to ask what I feel might be the next reasonable question to ask about this topic. I don't go to the field anymore but would like to try to keep a bit sharp about these things. So, if my questions sound like I might already know the answer to them, I may have known at one time, but have forgotten the answers from lack of use. Thanks in advance for any info you might provide!

I love this website.
Jim

[umd]

Jiminator writes "Is there any real use for any delta T reading?"

Good question. I think the answer is yes. Heeding the advice of Andy regarding "rules of thumb" I think that on a system which is known to be properly installed and is operating at "steady state" conditions, delta T provides a good initial indicator of the overall performance. Here in the Northeast during the months of July and August we can generally "assume" that a home which is continously conditioned will have a relative humidity of around 50%. Bearing this in mind we might look for an initial delta T of 15 to 18 degrees. If the system has been out of service, the ambient RH will probably be higher and on start-up the delta T may be lower.

My point would be this: It is far more important to understand the theory behind what goes into the delta T of an evaporator coil than to look for "rules of thumb" to guide us.

Again, sorry if my initial post created any hard feelings (which I believe it did).

[docholiday]

I agree completely with UMD. His point is accurate and he makes yet another great case against rules of thumb.

Sensible capacity can be calculated using the temperature rise method and compaired to the mfg sensible capacities found in the literature if in fact you were under the ARI conditions used. However, as he said, latent heat has priority and will cut away from the sensible capacity of the system which in turn affects the delta T.

[mark beiser]

This reminds me of a problem I had with a customer that had a "friend in the buisness" that was advising her. Unfortunatly the "friend in the buisness" was a rule of thumb kinda guy that had no education in psychometrics.

You could check a system on a low humidity day and find a delta-t of 21º, then check it again on on another day with the exact same indoor and outdoor temperatures, but with higher humidity inside and find a delta-t of 17º.

My favorite are when home inspectors come into an 85º+ house, turn on the AC for 10 minutes and claim there is a problem with the AC system because the delta-t is only 12º or so. Knowing psychometrics, I read the report and think "DUH!", But most people don't understand psychometrics, so it can be interesting trying to explain why the delta-t is low on an invoice in 3 lines or less.

[ct2]

Hey UMD:

I am a student in this busniess. I come here to learn. I dont have all of the answers and often ask some relativily simple questions that I should know the answer to- but dont-

It sounds like you have some knowlege to share with us but with 24 posts you mite sit back and figure out who is who here because there are some that I would bet that know enough to make you look like a student

[Reliant Air]

Originally posted by umd
I am a little confused...isn't this a web site for professionals in the field. The very question indicates the author knows nothing about HVAC...

Just my opinion.

No it's not. anyone that is interested in HVAC accept DIYers are welcome here from what I understand. I think the pro sections are a little more exclusive.

Since your a confused I copied the part of the rules that you seemed to have missed.

Here you go:

HVAC Pros, please note; home owners are welcome to ask questions here, if you do not wish to be helpful that is fine but do not intimidate, obstruct or criticize those that wish to be more helpful than you. If you do not like a particular thread or topic, do not reply.

comments like yours are what discourages the lurkers from posting.