A/C location of expansion device vs line temps.

Many HI tests and books talk about the temperature of the liquid line and the suction lines of operating units.

Many resources show pictures of system components and they all show the expansion device as being located inside the house coil (air handler unit).

The location of this expansion device will obviously determine the observed temperatures of the refrigerant lines at the outside unit.

All of the A/C units I inspect have the larger refrigerant line insulated and cold at the outside unit (condenser). This conflicts with most of the HI training data I find.

I am interested in knowing why there is conflicting data out there within the HI training world. Has the location of the exp. device changed at some point?

I know how these systems work please do not post links to that.
My question is why does most of the training pictures and text conflict with each other and real world conditions?

NACHI used to have a nachi.org/tips/xxxx.gif pictures etc. This is where some of the conflicting info was noticed. Check your PC’s to see if you saved 1211.gif and 1213.gif

Other places are on tests etc. where the “correct answer” is wrong based on the systems I see.

Can you post some of this misinformation?

I have not seen what you are talking about.

There are one or two old (rare) units that have the TXV at the outdoor unit and the line set temps are different. These are few and far between around here.

For clarification: As far as what the TXV regulates, it is the saturation temperature at the outlet suction line in the indoor AHU.


Some of these pictures are almost identical to 1211.gif and 1213.gif that used to be on nachi.

This stuff is correct, (except that a compressor does not pump liquid out of it) but it is confusing the way it is described.

It shows that the liquid line is “cool” and the suction as “warm”. This is “relative” to the coil it came out of.

Liquid refrigerant is cooler (100 degrees is not cool) than the hot gas (150) entering the condenser coil.

The suction line gas is "warm"er than the flash gas at the TXV (20 degrees ?). That 20 is deceiving as well. The saturated temperature of the gas in an evaporator is in the 40 degree range, not 20. 20 is at the “flash point” where the liquid gas enters the evaporator. This is not really accurate either because the cold flash gas reacts with the 100 degree liquid refrigerant and cools it to about the saturation temp within the coil. Temperature and pressure are relative. 70# psig R-22 = 40 degrees. Raise the pressure and the temp goes up. Lower the pressure and is gets colder.

Latent heat of evaporation and latent heat of condensation is where the “work” is performed. Massive amounts of Heat is absorbed (in the evaporator) and rejected (in the condenser) when the refrigerant changes state (liquid - vapor). The pressure changes after the TXV and after the compressor, combined with the air flowing through the coils which absorb or reject BTU’s cause the change of state.

The suction line, labeled in the drawing as warm, is really cold to your hand. No?


These are the pics that you are seeking…

The 2nd and 3rd picture above shows warm suction lines at the outside condenser. That is wrong, suction lines on A/C units are cold after just a short period.

Thanks for the pictures posted it helped me to finally explain the issue.

The real issue is that many HI tests were derived from errors like this.

Maybe Gerry can double check nachi’s test too.

I have found on heat pumps, the easy way to remember the correct temperature is that the insulated line (larger tube) should be cold in the summer and hot in the winter.

Yes, these are even worse. The use of “hot” and “cold” is not good terminology.

It makes it look like the “cold liquid” is doing the cooling and becomes warm gas, which becomes hotter gas.

However, as I posted above, this terminology is “relative” (meaning, in comparison with a known). Actually the suction gas at the outlet of the evaporator to the compressor is warm. It’s 55 (+) which is warm compared to ice. It feels cold compared to your hand which is 99.

One of the problems is that there is no such thing as “cold”. You have “heat”, more heat and less heat. At what temperature does it become cold? We know when water freezes and when it boils. But we don’t know when it becomes cold or hot. These are terms used to describe “comfort” which has yet to be plotted with 100% accuracy for all people.

Which answer did you choose on the poll in this hvac section?

I did not want to get “picky”!

What is a “hot” day?
System design temps here in Nashville is 95-98 degrees (we will not bring in the “latent heat” load issues here). Anything over this temp is outside the design condition (IF the system was designed in the first place!) of the equipment. In which case the answer would be “Warm - warm”.

My answer was Cool Suction - Warm Liquid line.

Bruce, keep thinking as you do and you will be OK.
If, on the other hand you want to “know” what is going on, I can elaborate in great detail (why all of this stuff is not worth the weight of the paper it is printed on).

When the EPA certification testing was initially required, there were answers on the test that were WRONG and to get it RIGHT you had to answer the question WRONG! That is the way things ARE sometimes. We must go on with what we have or we will NEVER get there!

It is good that you see the issues at hand. It means you UNDERSTAND what is going on! Really. most tests give you sufficient latitude to get a few questions wrong and still pass the test. You are right in bringing this up though. In a perfect world, “partial credit” should be given for answers in the “Gray Area”!

One such example that pisses me off to this day (as I watch CSI and NCIS) was when I was in Special Agent School studying pathology. In the final test we were given a “close up” photograph and were required to determine the “cause and manor of death”. I spent three sleepless days studying with other very competent Agents for this test and I got an almost perfect score. This infuriated the Instructor who was known world wide for his “Impossible” test. My answer was “Small caliber, high velocity gun shot”. “His” correct answer was “Large” caliber, high velocity gun shot.

What is Large and Small Caliber?
Caliber is a “measurement of size” not power.
To add to the moment, we were on the FBI combat pistol qualification course (which I missed maxing out on two attempts) when we were given the results of the pathology exams. I rebutted the Large -Small caliber issue and offered to “demonstrate” first hand! Offer was declined. My correct answer was also declined. So, answer is… life sometimes sucks!

This is a good topic, as are electrical issues posted on the board. There is a lot of misunderstanding of the laws of physics and thermodynamics. But, that’s OK. To question is “GREAT”! It expands the mind. That is why I hang out here. :slight_smile:

Thanks !

I hope the inspectors that chose the warm suction line, cool liquid line realize their mistake. The whole thing is about “what an inspector will feel when he touches those lines”

It is in the 90’s here today and my suction line is very cool to the touch and really would be considered cold as a cold beer.

Think about a system like this, if the suction line was actually warm then that would mean that all of the “cold” was released in the airhandler which if so would mean that part of the evaparator coil is warm which is no good for cooling a house.