Hot Liquid Line on AC

[Kenny Rigler]

Hey Guys-

I've searched for awhile now trying to find an answer for this and have yet to find anything substantial...so please forgive me if this is an elementary question. On todays inspection the liquid line for the AC was pretty hot. The suction line was beer can frosty but the liquid line was like coffee cup hot (to keep the drink analogy going). Everything I've read says warm not hot. Does hot indicate a problem? Thanks.

Kenny

[Kenny Rigler]

And by the way the outside temperature was in the mid 80s.

[Bruce A. King]

See this thread, I agree with one of those posters, not enough information

http://hvac-talk.com/vbb/showthread.php?t=172873

[Wayne B. Wilson]

If the Temperture was 80 then the liquid line would be approx 110 to `115 . Depending on what refrigerant it was and if the coil was clean and so on and so on.

[Kenny Rigler]

So a hot line is not a clear indicator of a problem? If I knew exactly what more information to provide, I could try. But I use the KISS method when checking the HVAC as I'm not an expert in it. I didn't stick my thermometer on there to check the temperature of it so I can't say for certain. From my past experience, even on 100+ days, the liquid line has always been warm, but not hot like this. It's not like it was scalding hot, but it was hot enough I wouldn't want to hang onto it for more than a few seconds.

[Wayne B. Wilson]

How was it cooling, did it have a variable speed condenser fan motor? if it was cooling what was the output temperature. was the air flow good through the coils i see a shrub close by.

[David A. Andersen]

Wayne, you're confusing air discharge at the condenser with liquid line temperature.

Kenny, you do have a problem. The outside coil is dirty. Nothing to get excited about in a home inspection report, just recommend annual servicing.

You should not feel any temperature rise above the temperature of your and when the temperature outdoors is below 95°F, with the indoor air temperature stabilized at 70°F.

If it feels hot, that high temperature is going back through the system and reduces its allcapacity to cool. The efficiency loss comes from the initial refrigerant flash that occurs at the metering device which must use up enough refrigerant to lower the liquid refrigerant from what is in line to the evaporating pressure/temperature which is 40°F. So in essence, if you have a 120° liquid line, you must flash off liquid refrigerant toblower the refrigerant from 120°F to 40°F. The closer you are to 40°, (you'll never get there by the way) the less capacity loss that you must deal with.

Also refrigerant temperature is proportionate with pressure. If you have high temperature, you have high pressure. When you compress refrigerant gas at the compressor piston head you lose capacity because high pressure must expand back to the neutral pressure plane in the piston to a point where it can begin to suck!

If the pressure is lower, it begins to suck faster on the down stroke of the piston.

Brian Kelly can accuse me of conjecture now...

[Wayne B. Wilson]

Sorry David my bad, It is hard to type what i was trying to explain BTW you did well. I know what i check and where. I did Refrigeration for 23 years . I am more hands on than a explainer. Anyway he got the help he needed. Thats why i asked him about the coil.

[Doug Edwards]

Kenny,
I have to admit you got your head in the right place though. I can tell from your posts you are thinking and using your hands and eyes to observe all the indications and you recognized something wasn't right. Good catch!

[Wayne B. Wilson]

Yes it was Ken A very good catch.

[Kenny Rigler]

Thanks guys, I appreciate the feedback. After the recent replies in the thread on AC differentials, I try to keep the thermometer in the bag and look for other indicators for the functionality of the AC first. Wayne, I'm trying to follow your advice and use the KISS method you described. Now that I'm not using the thermometer as much, I realize how much I kind of blindly used it before...just simply going off of what the differentials gave me. Without it, you have to be a little more sensitive to the other indicators which I think is a good thing.

Originally Posted by dandersen

You should not feel any temperature rise above the temperature of your hand when the temperature outdoors is below 95°F, with the indoor air temperature stabilized at 70°F.

This is good to know David and something practical that I can go off of. I appreciate your time and explanations.

Kenny

[David A. Andersen]

Kenny,

In spite of my previous post about using the Delta T, if you are going to attempt HVAC evaluation without obtaining certification to do all that's required, I recommend that you still use your thermometers. Just don't solely rely on them. You must make a lot of assumptions, but if you're paying attention to other indicators (as you apparently are) you can get close to the mark.

Your hand temperature is in the 95-98° range. Any temperature variance can be felt. The sensors in your hand are as sensitive and accurate as any thermal imaging camera (you just have to learn to recognize those temperature signals to your brain)!

Suction line temperature should feel like a canned soda from the refrigerator.
Now what temperature is a refrigerator? 35°

What is the operating temperature of an HVAC evaporator coil? 40°
which can be converted to pressure 70 PSIG.

Close aren't they?

The liquid line temperature is relevant to the outdoor air temperature.
On a 90° day the liquid line may be 100° if operating properly. Your hand temperature is about 98°. You're not going to feel much heat transfer are you?

What Wayne was trying to relate; the saturation temperature of the refrigerant in the condensing unit is generally 30° higher than the outdoor air temperature (average system design). However, by the time the liquid refrigerant leaves the condensing unit, we want it as close to the outside air temperature as possible (this is what high-efficiency equipment does best). Seeing as we don't know what the saturation temperature or pressure is, this information is not helpful to this case. We could note however that if the liquid line is excessively hot, the number of degrees above ambient temperature is relative to the saturation temperature and pressure within the condenser. Higher liquid line temperature equates to excessively high refrigerant pressures in the compressor. This consumes a larger amount of energy and significantly reduces the capacity of the compressor to pump sufficient volumes of refrigerant gas.

High temperature liquid line will also correlate with the condenser air discharge. A dirty outdoor coil restricts airflow and the air that does leave the unit is a much higher temperature. You can feel this with the back of your hand. I feel it on my face as I'm walking up to the unit and confirm by feeling the liquid line.

Once we realize that the HVAC system is functioning to apparent capacity, we should check airflow in the house at the supply and return registers. Listen for excessive noise at the return grill (air velocities greater than 500 ft. sec causes the face of the grill to resonate). This indicates inadequate return design.

Now comes the thermometer!
After conducting the above steps and have determined that the suction line at the compressor is sufficiently cool, we can assume that the saturation temperature of the coil is 40° (system design). Air discharging from the coil (depending on bypass factor and moisture on the coil) may start out at 50°. What is your supply air register discharge temperature? You can compare temperatures from register to register for variance. You can determine excessive heat on the air duct system in an unconditional space.

Inadequate airflow would give you a reading closer to 50 and excessive airflow higher than 65.

Air duct leakage between the filter grill and the evaporator coil will cause discharge temperatures to be higher then expected. This is a very common occurrence which will dramatically skew the normal Delta T testing procedure everyone uses. The air temperature going into the return could actually be much colder than the air temperature entering the evaporator coil due to air leakage of the return duct system. I find this condition in approximately 60% of the homes I inspect. 90% have air leakage, but 60% of them are significant enough to significantly cause adverse comfort conditions in the house.



In this thermal scan you can see the obvious air leakage in the ceiling, but note the increased temperature spots on the face of the return grill. This is the air entering the return from the attic which can be 130°. That offsets your Delta T. scenario significantly, does it not?



Here is air being pulled through the wall from the attic to the return.

So as you can see, taking temperature readings does not determine that equipment operation is adequate. There may be defects elsewhere in the system design. These defects can not be evaluated with a thermometer so we are recommending things blindly.

caution: these testing assumptions can not be conducted on equipment that is not in a stabilized condition. Home inspectors entering a vacant house and turning on the air conditioner for the first time may have to wait several hours for stabilization to occur.

[Tom Leahy]

Great post David. Thanks for sharing your knowledge!!

[Jeff Merritt]

I have a question, with the new R-410-A starting to show up do the same methods hold up, I know they run at a higher pressure.

[David A. Andersen]

The pressure/temperature conversion chart is different because of the type of refrigerant used, but the methodology is basically the same (as far as a home inspector is concerned).

New refrigerants operate a little different in reality, their change of state (liquid to vapor) occurs over a wider temperature differential but the conditions should feel the same to you at the points where you normally test with your hands-on method.

When refrigerant changes state, a large amount of heat is absorbed or rejected. This is what you're feeling. Regardless of refrigerant type, the refrigerant should be sub-cooled by the time it leaves the condenser and should have sufficient capacity after it completes the circuit and returns to the compressor to assist in cooling the compressor motor.

The only thing that really changes is the pressure and that is the one thing home inspectors cannot test without certification.