Hot Liquid Line on AC

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.


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

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

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.

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.

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.

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…

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.

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!

Yes it was Ken A very good catch.

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.

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



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.

Great post David. Thanks for sharing your knowledge!!

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.

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.

Good Reading here as well

I usually catch liquid line’s that are scorching hot (dirty coil/poor performance at condensing coil) or the liquid line is ice cold like the suction line.

I was at one a few days ago that had both, where the liquid line would go from scorching hot to “beer can cold” :slight_smile: and back and forth…

My feeble HVAC mind suggested a blockage ie… drier/moisture etc… as the blockage in the liquid line would free up momentarily and the urine ridden, dirty, lint ridden condensing coil would allow the liquid line to heat up again until the blockage showed up again… and back and forth.

I know the tech that is to fix it, maybe I’ll call and ask him what it was.

As HI’s we get curious about the “why” of things, but the reporting part is generally much the same…

“The AC system needs service or repair for reasons indicated in the report”

Most of the conditions we find are, IMO, related to poor (if any) maintenance.

David A., quick question… I had read in one of the articles on the link I posted about the use of Meghometers… I don’t think I’ve ever seen a Tech use one before, have you?

This time of the year, some of the tech’s around here are in such a hurry, I think the truck is still running when they show up at the door…“There’s a bubble in the sight glass… needs freon and a hard start capacitor” “Bye”

That hot/cold scenario that you’re describing is what I call “hunting”. It usually has to do with a unit that has a thermal expansion valve as a metering device. It is associated with refrigerant starvation (for whatever the cause). The thermal expansion valve senses the temperature of the suction line, when it gets warm the valve opens up wide and floods the evaporator with refrigerant. When there is not enough liquid refrigerant in the condensing unit to maintain a constant load, hot gas abruptly discharges through the condenser and down the liquid line without adequate sub-cooling.

There can be other issues concerning this scenario as well, but as you say we are limited to what we try to evaluate just simply try to understand it. Also as you said, insufficient data!

As for your question; between you and I a Megometer is an HVAC engineers “crystal ball”!

Yes I do use them ( on all old HVAC equipment I inspect during a home inspection w/ HVAC diagnostic ancillary inspection).

The refrigeration circuit (including the compressor) is a closed system that you must have certification to access. Contaminants in the refrigerant occur for many reasons, the Megometer can identify the presence of these contaminants in the refrigerant system as well as determine the installation capacity of the compressor motor windings. As the compressor starts, is under a considerable load and there is considerable flexing of the windings of the motor. This eventually breaks down and causes leakage which the Megometer can detect and identify.

In cases where contamination is the cause, there are numerous mitigation processes that are available and not that expensive that will stop the erosion of the compressor and extended its expected life by stopping the accelerated deterioration. Contaminants cause coking of the oil which turns to an acid which eats away at the windings of the compressor. Even when the insulation is still intact, contaminants in the refrigerant and oil will trigger a bad meter reading on the Meg. Additional testing of the refrigerant oil/refrigerant can be performed to determine whether it’s contamination or damage to the insulation on the compressor (which cannot be repaired). If it is contamination, mitigation practices will alleviate some of the situation.

My first Meg, I confiscated from the company’s tool locker along with numerous other test devices that nobody knew how to use. Last year I purchased my third Meg. They are not cheap, but I consider them a worthwhile investment for how I do home inspection.

As far as bubbles in a slight glass, ensure you realize that this means absolutely nothing as far as refrigerant charge! Here I go again! Another practice that is futile!

You never charge by slight glass!

If you don’t perform adequate analysis on the equipment to determine if a flashing slight glass is appropriate or not, you will overcharge the system.

I don’t know how many commercial 25 to 150 ton air conditioning compressors that I have come across where refrigerant was added because the slight glass was flashing. By the time I got there, the oil slight glass in the compressor was foaming and the compressor was shutting down on low oil pressure safety devices. Excessive refrigerant charge was washing the compressor oil out of the crankcase tripping the oil pressure safety switch. I remember many cases where I removed as much as 60 to 80 pounds of refrigerant from the system to get it back online again.

Hard start capacitors are another touchy subject. There are some cases where hard start kits are required because of system design. When Lennox came out with the first Copeland scroll compressor, they thought it was indestructible. No accumulators on heat pumps. No pressure equalization safety measures while using accurators versus internal equalizing thermal expansion valves. They refused to let us add hard start kits. They required compressor replacement under warranty!

As a home inspector and you show up to a home inspection and see a hard start Kit (a start capacitor with a potential relay) what is the reasons for its installation?

In many cases when the equipment is old, it takes a lot more to get the compressor started because the bearings are wearing and other issues. In some cases the brand-new equipment should have had one from the factory but they didn’t put it on.

At least note in the inspection report that these devices have been added as an aftermarket product and it just may be an indication of future failure of the equipment due to the age of the equipment.

Please note: I never recommend replacement of equipment because of its age. If you haven’t noticed I do not respond to “what is the age of this equipment” request on this website.

This is all very educational!

But the AC cool the home or it doesn’t. At my last inspection, it didn’t and was reported as such.

Recommended an HVAC technician for further evaluation…

The unit was Evcon serial #941014020 & model # DRHS0301BB

My guess: heatpump 1994 built, 2 1/2 tons which is most likely oversized for a small bungalow in my area. Return air restriction may also have been a factor.


Unfortunately Marcel, the client had to pay for a service call that was totally unnecessary.

Blocked filter restricted airflow resulting in freezing the evaporator well which stopped airflow and cause the temperature in the house to rise.

If you simply had the homeowner remove the dirty filter and the ice from the coil (by turning on the blower fan to the on position) I’m sure everything would have worked just fine after that.

Sometimes knowing a little bit more can go a long way…


I know that and so did the client…why did you think that I posted that pic of a clogged upside down filter…