Liquid Line Temperature

Bryan goes over the basics of liquid line temperature. He explains how we measure it, what it can tell us, and what appropriate ranges tend to be.

We typically measure liquid line temperature with a temperature clamp outside at the condenser. Normally, the liquid line temperature shouldn't change very much; even in cases when you have exceptionally long line sets or if the liquid line has been routed through hot areas, you shouldn't see much more than a two to three-degree difference throughout. By comparison, the suction line has a lot more variation from start to finish.

The liquid line can't be any colder than the medium to which the condenser is rejecting its heat (the outdoor air). In other words, the liquid line can't be cooler than the outdoor air in a traditional residential split A/C unit. If the liquid line is colder than the outdoor temperature, check to make sure that your probes are in the correct places (e.g., not in direct sunlight).

If you detect a significant temperature difference or if the liquid line is colder than the outdoor air, you may be dealing with a restriction between the condenser and wherever you're measuring. Restrictions may be in the line itself, in the liquid line service valve, or in the liquid line filter-drier. Elevated liquid line temperatures may indicate a restricted condenser coil or overcharge.

We can determine the appropriate liquid line temperature range by looking at the condensing temperature over ambient (CTOA), which is the difference between the condensing temperature (saturation inside the condenser) and the outdoor ambient temperature. Older systems tend to have higher CTOA, and newer high-efficiency systems tend to have lower CTOA.

Comparing the liquid line temperature to the ambient temperature is called the "approach" method. The absolute highest temperature that the liquid line could be is the CTOA, and the lowest it could be is the outdoor ambient temperature. To find out what our liquid line temperature should be, we also have to look at our subcooling. You can subtract the subcooling from the CTOA to get your ideal liquid line temperature. Of course, there is some guesswork involved in this method.

If the liquid line temperature is much higher than expected, then it may be time to connect gauges.

It's also worth noting that refrigeration systems will often have higher CTOA than HVAC systems. However, we're also trying to reduce the condensing temperature to reduce compression ratios and get more efficiency out of our systems.

[JoeB]

I especially appreciated this episode, thanks.

[phxsundevil]

THANK YOU!! I am not an HVAC technician and do not have plans to become one, just a DIY homeowner. I thought my liquid line felt "too hot" and crawled over so many forums that turned me in circles. Your video summarized and explained everything so simply in 11 minutes! I'm glad I found your channel as I will return to it in the future.

[leekazan]

Been doing HVAC for a few years now and I still am learning new things everyday thanks to you!!

[truthsmiles]

9:35 - One small quibble: As I'm sure you probably know, Lennox uses (used) approach method charging, which is awesome. The instructions specifically say, "add refrigerant to lower the approach temperature or recover refrigerant from the system to increase the approach temperature". Indeed, I recently serviced a unit with a bad capacitor and after replacing it, I checked and found the approach temp to be around 1°F. So, following the instructions, I recovered just over a pound of refrigerant, to bring the approach temp to exactly 6° as called for.
Aside: The homeowner told me she had it serviced the previous year by a tech that "didn't seem sure whether it needed freon, so he added some to be safe". Sounded like he was looking for a subcooling number, which isn't provided on these units. Using the traditional subcooling measurement, it showed about 11°F before I removed the charge, and about 6° after. So, not too surprising to find it overcharged from "guessing".
EDIT: Awesome video otherwise. I love these theory discussions - they make us all smarter! :-)

[Niveous]

Me and some other students worked on a remote condenser that was just... burning radiant hot. We had a lil' fold out canopy. It was a HUGE life saver. Not even measurement taking, just being able to get inside the fans to clean them without burning our arms. Radiant is no joke.

[mike-ypuk]

I just bought that field piece set so I can start checking stuff out. I like the little facts that only come from a pro that has teaching skill. Teachers know the details help to make it fun learning.

[dietrichschaefer]

Extremely helpful to me. Thanks for bringing this episode to us.

[escuelacerogrados]

Excellent explanation about subcooling, but it is important to mention that the subcooling calculation only applies to equipment that has a TXV.

[actechformallyyomama]

The term “gauge up” should not be taught. Every time you put gauges on a system you loose refrigerant. If this continues you will find the system low on charge and start looking for a leak that doesn’t exist. This is especially bad on package units. My gauges are the last tool I grab for diagnosing. Very good video. All techs should watch and learn from this video.

[gyrgrls]

It is instructive to note that long lifts (vertical rises) can hamper subcooling at the throttle or expansion valve. This is one reason many small refrigerator/freezers will skimp on the metering device, while helping to ensure good subcooling by using a makeshift heat exchanger consisting of a capillary tube liquid line inside or alongside the the suction return line. Kind of an ingenious design to cut cost, if you ask me.

[jimdamiani]

You are a fantastic teacher and communicator.

[mjshuja]

I LIKE YOU A LOT, YOU VERY KNOWLEDGEABLE HUMBLE PERSON👍👍👍👍

[Stuart]

Thank you. This is a wonderful explanation and very helpful to gaining another quick way of checking out a system.
I don’t know that we even talked about sub cooling in my trade school in the late 80s.
I always learn something new from your videos and they clarify concepts and techniques. Have you considered having a formal online hvac trade school? I think the course content is already present in your videos. You would just need to sequence them and add questions.
Some of the questions could come from your current fans and followers about actual on the job challenges.
Sometimes the issue could be ethical concerns. Once I directly told the customer why a motor had failed who was looking over my shoulder while I worked on it. A previous technician had shorted the leads together in a wire nut that we’re not being used for the different motor speeds and that burnt out the new motor. Technicians in my shop were upset that I had not made up a story to cover for that mistake. I believe that honesty is the best answer even though it has cost me my job a few times. I moved on to something that was more suited to me and was more beneficial financially. So even though at the time the results of being honest seemed to be bad, in the near future it really worked out for the best!

[dandaman]

I just tested these numbers with my Testo 550 and it's absoultly correct!
I'm amazed.

[harrystrohm]

Bryan Thank you for your time&videos it helps me tobe a better tech.

[nejatamrei]

Hi, I want to say thank you so much for sharing the information . I know about the time but you speaking very fast. Thank you again

[Prkour]

This man is a master HVAC TECHNICIAN and instructor

[jaredj]

Wow great thinking, I feel this could be useful for commercial refrigeration too. Often you don’t even have a service port to hook up to and if you use a piercing valve you really shouldn’t just leave it.

[reynaldoschorizo]

approach temperature 5:05 . i was looking for this definition. difference between line temp and outdoor ambient temperature. Thank you.

[thehvachacker]

Lennox has lots of condensers looking for 3 degrees of sub cooling. Now measurequick is always telling me my liquid line temp is too low. Radiant heat plays a big factor yet most Lennox xc17-25 subcooling is 2-6 degrees. If I charge the system to manufacturers specs I get a 20 degree split on evap coil.