Basic Air Conditioning Piping Recommendations – Liquid Lines


As promised (I’m back from vacation), this post will discuss Liquid Lines.  But before I do, I forgot to mention something during the discussion on suction lines that also applies to liquid lines.  When we talk about pipe sizes like 1-5/8 inch or  2-1/8 inch or 3/8 or 5/8 we are always talking outside diameter of the pipe.

I’ll give you this little way of how to remember this and then I’ll start on liquid line applications.  When you go to a supply house and you want to get fittings for the line set you need to know if you are in a refrigeration supply house or a plumbing supply house.  See, we measure our tubing by OD (outside diameter) and plumbers measure their pipe and fittings by ID (inside diameter).  So a 3/4 refrigeration fitting is 3/4 OD and a 3/4 plumbing fitting is 7/8 OD (3/4 ID).

The way I was taught to remember this — when going to either type of supply house — air conditioners sit outside the building so we measure everything in OD.  In the civilized world, plumbing is now all inside the home so plumbers measure all their pipe and fittings by ID. If you just remember this, you will never have a problem getting the right material for the job regardless of the supply house you go to [and it will help in doing proper refrigerant pipe sizing too :>) ].

With all of that said — now let’s look at liquid lines!  Liquid lines must be sized, like suction lines, to minimize pressure change. With liquid lines, there are two factors that come into play that actually creates the sum of the pressure change.  Like suction lines, you have the loss due to friction (tubing size, number of fittings, length, etc) but you also have pressure loss / gain due to the static head in a vertical line.  Liquid pressure loss reduces the amount of liquid sub-cooling at a rate of 1 degree for every 5 psi for R-410A. Sufficient sub-cooling must be maintained at the metering device (particularly the TXV) to allow proper operation.  If the liquid pressure drop is high enough to not maintain a column of liquid in the line at all times, liquid will begin to “flash” reducing the refrigerant flow to and through the indoor coil metering device (TXV).  Then, the whole effect begins to snow ball and as soon as flash gas starts in the liquid line, the rate of pressure loss increases and continues to increase as the amount of gas increases and so on and so on and so on! As we get more “gas” in the coil, superheat goes up ( and remember what Bruce said) and too much superheat kills compressors.

Properly sized liquid lines will minimize the pressure drop and system charge. Liquid line should be sized as small as possible without exceeding the recommended pressure drop.  The other thing to consider is refrigerant velocity inside the line.  The MAXIMUM recommended liquid line velocity is 400 fpm. Velocities higher than this can and will result in higher than acceptable noise levels.

But now we also need to look elevation differences since this will affect a pressure loss/gain on the line. Let’s start with the indoor unit ABOVE the outdoor unit.  With this configuration, a common problem with the cooling is that the amount of liquid sub-cooling varies as operating conditions change. Under some conditions it is possible that flashing will occur in the liquid riser.  The liquid static LOSS can be calculated at 1/2 psi per foot of rise. When the indoor unit is above the outdoor unit, the pressure loss in the liquid line during the cooling cycle will limit the amount of elevation difference allowed.  Maximum loss is 30 psi.  So the static loss due to “head pressure” plus the friction loss  — line length, fitting, etc — limit the elevation difference in this configuration.

cond_below_evap

The opposite holds true with the outdoor unit ABOVE the indoor unit. When the outdoor unit is above the indoor unit, the static pressure GAIN in the vertical liquid line drop is also 1/2 psi per foot.  This may actually overcome the static loss and create a total system pressure GAIN.  A pressure gain will not hurt the system or its performance. The one caution here is, with an orifice metering device, you may need to reduce the orifice size or you could over-feed the coil and create too little superheat which leads to liquid “slugging” in the compressor. But a real benefit is, on cooling only systems, it may even be possible to REDUCE the size of the liquid line.  The static gain in the vertical drop will offset the increase in the friction loss caused by the smaller tubing size.  In addition, this produces an even greater benefit — the reduction in total system charge due to the smaller liquid line will actually enhance the reliability of the system.  The compressor has less refrigerant to “push around” so it does not have to work so hard.

cond above evap

So the rules are —Liquid Line Total Pressure changes allowed

  • R-410A = maximum of 60 psi GAIN due to elevation differences (outdoor unit above indoor unit) minus static loss
  • R-410A – maximum of 30 psi total system LOSS due to elevation differences (indoor above outdoor unit) plus static loss.
  • 1 ft of height = ½ lb of pressure drop / gain.
  • Maximum Liquid Line Velocity to reduce noise = 400 fpm – Minimum 100 fpm
  • Liquid Line Pressure loss due to friction  (More pipe length = more pressure drop)
  • Liquid Line Pressure loss due to static {More height = more pressure drop =Loss of Sub-cooling)

As with suction lines, here is a “cheat sheet” to help you as an aid when servicing or quoting liquid lines.  Again, this is just “rule of thumb” sizing and should not be used in place of an accurate line sizing program.

Liquid Line Selection Chart: The Chart below shows the line sizes that can be selected for each tonnage of unit and the maximum equivalent length and maximum rise of the line.  The maximum actual line length is 200 feet. Equivalent line lengths would include elbows and other components that would increase the equivalent length.

liquid line chart

Multi Stage Refrigeration Systems: When sizing the liquid line for a system with either a 2 stage scroll compressor (residential) or when a single refrigerant system uses staging for capacity reduction (commercial). ALWAYS Calculate the liquid line for the Maximum tonnage rating of the unit .

It is alway best to try to keep the line length to as short as possible — both liquid and suction lines. This will give you maximum capacity and eliminates a lot of issues that can occur with longer line length. If you have line lengths over 75 feet, there are additional things you need to add to the system:

  • Crankcase Heater — A crankcase heater will warm the compressor sump and prevent the refrigerant from migrating to the compressor in the off cycle. Needs to be added if one is not present.

  • Non Bleed TXV’s on all ID Coils  — Prevents refrigeration from bleeding into the low side of the system through the evaporator in the off cycle.

  • Hard Start Kit — On Single phase units a hard start kit is necessary to increase the compressor starting torque anytime a TXV is used in a system. This is necessary to overcome the pressure difference across the compressor.

  • Cooling Only Units — require liquid line check valves placed in the liquid line near the condensing unit. This is used to lock the refrigerant in the liquid line between the TXV and the condensing unit to reduce off cycle migration.

On long line application you may need a suction line accumulator or an oil separator. You may need to add “hot gas by-pass” or a capacity reducing control like a Rawal Valve. This all goes back to the proper design of the system.

I think by now, you should have a pretty good idea about how sizing the refrigerant line set impacts the operation, noise level and performance of air conditioners.  It is one of the most often overlooked parts of the Air Conditioning SYSTEM.  Hopefully, the charts I’ve included will help as a SERVICE TOOL to see if the line set is a possible issue with a particular installation giving you problems.

Keep in mind that, as the title of these posts says — BASIC air conditioning piping recommendations.

(a lot of the charts and information presented in this blog were taken from published piping guides from different manufacturers. Some information was courtesy of Ed Schmidt – CM, NATE, Johnson Controls CPS and Bruce Porter  Manager UPG Field Service. Bruce has written many application bulletins for UPG and is well-respected for his expertise in the HVACR field.  He also teaches HVACR at Metro Technologies Center HVAC). Thanks to both for allowing me to use some of their materials and ideas!

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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20 Responses to Basic Air Conditioning Piping Recommendations – Liquid Lines

  1. Whit Perry says:

    If people aren’t learning it’s their own doggone fault because you are sure putting some great information out here for everyone… Not only do I continue to learn but you give me new ways to teach, and to make it easier for my student to remember….

    • Whit — thanks again for the compliment. All I’m trying to do is pass on stuff I’ve learned over the years that made my job easier and hopefully does the same for the readers of this. And the amazing thing is I, myself, am still learning and I hope that never stops.

  2. Bruce Porter says:

    Mike,
    Excellent article ! The one thing i would like to add is after talking to many types of technicians what does all this really boil down to with regard to operation? We all went to a trade school or learned it from “daddy” or a mentor so for some of us we are as good as our training, That being said one thing that we must do is to put back on our “refrigeration basics hat” because at the end of the day what we are really doing is managing refrigerant, plain and simple. When in liquid phase you can do literally anything as long as you keep it in liquid phase, Pump it, store it , steal it , use some of it somewhere else for something else entirely. As long as the compressors use capacity is not higher that it’s rating then the compressor does not care where it gets it’s refrigerant from and if superheat is kept in line it will always survive its installation. Proper cyclic rates and oil return are going to be impacted also when a systems refrigeration dynamics are not explored thoroughly enough. When in doubt reach out!

    • Bruce — Thanks for the information — you are correct — it is all about managing refrigerant — my next article is on Rawal valves as an extension of my post on hot gas by-pass.

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    about that.

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  8. kevin says:

    i have a question sir in the calculations of the size of the piping…..can you help me..?plsss response

  9. Samad says:

    Hi, your article is very good.. but for me , its very difficult to understand because i have no knowledge of HVAC. My question is that., i have bought a brandnew 1 ton Inverter Split AC , my room is on ground floor but my outer unit is fitted on above 30FT.. will it harm my compressor due to 30Ft of Length Piping…?? waiting for your answer

    • Inverter systems are great as they adjust to load conditions. Because of this, line sizing and length are critical. You need to check with your manufacturer their specific recommendations.
      I would really recommend you get a trained professional to do the installation.

  10. Bob Cranfill says:

    I have a TRANE 2 ton air conditioner with the outdoor unit below the indoor unit.
    The line length is 50 feet.
    The size of the copper liquid line is 5/16.
    Is this large enough?

    • What is the elevation difference between the outside unit and the higher inside unit? What refrigerant arecyiu using? How many bends or els in the 50 foot length?
      These are all things your installer should have looked at and sized the line accordingly.
      If it is too small, the most common side effect would be noisy lines due to the higher velocity of the refrigerant in the line.

  11. Scott says:

    Hey guys ? If installing apx 1 foot of flex hose in both hi. Lo. Side to eliminate pipe cracking as in an Rv ac unit. If the 1/4 hi side. Had a flex line installed that was 3/8. With 5/16 I’d on the flex section. Would that be ok. It’s a dual circuit dual compressor 27k btu unit. The hard lines did crack and the co went out of business sometime but they did install flex on their last few units !

  12. Martin kimemia says:

    Nice insights there.

    I am installing a 14kw Outdoor unit 72 m away from the Indoor. Part of the length is a 23 m veritcal with the ODU on rooftop. Manufacturer recommends 50 m total and max eleveation of 30m

    Advise how best to install. piping is 3/8 and 5/8

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