Why is 3/8″ Liquid Line So Common? Liquid Line Sizing

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You may have noticed that in 5-ton and under equipment 3/8 liquid lines are generally the norm. We went to a job recently where the system had a 1/2″ liquid line and it got me thinking about the ramifications of going larger or smaller on the liquid line.

Liquid Line Basics

The liquid line should be full of liquid with additional subcooling to prevent flashing due to pressure drop from the length, rise, fittings and filter/drier. Because liquid refrigerant is much denser than vapor the liquid line contains a relatively large amount of refrigerant compared to the much larger vapor line.

Even small changes in liquid line size can have a big impact on refrigerant velocity in the liquid line as well as the amount of charge contained in it. This is why we see a big variation in suction line size but very little change in liquid line in residential applications.

Pressure drop in the liquid line is only a concern when it results in flash gas or when it results in an unacceptably low pressure drop across the metering device. Flashing occurs when the refrigerant pressure drops to the point that all of the design subcooling is “used up” and the refrigerant in the liquid line begins to boil off. Up until that point, the pressure drop will result in negligible temperature drop.

From the Lennox Design and Fabrication Guide

Liquid Line Sizing Factors

We need to size the liquid line with the following factors in mind

  • Keep the velocity low enough to prevent noise
  • Minimize pressure drop to prevent flashing
  • Do not oversized the liquid line to prevent excess refrigerant charge

At first, it may seem like bigger would be better on the liquid line but that isn’t the case. An oversized liquid line can lead to a lot more refrigerant charge which will result in a greater likelihood of off-cycle refrigerant migration and flooded starts in addition to the cost associated with more charge for no good reason.

Lennox Liquid Line Chart

Our goal should be to use the smallest liquid line size that will still reliably provide a full line of liquid to the metering device under all load conditions that the system will be reasonably operated under. Luckily for us, we don’t need to guess as the manufacturers provide us guidelines for liquid line sizing.

Carrier Liquid Line Sizing Chart

Vertical Pressure Drop / Gain 

In general, on an R410a system, we don’t want more than about a 35PSI pressure drop in the liquid line otherwise we run the risk of flashing. When the condenser is LOWER than the evaporator the liquid line pressure loss is about 0.5 PSI per foot of vertical rise which limits the rise to around 60′ for R410a systems by the time you consider the other pressure drops.

If the condenser is ABOVE the evaporator then the pressure actually increases the longer the vertical separation allowing the liquid line to be downsized in some cases.

Each manufacturer has their own piping guide or has the details in the install instructions or the product data. In most cases 3/8″ liquid line is a safe bet but just like the suction line there is some wiggle room depending on the system and the specific application.

Here are some great guides

Carrier Guide

Lennox Guide

Johnson Controls / York Guide

— Bryan


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