Tag: liquid line

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


The primary role of setting an appropriate level of subcooling is to ensure that we deliver a full line of liquid refrigerant to the metering device.

We want to do this at –

  • A pressure differential required by the metering device
  • At a temperature and pressure no higher than required for maximum capacity and efficiency

But most important is that it is 100% liquid with no “flashing” or bubbles when it hits that metering device. Any amount of refrigerant that is already vapor when it hits the metering device is wasted energy and unwanted turbulence leading to noise and additional pressure drop.

We are generally safe to set the subcooling level listed on a the system data tag or the old 10° rule of thumb when you have nothing else to go  on.

We need to consider the adjusting the target subcooling in the following cases  –

  • Long lines or tall risers
  • Liquid lines run through high temperature environments

As soon as the pressure or temperature of the refrigerant in the liquid line hits the saturation point, bubbles will begin to form and the the dreaded “flashing”.

Let’s consider an example –

R410a system with a 110° liquid line saturation pressure (368 PSIG) with 5° of subcooling at the condenser so the liquid line is 105° but it’s a 100′ run of line with 20′ of rise and then through a hot attic that is 120°

First we can estimate the pressure drop of the rise based on the York / Johnson Controls rule of 1/2 psi of drop per ft of rise so this means we would see a 10 PSI pressure drop in the riser alone. Depending on the size of the liquid line there would be an additional pressure drop but it would not be significant so lets just estimate a 15 PSI total pressure drop and 2° of sensible heat gain into the liquid line due to the hot attic.

This would mean the liquid line would now be 107° and the liquid saturation temperature would also be at 107° due to the pressure drop from 368 to 353 PSI.

In other words the refrigerant could now begin flashing

In long long line applications Carrier instructs you to charge to 10° of subcooling or the listed subcooling whichever is greater becasue at 10° you have enough wiggle room to deal with most residential / light commercial situations.

In heavy commercial applications there are routinely longer line runs and the actual field pressure drops and temperature gains must be calculated to ensure flashing will not occur in the liquid line. Often this requires a higher subcooling.

— Bryan


Diagram above by Carrier 

It’s really easy to put a liquid line drier in the proper location, it’s still more common that it gets installed in the WRONG location , namely, right at the condensing unit (OK it isn’t that big of a deal but for dramatic emphasis). Installing at the indoor coil is good practice for two main reasons.

#1 – It better protects the metering device (expansion valve or piston) from anything that may be in the liquid between the outside and the inside

No matter what, when you first put a unit in Service, you are either releasing the charge on the liquid line first or adding pressure in the liquid line. This means if anything is in the liquid line it is going to hit the indoor metering device first and putting the drier inside better protects the valve.

#2 – It won’t turn into a rusty mess and start leaking after a few years

This is pretty simple, so to make this tech tip a bit more in depth here are some other drier best practices.

  • Don’t “sweat” out an old drier. When you heat an old drier the moisture it has previously absorbed is driven out of the drier and back into the system. Cut it out instead.
  • Use the right type and size. Different driers have different purposes and vary in capacity. If you have a heat pump make sure to use a “bi-flow” drier. If you are mitigating a burnout ensure you are using a burnout suction drier. Make sure the capacity of the direr matches the capacity of the system, this will take a bit of reading. However for residential systems you can use 8 cu/in on small tonnage systems only. To be safe I would generally stick with 16 cu/in liquid line driers (chart by Parker / Sporlan)

  • Don’t burn the paint on a drier when installing. Not only will it look ugly, it will be more prone to corrosion. Use a damp cloth or other heat control methods.
  • Point the arrow in the correct direction. Suction driers point towards the compressor and away from the evaporator. Liquid line driers point toward the metering device and away from the condenser.
  • A liquid line drier goe in the liquid line NOT in the discharge line. The discharge line is between the compressor and the condenser. The liquid line is between the condenser and the metering device.
  • Flow nitrogen while brazing and pull a proper vacuum. Both of these practices are more important than whether a drier is inside or outside.
  • Remove all old line driers and install a new line drier whenever the system has been open and exposed to the atmosphere. Sometimes the old ones were in the wrong place, if so, go ahead and straight pipe them and install your new filter / drier in the proper location.

— Bryan


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