What is split system line sizing and why is it important?

Poorly sized lines can cause issues with system operation or complete system failure.

The size and layout of the piping connecting your indoor and outdoor units in your split system will affect how well your system can operate. 

Each line needs sized for two main criteria:

  • Velocity for oil return
  • Temperature loss

In general, smaller pipes have higher velocity and higher temperature loss.  Larger pipes have lower velocity and lower temperature loss.

Velocity is important for oil return to the compressor.  Running a compressor without oil will result in an expected life of days instead of years.  If the oil gets trapped somewhere in the line set and does not make it back to the compressor then the compressor will fail.  Replacing the compressor without fixing the piping problems would just result in another compressor failure.

For a refrigerant system with both liquid and gas phases, temperature and pressure are linked.  In turn, pressure drop along the line set can be listed as either psi, or temperature loss which is a measure of °F change in the saturated suction temperature due to pressure effects.  As pressure loss increases, temperature loss also increases, so they are treated as largely the same thing.  Many rating systems are based on temperature loss so that new rules do not have to be set up for each refrigerant.  Temperature loss can be used in the cross plot to determine the system's capacity with the line losses.

Refrigeration systems operate with certain ranges of pressure differences between the indoor and outdoor portion of the system.  The outside portion is hot and high pressure (to reject heat to the warm outdoors) and the indoor portion is cool and low pressure (so it can cool the air to the space).  The liquid and suction lines along with the expansion device and compressor help maintain this pressure differential, which allows the refrigeration cycle to work.

The compressor, which causes the refrigerant to flow through the system in the first place, has maximum pressure that it can run to.  The pressure loss of a poorly designed line set can force a system outside of acceptable ranges and cause the system to shut down. 

A detailed example would be nuisance high head pressure trips, which can occur when the reheat line has too much pressure loss (among other reasons).  During the flush cycle that keeps oil from getting trapped in the reheat lines, the reheat coil is forced to handle all the hot refrigerant of the system.  If the reheat coil is pushed to hit 135°F and the reheat line adds another 20°F of temperature loss, then the compressor will see a saturated discharge temperature of 135 + 20 = 155°F which is too hot for the compressor and enough to trip the high head pressure switch and shut down the condensing unit for system safety.   This typically requires a manual reset to help identify the problem.  Because the issue occurred during a flush cycle, it might not show any issues during the normal operation.

Line sizing is not as simple as choosing the smallest pipes for high velocity or the largest pipes for low temperature loss.  There are additional concerns including noise issues, overall refrigerant charge, potential for nuisance trips, and capacity loss.

If you are purchasing AAON equipment, then you can contact your AAON Sales Representative with any questions regarding line sizing.

What pipes make up a split system line set?

Liquid Lines

Generally the smaller pipe that is warm to the touch.  The rules for sizing liquid lines are not extremely strict because liquid refrigerant is much better at carrying oil than vapor refrigerant that you see in the other lines.  On liquid lines that rise multiple stories (floors) you can get pressure drop due to the height of the liquid column that can cause the liquid refrigerant to flash to a vapor before it gets to the Thermo Expansion Valve (TXV).  Flashing in the liquid line can also occur on systems with long line sets and undersized liquid lines.  This causes fluctuations in the capacity of the system as the TXV gets hit by bubbles of vapor.  AAON does not allow split systems to have more than 70 feet of elevation difference, partially due to this issue.

Suction Lines

Insulated because they are cool to the touch when the system is running.  The insulation keeps moisture from collecting on the pipe and then dripping and damaging nearby surfaces.  When the suction is flowing downward or horizontally the velocity only needs to be high enough to keep moving it along the pipe.  Vertical piping where the suction flows up have higher velocity requirements and sometimes requirements for traps help ensure oil return.

Discharge Lines

Pull hot gas right off the compressor discharge (thus the name) and can be used as hot gas reheat or hot gas bypass lines.  Heat pump systems have a line that acts as both suction and discharge depending on what mode the system is operating in.  In general, systems with hot gas reheat, hot gas bypass, and heat pumps must have shorter line sets than simple cooling only systems.