How to Pipe a Single Circuit A/C with a Dual Circuit Evaporator Coil

If you’re installing a split commercial DX system, how do you know what kind of circuiting you need? And how do you install the piping and refrigerant specialties if there are more indoor circuits than outdoor circuits?

The two most common circuiting options are a single circuit system (known as 2-pipe) or a dual circuit system (known as 4-pipe). Circuiting doesn’t always affect staging. A single circuit condensing unit can have 2 or more stages of cooling, for example. Single-circuit systems can have a variable speed/digital scroll compressor or tandem compressors that are piped together, obtaining 2 or more stages. 

With a dual-circuit system, it is likely that there are (2) independent compressors that each have their own independent circuits. One benefit of a dual-circuit system is that if one compressor fails or burns out, the second circuit won’t be affected by the first and can continue to handle half of the load. The caveat with a dual-circuit system is the added cost and labor to install twice as much refrigerant piping. Due to the added upfront cost, single-circuit systems are often favorable.  

Considerations for single-circuit systems with two stages of cooling:

  • When the system is running on first stage cooling there will be half of the capacity running through refrigerant piping that is designed for full capacity (assuming an even split tandem compressor rack). This means that the suction line velocity at first stage cooling will be half of the velocity at second stage cooling.
    • For example, if you have a 10-Ton split system with a 1-5/8″ suction line size, you will have a suction line velocity of about 1,120 fpm at full capacity and about 560 fpm at half capacity. At 560 fpm, you will likely have issues with oil return to the compressor causing premature compressor failure. We recommend designing your suction line velocity to be at least 1,000 fpm or above (for suction risers) to ensure oil return to the compressor.
    • Designing the suction line to be 1-3/8″ yields suction line velocities of 1,140 fpm and 2,280 fpm at first and second stage, which works great. 
  • If the DX coil is a single-circuit coil and the system is running at first stage cooling, there won’t be as much dehumidification as a dual-circuit coil (assuming a single speed constant volume fan). In a similar concept to the previous bullet point, in first stage cooling, there will be half of the capacity running through a coil that is designed for full capacity. This means that in first stage cooling there is a much larger surface area for heat transfer relative to the amount of refrigerant running through the coil. Due to this, the refrigerant absorbs heat too quickly and doesn’t allow the coil to get cold enough to cause condensation on its surface. Imagine a large vase with a cup of ice water in it compared to a small glass with a cup of ice water in it. The latter is going to have much more condensation relative to the amount of exterior surface area. Without condensation on the coil surface, no moisture is pulled out of the air stream and the space remains as humid as it was before. 
    • If a dual-circuit coil is used and the system is running in first stage cooling, only half of the coil will have refrigerant running through it. This gives the refrigerant much less surface area for heat transfer and allows the coil to get much colder. A colder coil creates more condensation on its surface and allows for more dehumidification of the air stream. Note that if there is too little surface area for heat transfer on the coil (or too little airflow over the coil to cause heat transfer) then the condensation could freeze causing a solid block of ice to consume your evaporator coil. Read on to learn how to install the refrigeration specialties for a single-circuit system with a dual-circuit coil. 

How to Install Piping and Refrigerant Specialties for a Single-Circuit A/C with a Dual-Circuit Coil

Transitioning the refrigerant piping from a single-circuit system into a dual-circuit coil only requires a few extra steps. In this example we are looking at an intertwined dual-circuit DX evaporator coil. See the attached drawing for more information.

  • The liquid and suction lines should be ran as a single circuit as long as possible, all the way to the air handling unit. 
  • The liquid line filter drier can be installed near the condensing unit or near the evaporator (contractor/owner’s preference). If it is installed close to the evaporator, make sure that it is installed upstream of the ‘T’ and upstream of the sight glass. 
  • The sight glass must be installed downstream of the filter drier and upstream of the ‘T’. 
  • A ‘T’ must be installed at both of the liquid/suction lines to create 4 total pipes that go to the evaporator. The pipe sizes do not need to be reduced. 
  • One or two solenoid valves/coils can be installed on either of the liquid lines feeding the evaporator.
    • Only one solenoid valve/coil is required – when the unit is running on first stage cooling, the solenoid valve will close causing half of the evaporator coil to be cut off from the refrigerant flow. When the unit runs on second stage cooling, the solenoid valve will open and the entire evaporator will have refrigerant running through it. 
    • A second solenoid coil can be installed on the other liquid line and controlled to open when first stage cooling is ran (contractor/owner’s preference). York condensing units have (2) solenoid coil outputs for this option. 
  • Both liquid line coil connections will have TXV’s installed just before the distributors.

That’s it! Remember that R-410A split systems do not require suction traps as long as minimum suction velocities are maintained. 

Let us know how you prefer to install dual-circuit coils in the comments below!

Hank Kutilek
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