A walk-in refrigerator with a remote refrigeration system includes an evaporator coil that requires a drain. The typical installation is an indirect drain (terminating above a funnel drain or floor sink to provide an air gap).

Location: USA and Canada


I have recently seen installations and plans calling for an S-trap in the indirect drain line BEFORE the funnel drain/air gap.

[Evap Coil] ===/ /=== [S-Trap][Air Gap][Funnel Drain]====>

drain schematic
Image Source: HeatCraftrpd.com

My understanding was that traps provide a barrier of water that blocks gasses from entering the room through the drain.


What purpose does a trap serve immediately before an air gap?


1 Answer 1


It's preventing air movement through the drain pipe. Without it, cold air from the refrigerated space will constantly flow out the pipe, and warm air from outside will constantly leak in - wasting your refrigeration energy/expense.

So, it's doing exactly what you understand, but your understanding was perhaps limited to the case of "sewer gas" while in this case that's handled beyond the air-gapped drain, and the "gas" being prevented from moving is air - warm or cold.

  • 1
    You can also get negative pressure preventing the condensate from flowing out. Instead is will accumulate in the unit and eventually leak inside the building.
    – JimmyJames
    May 3 at 17:05

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