Condensate drains and traps

Improper condensate trapping can cause a lot of property damage. It’s necessary that a drain line is installed correctly to avoid damage to a residence or business.

Evaporators coils are installed to allow a blower to either push or pull air across the coil. The pressure, either positive or negative applied across the coil is also the same applied pressure to the drain line connected to the drain pan of the evaporator coil.

When air is pushed across the coil, it’s considered to be under positive pressure. Since the coil is under positive pressure, the drain line will also be under positive pressure.

When air is pulled across the coil, it’s considered to be under negative pressure. Since the coil is under negative pressure, the drain line will also be under negative pressure.

The idea of the condensate drain trap is to use the weight of the water to stop the flow of air produced by the blower from being sucked into (negative pressure) into the evaporator coil or from being pushed (positive pressure) out of the drain line during operation.

The positive pressure evaporator coil

When the blower blows across the evaporator coil, the coil would be considered to be under positive pressure.

As the blower creates a positive pressure moving air across the coil and through the ductwork, it creates a negative pressure pulling air through the return side.

The positive pressure coil is most commonly paired with a gas/oil furnace. Placing an evaporator coil upstream (before) of the furnace heat exchanger could cause premature failure of the heat exchanger due to moisture damage from the evaporator coil.

Since the positive pressure condensate line is under positive pressure, any condensate sitting in the pan is forced through the drain line with or without a trap.

The downside of not adding a trap to a positive pressure condensate drain line is the minor efficiency loss of air pushed through the drain line rather than being sent to the conditioned space. Adding a trap reduces those losses as the weight of the water acts as a barrier.

It’s important to note that the outlet side of the trap needs to be lower than the inlet otherwise water will backup once the trap is full.

The negative pressure evaporator coil

Using a drain trap becomes very important when you have a negative pressure evaporator coil. When a blower is pulling air across the evaporator, it creates a negative pressure across the coil and positive pressure on the ductwork.

The negative pressure evaporator coil is most familiar with heat pumps and cooling only systems usually found in attics.

The negative pressure applied to the coil will not allow the condensate in the pan to flow through the drain until the blower shuts off. By this time, there could be too much condensate in the pan for the drain line to handle or worse the pan could overflow before the blower even shuts down.

The air being sucked in through the drain can even splash condensate through the ductwork causing damage to the structure or nearby electrical components.

No drain trap installed causing negative pressure through the drain pipe.

Using a drain trap allows condensate to flow naturally

Adding a Vent

Adding a vent to your drain line is important done to prevent airlock caused by sagging drain lines, double trapping, or sharing a drain such as a high-efficiency furnace and AC evaporator coil.

It’s crucial the vent be installed after the trap or it’s like the trap doesn’t exist at all.

Incorrect vent location makes having trap irrelevant.

Vent added after drain trap for the correct drain line run.

Drain piping should be sloped 1/4″ per foot towards discharge location without any sagging. Anytime you have multiple traps on the same drain line, you have a double trap, and this includes sagging drain lines as they can act as a drain trap.

Once water flows from the first trap to fill the second trap, a column of air is locked between the two. Air weighs less than water and rises which stops the flow of condensate from the first trap.

If you add a vent between the two traps, the air has an escape and condensate will flow.

Common Best Practices
  • Dispose of condensate to an appropriate location. Don’t drain to a walkway, anywhere there’s a chance for the line to freeze or around the foundation of a structure.
  • Using a 3/4″ drain line reduces the chance of a clogged drain which can cause property damage or damage to the unit.
  • Add a clean-out that’s accessible without having to cut the drain line.
  • Support the drain line with strapping to keep it from sagging or being knocked around.
  • Add at least 1/4″ slope for horizontal runs to assist in drainage.
  • Add a vent after the vent and before a common drain.

Clearing Drain Piping and Drain Traps

It’s important to note that any appliance equipt with a pressure switch must have the hoses removed from the switch to avoid damage to the diaphragm of the switch when blowing out or vacuuming the drain line or condensate trap.

There’s a number of high-pressure drain sprays or nitrogen that can be used to clear a drain quickly. The most common way is to simply blow the drain out manually or to use a vacuum to suck everything from the system.

Blowing out a drain will free up obstructions, but you should always flush the line after to clear out any gunk left or the line could plug again fairly quickly.

Blowing Out the Drain Line

Blowing out the drain line only requires a piece of 3/8″ condensate pump hose and a bit of electrical tape to make a seal inside the PVC.

Once inserted, block off air from entering into the condensate trap of the appliance if applicable, and blow out debris in the drain line.

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