Running Trap Vs P Trap for HVAC – Which Is Better?

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You just had your HVAC equipment commissioned, and you are concerned about the condensate drain trap the installer will use. You are in a dilemma about which would be a better option. A P trap or a running trap? Don't fret! We have done the research and consulted the experts to give you a precise answer.

The P trap and running trap essentially serve the same function: to seal off condensation gas from coming into your living space. However, the P trap has a better design and features that can work perfectly well with your HVAC equipment.

The P trap and running trap can be used with condensate drain lines. In recent years, P traps have become widely used and proven to be better for HVAC equipment. Let's dig into its features and mechanism and uncover why it has become a better option for your HVAC equipment. Keep reading!

black ABS plastic U bend trap for a residential plumbing and sewer system, Running Trap Vs P Trap For HVAC - Which is Better?

Why Traps Are Important in Condensate Drain?

The trap uses a column of condensate to prevent the movement of air from moving into or outside of the evaporator coil chamber or the air handler unit (AHU) during the operation of your HVAC equipment. They should be installed in a way to seal off the air passing through while at the same time allowing the fluid to drain continuously.

If gasses are not managed well, it will lead to the formation and spread of microorganisms into your living space. It ultimately impacts the quality of indoor air. This is where condensate drain traps play an important role.

drainage lines from your furnace

How Do Condensate Traps Work?

Inside the HVAC unit, the boiler collects water during the condensation process. The water collected empties into a condensate drain pan. During regular operation, the water continuously flows into the drain pan. So that the pan will not overflow, it should have a drain pipe attached to it.

When the water reaches a certain level at the pan, it should empty and continuously flow through the condensate pipe, directed towards the drainage sewer.

condensation drain has become plugged and must be cleaned

The P trap or running trap is a device that should be installed at the condensate pipe. Specifically, the condensate trap has the following functions.

  • Collects and releases the condensate from the boiler
  • It prevents gases or vapors from escaping during the combustion process
  • Seals off the sewer gases from backing or entering your home through the pipe
  • Catches solids that may otherwise clog the drain pipe

The trap is a depressed U-shape portion at the condensate pipe. It is purposely shaped to perform its functions. Due to its shape, the water automatically creates an air seal to prevent sewer gases from entering through the outlet pipe while at the same time stopping the vapors from escaping through the inlet pipe.

The condensate trap prevents toxic gases from entering the pipe while regulating water flow out of the boiler. The passage of air can completely stop while condensation can continuously flow.

This demonstrates that the condensation process of your HVAC is the main component that enables the drain trap to work efficiently. If the trap fails to perform its function due to inadequate condensation or any other mechanical problem, then combustion gas smells and may diffuse to enter your living space.

Due to its essential role and healthful benefits, installing a condensate trap at the drain pipe is now included in the building code.

Comparing Running Trap and P Trap

Now that you have learned how condensate traps work and their essential roles in your HVAC system, the P trap and running trap stand out differently from each other. Each has its unique features that affect its usefulness. We discuss them in further detail.

Running Trap

Running traps come as one-piece units. The pipe thickness is Schedule 40 and is made of PVC [polyvinyl chloride]. It is ¾ inch in diameter and 10 inches in length. The inlet and outlet connection types are solvent weld.

Running traps are best suited for air conditioner condenser drain lines. Since they are sold as a one-piece, you save on fittings, welds, and sealants. Due to their simple design, it can be easily installed, thus saving you on labor costs.

Condensate running traps are criticized, though, that they may not be the best fit for condensate drain lines because the depression is shallow. A shallow trap brings many disadvantages, as listed below:

  • The trap height may not be enough to cope with increased negative plenum pressure in the air handler unit.
  • The water capacity may not be enough to cope with the column of condensate dropping down from the drain pan.
  • The low water level could not effectively function as an air sealant,
  • Due to the low capacity and water level, there is a tendency that water will be siphoned out of the trap down the drain.
  • Due to low water capacity, freezing may occur faster, thus inducing pipe clogging.
  • Accumulation of sludge, algae, and other debris at the trap bed is higher due to less water flow pressure.

Check out this running trap in Amazon.

P Trap

The pipe specifications are similar to that of the running trap. The pipe thickness is Schedule 40 and is made of PVC [polyvinyl chloride]. The bent pipe diameter is also ¾ inch for residential use, which proves appropriate.

What significantly differs is how it is designed and installed to be an integral component of your HVAC system.

Due to its cost, ease of installation, and simple design, the P trap has become the HVAC industry standard.

Check out this P trap on Amazon.

Design Features of a Drain System with P Trap

With proper design and installation, coupled with system start-up procedures and periodic preventive maintenance of your HVAC equipment, you can maximize the usefulness of the condensate drain P trap.

Below are the features of a properly designed condensate drain system:

Condensate Drain Pan

  • It is made of stainless steel as condensate filtrates may contain corrosive elements.
  • It is positioned a minimum of 12 inches downstream of the evaporator coils.
  • The positioning is positively sloped towards the drain outlet.

Condensate Drain Pipe

  • The outlet drain height is lower than the inlet drain. If not, water will back up if the trap is full.
  • The plumbing code requires that the downward slope of the drain line should be 1/4 inch per foot towards the discharge area.

Condensate Trap

  • The water reservoir is sufficiently deep to ensure the water will not evaporate readily.
  • The u-bend is insulated from freezing, especially if the location experiences winter's extreme temperature.


  • The vent is installed after the trap. Without a vent, the water inside the trap gets sucked, and gases can enter your room.
  • The vent prevents airlock that may be due to sagging of the drain pipe.
  • The vent is installed between two traps for multiple traps in a single pipe.

Check out this condensate drain pan on Amazon.

Common Best Practices

Here are the best practices in the HVAC industry related to maintaining an efficient condensate drain system:

  • Condensate is discharged appropriately. It should not be on walkways since condensate could be a health hazard. It should not also be near the foundation of structures.

Drainage piping for condensed water attached to the air conditioner

  • The ¾" diameter drain pipe is good to reduce clogging, which might otherwise cause damage to your property or your equipment.
  • The ¼" slope for drain lines is good enough to let water flow smoothly.
  • The pipe clean-out is added in areas where it is accessible and convenient in case of troubleshooting and repairs without getting the whole drain line cut.

repair of plumbing in the house

  • The drain line is sufficiently supported with strapping to avert sagging or being bumped into. This ensures that the pipe gets anchored and not easily displaced.
  • Adding a vent before a common drain.

Final Thoughts

black ABS plastic U bend trap for a residential plumbing and sewer system

P trap and running trap are commonly used as condensate drain traps for HVAC. The running trap is recommended for air conditioner drains. However, it is widely criticized because of its shallow bend, which may not be enough to withstand negative pressure during normal operation. Other disadvantages include low trap height, insufficient water reservoir, and water siphoning.

Because of the running trap's shortcomings, HVAC specialists recommend the P trap, which is a better alternative. Because of this, the P trap has become the industry standard.

P traps are better in design for evacuating water from the HVAC system without allowing the inflow of outside air. This ensures that your home's conditioned air is not diluted by external air. From this perspective, P traps have remained the essential key to improving air quality indoors.

Hopefully, you find this post informative. We have more posts for you on related topics:

Should Air Be Coming Out Of My Condensate Drain Line?

Does the Furnace Drain Need a Trap?

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