Q&A - Drain Traps & Static - Short #209 · HVAC School - For Techs, By Techs summary

Key takeaways

Properly sizing condensate traps is crucial in negatively pressurized systems to prevent overflow.
The rule of thumb for trap depth recommends it should be twice the maximum return static pressure observed.
Designing a trap with a bit of extra depth can help preemptively address potential issues before they arise.
Engagement from the community is valuable for ongoing learning and sharing knowledge in the HVAC field.

Summary

Introduction and Context

In this brief Q&A episode of the HVAC School podcast, host Bryan dives into a question posed by Steven from the Comfort Squad regarding condensate traps and static pressure in negatively pressurized systems. As an introductory note, before addressing the technical question, Bryan expresses gratitude toward the podcast's sponsors.

Understanding Trap Depth and Negative Pressure

The discussion centers around the common issue of nuisance calls related to retrofit jobs where secondary drain pans overflow due to negative pressure issues. Bryan explains that a common cause for this is the high static pressure in the air handler, which affects how water is managed in the drain line. He elucidates how properly sizing the trap depth is critical: for a system with a maximum return static pressure of 0.5 inches, a one-inch trap is typically sufficient. However, there might be variations in static pressure, making it prudent to design traps with a safety margin. Generally, a two-inch trap is recommended to ensure efficacy in most residential applications.

Guidelines for Trap Sizing

Bryan elaborates on the rules of thumb for estimating trap depth based on return static pressure. He emphasizes that an inch of trap depth for a 0.5 static pressure is a guideline, and if higher return pressures are anticipated, traps should be constructed deeper. A well-prepared trap can prevent issues with water backflow, and it's advisable to build with a larger size when possible, stressing that oversized traps won't typically create issues, provided the drain installation is done correctly.

Final Thoughts and Community Engagement

Wrapping up the episode, Bryan thanks Steven and the Comfort Squad for their insightful question and encourages listeners to engage by submitting their own queries for future episodes. He stresses the importance of community knowledge sharing in overcoming technical challenges faced in HVAC installations.

Show notes

In this short Q&A podcast episode, Bryan answers a question about drain traps and static pressure. This question was sent by Stephen with The Comfort Squad when he noticed an uptick in nuisance calls on retrofit systems that kept having their secondary drain pans filled.

In negatively pressurized systems (i.e., straight-cool or heat pump units, not gas furnaces), air can pull water back into the drain line. The evaporator coil is under negative pressure because the blower is on top of the evaporator coil.

The rule of thumb for trap depth is that it should be greater than or equal to twice the return static pressure (which is why a 1" deep trap is typical for systems with 0.5" return static pressure). However, return static can have some variation, and it's also not a great idea to use the TESP for this measurement because the return static could be greater than the supply. It makes most sense to account for the highest possible return static you will see.

1" is only sufficient if 0.5" return static pressure is the most you will see, so bigger is usually better in all cases. If you can install a 2" or 3" trap, you'll typically have your bases covered without having an excessive trap.

We also wrote a tech tip about this topic: https://hvacrschool.com/drain-trap-depth-and-negative-pressure/

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Q&A - Drain Traps & Static - Short #209