We recently received a Speakpipe question for the podcast about a common nuisance call on straight-cool A/C units and heat pumps. The systems, often retrofit units, were backing water up into the secondary drain pan and overflowing it. This issue isn’t happening on furnaces, which are positively pressurized, which means we have to take a look at the effects of negative pressure on the drain and how we can design our trap depth to mitigate those issues.

This tech tip will explore this common problem in systems with standard P-traps. We’ll look at the relationship between static pressure and trap depth, and we’ll also share a solution to this nuisance call that’ll work for most cases.

Static Pressure

We calculate total external static pressure (TESP) in different ways on furnaces and fan coils due to the arrangement of components inside the indoor unit. The way we think about TESP isn’t the only thing that changes, though; the coil and blower placement dictate the drain static pressure that has to be overcome.

In systems where the coil is over the blower, such as in furnaces, the positive pressure pushes air out of the drain. However, in systems where the coil is underneath the blower, such as in most heat pumps, the blower position puts the drain under negative pressure, which “sucks” air back into the drain.

The drain will have to be designed to overcome that pressure, and a trap that’s too shallow will allow the air to come back in and the condensate to back up.

Trap Sizing Rule of Thumb

Let me preface this section by saying that you should always heed the manufacturer’s recommendations over simple rules of thumb. You may have some exceptional cases where a rule of thumb won’t apply and can do more harm than good.

That said, there is a rule of thumb for residential heat pumps and straight-cool A/C units.

Since 0.5”w.c. is a common return-side static pressure target, we see many traps that are one inch deep to help overcome that pressure. The trap depth (in inches) is double the static pressure (in inches of water column). Even though correlation doesn’t necessarily equal causation here, the rule of thumb follows this same formula for other return static pressure readings. For example, if the return-side static pressure is 0.7”w.c., the trap depth should be at least 1.4 inches.

Now, it’s worth noting that static pressure will vary based on things like filter restrictions, especially in systems that use ECM blowers. It can change a bit over time, so be careful not to get too comfortable with a low return static and end up undersizing your trap; the static pressure can rise later.

Bigger is Better

There isn’t much harm in making traps bigger than the rule of thumb. The main thing that’s important is that the trap outlet is at least one pipe dimension (e.g., 3/4”) lower than the inlet.  

In residential HVAC, there are very few circumstances where the trap would need to be any deeper than two inches. If your return static ever reaches 1”w.c., you’ve got bigger problems to address than trap depth.

Remember, the issue has to deal with negative pressure and is unlikely to be an issue in systems that have their drains under positive pressure. Simply stated, more negative static pressure requires a deeper trap. Oh, and make sure to cap your cleanouts! 

P.S. – You can ask us questions that we might answer on our podcast and/or in a tech tip! Submit a voice recording at https://www.speakpipe.com/hvacschool.