Plumbing Question - Direction

[MjOB]

I feel like this is a stupid question, but I conceptually can't figure it out (But I am awful at plumbing and physics).

I plan on having a Varios-8 for my return pump and running it through a Pentair 80 Watt UV sterilizer, positioned horizontally. I want 450-600GPH to run through it for Protozoa.

If my setup is this:

I plan to use gate control valves (pink) after the tee in both directions, and a flow meter going into the sterilizer (brown), to ensure I have the correct flow.

However, if I have ~500-600GPH going through the UV sterilizer, and the remainder from the Varios-8 going left horizontal towards the DT (left), why wouldn't the water try to go down at the place I circled in green (going into what should be "outlet" of the sterilizer)? Wouldn't the flow and pressure at that point from the right be greater than what's exiting the sterilizer, and thus result in the water actually going downwards?

Or am I being an idiot and I don't need to worry about this, and my design should work?

 

[Reefahholic]

I feel like this is a stupid question, but I conceptually can't figure it out (But I am awful at plumbing and physics).

I plan on having a Varios-8 for my return pump and running it through a Pentair 80 Watt UV sterilizer, positioned horizontally. I want 450-600GPH to run through it for Protozoa.

If my setup is this:

I plan to use gate control valves (pink) after the tee in both directions, and a flow meter going into the sterilizer (brown), to ensure I have the correct flow.

However, if I have ~500-600GPH going through the UV sterilizer, and the remainder from the Varios-8 going left horizontal towards the DT (left), why wouldn't the water try to go down at the place I circled in green (going into what should be "outlet" of the sterilizer)? Wouldn't the flow and pressure at that point from the right be greater than what's exiting the sterilizer, and thus result in the water actually going downwards?

Or am I being an idiot and I don't need to worry about this, and my design should work?

Water will take the path of least resistance and if the UV always has pressure from the RT side forcing water to the left out of the outlet, then you should be fine. Otherwise install a one-way check valve in between the T and the UV outlet. That will prevent water from going backwards if it tried.