What flow rate should an RDWC system have?

Why

Seven turnovers means the main circulation loop actually moves seven times the system's operating water volume each hour.

That gives a nominal turnover time of about eight and a half minutes. It doesn't mean every gallon completes a lap, or every bucket finishes mixing, in that time.

The goal is to make every plant site behave like one reservoir.

Too little circulation lets differences in nutrient strength, pH, temperature, and dissolved oxygen persist between sites.

But more isn't automatically better.

With turbulent flow in the same plumbing, friction loss rises roughly with the square of flow.

Push more water than the returns can handle, and plant-site levels can rise while the control reservoir draws down. Mature roots can make those restrictions worse.

A bigger pump doesn't fix a blocked return.

So how do you size the pump?

Add up the water actually circulating in the plant sites, control reservoir, and plumbing. Exclude a static top-off tank.

Multiply that volume by seven.

At that target:

Thirty-five gallons needs about 245 GPH. Sixty gallons needs 420. One hundred gallons needs 700. And 190 gallons needs about 1,330.

Those are delivered-flow targets, not the numbers to look for on the box.

For that 60-gallon example, a pump rated for a maximum of 350 GPH cannot meet the target. A 500-GPH model is only a candidate: check whether its pump curve delivers at least 420 against your system's resistance.

Account for pipes, fittings, valves, chillers, filters, and any net lift.

The roots don't read the box.

Then verify the system while it's running.

Measure actual flow if you want to confirm turnover. Check for stable water levels without excessive differences, and compare pH, EC, and temperature at the control reservoir and distant sites after normal mixing.

Recheck as roots fill the system.

And circulation alone doesn't guarantee adequate oxygenation. However you aerate, maintain adequate dissolved oxygen around the roots.

Size the pump to make the system behave like one reservoir.