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We have a problem with the condensate drain line on the air conditioner and water softener. It is the only basement drain in the lowest building in the whole town. It occasionally backs up.

The solution was to install a check valve before the trap. That should work fine because condensate and softener purge are very clear/clean liquids, and unlikely to clog it with soapy water or hair. However, the design of the available check valves has the flapper flap downward, so it hangs open at all times. The flapper is not particularly lighter than water (it's not a "float").... So if backflow comes slowly enough, it won't lift the flapper upward to close the valve, and slips right by it.

I was thinking, "OK, make the trap taller, and mount the check valve in the upward-flow side of the trap". That will make the flapper shut by default unless pushed upward by condensate/flush flow. However, it will be immersed 24x7. Alternately, I could turn the pipe flat horizontal and mount the flapper so gravity pulls it shut naturally.

Is any of these a good plan? Or would another idea work better?

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Use a "float ball check valve" (assuming here that your pipe is vertical). enter image description here

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