return line issues

[Poconos]

An interesting idea with the trapped air in the pipe. Let's start with the pump and filter. Makes and Models? Bubbles are a clear sign of an air leak but when the system reaches steady state, the condition after 30 minutes or so with the reduced flow in the one return, you should see bubbles out the normally functioning jets and see them in the pump basket. It can't be a big leak or the pressure would drop. The 18 psi does seem a little high for a clean filter assuming sand. If it is trapped air in that one pipe, covering the other two jets should blow it out and that should be clearly visible as the air is purged. For another piece of info you may try removing the eyeballs just to see what happens. Those things present quite a restriction and I bored mine out some to increase flow.

The buggy part of this problem is something appears to have changed. Plumbing usually doesn't change so that leaves the filter and pump. Consider the pump. Check the impeller to make sure nothing is tangled in there. Been many instances where people find stringy things that got past the skimmer and pump baskets. Otherwise nothing should change unless the pump overheated somehow and was deformed, but unlikely since the pressure is 'normal'. So let's consider the filter. Need to know more as I asked above.

Al

[Searcher]

The buggy part of this problem is something appears to have changed. Plumbing usually doesn't change so that leaves the filter and pump. Consider the pump. Check the impeller to make sure nothing is tangled in there. Been many instances where people find stringy things that got past the skimmer and pump baskets. Otherwise nothing should change unless the pump overheated somehow and was deformed, but unlikely since the pressure is 'normal'. So let's consider the filter. Need to know more as I asked above.

Al

Al, as an after thought, I would venture to guess that his filter may be marginal for the pump that is in place. Since a too big filter is excellent compared to a too big pump that causes horrors; that may be yet another issue.

I wish the OP would have posted pool size in gallons, pump size, pipe run lengths etc. It sure helps to diagnose stuff like this.

As for an air lock, you should see it on a hydronic heating system that is not piped properly, but with that and most pipes exposed, it is easy to diagnose.