I recently bought a new house and I was having a problem with one of the AC units randomly shutting off after it had been running for a while. Since it's a new home I called the builder and he came out and took a look and determined that the people who installed the AC installed the drain for the drip tray with a slight up hill slope to it, so the drip tray never fully drains.
The problem was that after the AC ran for a while the drip tray wouldn't drain well because of the problem with the drain and the safety float valve would trip and shut down the AC.
The builder's solution was to raise the float valve up so it takes more water in the drip tray to trip the safety switch. This allows the drip tray to drain without tripping the safety switch, but it also means that there is standing water in the drip tray pretty much all of the time. It doesn't look like it's in danger of overflowing at all, so I'm not worried about that, but I am concerned about having water in there all the time.
Should I call the builder back and have him fix the drain line, or is it not really a problem to let water sit in the tray like that?