The relationship between pounds per square inch (PSI) and feet of water (ftH2O) can be determined using the following formula:

```

ftH2O = PSI * 2.31

```

To calculate the feet of water that 15 PSI represents, we can substitute the value of PSI into the formula:

```

ftH2O = 15 PSI * 2.31

ftH2O = 34.65 ftH2O

```

Therefore, 15 PSI represents approximately 34.65 feet of water.