|To calculate the PSI reading on the pool's filter gauge, you need to determine the entire head in feet on the pressure side or the head in feet on the vacuum side. Once you know the total head in feet, you divide that by 2.31 to determine PSI. For simplicity sake, the attic solar pool heater will typically add only 4-6 pounds pressure to the gauge. Therefore, if the gauge reads 13 PSI without the attic solar pool heater, it will typically read 17-19 with the attic solar pool heater installed. Some solar pool heater installations will read in the mid 20's. You will need to determine what the new "clean filter" PSI reading is so you will have an idea of when to clean the filter the next time. The Guide to Swimming Pool Heating also has additional pressure and technical information at the back.