Question
how may rainbird series 1800 can be installed per zone on a system with a well that has 20-25 gpm of pressure?

Answer
Hey Job, each head on the system could or would use a different nozzle depending on how far you need it to throw, and also what patternit need to throw (90 degrees, 180 degrees, 360 degrees etc.).What you need to do is get the performance chart on the different nozzles, to determine how many GPM's each nozzle uses. For instance, lets say you have a row of fulls down the middle of the yard, and you space them 15 feet apart, and you want them to be a full pattern(360 degrees). You look at the chart and see that at 40PSI they put out 4 GPM's each. That would mean that if you put 5-heads with 15f (full,360 degree) nozzles on each head, this zone would use 20GPM ( 5x4GPM=20GPM). GPM stand for gallons per minute, not pressure. If you use a 12 foot nozzle with a 180 degree pattern, it might only use 1.5 GPM, so you could put 12 heads on this zone (12x1.5GPM=18Gpm). I hope this is helpful, either check on the Rainbird website or the place you purchase your heads, and you can get a performance chart. Good luck and have a great day.