I'm about to replace some I-20 hunter heads (4 or 5 per zone) but I have no idea which nozzle to use? I got 12 nozzles with each head (4 low angle & 8 standard). Do I need to know the gpm or pressure? ......I don't have either.
Each zone only has a certain amount of water available, measured in gallons per minute.
You need to know this to decide how to set your nozzles.
Since you already have I-20s installed we'll assume you have enough pressure and flow.
In that case you have a couple of options.
First is look at the old ones and see what they have, match.
Second, measure from one head to another, assume you have a certain amount of pressure (say 50psi) and choose the nozzle that fits that distance. For example, at 50 psi nozzle # 3 will go about 38' (always figure the distance listed is maximum under ideal conditions.) The chart is located HERE
Finally, start with the low end nozzles on all of them, see how they work, change out if needed.
Knowing the psi would help a lot.