Reddit mentions of Underhill A-SHG-160K Sprayhead Tester Adapter with Pressure Gauge for Female Nozzles

New House? 21GPM? It sounds like you will have BIG problems forever unless you make the builder fix it.

You are a victim of the builders irrigation installation crew. They are horrible in most instances. My bet is they used regular VAN nozzles if we are talking about sprays. If they is the case you can probably just replace those with HEVANS or MPR's.

You probably dont have head to head coverage either.

It is highly unlikely you have the capacity to run 21 GPM in a residential area. Even when I have 65PSI and 5/8 meter, I try not to go over 15GPM.

Even though you see your heads running, they will probably never water efficiently with sub-optimal pressure.

Further more, 21GPM most likely does not leave any water for the house. Turn on the high zone, go inside turn on the sink, and observe for pressure change.

A properly designed system will not impact your water pressure inside.

20 minutes 3 times a week is probabaly too much if your talking about sprays. The actual nozzle being used and what is being watered is what determines the time needed. For instance a rainbird VAN nozzle may only need to run 4 minutes per day, but an HEVAN or an MPR probably needs 10 minutes. The type of nozzle is listed on the top.

In any case, if you want to run sprays 20 minutes per day, set your timer to run two start times for 10 minutes each so you waste less water to runoff.

Put one of these gauges on the farthest head and check the pressure. Compare to what is recommended in the manufacture manual. Anything plus or minus 10 pounds from optimal should be fixed. 10 pounds is generous. I try to stay within 5 - 7 pounds.

Check this out at Amazon.com
Underhill A-SHG-160K Sprayhead Tester Adapter with Pressure Gauge for Female Nozzles https://www.amazon.com/dp/B00EE3OR5S/ref=cm_sw_r_other_apa_i_iaVJDbMAKWAX1