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I have one of those indexing valves for my sprinkler system as is common in Florida. The system is pump-fed. Recently, all the zones come on at the same time and as a result the pressure is greatly reduced.

I disassembled it and re-seated the cam and it worked for a bit.

According to the troubleshooting guide of my valve (https://www.krain.com/6000-series-indexing-valves) it could be that there are too many heads on a zone (we recently added some heads to a zone).

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The manual says a solution is to:

N: Reduce the number of heads on the zone to obtain the proper sprinkler operating pressure.

How can one determine the proper number of heads for a given zone?

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It appears that your indexing valve relies on downstream pressure for the valve to seat properly. Too many sprinklers reduces the pressure between the indexing valve and the sprinkler heads.

You ask:

How can one determine the proper number of heads for a given zone?

The true answer is that the system should have been designed to provide adequate water volume delivered at the listed minimum operating pressure for those sprinklers. This is usually a calculation using static and flowing pressure readings with consideration of pipe diameter(s), lengths, and system configuration, accounting for friction loss.

In your case, I would suggest a simpler method: remove one sprinkler and cap the riser, then see if the indexing valve works. If not, continue to remove and cap sprinklers until it does. When it does, make sure the sprinklers are spraying correctly; just because there is enough back-pressure to allow the valve to operate does not mean your system is necessarily at the pressure required for optimum sprinkler head performance.

I would also suggest you look into a higher capacity pump, if you absolutely need to run more heads.

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