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Starting Member

Posts: 1

Location: Utah


Saturday, July 22nd 2017, 1:40pm

Pressure Vs Volume: Who is right?

I have a unique situation I could use some outside perspective on. My elderly mother has secondary water and as an original stockholder in the water company is allowed a 1 & 1/2 inch gate valve. Over 40 years ago my father installed a 1 & 1/2 inch black poly main line of about 75 feet from the street to the back of the house and from there branched out to various spigots using white PVC pipe. My father died in '91 and in 2000 my family installed a full sprinkling system to my mother's 3/4 acre lot to avoid her pulling hoses in old age. One of my brothers was at this time a sprinkling system contractor and apparently knew that with a 1 & 1/2 inch main line feed we were good to have up to 7 Rain Bird Maxi Paw sprinklers per station. From 2000 until 2016 we never had any problems with pressure, volume, sprinkler functionality or coverage. Either due to age or through faulty pipe (my brothers suspect faulty pipe) we began having yearly breaks in the black poly main beginning about 25 years ago. They were difficult to splice in and repair with the depth of the pipe being about 18 inches down. Last year I decided we would finally replace this black poly main and be done with it. There is a young man I work with who is also my neighbor and a member of my church congregation. As part of a side job he installs sprinkling systems in the summer. I'm getting older and have a bad back so my mother suggested a contractor for the job and I chose this young man out of ease and to give him the work. He joined me in examining the existing system beforehand and commented that he was surprised to see a 1 & 1/2 inch main line installed in a residential setting and that he would probably just go with a 1 inch line when he replaced it. (At this point I did not know about my mother being grandfathered into a right to have a 1 & 1/2 inch, nor did I have any personal knowledge or understanding in pressure versus volume) I did share my concern with this young man that maybe he better just stick with a 1 & 1/2 inch line because that was what was already there but he said that he just couldn't see any reason for such a big line and that if anything the system would work better than before because the reduced sized main line would mean higher pressure. He was the professional so I trusted in his judgment. He installed a 1 inch line with an inline filter. I knew we had a problem, the first time we ran the system because the pressure seemed very low and many of the Maxi Paw heads had ceased to function. (mostly older ones that up to this point had no problems) I contacted the young man and told him that he should have stuck with the 1 & 1/2 inch line and told him the problems we were having. He maintained that with a smaller line we should have better pressure and should have no problems and that there must be something else at play. Each station has to run for over an hour now and there are still areas the sprinklers won't reach anymore that are being burned to a crisp. I just contacted this young man again and told him I wanted him to help me replace the 1 inch line with another 1 &1/2 inch line. He is still insisting that there must be some other explanation because he's installed lots of systems on 1 acre lots with a 1 inch main line with no problems. I am looking for feedback to at the very least put my mind at ease one way or the other. Thanks.


Supreme Member

Posts: 5,331

Location: Metro NYC


Saturday, July 22nd 2017, 3:26pm

Replace the one inch line. You have been misinformed. (and for all you know, your contractor was misinformed, since the smaller-line-more-pressure fallacy does seem to get around) This is absolutely not negotiable, so long as the original heads are in use.

The science behind this is simple. Water pressure in a pipe is reduced by friction of the flowing water. Friction is a function of water velocity inside the pipe. Replace a 1-1/2 inch pipe with a 1 inch pipe, and the water velocity must more than double in order to have the same flow. More than double the velocity means way more than double the friction loss. Science.

By the way, the Maxipaw heads are very tolerant of particles in the water supply, so they derive no benefit from the added inline filter.

This post has been edited 1 times, last edit by "Wet_Boots" (Jul 22nd 2017, 6:26pm)

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