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The outdoor spigot of my backyard is 3 GPM (gallons per minute), without a hose attached. I filled 8 cups in 10 seconds. Is this low, expected or high? It seems low to me.

Looking at the plumbing under the house (everything is copper), I noticed that it switches from 3/4" to 1/2" before it gets to the kitchen, and proceeding to backyard. Would this transition be a cause of a reduced flow rate? I was under the impression that pipe should be 3/4" the whole way until it finally gets to the fixture (sink, spigot, toilet, etc). The pipe run from front-of-house/sidewalk (water entrance) to backyard spigot is about 55', so I guess maybe 25' is 1/2" pipe.

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  • 14 gal per minute is the most that can flow through a 1/2 pipe. If you have 20 lbs of pressure or 100 you will get 14 gpm. If you are Getting less than that you have a smaller than 1/2” opening in the plumbing or less that 20 lbs pressure. – Kris Nov 16 '20 at 21:44
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    Where did you get that from, @Kris? It makes no sense to me. Pressure absolutely affects flow rate. – isherwood Nov 16 '20 at 21:53
  • @isherwood yes pressure effects flow rate up to a point. Once the max flow is reached no amount of pressure will increase flow rate. At 20 psi a 1/2 inch pipe will flow 14gpm At 100 psi it will flow 14gpm but you could fill the bucket from across the yard – Kris Nov 16 '20 at 22:06
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    I'm not buying it. Every chart I see shows pressure going up more or less linearly. Please cite your source. – isherwood Nov 16 '20 at 22:07
  • Working on swimming pools taught me this. It is of no use to upsize your pump to greater gpm capacity than your piping can flow. You will get much more turbulence but not turnover the pool any faster – Kris Nov 16 '20 at 22:16
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Most house hold well test require 5gpm. This flow rate is needed for many lenders, so 3 GPM is low. The well flow test are usually done for 3 hours non stop.

There can be many factors that reduce the flow. Galvanized pipes are the worst, they fill up and end up with small channels inside the pipes limiting your flow , you put a pressure gauge on the spigot and have 60+ psi but when you open the flow it drops to 20, it happens I have picked several short buys and had to change the plumbing out. Debris in the valves can also limit flow Last debris in the foot valve is another problem for wells

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  • I’m assuming the OP is on municipal water since they speak of the sidewalk as source? – Kris Nov 17 '20 at 0:40
  • Kris is correct, this is city water. – dabi Nov 17 '20 at 1:05
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Most cities do a very intuitive yet not quite scientific test. They pick your first full opening - and near me these are usually hose bibs and to meet the water company threshold for flow rate you should not be able to hold your thumb long on the faucet.

Prove that you can hold your thumb on it and the water company will come out and do a thorough test and possibly blow out your lines or look for issues.

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