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Discussion Starter · #1 ·
I'm filling out a RainBird design request form so that they can give me a recommended layout for a sprinkler system for my house, but I need to figure out the flow rate that I will provide to the sprinkler system.

I have 3/4" pipe coming into my house, but every faucet in my house including the hose connections outside uses 1/2" pipe. I would like to tap into the 3/4" pipe for the sprinkler system since the flow rate that I measured out of the 1/2" hose connection only provides 8.89 gpm, which is barely above the minimum 8 gpm required by RainBird. The problem is that I don't have anywhere to measure the flow rate out of the 3/4" pipe.

Seems like there has got to be a way to estimate the flow rate of the 3/4" pipe, knowing the flow rate, velocity and pressure of a 1/2" pipe in the same house.

Pressure: 58 PSI
Velocity @ 1/2": 14.71 Feet Per Second
Flow Rate @ 1/2": 8.89 GPM

I can think of two ways to estimate this, but I don't know if they are even in the ballpark or not.

#1: since 3/4" pipe is about 67% larger than 1/2" pipe, I could multiply 8.89 * 1.67 to get roughly 14.8 GPM. This seems to neglect too much to be even roughly accurate, but I could be wrong.

#2: use a flow rate calculator, such as:

http://www.1728.org/flowrate.htm

with 3/4" pipe and the same 14.71 FPM velocity that I calculated with the 1/2" pipe. The problem with #2 is that I don't know if velocity is constant across multiple pipe sizes at the same height even though pressure is constant at the same height. The GPM calculation with method #2 comes out to 20.3 GPM.

Any thoughts?

Thanks,
Adrian
 

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Fastest way is extremely high tech. It is what I used when designing residential irrigation systems.

You will need a bucket that you know the size of and a stop watch. A five gallon paint can will give you nice numbers to work with.

Close of all running water in the house---bath and kitchen faucets, showers, washing machine, dishwasher, etc.

Place the bucket under an outdoor hose spicket. Crank it open all the way and start the stop watch. Click the stopwatch off when the bucket is full. Extrapolate the number of seconds it took to fill the bucket out to the number of gallons that would have flowed in a minute. There you have it. An accurate enough flow rate calculation for a sprinkler system.

Hope you are planning drip irrigation for everything but turf. It waters in gallons per hour, not per minute, and puts water just where you want it.
 

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Discussion Starter · #3 ·
Fastest way is extremely high tech. It is what I used when designing residential irrigation systems.

You will need a bucket that you know the size of and a stop watch. A five gallon paint can will give you nice numbers to work with.

Close of all running water in the house---bath and kitchen faucets, showers, washing machine, dishwasher, etc.

Place the bucket under an outdoor hose spicket. Crank it open all the way and start the stop watch. Click the stopwatch off when the bucket is full. Extrapolate the number of seconds it took to fill the bucket out to the number of gallons that would have flowed in a minute. There you have it. An accurate enough flow rate calculation for a sprinkler system.

Hope you are planning drip irrigation for everything but turf. It waters in gallons per hour, not per minute, and puts water just where you want it.
Thanks for the reply. This is exactly what I did on my hose valve, and that's where the above numbers came from, but the hose valve only uses 1/2" pipe and my sprinkler is going to use 3/4, so I need a way to determine the flow rate of the 3/4" pipe which has no direct opening for flow rate measurement.
 

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Is your water supplied by a well, or city water. Part of what you need to know is can your water source deliver what you need.

3/4" inch copper can deliver twice as much water at a fraction of the pressure drop as 1/2" copper.

EG: 1/2" copper type K at 100 foot and 3 GPM has a pressure drop of 7.4 PSIG. 3/4" copper type K at 100 foot and 3 GPM has a pressure drop of 1.4 PSIG.
 

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Discussion Starter · #7 · (Edited)
Thanks for the link sdsester. By just looking at the 3/4" flow rate alone, the chart gives me confidence in the #2 calculation that I used. The pressure range on the chart is pretty wide though. Seems like there would be a huge difference in flow rate from 20 to 100 PSI. The chart also seems to indicate that velocity is constant (12 f/s in their case) across varying pipe size, probably as long as the pipe of interest is the bottle neck and not the supply line as beenthere indicated.

The chart over-estimates my flow rate at 1/2" by about 63.5%, so if I assume the same chart would also over-estimate my 3/4" pipe by 63.5% then:

23 GPM * 0.635 = 14.605 GPM

So then that number corresponds more with my #1 calculation.

Maybe it would be best to just use 14 GPM as my design flow rate. Seems like it would be better to under-estimate than to over-estimate. I'm thinking that under-estimating would just mean more zones, but over-estimating would mean that there would be too many sprinkler heads per zone and the water won't spray enough out of each head. Is that correct?


Also, beenthere, I have city water at my house (located in Colorado). How would I determine if my water source can deliver what I need?
 

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Discussion Starter · #8 ·
Actually, I just thought of a way to determine what I need. I can open both the front and back garden faucets simultaneously, measure the individual flow rates and add them together. The combined flow rate should tell me the flow rate limit of the house (due to the 3/4" main pipe). And if two isn't enough, I could always use one or two of the bathtub faucets in addition.

Does that sound like it will work?
 

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Don't loose too much sleep over this. The irrigation designer is going to error on the conservative side anyhow and should provide you with a triangular sprinkler head layout---this is the best coverage with more than sufficient overlap to water evenly. Your circuits will likely run out of water pressure before flow rate.

If you go with drip for everything but turf, your flow rate is almost irrelevant as the drip circuits will be set-up logically (backyard circuit on the back of the house, etc.) rather than based on pressure or flow rate. Remember drip waters in gallons per hour and you can create I really nice system working with emitters in the .5-4 gallons per hour range. They do have little mini rainbirds and things for groundcover that draw 12gph but I never found a real reason to use them.
 

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You're going to have to tee into the 3/4" line for your irrigation line anyways.... why not do it now and install a valve, then you've got your 3/4" line to run the flow test on?
 

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Discussion Starter · #12 ·
I was thinking about that, but the main line for the house is in my basement behind a finished wall. I will have to tear the wall out to do the plumbing anyway, but I was hoping to wait until I'm ready to do the whole thing before removing the wall because it will probably be 2 or 3 months before I begin the sprinkler project. Plus I imagine if I did do the test in the basement, I'll probably get tons of water all over the carpet down there.

Thanks for everyone's help
 

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I was thinking about that, but the main line for the house is in my basement behind a finished wall. I will have to tear the wall out to do the plumbing anyway, but I was hoping to wait until I'm ready to do the whole thing before removing the wall because it will probably be 2 or 3 months before I begin the sprinkler project. Plus I imagine if I did do the test in the basement, I'll probably get tons of water all over the carpet down there.

Thanks for everyone's help
Why not cut a small hole in the drywall get a shark bite 90 and some pex. Have a helper turn the main on when your ready. Before you cut the main, drain all the water that you can with sink faucets. Have a shop vac running to suck any water left in the line when you cut it.
After your test just put a coupler on, and you're good to go.
 
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