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Saturday, March 15th 2014, 3:22pm

PVC to Poly Conversion


I am researching and purchasing the parts for a new installation and currently working on the valve manifolds. My research has led me to believe the common method of converting from PVC downline of the valve to the poly is with a PVC to poly adapter (slip or threaded x barb).

My concern is that the barbs I have found are reducing from 1" on the PVC side to 3/4" on the poly side (obviously - the barb needs to fit inside the 1" polypipe). Does this now effectively create a 3/4" flow rate for the system downline of the valves? The adapter I currently have purchased is from Lowes: LASCO 1-in Dia PVC Sch 40 Adapter (
Item #:
| Model #:

If the system is actually effictively 3/4" due to the barb adapter, is there a better way to make this connection? My system will be 1" for the entirety of the lines and valves - from the well through the laterals.

Central Irrigation

Supreme Member

Posts: 371

Location: Central Minnesota


Sunday, March 16th 2014, 9:36pm

Relax....using 1" barb fittings with 1" poly is completely acceptable. The amount of pressure lost through a MPTxbarb fitting is very minor and negligable. Any fitting that changes the direction of flow, like elbows and tees, can cause above average pressure loss, and should be used sparingly.


New Member


Monday, March 17th 2014, 12:03am

Ok, thanks for the information. I am concerned more with the flow rate, unless pressure is the bigger factor. I have in my head that the narrowest passage for the water becomes the maximum flow rate for the system or is that incorrect? So, if my system is 1" but I have that fitting that is capable of 3/4" flow rates, down line from that point wouldn't the maximum flow then be equal to the maximum flow of a 3/4" system?

I calculated the flow rate before the PVB (the system is installed only to the point prior to the PVB at this point) at approximately 25 gpm and a pressure with one faucet and one shower turned on at 50 psi. I've planned the zones to a maximum of 12 gpm, based on information I've seen that 1" polypipe can handle 13 gpm max. The valve manifolds will be PVC and after the valves will convert to polypipe. I guess what I'm looking for is with that PVC barb fitting can the system still accommodate the 12 gpm zones that I have planned? Based on your comment that the fitting is acceptable I suspect it is, it's just that my biggest fear is overextending my zones to the point that it doesn't work so I appreciate your insight.


Supreme Member

Posts: 5,331

Location: Metro NYC


Monday, March 17th 2014, 7:04am

You can confirm that this is not an issue by working out the math for pressure lost through the insert adapter (bring a lunch)

A more common-sense approach is to consider the average sprinkler system fed from a basement water meter in a home fed by a 3/4-inch supply line. Even though the supply line is 3/4 inch pipe, the system will usually employ 1-inch poly, and any insert fittings in the water pathway are simply the equivalent of a bit more length in the existing 3/4 inch supply line.

Central Irrigation

Supreme Member

Posts: 371

Location: Central Minnesota


Monday, March 17th 2014, 8:35pm

I did the 12gpm you will lose a staggering 0.017psi thru a 1" poly fitting. Which is only 0.007 psi more than the 1" poly itself.

Your job in installing a system is to moderate pressure loss. Flow rate will dictate pressure loss. The more water you try to floww thru a 1" pipe, the more pressure you will lose.

This post has been edited 1 times, last edit by "Central Irrigation" (Mar 17th 2014, 9:03pm)


New Member


Monday, March 17th 2014, 9:36pm

Ok, great, so as per your first reply I can "relax" lol. Thanks for the help, both of you!

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