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Gpm Of Pex Pipe
Pex Sizes Guide
Pex Sizes Gpm Vs
Pex Gpm
Pex Sizes Gpm Chart

So a 3/8 copper line is currently code compliant at 1.5 GPM, while the same nominal PEX is allowed to deliver 3.0 GPM at exactly 10 FPS. Gerald, we have done this successfully on multiple projects with up to 2.5 GPM fixtures with very happy occupants who don't have to wait or waste unnecessarily. You might consider going 1/2' on the tub. PEX tubes range in diameter from 3/8-inch to 1-inch for residential applications. For adequate water flow, it's a good idea to install 1/2-inch or 3/4-inch PEX for your main lines. The standard when replacing old water supply lines with PEX is to use the same size.

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How to get more water 'pressure' or how to improve water flow rate (gpm or lpm).

This article series explains how to improve water pressure or flow rate and how to improve the total quantity of hot water available from your water heating system.

Page top sketch illustrating the effects of changes in pipe diameter, courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool firm.

We also provide an ARTICLE INDEX for this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Install Larger Diameter Water Supply Piping to Improve Hot Water Pressure & Flow

As we discussed at WATER PIPE CLOG REPAIR, and as we illustrate with Carson Dunlop Associates' sketch shown here, installing larger diameter water supply piping makes a large difference in the water flow rate.

In fact you can improve hot water flow in a building by replacing only part of the supply piping - perhaps that portion which is easily accessible. Installing larger water supply piping feeding the water heater may alone improve the hot water pressure and flow in the building.

[Click to enlarge any image]

Watch out: if your hot water is provided by a tankless coil, increasing water pipe diameter may not be of much use, and it could make matters worse!

A tankless coil (and also an instantaneous water heater) is normally rated by its manufacturer as capable of increasing water temperature to a desired level only if water flow through the coil is limited to a specific rate, perhaps 5 gpm. Flowing water through the tankless coil or instantaneous water heater too fast will mean that the water temperature may be too low at the fixture.

In sum, larger diameter piping increases water pressure and flow. It does not increase the total volume of hot water that is available from a water heater. This improvement may be of most value where water pressure is poor and where water piping has previously become clogged by rust or mineral deposits.

You may regain some of this loss by insulating hot water supply piping or by setting water heater or boiler temperatures higher as well as by an adjustment at

the HOT WATER MIXING VALVE.

Impact of Changing Pipe Diameter on Water Flow Rate

Question: how much water pressure will we lose if we replace copper pipe with PEX of the same nominal size

I have municipal water and a combination furnace. The bathroom shower has separate hot and cold faucets and a diverter from tub spout to shower head.

Recently I noticed that I was using less and less cold water to moderate the hot but only in the shower. The hot in the bathroom sink was still scalding so I knew it wasn't the thermostatic control valve at the furnace.

The only thing I could think it could be was the shower control unit. My thought was that even though the cold was off there was still some cold water somehow mixing with the hot. It had got to the point where you could comfortably take a shower just in hot water while the hot in the sink would scald my hand after a second.

The only logical explanation I could come up with was a failing control as suggested above.

So I have cut out the old unit and replaced it with a Glacier Bay (Home Depot) similar unit. I also replaced the cut out copper with pex and used shark bite fittings to complete the job.

Well, the hot water problem has gone but now I have lower pressure from the shower. I haven't changed the shower head itself, just a few feet of copper to pex, shark bites instead of soldered unions and the fitting itself. I can't say if the tub spout has also been negatively affected.

Do shark bites or pex piping reduce flow ?

Could the new fitting reduce flow, after all, it's a cheapo from Home depot ?

If the water is too hot, should I adjust the thermostatic control at the furnace so I can turn the hot water up in the shower and so increase the volume at the shower head ? That said, it was higher pressure in the old unit so where has that pressure gone ?

Reply:

Shark Bites won't make a notable difference in flow rate; they very slightly crimp the tubing. But as we'll discuss, the smaller ID of PEX tubing may constrict water flow up to ytour shower head.

If the internal diameter of the Pex piping were as large or larger than the piping that you removed and I would not expect it to make a difference in the flow rate. But of course we know PEX has a smaller ID than copper of the same nominal size.

Generally, before ripping out the PEX to go to the next larger size you may want to be sure other easier obvious fixes have been done.

Have you checked the shower head itself? Often mineral scale or debris clog the shower head; cleaning that may be enough.

Reader follow-up:

2020/01/02 Will said:

I have not, it didn't really make sense to me that when changing the fitting the shower would lose pressure because of a part I didn't change.

I'll put a new one on tonight and report back :-)

Also, PEX 1/2' seems to maybe have a slightly smaller internal diameter than copper 1/2' due to the thickness of the pipe wall.

Moderator reply: Calculating the reduction in pipe cross-sectional area when changing from Copper to PEX = Reduction in Water Flow Rate

Will

Thank you for asking a great question: how much difference to water flow does changing from Copper to PEX make if the nominal pipe sizes remain the same.

At WATER PIPE CLOG REPAIR we explain that as illustrated with Carson Dunlop Associates' sketch, installing larger diameter water supply piping makes a big difference in the water flow rate.

I am (sorry to say) far too familiar with the effects on water flow rate (popularly called 'water pressure') of reductions in the diameter of a supply pipe, thanks to a lazy local Poughkeepsie plumber who used 1/2' instead of 3/4' ID PEX on a job.

That reduction in diameter when changing to PEX or when including ANY PEX in the piping run can show up as a complaint of a weaker shower flow just as you’ve suggested.

In general, the greater the percentage of smaller diameter piping in a water system the greater the reduction in flow rate, all other factors (such as pressure, total piping length, number of elbows, valves, etc. ) remaining the same).

The ID of copper pipes is ROUGHLY 1/8 less than the OD. But the actual copper piping inside diameter (ID) number varies as across types L and M the OD stays the same.

'Type K tube has thicker walls than Type L tube, and Type L walls are thicker than Type M, for any given diameter. All inside diameters depend on tube size and wall thickness.' - copper.org cited below

Nominal 1/2” K Copper: OD: 0.625” ID: 0.527” Wall: 0.49” Cross-Sectional Area: 0.218”
Nominal 3/4” K Copper: OD: 0.875” ID: 0.745” Wall: 0.65” Cross-Sectional Area: 0.436”
Nominal 1/2” PEX: OD: 0.625” (same as copper), ID: 0.485” (0.005” smaller than copper), Wall: 0.70” Cross-sectional area: (pi r squared = area of a circle & r= 1/2 diameter): (3.1416 x 0.2425 (squared)) = 0.185”
Nominal 3/4' PEX: OD: 0.875” (same as copper), ID: 0.681” (0.064” smaller than copper), Wall: 0.097” Cross-sectional area: .364”

Percent decrease in cross sectional area going from copper to PEX

Percent decrease in cross sectional area going from copper to PEX is about the same as the percent reduction in water flow through the piping, if all other factors are kept equal:

1/2” PEX = 15% less cross-sectional area & flow rate than 1/2” copper [ 1 - (0.185 / 0.218) x 100 ]
3/4” PEX = 16%