Question about flow rate and pipe diameter...

[TDawwg]

Hi there!

I am remodeling my bathroom and I'm hoping to get a higher flow rate to fill our bathtub quicker. Currently we get about 5Gpm.

The pipes to our water heater are 3/4" Galvanized Steel, and there are 15 foot runs of 3/4" galvanized off the water heater before the pipes transition to 1/2" PEX at the bathroom group. The 1/2" PEX from the Galvanized to the tub is about 10 feet.

Now, my question is this: Do I need to replace the entire run (including the run to the water heater) to get the benefits of increased flow? Or do I only need to replace the 1/2" pex with 3/4" pex for that? Accessibility is not a problem, so I could replace the whole system after the water heater easily.


I guess I'm just struggling to understand what benefit a diameter increase will do at any point if the rest of the pipes are confined to 3/4"

Also, I plan to replace all the galvanized at some point, but would rather put it off if I don't need to do it now!

 

[Reach4]

Now, my question is this: Do I need to replace the entire run (including the run to the water heater) to get the benefits of increased flow? Or do I only need to replace the 1/2" pex with 3/4" pex for that? Accessibility is not a problem, so I could replace the whole system after the water heater easily.

Water pressures add.

The flow is not determined by the smallest diameter. It's like resistors: a circuit with a 1000 ohm resistor in series with a 470 ohm resistor will have less resistance than a 1000 ohm resistor in series with another 1000 ohm resistor.

 

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[TDawwg]

Thanks for that analogy! Just to verify that I understand, that means (assuming the ID of the galvanized isn't rusted beyond the ID of the PEX) I would see benefit by only replacing the 1/2" PEX with 3/4" PEX. Right? Now, the valve for the tub is a 1/2" valve. Do I need to replace the valve with a 3/4" valve?

 

[Reach4]

Thanks for that analogy! Just to verify that I understand, that means (assuming the ID of the galvanized isn't rusted beyond the ID of the PEX) I would see benefit by only replacing the 1/2" PEX with 3/4" PEX. Right? Now, the valve for the tub is a 1/2" valve. Do I need to replace the valve with a 3/4" valve?

Same thinking applies.

Not all 1/2 inch valves or 3/4 valves are the same. A full port 1/2 inch ball valve might have less pressure drop than some 3/4 inch valves. However if th e valve is in the area being replaced anyway, might as well go to a new ball valve with less restriction.

Besides pressure drop, replacing galvanized with PEX means less iron in the water. PEX should be supported every 32 inches. I like the screw-type Talons, because they don't need a good hammer aim. I put a bit in my battery-powered drill. Center the screw into the joist to minimize weakening the joist.

 

[TDawwg]

Perfect. Thanks so much! That helps me a ton!