# Why aren't house pipes smaller?

I was trying to wash something in the kitchen sink and it occurred to me that I need very hot and high pressure water. Both of which should be served by smaller pipes - hot because it would take less time for hot to get to the spout, and pressure because smaller pipes causes higher pressure (I assume by experiments with my thumb and the garden hose).

So except for the bath, for which a high water volume is needed, why aren’t house water pipes a smaller diameter? This would mean both that hot water got to the sink faster, and that it was higher pressure and therefore better at cleaning things.

The water would also get colder quicker in a small pipe. Samll pipes are easier to clog with scale. It’s much easier to damage small pipes.

What makes you think that? I am no expert, but I should have thought that if anything the reverse would be true. Certainly a narrower pipe would deliver less water in the same time.

It works to a point to maintain pressure when other locations get turned on, but the feed is off a larger pipe that can supply the needed volume to both locations using the water. They do reduce the size of pipes that split off to other fixtures to maintain pressure. Get too narrow a pipe and the pressure can drop at the faucet because of the flow restriction. Using the smallest pipe that provides the needed output volume will take less time to get hot water than the pipe the is too large of a capacity, but those larger capacity pipes are needed before the splits to other faucets

I used to work on cartridges to mix hot and cold in plumbing fixtures. The output hole restriction to the spout was about 1/4 to 3/8 inches in diameter. Tubs may have a slightly larger hole. You want to make sure your water supply lines are no smaller than the faucet can handle.

You are correct in that hot water would arrive sooner with smaller pipes, since less volume flow is required to clear a smaller-diameter pipe.

You are incorrect re: pressure and pipe diameter. If we take it to a logical extreme, then an extremely small diameter pipe (say, 1/16" ID copper tube) from the street to your kitchen sink should result in the entire source pressure (city water supply is typically ~50 psi) being delivered to your kitchen sink. Clearly this is not the case: flow friction (over the entire length of that skinny-ass pipe) will dominate, and you won’t get but a trickle of water .

What you want out of your kitchen sink sprayer for blasting dried toast crumbs off of your plate is not high pressure water per se, but high velocity water. How do we get high velocity water? First, we have two boundary conditions to consider:

1. at the street, the water supply pressure is about 50 psi.
2. just downstream of your kitchen sink sprayer (when the water is released into the air), the pressure is 0 psi.

You need to convert the city water pressure into velocity efficiently, at the kitchen sink sprayer. To do this, you need to have high pressure available at the kitchen sink sprayer (just upstream of the exit point), which means you don’t want tiny pipes; you don’t want to piss away that supply pressure to friction along the entire pipe’s length.

You convert pressure to velocity at the sprayer by providing a restriction there and only there. Your experiments with the garden hose didn’t demonstrate that a skinny hose delivers high pressure; rather, they demonstrated that a large-diameter duct (here “duct” means either hose or pipe) with a restriction only at the very end provides a high-velocity jet of water.

This isn’t quite true. As you reduce the diameter the intensity of the flow will increase cleaning the scale off. However, this will lead to erosion corrosion of your copper pipes.

I wonder how much knowledge of fluid dynamics has been corrupted by people mis-interpreting the “thumb on the garden hose” experiment.

I remember my friends would insist that putting their thumb on the hose would make the water flow faster and hence get colder quicker for drinking. I tried multiple time to point out that making the exit smaller wouldn’t make the water in the hose flow any faster, but they never listened.

By “intensity”, do you mean velocity?

I’m skeptical that any significant de-scaling - let alone copper erosion - happens at flow rates typical of home plumbing.

I guess this has already been answered, but I would like to chime in.

I used to own a lawn sprinkler system business. Smaller pipes will not give you higher pressures at the outflow. It is a common misconception.

Static pressures remain the same in any size pipe. A smaller pipe will provide less dynamic pressure and you would be in worse shape.

Your best bet is to keep the pipes as big as is reasonable for the volume desired.

a smaller diameter pipe will carry a smaller volume of water, this results less heat flowing through the pipe and longer for the pipe mass to heat up. a smaller diameter pipe also has a greater surface area for the volume of water leading to heat loss.

insulating exposed hot water pipes makes a difference in the time to get hot water.

Erosion (mostly corrosion really, as opposed to mechanical erosion) can be a problem in smaller copper pipes due to higher flow velocities.