Originally Posted By: roconnor
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is a volumetric rate (usually measured in gallons per minute or gpm), while water pressure
is a force per unit area (usually measured in pounds per square inch or psi). They are not interchangeable.
Good water flow is a function of both good pressure [that can also be maintained over time ... called "residual pressure"], and adequate pipe size (or more correctly unrestricted pipe opening) without to many fittings or excessive length. While it is true that low pressure will result in poor water flow, having good pressure does not mean you will have good water flow. Undersized piping and corrosion inside pipes (particularly if ya have older galvanized pipe prone to corrosion) can restrict the water flow, even if you have good pressure.
is hard to measure unless you have special equipment, so most just get an indication of this by turning on two fixtures (like a shower and faucet) and then flushing a bowl. You can also get fancy and set the shower water temperature to like 105F, and then re-check the temperature after flushing the bowl to make sure it doesn?t go over like 120F which can cause scalding.
is fairly easy to measure, and is good to know particularly if a low water flow situation is suspected. However, be careful reporting both water flow and water pressure as they will vary somewhat depending on the time of day and demand on the municipal system. And if you are just hooking up a gauge to a hose bib or something, you are really measuring "static pressure".
Note that some consider 1/2? service pipe at 30 psi static pressure a minimum, but that is usually only marginally adequate for like two fixtures running at the same time in a smaller home. Many consider 3/4? service pipe at 40 psi static pressure a more reasonable minimum for a typical home (along with an 80 psi maximum), and that is what?s required by the IRC for new homes (IRC P2903.7 & P2903.3). But I think pipe size without deterioration is the most important thing.
Robert O'Connor, PE
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