Are you making changes to your home’s water system? or trying to diagnose a low-pressure problem? Starting with a basic understanding of how water flow works will save you a lot of headaches.
Small changes can significantly affect the “water pressure” you experience. This could be as simple as updating an appliance, adding a water filter, or changing your shower schedule.
In this article, I’ll demystify how household water flow works, how to plan for it, and fix some of the most common problems. But first, let’s clearly distinguish between water flow and water pressure.
Water Pressure vs. Flow Rate
At first glance, it might seem like water pressure, and flow rate are the same thing. They aren’t, even though many people use the terms interchangeably. They are related, though.
Water Pressure is a measure of the force pushing water through your pipes. It’s typically measured in PSI (pounds per square inch).
Flow Rate is the amount of water flowing out of a pipe over a designated period. In your home water system, this is measured as GPM (gallons per minute).
Which is more important?
For most households, water pressure is not a limitation. But flow rate can be.
Do you want to (comfortably) take a shower while the dishwasher is running?
In most cases, it’s impractical to increase the pressure of your water supply. For example, if you’re connected to a municipal water system, what you get is what you get. The pressure of the city’s water main becomes the pressure in your home supply line unless you install a regulator to lower it.
Installing a bigger supply line won’t change your pressure, either. But it can increase the number of gallons entering your household per minute.
That’s the flow rate, and it may be the most important factor when making changes or troubleshooting a problem.
City Water Pressure
Municipal water pressure is rarely a major problem, but most systems have temporary maintenance or outages. Usually, but not always, planned.
Most city water systems run at 45-75 psi. Anything below 40 psi would be considered “low.”
When connecting your supply line to a water main, the connected system adopts the water pressure of the main. If the main is 55 psi, so will your system.
Incoming pressure above 75 psi may put undue stress on the pipes, fixtures, and water appliances in your home. In many cases where the incoming pressure is high, a plumber may choose to install a regulator. This lessens the pressure that passes from the main supply to your home system.
Average Household Flow Rate
As a rule of thumb, the larger the diameter of your water supply line is, the more GPM can pass through it. Even small changes in diameter can create big differences in flow.
How many GPM does your home actually need?
For the majority of households, 6-12 GPM is sufficient. But it depends on your water needs during peak usage moments.
One family’s water system may need to support two simultaneous showers, laundry, and the kitchen sink. But a single-person home may only use one water appliance at a time.
In these two examples, the GPM needs will be very different.
The peak situation for the family I just described will require 3 GPM for each shower, 4 GPM for the washing machine, and 3 GPM for the kitchen faucet.
That’s 12 GPM, in total, at peak.
How To Calculate Your Flow Rate?
It’s best to start with napkin math and a few best guesses when calculating your flow rate. Yes, you can get more specific, but most of what you need to know shouldn’t take more than 5 minutes.
First, jot down how water is used at your home’s peak time each month.
Is it realistic that your peak usage could go up in the near future? Maybe your schedule is changing, so your water usage now overlaps with another family member, or you plan on adding a new person to the household. Perhaps a roommate, partner, or your parents are moving in.
If so, make your best guess about what your “peak” might look like after those changes.
Now, assign a GPM for each point of use at your peak moment.
- 3 GPM for a kitchen faucet
- 3 GPM for a shower
- 2 GPM for a dishwasher
- 4 GPM for a laundry washing machine
The GPM at each of these points of use in your home may vary slightly from the numbers I’ve recommended. Even so, I’d suggest using them as a baseline for napkin math before taking specific measurements. It will save you time and mess, and get you started on the right path.
Add them all together, and…viola! You now have a target GPM designed to support your household at the moment of its greatest need.
Factors Affecting Flow Rate
If you’re experiencing low water flow in your home, there are several potential causes. In most cases, the GPM out of your faucet has little to do with the water’s “pressure.”
Take water at a constant pressure and flow it through two pipes of different diameters. The resulting GPM will be vastly different.
As evidence of this, the NYC plumbing code shows that a 1” diameter pipe will flow at 8 GPM, but a 1.25” pipe will flow at 14 GPM. That’s a 75% difference!
The simple logic here is that if you want to move more water, get a bigger diameter pipe.
This is true for your supply line, which puts water into your household water system. And also for the smaller pipes that usher water from your supply line to individual points of use, like a shower head.
Limescale is a chalky buildup of minerals that drops out of hard water, especially in and around appliances like a hot water heater.
Even in areas that don’t have particularly hard water, like my home, you’re likely to find some buildup around faucets, fixtures, and in your tea kettle. Sometimes it appears like white chalk, and other times may have a pinkish hue.
Scale buildup in your water lines impacts their “usable” diameter.
I’ll draw from the example I shared in the last section to illustrate. Imagine that you are using a 1.25” supply line in NYC. It should output 14 GPM.
Now, imagine there is a buildup of scale inside the pipe that lessens the available space that water has to travel through. If it shrinks only .25” then your GPM drops to 8.
A small amount of limescale can make a big impact on flow rate.
Corroded metal pipes have a similar effect on flow rate as limescale buildup.
It won’t drop your water pressure, but the flow rate will decrease. Usually very slowly over the course of years..decades even.
This problem is common in older houses with galvanized steel piping. Unfortunately, the only solution is to replace and re-plumb the entire system.
The most common time for the flow rate to drop is during the morning hours when people are getting ready for their day. For most households, this is the “peak usage” window.
What does that mean? In plain language, it means household members are expending water more quickly than their system can replenish from a water source.
The simple solution is to rearrange your schedule so that fewer water outlets are engaged at once. Pause the dishwasher, or only take one shower at a time.
Suppose you’re using a pressure regulator to lower the input pressure of water flowing into your home’s water system. In that case, a faulty device can cause problems.
Most pressure regulators should be equipped with an analog gauge. See if anything looks out of place. If the reading is jumping around or has gone to 0, you may have found the problem.
Unfortunately, to replace a regulator, you’ll likely need professional help.
Water Meter Valve
If you’ve exhausted every other option, you might also check the inflow valve on your water meter to confirm it’s completely open.
In most cases, this device is the property of the city or your utility provider, so don’t tamper with it. If you notice the valve isn’t fully open, it may be worth calling them.
Problems with your meter valve are unlikely. Especially if everything was working fine until they suddenly changed.
What is considered a good water flow rate?
For the majority of households, 6-12 GPM (gallons per minute) is a sufficient flow rate. Your specific need will depend primarily on your water consumption during peak usage moments.
How much water do my household fixtures use?
A shower uses 1.5-2.5 GPM, a faucet 2-3 GPM, a dishwasher 2-4 GPM, and a washing machine 3-5 GPM.
What GPM is suitable for a shower?
A good flow rate for a shower head may be less than you think. The flow rate for shower heads in the US is federally limited to 2.5 GPM.