Water Pressure and Flow

Water Pressure Solid State Inspections

Running water is a must have in every home today but the amount of water that comes out of the end of the faucet is both a factor of flow rate and water pressure.


Water Pressure

Water pressure is commonly measured in pounds per square inch (psi) and the pressure is what makes water flow through a pipe. Gravity alone can create water pressure (think about diving to the bottom of a pool or ocean) and in some communities with water towers, the water pressure is based on gravity. In communities where water needs to flow against gravity, municiple pumps can be used to add pressure.


Very little water pressure is needed to deliver water to a tap. Hot water heating systems often operate at less than 20 psi. However, our lifestyles require a higher flow rate for washing dishes, washing machines, or taking a shower so household water pressures are more often between 50 and 80 psi. Anything over 80psi may damage pluming systems so in these cases a pressure reducing valve (PRV) is often used to lower municipal water pressure when it comes into the home.


Flow Rate

Psi does not measure the actual amount of water delivered. If you have ever put your thumb on the end of a garden hose to spray it further, you are raising the water pressure allowing it to spray further, but the volume of water remains the same.


Flow rate is the amount of water that comes out of the tap in a given time. Commonly defined rates are in gallons per hour or liters per minute. Flow rate is increased by water pressure at the source (which will increase the water speed) but also by the diameter of piping material.


Putting it Together

High water pressure in a small diameter pipe may result in high water speeds, but low flow rates. This is effectively what a low water use shower head will do or a pressure washer. If you however wanted to fill a bathtub with water, this would be a very slow way to do it as the volume of water is so low. Increasing the diameter of the pipe while maintaining the water pressure will deliver significant more water at the end of the pipe.


In many older homes, when you flush two toilets at the same time, there may be a noticable decrease in the water supply at the taps to wash your hands. Often people presume that the water pressure is low when in fact it is the volume of water available through an older 1/2 inch water supply pipe that may be the limiting factor. Modern homes have 3/4 or 1 inch water supply pipes which can deliver high amounts of water to meet household demands which may include additional suites and many bathrooms and kitchens. 


Pressure Reducing Valves (PRV’s)

Common Looking PRV

Often municipal pressures need to be higher than domestic as the municipal supply needs to deliver high pressure to taller buildings and fire hydrants. A pressure reducing valve (PRV) can be used to lower the water pressure coming in from the municipal source to a safe level for the homes fixtures and appliances. 


A PRV is most often located only a few feet inside the house right after the main home water turn off. PRV’s have adjustability so a plumber can get the water pressure adjusted correctly for the home. Household water pressure greater than 80 psi could harm fixtures and appliances so it is best to leave adjustments to the pro's.


What are Home Inspectors Looking for With Water Flow?

Some home inspectors will measure the water pressure at an outside tap or laundry tub during a home inspection. While this information is useful to see if the water pressure may be too high, it cannot adequately determine if the combination of pressure and flow rates are adequate for the home.


A common test for home inspectors is to run as many fixtures as they can at the same time and then to test if showers still engage. If the client is around, this is also a good test to show them as it will show the real life application of the water flow in the home. For some clients moving into older homes from newer ones, they may need to get used to a reduced amount of water supply as ‘normal’ for the older home.


Final Thoughts

Water pressure and flow rates are a complex puzzle for home owners to figure out for themselves. If you feel you are not getting enough water supply, consider calling in the experts to help diagnose and change your plumbing system to meet your needs.


By James Bell - Owner/Operator of Solid State Inspections Inc.