Updated October 28, 2026
Water always moves from high pressure to low pressure. Normally the water main in the street pushes harder than anything in your house, so water flows in. Backflow happens when that flips: the street pressure drops, or something in the house pushes harder, and water runs backward. A backflow preventer is the part that stops it, and in Northern Virginia it gets a yearly test, which we do for $250, normally $300, on our backflow testing page.
Think of a straw in a glass. Suck on the top and the liquid climbs up toward the low pressure. Your pipes work the same way, and that’s the whole trick behind backflow. Here’s what psi means, the two ways pressure sends water backward, and what pressure problems a backflow preventer can cause.
Backflow and water pressure: what psi means
Psi is short for pounds per square inch. It’s just how hard the water is pushing. Bigger number, harder push.
- The low end. Fairfax Water promises at least 20 psi at the water main in the street, and says it doesn’t promise the pressure will always be the same.
- The high end. Virginia’s home building code says static water pressure in a house shall be not greater than 80 psi. Where the street runs higher, a pressure reducing valve turns it down.
“Static” just means the pressure when nothing is running. Open a faucet and the number dips a bit. Backflow cares about the street side versus the house side: as long as the street wins, water flows the right way.
When street pressure drops: backsiphonage, the straw effect
Virginia’s waterworks rule calls the first kind backsiphonage: backflow “caused by a reduction in pressure that causes a partial vacuum, creating a siphon effect.” That’s the straw.
Loudoun Water gives the everyday causes: a drop in its system pressure, like a water main break or hydrant flushing. When the main loses pressure, every pipe hooked to it can start pulling backward. If the end of a garden hose is sitting in a bucket, a pool or a sink, that water can get drawn back in. Loudoun Water’s tip: never leave a hose end under water.
Lawn sprinklers are the big one on houses. The heads sit in soil that gets fertilizer and weed killer, and Loudoun Water warns that water can be siphoned back through the sprinkler head. That’s why sprinkler lines need a backflow preventer. More on the device itself: what a backflow preventer does.
$250 Yearly Backflow Test
The yearly test your water utility or county asks for, on a home's backflow preventer up to 1 inch. About backflow testing
When the house pushes harder: backpressure from boilers and pumps
The second kind is backpressure: backflow “caused by pressure in the downstream piping that is superior to the supply pressure.” Downstream just means on your side of the device. Your side is pushing harder than the street.
What can push that hard? Loudoun Water lists booster pumps, chemical feed pumps, boilers, elevated tanks and recirculating systems. On a house, think of a hot water boiler: its loop is pressurized and treated, and that water has no business going back to the street.
Can a backflow preventer cause low water pressure?
It can, on the line it protects. Every backflow preventer costs a little push, because water has to shove its spring-loaded one-way doors open on the way through. That’s normal. These aren’t:
- A check sticking partly shut, so less water gets through.
- Grit or scale caught inside, the same way a clogged screen slows a faucet.
- A shutoff valve left partly closed after winter.
Here’s the tell: if only the sprinkler is weak and the house is fine, look at the device on the sprinkler line. If the whole house is weak, the usual suspect is the pressure reducing valve. Prince William Water says a bad PRV can cause both high and low water pressure. Our pressure reducing valve post covers that one.
A device that’s spraying, dripping or cracked is a different story: what a broken backflow preventer looks like and what a leaking backflow preventer means.
Why a backflow preventer can push your water pressure up
This one surprises people. When a water heater heats water, the water grows a little. Without a one-way device in the way, that extra water just nudges back toward the street. Put a backflow preventer or check valve on the line feeding the water heater, and the extra has nowhere to go, so pressure in the house climbs every time the heater runs.
Virginia’s code covers this: where a backflow preventer or check valve lets heat raise the pressure, a device for controlling pressure has to go in, usually a small expansion tank by the water heater. Our pressure reducing valve post explains the tank and the 80 psi limit, and signs of a backflow problem covers the other warning signs.
How the backflow test checks the pressure inside
The yearly test is all pressure readings. A plumber hooks a gauge to the test cocks and checks that each one-way door holds back a set amount of pressure, and that an RPZ’s relief valve opens when it should. Virginia requires it every year, and here’s why backflow testing is required every year.
The full walk-through: what happens during a backflow test. What it costs: backflow test cost. The basics: what backflow is in plumbing. Local pages: Fairfax County, Loudoun County, Prince William County, Great Falls, Oakton, Aldie and Bristow.
Why SwiftPro makes the backflow test easy
You don’t need to know any of the physics to get it done:
- Pay $250 online, normally $300. Or call (703) 997-9757, or book a time online, whichever you like.
- We call you to pick the day. Tell us where the device is and whether the sprinkler is shut down for winter, and keep your notice handy.
- We test it and tag it. A SwiftPro plumber hooks the gauge to the device, reads every check and writes the numbers down.
- We send the report where your water supplier wants it. Loudoun Water, Fairfax County, Prince William Water and the town programs each take it their own way, and yours goes in the way your notice asks.
- If it fails, you get a written quote before any repair. Nothing gets fixed until you say yes, and the device is tested again once it is.
If the pressure in the whole house is off too, tell us when we call and we’ll quote the pressure reducing valve work in writing. What happens if the yearly date slips: what happens if you skip a backflow test.
Backflow and water pressure questions
What is backsiphonage?
Backflow caused by a drop in pressure on the supply side that pulls water backward like a straw, for example when a water main breaks or a hydrant is flushed.
What is backpressure backflow?
Backflow caused by your side of the pipe pushing harder than the street, for example from a boiler or a pump.
What does psi mean for my water?
Pounds per square inch: how hard the water pushes. Fairfax Water promises at least 20 psi at the main, and Virginia’s home code caps static pressure in a house at 80 psi.
Can a backflow preventer cause low water pressure?
On the line it protects, yes, if a check sticks, grit gets inside, or a shutoff is left partly closed. If the whole house is weak, check the pressure reducing valve first.
Can a backflow preventer raise my water pressure?
Yes, if it’s on the line to the water heater. Heated water expands and can’t go back to the street, so the code calls for a pressure control device, usually an expansion tank.
Does a water main break cause backflow?
It can. Loudoun Water names main breaks and hydrant flushing as causes of the pressure drop behind backsiphonage.
How much is a backflow test?
$250 for one home, normally $300, on a residential device up to 1 inch. Pay online and we call to pick the day.
Have your backflow preventer tested
Call (703) 997-9757, book online, use the form at the top of this page, or pay $250 online and we’ll call to pick the day. Everything about the test is on our backflow testing page, the rest of our plumbing work is on our plumbing page, and current deals are on our offers page.
SwiftPro Heating, Cooling & Plumbing tests, repairs and replaces backflow preventers, water heaters, sump pumps and water and drain pipes, and services furnaces, heat pumps and air conditioners, across Northern Virginia from Chantilly and Ashburn, with a separate plumbing team and a dedicated HVAC team. Family owned, BBB Accredited with an A rating, 24/7 emergency service, Virginia Class A contractor license 2705192276, and rated 5.0 stars across 1,130 reviews on Google, Angi, Yelp, the BBB and Facebook. Read the reviews.
