How To Inspect Your Own House, Part 7: Protect Your Water by Preventing Cross-Connections

The most important job of any plumbing system is to deliver clean water, and to keep that water clean from sewage and other contaminants.  This makes up the first and second of the basic plumbing principles in the Minnesota State Plumbing Code, which read as follows:

A. All premises intended for human habitation, occupancy, or use shall be provided  with a potable water supply which meets the requirements of the commissioner of health. Such water supply shall not be connected with unsafe water sources nor shall it be subject to the hazards of backflow or back-siphonage.

B. Proper protection shall be provided to prevent contamination of food, water, sterile goods, and similar materials by backflow of sewage. When necessary, the fixtures, device, or appliance shall be connected indirectly with the building drainage system.

A large section of the plumbing code is dedicated to protecting our potable water supply, so it's a good idea to check your own home for some of the most obvious violations and fix them.  Truth-In-Housing Evaluations have a big focus on protecting the municipal water supply, and homeowners are typically required to repair conditions that could contaminate the potable water supply.  I've already blogged about many of these conditions, so I'm pulling content from many of those past posts to put all of the information together into one post.

Check out the old video clip below for some good information about the importance of protecting the municipal water supply.  The first minute and forty-five seconds waxes on and on about who made the video possible, then they get to the actual content.

As mentioned in the video, any potential cross-connections are potential sources of contamination.


A cross connection between the water in a toilet tank and the potable water supply can be made if the toilet fill valve or toilet ballcock is improperly installed.  There are three most common types of toilet fill valves; the old style ball-cock, the new style fill valve, and the clam-shell type.  The ballcock relies on a floating ball to shut off the water supply when the water in the tank gets high enough.  The approximate height at which the fill tube connects to the ballcock is considered the "Critical Level" line, and it needs to be at least 1" above the top of the overflow tube.  The diagram below illustrates this.

Standard Ballcock Air Gap

By far the most popular type of fill valve today is the type with a float attached to the stem of the fill valve, which is shown in the diagram below.  Again, the critical level line needs to be 1" abot the top of the overflow tube.

Toilet FluidmasterBallcock

Here's a short video clip that I made many years ago, showing homeowners how to adjust the height of the fill valve in an existing toilet.  One quick warning though: the installation instructions for these fill valves actually says to adjust the height of the fill valve before it's installed, and that the little ring on the bottom shouldn't be moved at all, as this is what holds the assembly together under pressure.  Whatever.

A far less common type of fill valve is the clam-shell type, which sits at the bottom of the toilet tank.  These valves create a potential cross connection and just shouldn't be used.

  Clamshell toilet fill valveClamshell Toilet fill valve

While the risk of contamination to the potable water supply is fairly low, I suspect most people would prefer to not drink the water in their toilet tanks if they didn't have to.  While on the topic, here's a 'fun' photo I took during a home inspection a couple of years ago.  I flushed the toilet as I was filling the bath tub, and the filthy water in the toilet tank was siphoned into the water going into the tub, instantly turning the tub water black.

Contaminated tub water

Gross, huh?  That's why the guts of the toilet matter.

Outside Faucets, Utility Sink Faucets

Any time a garden hose can be attached to a faucet, the faucet should be protected from a cross connection.  On older faucets without a built-in vacuum breaker, this could be achieved by installing an external vaccuum breaker, as shown below.

Vacuum Breaker 1011

Note: the vacuum breaker shown above doesn't technically meet code in Minnesota, however, Minneapolis and Bloomington allow these vacuum breakers for their Truth-In-Sale of Housing / Time of Sale evaluations.  More on that specific topic here: Cheap Vacuum Breakers Don't Meet Code.

Every faucet I've seen on new construction homes for the past several years has come with built-in backflow prevention, and can be identified by a slightly longer stem.  The photos below show examples of faucets with integral backflow prevention.  These faucets don't need an external vacuum breaker.

Frost-free sillcock with integral vacuum breaker

Sillcock with integral vacuum breaker

Utility sink faucets with threads that will accept a garden hose should have an exterior vacuum breaker installed.  Don't install vacuum breakers at the hot and cold water valves for the washing machine though; washing machines are already built with an internal air gap.

Bath Tub Faucets

If you have an old bath tub with a faucet located inside the tub, such as the one pictured below, you have a potential cross connection.  Don't mind the height of the overflow; the overflow is not the same as the spill line, as a clogged drain would also mean a clogged overflow.

old faucet

One way to correct this issue is to replace the faucet.  A proper faucet for a clawfoot tub will have the opening of the faucet spout located well above the spill line of the fixture, as shown below.  I found a decent selection of replacement faucets at .

Code-Compliant Faucet

If you like the faucet you have or you just don’t want to change out the faucet, another option is to have dual in-line check valves installed on the water supply pipes.  These valves will prevent potentially contaminated water from flowing back into the potable water supply.  If you choose this method, make sure the check valves are accessible; when the next inspector comes through, they’ll probably be looking for them.

Bathroom Sink Faucets

Very old bathroom sink faucets that have openings below the spill line of the fixture create a potential cross-connection.  This usually happens with the type of faucets shown below, where there is a separate faucet for both the hot and cold water.  My tape measure is sitting on top of the spill line of the fixture to show the relationship of the faucet openings to the spill line.  The faucet openings should be at the same height as the top of my tape measure to create a 1" air gap.

Cross connection at bathroom sink

This is a bad setup to start with, because there is no way of tempering the water coming out of the faucets unless you want to fill the sink… and who wants to do that?

The best fix is to replace the faucets with a single faucet, which may also require replacement of the sink.  Another option is to install check valves on the water supply pipes.

One non-recommended way to fix this cross connection is to bend the faucets up a little bit to create an air gap.  I say non-recommended because there is a risk of permanently damaging the fixtures, but I bring up this fix because I've seen it done many times.

Hand Showers

Hand Shower

Hand showers that can sit in bath tub or shower water create a potential cross-connection.  Many hand showers already come with built-in backflow prevention, but not all.  The most obvious way to prevent a cross-connection at a hand shower is to replace the hand shower with a standard shower head that doesn't have a hose.

Vacuum Breaker for Hand ShowerIf you don't want to lose your hand shower, install a vacuum breaker with a 1/2" thread, as shown at right.  This device installs in-between the pipe coming out of the wall and the shower head.

As with most other potential cross-connections, installing dual inline check valves on the water supply lines that feed the shower would be an acceptable fix as well, but it's a lot more work.

If you live in Saint Louis Park or Bloomington, you’ll have to fix this when you have your pre-sale inspection done.  For Truth-In-Sale of Housing evaluations, Minneapolis only requires that handheld showers have a hook to hang on that will keep the shower head above the spill line of the fixture.


If the dishwasher drain hose is improperly installed, there's a potential cross connection at the dishwasher.  The Minnesota State Plumbing Code (section 4715.1250) actually says that dishwasher drains are supposed to have an air gap, which is this ugly thing that sits on top of the counter, as shown in the diagram below (courtesy of CodeCheck).

Dishwasher air gap

We don't see too many of those here in Minnesota though (thank goodness), because the plumbing code then goes on to say that a residential dishwasher is allowed to connect directly to a garbage disposer or sink tailpiece as long as "the drain line is fastened as high as possible under the countertop."  The diagram below shows a nice example of this:

High loop at dishwasher drain

Nevertheless, here's a more common installation that I found while doing a new construction inspection:

Improper air gap at dishwasher drain hose

Tsk tsk.  As you can see, the dishwasher drain hose isn't even looped as high as the bottom of the sink.  For more discussion on this topic, check out my old blog post on dishwasher drains.

Water Softeners

The discharge hose on a water softener needs to have an air gap at it's termination; it should never connect directly to any fixtures or drains.  The two most common places to terminate a water softener discharge hose are at a utility sink or a floor drain.  There are about a bazillion ways to create an air gap, so here are a few nice examples.  This first diagram shows the instructions that come with a water softener.  Easy enough.

air gap at water softener drain

The only problem with the setup shown above is that it will make a big ol' mess of the floor.  This next one is a pre-manufactured solution called the Handy GaP, which will prevent water from splashing all over.


If the water softener discharges to a laundry sink, simply figure out a way to make the hose terminate 1-1/2" above the spill line of the sink.  The photo below shows one example of this.

water softener air gap at sink


In Minnesota, the minimum type of backflow prevention device required for a boiler in a one or two-family home is a double check valve with an intermediate atmospheric vent (DCVIAV).  This valve must be installed on the water supply line that feeds the boiler.  The image below shows an example of this device.

Double Check Valve with Intermediate Atmospheric Vent

For anything other than a one or two-family home, a reduced pressure zone (RPZ) valve must be installed.  This valve is also needed if any chemicals are added to the system, such as glycol.

RPZ Valve

If you have a boiler at your home but you can't find one of the two valves pictured above, you ain't right.  This is a repair that's best left to a plumber.


These are the most common cross-connections that home inspectors find.  If you have any of these cross-connections in your own home, your "good citizen" duty is to fix them.  Next week, the topic will be inspecting your furnace.

Click on any of the links below to see the past topics in this series:

Author: Reuben SaltzmanStructure Tech Home Inspections


Written By

Reuben is a second-generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Sharlene has worked with Structure Tech since 2000 and Reuben has been contributing to her blog since 2008.

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