Is the Air Gap Really Needed for Dishwasher?


Question: The inspector who checked the home we’re selling reported that the dishwasher needs an air-gap device on the drain line. The problem is that we have no place to install an air gap because the hole on the back rim of the sink is being used for the water purifier.

Instead of an air gap, our handyman has installed a check valve in the dishwasher drain hose below the sink, but the home inspector says this does not comply with code. The handyman says air gaps and check valves are equally effective and that even a high-loop drain line can be used to prevent back-siphonage. Again, the inspector heartily disagrees. To us, this is a bunch of confusing terminology. Meanwhile, the people who are buying our home insist that this problem be corrected in an approved manner. How do we sort through all of this conflicting technical advice?

Answers: Your question covers a number of issues and misconceptions regarding dishwasher drain lines. Let’s take them one at a time:

* An air gap typically appears as a small chrome cylinder commonly found on the back rim of most kitchen sinks.


It is what plumbers call an anti-backflow device because it prevents sewage from flowing back into the dishwasher. Your inspector was correct in recommending that an air gap be installed. Although alternative methods are available, only an air gap is approved by the plumbing code, because it is the only method guaranteed to work in all circumstances.

* A check valve is also an anti-backflow device and will also prevent sewage from siphoning into your dishwasher. The problem with a check valve is that it is not foolproof. If a piece of food gets caught in a check valve, the valve can be permanently stuck in the open position, thereby defeating its function as a protective health and safety device. For this reason, a check valve is not permitted by code as an alternative to an air gap.

* The high-loop method of installing a dishwasher drain hose is commonly used by handymen and older plumbers. This method can be effective in preventing back-siphonage, but only with low-pressure, low-level sewage back-ups.

When sewage backs up under pressure, or when a back-up reaches the level of the sink rim, the high-loop method is ineffective and will allow raw sewage to flow into the dishwasher.


If your sink lacks an available hole for installing an air gap, a plumber can usually drill an opening where needed. Another way to provide a hole is to replace the standard type sink faucet with a post-type faucet, one that does not have the rectangular mounting plate at its base. Without the mounting plate, two holes will be exposed at the rim of the sink. One of these can be used for an air gap; the other can enable installation of a soap dispenser or a hand-sprayer.

‘Water Hammer’: All Bark With No Bite

Q: Whenever I turn off the shower faucet, a loud thump can be heard inside the wall. Even when I’m on the other side of the house, I can hear a knock in the walls when someone else uses the shower.

According to the home inspector who checked our house, this could possibly cause damage to the water pipes. I don’t want to spend money on a plumber unless it is absolutely necessary. Do you think I have a serious problem?


A: The thumping noise you describe is commonly known as “water hammer.” This condition usually occurs when pipes are loosely attached within the walls, especially in homes where the water pressure is high or where air is trapped within the water lines.

When a faucet is turned off suddenly, immediate stoppage of the water flow can jar the piping, due to the abrupt increase in pressure. When this happens, loose pipes can knock against the wood framing within the walls, causing a sound.

In most cases, water hammer is nothing more than a minor nuisance and is unlikely to result in any significant plumbing problems. If you’re willing to endure the noise, you can save yourself a repair bill.

If you’d like to minimize the knocking sound, reduction of the water pressure by means of a pressure regulator may help. A regulator can usually be installed for about $100. For a more specific evaluation of your particular situation, a licensed plumber should be consulted.


Computers Require Grounded Outlets

Q: My home was built in 1958, in the days when electric outlets were ungrounded. When I purchased the property last year, my home inspector mentioned that this could pose a problem for my computer, but I’ve always used a surge protector to make up for the lack of a ground.

I didn’t worry about grounding because I thought my system was protected. That turned out to be a costly assumption. Last week a spike in the power supply “cooked” my computer, in spite of the surge protector. Could you please explain why the surge protector did not save my system?

A: Computer users often assume that surge suppressors provide unconditional protection from electrical mishaps.


What every PC user should know is that suppressors are not magical devices. They cannot make an electrical power surge simply disappear. The laws of thermodynamics state that energy cannot be created or destroyed. It can merely be changed from one form to another. Electricity, for example, can be converted to heat, as occurred when you barbecued your computer.

When a sudden flow of high-voltage electricity enters the wiring in your home, it must be sent to a safe location to prevent fire or other damage. The purpose of the surge suppressor is to channel the power away from your computer. This is accomplished by sending the excess voltage into the Earth by way of a ground wire. If no ground wire is provided, the surge suppressor is useless.

Homes built before 1963 were typically wired without grounded outlets. Anyone using a computer in a home of this age would be well advised to consult an electrical contractor.

However, grounding deficiencies can be found in homes of any age. All it takes is a small wiring error at the time of construction. Therefore, verification of grounding for all outlets serving computers is a wise precaution.


It’s Time to Jettison Service Panel Lock

Q: The home inspector who checked our house advised us to remove the padlock from our electric service panel. We previously lived in a high-crime area and had a series of problems until we began securing the panel. What do you think about locking electric panels?

A: Padlocks are commonly used on electric service panels in large urban areas where security problems are prevalent.

However, for the sake of general safety, electric panels should remain readily accessible, which means they should be unlocked, in case of an emergency.


If a circuit breaker should need to be reset on a dark and rainy night, you wouldn’t want to be searching for a mislaid key. If an emergency, such as a fire, should necessitate turning off the electricity, immediate access to the main breaker could be crucial.

Got a question about any aspect of the home inspection? Send it to Barry Stone, Los Angeles Times, 540 Atascadero Road, Morro Bay, CA 93442. Queries can also be sent via e-mail to:

All questions will be considered for use in “Ask the Inspector” but cannot be answered individually.