The modern washing machine requires proper installation and understanding

It started a few years ago. First there were a few inquiries. Then they increased. Now it is a regular occurrence. All of the questions (and failures) have related to the same thing - the washing machine.

The first questions that surfaced were about washing machines that discharge to a standpipe with an air admittance valve for the vent terminal. The callers all cursed out the air admittance valves, saying that the manufacturers should have known that the valve wouldn't work on washing machine standpipes.

The callers always knew that it was the air admittance valve's fault because when they removed the air admittance valve, everything worked fine. With the air admittance valve in place, the water closet would bubble. None of the traps lost their trap seal with these original callers; it was just the annoyance of the bubbling.

Sometimes a person would be sitting on the can (typical caller's expression for a water closet) when the bubbling started. This was considered unacceptable for a new home.

As the calls increased, so did the different aspects of the same problem. All of a sudden, the callers said they didn't even know what an air admittance valve was, and, if it was a plumbing product, it surely was prohibited by their code. The common denominator of the callers was the washing machine; it was always connected to a standpipe, and it was always a new plumbing installation.

Personal Experience

Last week, my loving wife and I purchased a new washing machine because our old machine was 20 years old. It still worked fine, but we decided to replace it anyway. I am hoping this new machine will take me up to retirement. It didn't take a long time in the showroom to see what all the fuss was about.

A few years back, some genius came up with the idea of changing the pumps in the larger washing machines to high-speed pumps. Boy, do they pump out the water! The new machine we purchased pumps the water out at a rate of 17 gpm. That's right, 17 gpm! Surprisingly, we did not purchase the domestic washing machine with the largest pump. There are other machines out there that discharge at a rate of 21


If these numbers don't immediately register, let me review the drainage basics. We always assume that the maximum flow for a horizontal drain is half = full flow. The quantity of flow (gpm) can be calculated using the Manning Expressions. Some plumbing codes publish the values for various size drains at a given pitch.

Table 1 identifies the flow rate for half-full horizontal drains at various pitches. You may see slightly different numbers in tables that you have. The flow rates change based on the roughness factor selected and the inside diameter used (plastic pipe versus cast iron). I used the values that are published by the plumbing engineering community.

Plumbing codes permit either a 1-1/2- or 2-inch standpipe for washing machines. When you install a 2-inch standpipe, you normally connect it to a 2-inch drain.

According to the flow rates in the table, a 2-inch horizontal drain will not handle a discharge of 17 (or 21) gpm, even at a pitch of 1/2-inch per foot. I haven't seen too many drains pitched at 1/2-inch per foot lately.

If the drain is pitched at 1/4-inch per foot, the capacity of the drain flowing half full is 9.7 gpm. If a standpipe is discharging at a rate of 17 gpm, the drain will flow at full flow, which is double the half=full flow rate. If the discharge is 21 gpm, the flow exceeds the limitation of the 2-inch drain. The waste water commonly ends up rising in the vent pipe.

When these standpipes have a vent that terminates to an air admittance valve, the water cannot rise in the vent pipe. The air admittance valve is in the closed position under neutral or positive pressure. An air column is created in the vent pipe and the water has to bubble out somewhere. That could be the water closet. If the air admittance valve is removed, the water can then rise in the vent pipe again. Remember, however, waste water isn't supposed to rise in the vent pipe.

What often makes matters worse is when the appliance installer (not the plumbing contractor) connects the washing machine drain to the standpipe.

It is not uncommon to find a donut that allows the washing machine drain to be hard-piped to the standpipe. They do this to prevent the drain from leaking out of the standpipe. They probably know that the 2-inch drain is inadequate and the donut is the only way to prevent an immediate leak when they test the machine.

Why Closets Bubble

If you look again at Table 1, a 3-inch drain pitched 1/8-inch per foot has a flow rate of 20.3 gpm at half full flow. That means that any other fixture discharging to the drain will push the drain past the acceptable limits. What will happen then? The water closet will bubble.

Keep in mind that the further down the drain, the more the flow will attempt to level out and lower in the pipe. However, every fixture in the close proximity of the washing machine standpipe can experience problems.

It is not the plumbing's fault; it is the idiot that decided to boost the size of the discharge pump on the washing machine.

So what is a plumbing contractor to do? I have come up with three solutions to this problem. The first recommendation is for the homeowners to purchase a smaller washing machine that doesn't have a high-speed pump. To this suggestion I say, "Yeah, right!" Imagine me telling my wife she cannot have the best, top of the line washing machine. When we divided up the household duties, she said she would do the wash. To keep her happy (and doing the wash) I'll buy her whatever she wants.

The second recommendation (what I have done) is to discharge the washing machine into a laundry tray, rather than a standpipe. The water rises in the laundry tray and discharges into the drain at the normal rate of a draining sink.

The final recommendation is to connect the standpipe to a 3-inch drain as quickly as possible when roughing in the DWV system. Whatever you do, don't connect another fixture to a 2-inch drain receiving the discharge of a washing machine standpipe. Also, don't position a water closet too close to the connection of the 2- to 3-inch drain; bubbling will possibly occur.

Let's get back to the air admittance valve connection on a standpipe. I was curious as to whether the air admittance valve made matters worse. I asked a testing laboratory to test various piping arrangements with a water closet and washing machine standpipe with a super high-speed pump.

The only piping arrangement that did not result in the loss of the water closet trap seal was the system that had an air admittance valve for both the standpipe and the water closet.

The reason this system worked the best was due to the minimal amount of piping to the vent opening (air admittance valve). The vent piped outdoors had too great a pressure loss, resulting in the loss of the trap seal in the water closet. Again, the plumbing system was never designed for flow exceeding full flow conditions, and that includes the venting system.

If you have never encountered any of these complaints, count your lucky stars and keep praying. Just be aware of the various solutions when the problem arises.