WHAT IS the purpose of a bath tub overflow?

Answer 1

Home buyers and new homeowners believe that bathtub overflows will prevent their bathtubs from overflowing due to an unattended filling operation.

This belief is also held by many building inspectors and building code officials.

This belief is a myth.

Here is how bathtub overflows came to be:

100 years ago manufacturing molds for cast iron bathtubs had a hole for mounting a tub spout inside the tub. In other words, the original purpose for the hole was as a mounting point for the tub filler spout. It was later learned that water from the tub could back siphon through the spout into the water supply. Plumbing codes made this installation illegal. With the tub spout moved up on the wall, manufacturers were left with a hole that had no purpose. The molds for the tubs were so expensive that the manufacturers could not afford to change the mold to remove the hole. Their first idea was a solid plate mounted in the hole with a chain attached to the plate and a rubber drain stopper at the other end. Second idea: A trip lever to move the tub stopper up and down. The lever is attached to a rod which went down the backside of the tub and then under the tub to the drain where it attaches to the stopper. This assembly required a housing to enclose the trip lever rod. Once this housing was in place some trip lever plate manufacturers cut a slot in the plate. The slot allowed a very small amount of water to flow down the housing when the water level in the tub reached it. People later dubbed this assembly an overflow even though it would not prevent a tub from overflowing due to an unattended filling operation. There was no intention at the time for this to be a real overflow and no code work (standards creation, engineering calculations, testing protocol, or listing) was ever done to make this setup a real overflow. Nothing has changed from then to now - we are still installing trip lever housings - not overflows.

If the trip lever itself is not used then you have a useless trip lever housing collecting and growing mold and bacteria. Although using a trip lever only makes it worse, since the trip lever rod and springs increases the collecting and growing area.

So why do so many bathtubs still have the hole punched in them?

• By the time it was economically feasible to remove the hole from the tub mold the history had been forgotten and people were used to it.

• A generation of people heard the word overflow from the previous generations and assumed that is what it really was.

• People have a tendency to keep doing what they have been doing - so they kept making tubs with holes.

According to PMI (Plumbing Manufacturers Institute) no manufacturer currently makes bathtub overflows, only trip lever assemblies are made.

Many years ago the legal environment was much less intense. The fact that people commonly called trip lever assemblies "overflows" (because they saw that a small amount of water could go down the slot at the bottom of the trip lever plate) even though they really were not, was not such a big deal. The problem is that this practice has created a generation of people who believe that these things really are overflows when they are not, and the legal environment is now very hostile to a product being sold with a name that misleads consumers as to its true function.

If you disagree then I challenge you to come up with the documentation that these so called overflows are a real plumbing product in the same sense as any other plumbing product being what it claims to be:

• Show that a standards specification exists with ASME, ANSI, NSF, IAPMO, or any other approved standards making group.

• Show the documentation for the engineering calculations that specify the flow rates the overflow should be capable of handling and how this relates to the overflow "protecting against accidental flooding resulting from an unattended filling operation".

• Show what listing agency is providing a listing for such a product and what testing protocol they are using to verify that the overflow is meeting the standard.

• Show what manufacturer makes an overflow that has such a listing and stands behind the product with a warranty that the product will "protect against accidental flooding resulting from an unattended filling operation" according to the standard to which it is listed.

You will find that none of this exists.

Bathtub overflows that "protect against accidental flooding resulting from an unattended filling operation" do not exist.

I have checked with the Plumbing Manufacturers Institute and all the manufacturers are against the idea of creating a requirement for a real overflow in the code. I have also talked with many of the manufacturers directly. There is no manufacturer that makes a real overflow, they are all making the same thing that has been made for the last 50 to 60 years - trip lever assemblies, trip lever holes, trip lever housings. The strange thing is that these same manufacturers continue to sell this obsolete hole and piping as an overflow even though they know that is not what it is.

Here are a few other problems with trip lever housings that have cropped up in the legal system:

• The foam rubber seal on the back of the tub has a tendency to dry out and shrink slightly over a period of years. This creates a small gap where water can leak unseen and unknown into the compartment behind the tub, which creates a mold forest which then leads to lawsuits and huge remediation bills. Leaking overflows are a real problem, especially in two story houses where it creates extensive mold damage in the walls below.

• When a tub starts draining the water level in the trip lever housing rises to the same level as the water level in the tub, and as it does this it pushes the air in the tube out into the bathtub area and into the bathers face if they are sill in the tub. Since the trip lever housing tube is not self cleaning it can hold a lot of "gunk" and can harbor mold. I am hearing reports of lawsuits being filed by people claiming that mold spores are being pushed into their face and is spread in the air throughout the house, making them sick (and that for a long time they did not know what was making them ill).

I had a meeting with a plumbing engineer and he brought up some interesting points about bathtub overflows.

There are three sets of manufacturers that are involved in bathtub overflows:

1. The manufactures of the bathtub.

2. The manufactures of the waste assemblies that attach to the back of the bathtub.

3. The Manufacturers of the tub and shower valves that fill the tub with water.

These three sets of manufacturers do not talk to each other about this issue and none of them know what combinations of products are being put together. They also do not know what flow rates are going into the tubs and there is no limit set by any plumbing code. There is also no limit on the number of filler valves that can be installed on a bathtub. Flow rates can range from 6 gallons per minute up to 24 gallons per minute or more depending on what is installed and what the water system pressure is.

In many cases a bathtub will overflow even if the drain is left open. The tub filling system is a valve under 50psi to 80psi of pressure while the tub draining system is 1 1/2" pipe that is removing water by gravity. The tub filler will win every time.

Here are letters from AB&A and Lasco Bathware on the overflow issue:

http://www.gilbertplumbing.com/pdf/ABA_Overflow.pdf

http://www.gilbertplumbing.com/pdf/Lasco_Overflow.pdf

Recent International Code Committee decision on what the requirements are for a bathtub overflow:

http://www.gilbertplumbing.com/pdf/ICC_Overflow_decision.pdf

I have studied the code interpretation made by the ICC officials and it appears that they did not research the issue but acted upon the myth of bathtub overflows. They believed that someone at some point in the past had done all the engineering and code work. This is a good example of the power of myth influencing those in positions of power and regulatory responsibility in such a way that it maintains and strengthens the myth.

This is a quote from an email I received from a member of the committee that rendered the decision (after I sent him information similar to what is posted here, although more extensive):

"Subject: RE: ICC Bathtub Overflow Requirement

Thanks for the info. It is quite thought provoking. I hope you will be attending the hearings in Baltimore next month to present your views to the IRC-PM committee.

To quote from Mark Twain:

It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.

PS: I was a member of the 'anonymous committee' that rendered that interpretation."

The Uniform Plumbing Code published by IAPMO does not require bathtub overflows. IAPMO recognizes that real bathtub overflows do not exist and therefore it would be pointless to require them.

Ward Gilbert

http://www.gilbertplumbing.com

Answer 2

To make sure the bathtub does not overflow.

There is a long discussion of this for another question and I do not intend to repeat that here. A properly installed unit will drain some water from an overfilled tub down through the waste water drain. This is done under gravity and is not guaranteed to keep up with the pressurized water supply that fills the tub. This contradicts the assumption that it prevents overflow.

Any four-year-old can demonstrate this empirically, fifteen years before learning about proof by contradiction in an undergraduate mathematics class.