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Apr 09, 2023

Understanding Toilet Design and Efficiency

Synopsis: Toilet design took a big step forward after 1992, when new

Synopsis: Toilet design took a big step forward after 1992, when new water-efficiency standards lowered the gallons per flush (gpf) from 3.5 to 1.6. After some missteps in the short term, manufacturers have designed toilets that get the job done, and some even do so with less than 1.6 gpf. This article provides information on basic toilet design and different toilet styles, and it identifies the toilet models that plumbers recommend most often.

Plumbers have long memories, and many still think about the problems they encountered and the callbacks they received when the first low-flow toilets came on the ­market. The impetus for the flood of new products was the 1992 Energy Policy Act, which set unprecedented water-efficiency standards for toilets, faucets, and showerheads, requiring that toilets cut their water volume from 3.5 gal. per flush (gpf) to just 1.6 gpf. The first response by most manufacturers was to put less water into existing models, with results that seem predictable in retrospect: The "new" low-flow toilets didn't work well, ushering in the double-flush days that many plumbers and consumers still recall, as well as a windfall profit for plunger manufacturers.

The short-term answer for some U.S. customers was pressure-assisted toilets, which use air pressure to push water and waste through the toilet trap and the drain line below it. These models, however, are more expensive than the simple gravity-fed toilets that most of us own. Toto, on the other hand, which had long before re-engineered its toilets for Japan's established water restrictions, was well positioned to capitalize on the initial failings of North American models and quickly came to dominate the U.S. market with its line of dependable, low-flow, gravity-powered performers.

Forced to follow suit, domestic manufacturers threw out their old models and began their own re-engineering. Hydrodynamics is a complex business, and engineering advances have steadily improved toilet performance since those early days. That's good news for today's consumers, who have a broad array of looks and features to choose from, with performance guaranteed for most major brands and models. In fact, the engineering is so good that 1.28 gpf is the new unofficial standard for gravity-fed toilets, with some high-performing toilets using as little as 0.8 gpf.

An average person flushes the toilet five times a day, meaning that a 1.28-gpf model will save 11 gal. or more of water per person per day when compared to yesterday's water-hogging models, asking that much less of your well pump or local water utility. The amount of wastewater created also goes down correspondingly, and the savings build up quickly in many cities and towns.

If you’re wondering whether your toilet is a low-flow model, you will likely find its date of manufacture stamped on the back inside wall of the tank or the underside of the lid. If the date is earlier than 1994, it was manufactured before the new EPA rules kicked in and likely uses at least 3.5 gpf.

I interviewed quite a few experienced plumbers and leading manufacturers for this article, and all agreed that today's low-flow, high-efficiency toilets perform as well or better than yesterday's thirsty thrones. So whether you go with an economical two-piece workhorse or a sculptural one-piece model, you’re likely to get great value and a phenomenal flush.

Aside from how much water they use, toilets vary in two main ways: how they are constructed and how they flush. The following guide represents the most popular elements and features on the market. Prices and premiums cited are average retail figures for high-quality toilet models.

The simplest and least expensive type, as well as the easiest to repair. Very effective models available from 1.28 gal. per flush (gpf) down to 0.8 gpf.

Flushes more water (usually 1.28 gpf) for solid waste and less (usually 0.9 gpf) for liquid waste. Buttons can be confusing to users, leading to repeat flushes. There are more parts that can fail, and parts are more difficult to replace.

If space allows, a taller, more elongated toilet is more comfortable for most users. Tight door swings are the main reason people choose round-front bowls.

Extends an extra 2 in. to 3 in. from wall than round-front bowls

Narrower tank (from front to back) allows same extension as round-front bowl; not available for all toilets; tends to be pricier

Normal tank, minimal extension

An integrated tank and bowl looks cleaner and more modern, and it is easier towipe clean.

Price: $300+

The traditional style is a separate tank and bowl, which is less expensive.

Price: $200+

The size of the standing-water surface varies among models, but larger is better, helping to keep waste from contacting the sides of the bowl. A good baseline size is 6 in. wide by 8 in. long, front to back. The volume and size of the standing water are determined by the anatomy of the toilet and can't be increased by adjusting the amount of water in the tank.

With the simplest flush mechanism, gravity-fed toilets are the best value by far, both in initial cost and in maintenance. Their internal parts can be replaced by almost any DIYer. More important, these toilets work better than ever while using less water per flush.

These toilets use compressed air generated from water pressure to create a more forceful flush, which helps push paper and waste through compromised drain lines. The pressure chamber fits into a standard toilet tank. Many models offer this feature in their standard lines, but it comes at a premium, adding $150 to $300 to the cost.

We have two engineers to thank for the incredible effectiveness of today's toilets. The national standard implemented in 1994 was enforced by an ASME/ANSI performance test that clearly wasn't identifying poor performers. The main culprit was the test media made with plastic granules and balls, which perform quite differently than human waste and wadded paper. So Bill Gauley and John Koeller, engineers already working in the field of water efficiency, developed a better test that used much more realistic test media made from a combination of soybean (miso) flour, salt, and water.

Twenty-two organizations in Canada and the United States contributed funds for Gauley and Koeller to test 50 popular toilet models using their new protocol. After hearing that many of those models had failed the new test, a number of toilet manufacturers re-engineered their failing toilets and sent them back to Gauley and Koeller for retesting. Soon manufacturers were sending every new toilet they developed, agreeing to pay a nominal fee for the service.

The test—called MaP, which stands for Maximum Performance—has become a North American standard used by around 180 toilet brands. Today there are 5,000 individual models listed on map-testing.com, with specific data for how much waste each toilet can clear with a single flush (tested up to 1000g, which is more than four times the average amount produced by an adult male).

Using a miso-flour paste that closely emulates human waste, the Maximum Performance (MaP) test has been widely adopted by toilet manufacturers, with independent test results available at map-testing.com. Toilets that successfully flush 600g to 1000g of waste in the MaP test are cited as "highly recommended" on the website.

Most major manufacturers now use their own MaP-testing facilities to develop and improve products. However, to be included on the official MaP list, all must send their toilets to independent MaP-approved labs in North America or China to be tested. This makes map-testing.com an incredible resource for consumers. The website not only has the independent MaP test results, but it also has lots of other helpful info on toilet types and features. It should be a reference for anyone who's in the market for a new model.

Sharp-eyed consumers will also notice the WaterSense label that the U.S. EPA places on certain toilets. That certification is based on a simple pass-fail test that falls well short of MaP testing, so stick with map-testing.com when checking flushing performance.

Whether you’re looking for an effective toilet at an economical price or you’re willing to pay more for the latest features, there are some other critical specs to nail down.

For a start, the MaP test only gauges flush capacity, ignoring the other major job of any effective toilet: cleaning the sides of the bowl. In North America and Japan, water enters the bowl in two ways. One is at the bottom, through a small opening that creates siphonic action in the trap, pushing and pulling the water and waste down the drain line. The other involves tank water being diverted to the rim of the bowl, where it washes down the sides in an attempt to clean away marks from wayward waste. In short, consumers want to avoid not only the plunger but also the brush.

While methods for washing the sides of the bowl are also being improved these days, it's hard to say which manufacturer's method works best. One thing plumbers and manufacturers agree on, however, is the importance of the standing water.

Experts agree that a larger area of standing water will keep more waste off the sides of the bowl. Manufacturers are adding other improvements designed to clean those sides when they do get marked. As for cleaning the user's bottom, bidet seats are one of the hottest toilet trends of all.

Toilets send some of their water straight into the trap to flush waste, and the rest goes down from the rim to wash the sides of the bowl. Some of the latest toilets, like the Gerber Avalanche, send that bowl-washing water straight forward from the back in one or two powerful jets so it travels faster and farther, washing the sides multiple times as it spirals downward.

Chlorinated tablets and pucks can degrade internal toilet components and void warranties. Kohler's new ContinuousClean toilets use common cleaning pucks but isolate the chlorinated water from tank components, sending it directly to a cyclonic bowl-washing jet. The ContinuousClean system dispenses a consistent amount of toilet bowl cleaner with every flush to keep your bowl cleaner between flushes.

Remarkably effective and relatively easy to add if there is a GFCI-protected outlet nearby, a bidet seat can fit onto any toilet. Retailing around $500, Toto's top-rated Washlet+ system has all the bells and whistles, including a heated seat, heated water, warm-air drying, a self-cleaning feature, and presets for multiple users, all controlled with a handy remote. Models with fewer features are much less expensive.

Of the 1.28 gal. of water in most of today's tanks, a portion of that needs to be left in the bowl to catch falling waste. Obviously, the larger the surface of that standing water, the better the chance it has to catch waste, keeping it off the sides of the bowl. So the best toilets not only flush and wash the sides of the bowl effectively, but they do it with less water, saving as much as possible for the water in the bowl.

Most major toilet brands list the size of the water surface for each of their models, but you might have to dig deep into the spec sheets to find it. If you can't, try asking your local plumbing supply house for more info.

There are two main shapes for toilet bowls—round and elongated—which are standardized to make it easier to match replacement seats to bowls. Elongated is always the first choice if space allows, as it is more comfortable and effective to use. However, round bowls, which are about 2 in. to 3 in. smaller, are sometimes needed to accommodate tight clearances, such as for a door swing.

A newer toilet design, called "compact elongated," narrows the tank from front to back to allow a partially elongated bowl that takes up the same amount of space as a round one. These toilets are offered by multiple manufacturers as part of various lines, and they tend to be a little pricier.

The traditional height for the rim of the bowl is 14 in. to 15 in., with the seat adding another 1 in. to 1-1/2 in. But taller toilets, ushered in by ADA rules, have proven popular with all types of users, including those who want to age in place. Called "comfort height" across the industry (with other names applied by specific manufacturers), these toilets have seat heights between 17 in. and 19 in. The taller structure adds $100 or so to the price of most toilets. Experts point out that a portable footstool, such as a Squatty Potty, can be kept nearby if someone in the home prefers to have their legs positioned higher.

Manufacturers sometimes save money by including cheaper seats with good toilets, or by selling them separately, so be sure yours has solid construction and a soft-close feature. Manufacturers like Bemis specialize in high-quality seats made to fit most toilets.

Dual-flush toilets, developed first in Australia to reduce water usage, made some inroads in the United States as part of the attempt to reduce water usage. These toilets have separate buttons for flushing liquid waste with less water and solid waste with the full tank of water. But single-flush toilets use so little water now—down to just 0.8 gpf—and do it so successfully that dual-flush toilets aren't worth the trouble, according to most experts.

Aside from being more expensive than single-flush models, dual-flush toilets have much more complex mechanics, with more components sitting in the tank water waiting to fail, at which point they are difficult to replace by nonprofessionals. Studies also show that the internal seals tend to fail, letting water drip constantly through the system. These factors are likely why dual-flush toilets make up less than 20% of all new toilets sold.

Beyond the standard two-piece designs are a host of beautiful new looks and user-friendly features. High-tech "smart toilets" use built-in technology to do fancy things like play music and adjust seat temperature. While not every new feature and style is worth its price premium, experts agree that the following trends are here to stay.

Price: $300 to $1500

Mounted above the floor, with the tank usually hidden in the wall, these toilets offer a number of unique benefits. For one, they take up the least space in a tight bathroom and can be mounted at any height. They eliminate the caulked gap at the bottom of a floor-mounted toilet, and they leave the floor wide open and accessible for cleaning. Their flush button can be placed anywhere you like—even by the door so you can hit it on the way out. Finally, some of the latest tank designs can fit into 2×4 walls (instead of requiring 2×6 construction).

Wall-mounted models also come with the most caveats. Not only are they much pricier to purchase up front—up to three or four times the cost of a basic two-piece toilet—but they are also three to four times more expensive to install, as they require fortified framing, separate trim for the flush button, and, if you’ve done it right, a panel for accessing the tank and flush mechanism when they need maintenance or replacement. Installation instructions can be complex, and pros and manufacturers recommend reading them carefully.

Pros

Cons

Price: $1500 to $15,000

The ultimate in looks, convenience, cleanliness, comfort, and performance, smart toilets combine a powered flush (which eliminates the tank) and a sleek, low-profile exterior with a number of high-tech features, usually including automatic flush, a hands-free seat and lid, a nightlight, and high-end bidet functions. Other available features include UV sanitation, air freshening, and warm, ergonomic seats.

Price: Adds $100 to $300

If you are going for a modern, Zen-like look in the bathroom, consider a toilet with an integrated skirt. This can hide the trap and bolts, which not only makes the toilet look cleaner but also makes it easier to clean. The most sculptural and the easiest to clean of the skirted models are one-piece toilets that eliminate the gap between tank and bowl. These designer models can retail for two to three times as much as standard toilets, however. If you don't have the budget for a one-piece toilet with an integrated skirt, similar less expensive skirts are also available on the base of two-piece toilets. An integrated skirt usually requires an additional boot or clips to hold it in place—as well as the toilet's main attachment bolts—so read the instructions carefully.

Pressure-assisted toilets use air power to ensure a successful flush with less water. However, they share some of the same drawbacks as dual-flush models, including initial cost—in this case, up to two times the price of a similar gravity-fed model. They also have far more complex components, which are costly to replace, and they tend to be louder than gravity-fed types.

With today's gravity-fed models being so well engineered and effective, it no longer makes sense for most people to choose a pressure-­assisted model. The one exception is for homes with drain-line carry problems that require a particularly strong flush—but even in that case, most experts recommend other solutions. Drain-line carry refers to the distance waste needs to travel to reach the sewer, and it depends far more on the condition of the drain line than the power of the toilet flush, which subsides quickly along the way.

A number of things can adversely affect a drain line's ability to carry waste and water along at sufficient speeds to prevent fouling and clogging, but the main one is insufficient slope. This is often caused by an old line being lifted in spots by tree roots. And once waste starts to gather in the low areas, flow is hampered even more. Old cast-iron drains or plumbing can also be a problem, as waste and toilet paper tend to stick to the rougher surfaces, narrowing their diameter.

The lower overall flow from today's water appliances has only worsened drain clogging. Interestingly, according to recent studies, the type of toilet paper used has a significant effect on drain-line carry, with thinner, weaker paper working much better due to its lower tensile strength. A simple change to single-ply might do the trick for recalcitrant drains.

If your new toilet still won't flush properly after a paper change, experts suggest a close look at drain conditions before assuming a pressure-assisted toilet is needed. The first thing a plumber will do is send a camera down the drain to look for problems. If one is found, the next move might be using a drain jetter—a sort of pressure-washer for pipes—to blow out built-up waste. And if that won't do the trick, the lines will need to be replaced altogether—a common move in old houses where cast-iron plumbing is often replaced with low-friction ABS or PVC.

Most new toilets are used to replace older ones, and many of these replacements are part of some sort of bathroom renovation. While almost any new toilet can be plumbed into new construction, existing plumbing and bathroom layouts come with an additional set of factors and realities.

As builders and plumbers know, the rough-in dimension is the distance from the edge of the drywall to the toilet mounting bolts that mark the center of the floor drain. The standard distance is 12 in., but 10-in. and 14-in. rough-ins are also common. For new construction, you can design the layout from scratch and place the drain as needed. But when you’re replacing an existing toilet, it's a lot less costly to use the existing drain than to relocate it.

Toilet manufacturers address this issue in one of two ways: They either make various toilets in the same line with different rough-in dimensions, or they add plastic flanges that guide the wastewater to various drain locations. In cases where only the toilet is being replaced and not the flooring, there will be a clean, unworn area where the old toilet was mounted, which will stand out from the rest of the floor. So it's important to measure to make sure the footprint of the replacement toilet is large enough to hide the old one.

Finally, make sure you check how much room you have for the bowl. Small bathrooms can make it impossible to fit an elongated bowl into the toilet area without getting in the way of a nearby door swing, for example.

Many plumbers carry a basic single-flush, two-piece, gravity-fed model in their trucks for replacing poorly performing toilets with a workhorse model that will make customers happy. The Gerber Viper and the Toto Drake are the two models carried on the most calls—shown here in their 1.28-gpf configuration with the elongated seat and ADA height that most customers prefer.

GWS21518

Price $435

Plumbers praise this proven yet economical performer. Specially angled rim jets provide excellent bowl-washing ability, and MaP scores are top-notch.

CST748CEMFG

Price $480

With decades of proven service, this model is the one most favored by very experienced plumbers. Toto continues to improve performance and add models to the Drake line.

K-31621-0

Price $360

Another favorite among trade pros is the Cimarron line from Kohler, which combines sleek looks with top MaP scores.

Because the vast majority of new toilets are sold to replace old ones, plumbers tend to carry a single trusted workhorse in their trucks for repair customers who don't have an attachment to a specific model or look.

The experienced plumbers I interviewed cited just a few models they carry in their trucks—all single-flush, gravity-fed, two-piece versions. Not surprisingly, their favorites all have high MaP scores.

Better yet, the suggested product lines come in various sizes and configurations, including elongated, comfort-height bowls, and with various rough-in dimensions. With 1.28-gpf models that cost $300 to $500 at retail, depending on features, you won't have to spend much to get a toilet that will look and work great for a lifetime while also saving water. The two lines mentioned most often by pros were Gerber Viper and Toto Drake, both of which are only available as two-piece models. Others included American Standard's Champion and Vormax lines, Gerber's Avalanche toilets, Kohler's Cimarron and Highline models, and the 0.8-gpf No Clog models from SSI (Sustainable Solutions International).

Whether you are replacing, renovating, or building new, these are likely the best-value models you’ll find. They are guaranteed to leave a clean, empty bowl with each flush, no matter how much turkey your uncle eats at Thanksgiving, or how much paper your kids flush down the bowl.

Asa Christiana is a contributing editor to Fine Homebuilding. Photos courtesy of the manufacturers.

Drawings: Kate Francis

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