When it comes down to it, there are really only two types of flush systems in modern toilets: those that depend on gravity alone and those that employ the addition of pressure to make the flush more efficient. To understand the flush valve in a toilet tank, you have to first appreciate that it functions independently of the fill valve, although it's easy to get them confused. When you press the lever or button to flush the toilet, the flush valve lets water flow into the bowl, and after that process has completed, the fill valve engages and refills the tank.
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As water conservation becomes an increasingly important global issue, the best flush systems are the ones that do their job with the minimum amount of water. Manufacturers have developed increasingly innovative ways to do this, and it's now possible to identify several different types of flush valves. The familiar flapper-style valve is still probably the most common, but even it has changed quite a bit since the days of the old "water waster" toilets that used as much as 5 to 7 gallons of water per flush instead of the now federally mandated 1.6 gallons. Pressure-assist valves are newer, more efficient and, unfortunately, noisier than gravity-flush valves.
How a Toilet Flushes
When you flush a toilet and release water into the bowl, it's true that the water pushes the solid and liquid waste into the trapway, but that's just the beginning. As illustrated by Professor Toilet, some toilet trapways have a high arc near the rear of the toilet, and when the water reaches the crest of this arc and begins to fall, it creates a suction that does the bulk of the work of clearing the waste out of the bowl. Not all toilets have this type of trapway, and for them, the flush relies on suction created in the waste pipe, not the toilet itself. So, if the waste pipe is too large, poorly designed or partially blocked, the toilet won't flush well.
The suction effect is the principle behind gravity-flush systems, and to make sure it happens, water has to flow quickly from the tank to the bowl. Most toilet bowls have evenly spaced holes in the rim for this purpose, and when the flush mechanism releases water, it flows under pressure (a function of the toilet design) through these holes, generating water jets that clean the sides of the bowl during the flush. Washdown flush systems, found mostly in European and Asian water-saving toilets, employ this technology, but the water isn't pressurized and doesn't siphon. Instead, it exits through a large trapway entirely under the force of gravity.
Toilet manufacturers use pressure assist in one of two ways. In the positive-pressure version, a cistern in the toilet tank collects water during the refill cycle and stores it under pressure, and the pressurized water is released into the bowl when you flush. In the negative-pressure version, the cylinder in the tank is connected to the trapway by an air passage, and this configuration creates a vacuum when you flush that pulls water out of the bowl.
The original and still the most common of flush systems, gravity-flush systems feature a spillway, or siphon hole, in the bottom of the toilet tank that is covered by a retractable, water-tight seal. A rubber flapper is the most common type of seal. It's connected to the flush handle by a chain, and when you push the flush handle, it lifts to allow water into the spillway, and the pressure of the flowing water keeps it open until the water tank is empty.
Some toilet manufacturers, such as Kohler, have replaced the rubber flapper with a canister that makes a better seal and lasts longer, and because it occupies more space inside the tank, it reduces water usage simply because there's less room in the tank for water. The American Standard Champion universal replacement flush valve, rated the best gravity-fed flush valve by Toilet Verge, is a canister-style flush valve that you can install on any toilet that uses a flapper-style valve.
The introduction of the canister makes possible the widening of the spillway in the bottom of the tank, and the overall effect can be dramatic. Some canister-style toilet tanks empty almost instantaneously, sending water through the bowl and into the waste system so quickly that nothing can be left behind. For example, the Kohler Corbelle, which is part of the Revolution 360 series, is a two-piece toilet with a precision-engineered bowl, tank and trapway to maximize water flow.
A positive pressure-assisted toilet has a standard toilet tank, but when you look inside, instead of water you'll find a sealed plastic cistern. The cistern is filled with air, and when water is introduced during the refill cycle, it compresses the air, which acts like a spring to eject the water forcefully when you flush the toilet. The force of the water provides complete removal of liquid and solid waste and thorough cleaning of the sides, all while using less water than a gravity-flush toilet.
Inside the porcelain tank of a negative pressure-assisted toilet, you'll also find a second sealed cistern, but this one works differently. It's filled with air and is connected to the rear of the trapway by an air passage. When the tank fills with water, compressed air is forced into the trapway, where it puts pressure on the water in the trap and the bowl, raising the water level in the bowl somewhat. When you flush and water rushes out of the tank and into the bowl, it pulls behind it the air that was in the trapway, and the resulting suction in the trapway is an added force that works to empty the bowl using as little as 0.8 gallons of water.
The main drawbacks of pressure-assist flush systems, besides the fact that they are more difficult to repair than gravity-flush systems, are that they are noisy and more expensive, but they do have one advantage. Because the water is in a sealed tank inside the toilet tank, it doesn't cause condensation or sweating on the outside of the tank, so the toilet remains mold-free.
An invention that came from Down Under in 1980, the dual-flushing system was developed in response to Australia's perennially dry conditions, and to make it work, it originally required a completely redesigned toilet. Nowadays, however, you can install a dual-flush cistern (such as the Duo Flush from Fluidmaster) on virtually any toilet and convert that toilet to a dual-flush one that reduces water consumption, which is a testament to how far toilet technology has advanced.
Dual-flush systems vary, but the ones that work with conventional toilets take advantage of siphon action. They typically have two buttons or a lever with two positions, and one button or lever position gives a full flush, while the other produces a partial flush, which is all that's necessary to clear the bowl of liquid waste. A plunger in the cistern directs tank water upward through a siphon tube from where it goes into the bowl. When you select a partial flush, air is introduced partway through the flush to break the siphon and stop the water flow, whereas the full-flush option uses all the water in the tank.
When it works properly, a dual-flush toilet can reduce the amount of water you use significantly, but beware of bargain models. This type of toilet can be so miserly in its water allocation that you have to flush two or three times instead of just once, and you end up using more water instead of less. You probably won't have this problem if you choose a higher-priced model.
Since Congress mandated low-water usage toilets with 1.6-gallon flushes in 1992, all subsequent toilets have been water-saving toilets compared to their predecessors, but some models go beyond the federal limits. For example, Japanese manufacturer Toto improved on the gravity-flush design by replacing the holes around the rim of the bowl with a pair of high-pressure nozzles. Water from the jets swirls around the bowl during a flush, creating an efficient flush and completely cleaning the sides of the bowl while using only 1.2 gallons of water per flush instead of the standard 1.6 gallons.
Toilets of any type that are certified by the EPA's WaterSense program meet specific water-use and performance criteria. Certified models use no more than 1.28 gallons per flush, which is 20 percent less water than the current federal standard of 1.6 gallons per flush.
Composting toilets are true water savers in that they don't use any water at all, but if cleaning out the toilet — which is a job that eventually has to be done — doesn't appeal to you, you'll probably want to stick with a model that at least uses some water. A tankless toilet that uses a pressure valve to spray water directly from the water pipe into the bowl, such as a Sloan Flushometer, is an option that might remind you of your high school lavatory, though more sophisticated versions are found in designer bathrooms. It can be activated manually or by means of a motion sensor, and it can complete a flush using just 1.1 gallons of water.
Do You Need a Pressure-Assisted Toilet?
Pressure-assisted toilets definitely save water, and because they are so efficient, they can be smaller than gravity-flush models and fit more easily in small bathrooms. One of the biggest drawbacks of retrofitting all your gravity-flush toilets with pressure-assisted mechanisms is the expense because a typical gravity-flush toilet costs between $70 and $200, whereas a pressure-assisted one can set you back as much as $700, according to data on Networx. While you can install dual-flush systems on regular toilets, the same isn't true for pressure valves because they have to be integrated into the toilet design.
Besides being noisy, pressure-assisted toilets present two other problems for the homeowner. The first is that repairing one takes a level of plumbing skill that is beyond what most DIYers possess, so when something goes wrong, you'll probably have to call a plumber, and the repair is likely to be expensive. The second problem is that children and people with disabilities may experience difficulty operating the flush mechanism.
Older homes with poor plumbing, in which a gravity-flush toilet doesn't work as well as it should, can benefit from a pressure-assisted toilet, especially if it gets a lot of use. In modern homes with good plumbing, though, purchasing a pressure-assisted toilet is like buying a $200,000 Lamborghini when a Porsche would do, and make no mistake — contemporary gravity-flush toilets are more like Porsches than Chevys, Hyundais or other lower-cost vehicles.
- Water Footprint Calculator: Toilet
- Professor Toilet: Flushology 101 How Toilets Flush
- Toilet Verge: 7 Best Toilet Flush Valves 2020 – Reviews & Guides
- Best Toilet Guide: Best 4 Flushing Systems for Toilet
- ZDNet: With Vacuum Assist, Niagara Conservation 'Stealth' Toilet Manages 0.8 Gallons Per Flush
- Networx: Pressure-Assist Toilets: Benefits and Costs
Chris Deziel is a contractor, builder and general fix-it pro who has been active in the construction trades for 40 years. He has degrees in science and humanities and years of teaching experience. An avid craftsman and musician, Deziel began writing on home improvement topics in 2010. He worked as an expert consultant with eHow Now and Pro Referral -- a Home Depot site. A DIYer by nature, Deziel regularly shares tips and tricks for a better home and garden at Hunker and Family Handyman.