The 1.0 GPF Myth: How Tankless Smart Toilets Flush With Less Water

For decades, the standard toilet flush has hovered between 1.6 and 1.28 gallons per flush (gpf). So when you see a high-end smart toilet like the Kohler K-77795-2MB Eir (ASIN B0D4TGCDNF) claiming a “Dual flush… of 0.8 or 1.0 gpf,” it’s natural to be skeptical.

How can a toilet use less water but achieve a better flush?

The answer isn’t just “smart” technology; it’s a completely different engineering philosophy: the tankless system. This design is the key to the Eir’s striking minimalist look, its water-saving flush, and—as we’ve explored with other smart toilets—its single biggest installation hurdle.

The Old Way: Gravity (The Tank)

Your traditional toilet is a simple, reliable machine that runs on gravity. It has a large tank on the back that holds a “reservoir” of 1.6 (or more) gallons of water. * How it works: When you pull the handle, a flapper opens, and the entire volume of that stored water falls into the bowl, creating a siphon that pulls the waste down. * The Limitation: It’s slow, bulky, and uses a fixed, large volume of water.

The New Way: Flow Rate (The “Tankless” Engine)

A tankless toilet, like the Kohler Eir, has no reservoir. It is, as the name implies, a “compact, streamlined one-piece” unit. * How it works: It’s connected directly to your home’s main water supply line. When you flush, an electric valve opens, and the toilet uses the raw pressure (PSI) and flow rate (GPM) from your pipes to create a powerful, siphonic “turbo-flush.” * The Analogy: A tanked toilet is like dumping a bucket of water into the bowl. A tankless toilet is like attaching a fire hose.

This is precisely why it can use so little water. The flush is significantly shorter but far more powerful. The 1.0 gpf of pressurized water, when channeled correctly, creates a more effective siphon than 1.6 gpf of slow-moving gravity-fed water.

This effect is amplified by another feature listed for the Eir: the “Bowl is automatically misted prior to use.” This “pre-mist” wets the ceramic, preventing waste from adhering and allowing the 1.0-gallon “fire hose” flush to clean the bowl completely.

The Hidden Hurdle: The Return of PSI vs. GPM

This powerful, water-saving flush comes with a non-negotiable requirement, the same one that affects high-end bidet performance: your home’s plumbing must be up to the task.

As we’ve explored in our analysis of other tankless smart toilets, there are two key metrics for your water:
1. PSI (Pressure): The force of the water. (Most homes have adequate PSI).
2. GPM (Gallons Per Minute / Flow Rate): The volume of water that can be delivered at once.

A tankless toilet’s 1.0 gpf flush and its bidet demand a high GPM. They need a “multi-lane highway” of water. If your home has older, 1/2-inch pipes, you simply cannot deliver the volume of water fast enough for the siphonic action to work, even if your pressure is high.

This is the central trade-off of the entire tankless smart toilet category, whether from Kohler, Toto, or other brands. You get the sleek, tankless design and incredible 0.8/1.0 gpf water savings, but only if your home’s plumbing can provide the high-volume flow rate that “fire hose” requires.

Conclusion: A Feat of Engineering

The 0.8 / 1.0 gpf flush of the Kohler Eir (K-77795-2MB) is a feat of modern engineering. It represents the pinnacle of residential water conservation, saving thousands of gallons per year compared to older toilets.

It achieves this by replacing the passive, bulky gravity of a tank with the active, powerful flow of your home’s water line. Before investing in this next-generation technology, it’s essential to ensure your home’s “highway” is wide enough to support it.