Short definition
A dual-flush toilet has two flush options — full flush at about 1.28 gpf for solids and reduced flush at about 0.8 to 1.0 gpf for liquids — activated by a two-button or split-paddle handle. Average household use lands near 1.0 gpf, the lowest mainstream toilet category. WaterSense-labeled and rebate-eligible across most of WA.
What it is
Inside the tank, a canister-style flush valve (or a two-stage flapper) opens to one of two depths depending on which button is pressed. The full button releases the entire tank; the reduced button releases only the upper portion. WaterSense compliance requires the reduced flush at 1.1 gpf or less, the full flush at 1.6 gpf or less, and the effective (mixed-use average) volume at 1.28 gpf or less.
Two ways to get one:
- Factory dual-flush toilet. $200 to $700 (most popular models around $300). Includes both flush valves and the matched canister.
- Dual-flush retrofit valve. A drop-in flush-valve replacement (Fluidmaster Duo Flush and similar) that converts an existing 1.6 gpf toilet to dual-flush. $25 to $45. Compatibility with older 2-inch flush-valve toilets varies — research before buying.
Why it matters to a homeowner
For WA homeowners with a pre-1994 toilet (3.5 gpf or higher), replacing with a dual-flush 1.28/0.8 model can cut household water use by roughly 13,000 gallons per year (EPA estimate). Saving Water Partnership offers $100 toward a WaterSense-labeled replacement of a pre-2011 toilet. Washington Water (Pierce County) offers $50 for MaP 600+ models. Combined with the savings, payback runs three to five years on most installs.
The retrofit kit is a cheap experiment for a working 1.6 gpf toilet — but for a pre-1994 toilet, full replacement is almost always the better answer because the older bowl design wasn’t engineered for low-volume flushing in the first place.
The trap: homeowners who always press the full-flush button. The savings only work if you actually use the reduced button when it makes sense. Some dual-flush toilets default to the full flush; others split the buttons clearly. If you’re shopping, look for a toilet with the smaller button on top (clear ergonomic cue for liquid).
Common failure modes
- Membrane / canister flush valve fails. Dual-flush stops working. Replace the canister ($30 to $60).
- Half-flush button stuck. Linkage issue; clean or adjust.
- User confusion. Always-full-flush defeats the savings.
- Retrofit kit doesn’t fit. Older 2-inch flush-valve toilets may not accept a retrofit cleanly.
Common variants
- Factory dual-flush (purpose-built) vs. dual-flush retrofit valve (DIY add-on).
- Two-button vs. split-paddle vs. up-down handle. Same idea, different ergonomics.
- Dual-flush vs. pressure-assist vs. gravity HET. Different mechanisms achieving similar savings.
Washington note
Dual-flush is fully covered by WA utility rebate programs as long as the toilet is WaterSense-labeled and meets MaP 600+. Saving Water Partnership (Seattle, Tacoma, Bellevue, Everett, and the Cascade Water Alliance utilities) currently offers $100 toward a pre-2011 toilet replacement; verify the figure on their site before publishing public copy as program funding shifts year to year. Spokane and east-of-Cascades utilities run smaller or intermittent rebate programs — check the local utility’s conservation page.