Tankless Reverse Osmosis Brands and Their Models, Sorted by Rated Output

A household that wants purified water on demand, with no storage tank under the sink, is well served by the Viomi V8 PRO, a tankless reverse osmosis (RO) system rated at 800 gallons per day (GPD). The models with a published daily rating range from 500 to 1200 GPD, and they share one basic design: a pump pushes tap water through the membrane as purified water is drawn, so there is no storage tank to fill or wait for.

Where the brands differ is in stage count, pure-to-drain ratio, faucet display and remineralization. Viomi's own range spans three output levels, from the Viomi V6 PRO at 600 GPD to the Viomi M1 at 1000 GPD, which adds a remineralization stage. Viomi also sells the Viomi B6W, a 600 GPD, 7-stage tankless system that works with a standard faucet.

Tankless RO Brands by Output Tier

Besides Viomi, tankless RO for the kitchen sink comes from iSpring, Waterdrop, Frizzlife and APEC, and Culligan offers one through its local dealers. Every figure is the brand's published rating for that model.

The tier number is each model's rated daily production. The tier comes into play at home when a lot of water is drawn at once: a tankless system makes water as it is drawn, so a higher rating puts more production capacity behind filling pitchers, coffee makers, stock pots and reusable bottles, with no storage tank to wait on. APEC also makes the three-stage ROTL-600 and ROTL-600PH, the latter with a mineral cartridge. In the 600 GPD row, Waterdrop's K6 is an under-sink tankless unit that also dispenses hot water, and Culligan sells and installs its Aquasential Tankless RO through local dealers. Our comparison of tankless and tank-based RO brands covers how the two designs differ.

How the Tankless Lines Differ in Ratio, Faucet and Minerals

Pure-to-drain ratios differ from brand to brand, even at the same output. The Viomi V6 PRO, Viomi V8 PRO and Viomi M1 all run at 3:1, which works out to about one cup of water to the drain for every three cups of purified water. Waterdrop reaches the same figure on its G3P800 and X12, as iSpring does on the RO1200.

Both Frizzlife models and Waterdrop's G5P500, G5P500A and G3P600 run at 2:1, and the iSpring RO600-BN at 2.5:1. In household terms, the ratio sets how much tap water each fill uses: at 3:1, four gallons of purified water for a day of cooking and drinking send about 1.3 gallons to the drain.

At the tap, a readout puts TDS and filter life in view at each pour, which makes the faucet itself the place to check on the system. Viomi's two PRO models show both in real time on an LCD smart faucet, and Waterdrop's G3 models and the Frizzlife PD600-N put the same readings on their faucets. Measuring out a pot or pitcher is a different task, and the HD touchscreen on the Viomi M1, which shows TDS too, handles it with one-touch presets of 10 oz, 40 oz and 700 oz. The two-handle iSpring RO1200 takes yet another approach, pouring carbon-filtered rinse water from one side and RO water from the other, with inlet and outlet TDS in RO mode. Away from the tap, the Viomi app keeps usage history and replacement reminders.

Remineralization is available at every output level, from 500 to 1200 GPD. On the Viomi M1, selected minerals return after the membrane for a smoother taste, while the iSpring RO1200AK adds a dedicated mineral filter.

How a Tankless RO System Makes Water on Demand

A tankless system replaces the storage tank with a pump and a high-output membrane, and tap water passes through each part in turn. Cold water enters through a feed-water adapter on the supply line and meets pre-filtration first: a sediment and carbon stage, often combined in one cartridge, takes out particles and chlorine so the membrane sees cleaner water. On the Viomi PRO models and the Viomi M1 that cartridge is the PCB filter, and on the Viomi B6W it is the PPC filter. On the three-stage iSpring RO500-BN, for instance, a 2-in-1 sediment and carbon cartridge comes ahead of the RO membrane and a carbon block. The system's own pump, such as the built-in booster pump on the iSpring RO600-BN, then raises that water to the pressure the membrane needs, which is why production begins the moment water is drawn.

Pressurized water then passes through the RO membrane, where the stream splits. Purified water heads for the faucet, and the concentrate carrying rejected salts goes to the drain line, in the proportion set by the pure-to-drain ratio. Depending on the model, a carbon polish or a remineralization stage (Viomi M1, iSpring AK models, Waterdrop X12, Frizzlife) finishes the water, which then flows to the RO faucet, or to a connected refrigerator or ice maker on models that support one, with no storage tank between the membrane and the tap. Many tankless models also rinse the membrane automatically to keep it in good condition.

On smart models, the faucet display gets its readings from the main unit. The Viomi V8 PRO and Viomi V6 PRO show TDS and filter life right at the tap, with no need to open the app.

How the Viomi Models in the Grid Compare

Within Viomi's range, the Viomi V6 PRO, Viomi V8 PRO and Viomi M1 share 9 stages, a 3:1 ratio and a live-display faucet backed by the Viomi app, which tracks usage and sends replacement reminders. Between the two PRO models, the rated output is where they part: 600 GPD or 800 GPD in the same slim body, 16.3" long, 5" wide and 12.8" high, with a spare PCB filter in the V8 PRO's box so the first pre-filter change is covered from day one. What the step from 600 to 800 GPD gives a household is extra production capacity for big fills, since both models make water as it is drawn.

The finishing stage and the faucet are what set the M1 apart. It puts selected minerals back into the water after the membrane for a smoother taste and pours from an HD touchscreen faucet, and its filter intervals stretch to up to 2 years for the PCB filter and up to 4 years for the RO filter. A standard faucet, either the one in the box or a compatible faucet of the household's own, is the approach on the Viomi B6W, which brings 7-stage filtration to the 600 GPD level.

These four install with the parts in the box and come with a step-by-step installation video. The kit for the Viomi V8 PRO and Viomi V6 PRO covers the purified-water faucet, feed-water adapter, drain saddle, purified-water tubing, tee fitting and thread seal tape. For model-by-model detail on tanked and tankless under-sink systems across more brands, see our guide to under-sink RO brands and models.

FAQ

Can a tankless RO system also feed a refrigerator or ice maker?

The four Viomi models in the grid all do. The Viomi V8 PRO, Viomi V6 PRO, Viomi M1 and Viomi B6W each connect to a compatible refrigerator or ice maker with standard tubing, and the Viomi M1 runs that connection through Multi-device Mode in the Viomi app.

Does a tankless RO system need an electrical outlet?

It does, because the pump that pressurizes the membrane runs on electricity, and that pump is what lets a tankless system start making water the moment the faucet opens. The Viomi M1 plugs into a standard electrical outlet, and the Viomi V8 PRO and Viomi V6 PRO ship with a power supply adapter in the box. The iSpring RO1200, for one, lists an electrical outlet among its requirements.

Do tankless and tank-style RO systems take the same replacement filters?

Each tankless system uses cartridges designed for it. Viomi sells every model's replacements by name on its own site, such as the PCB and RO800 cartridges for the Viomi V8 PRO and the PCB and RO1000 cartridges for the Viomi M1.

What does "auto flush" mean on a tankless spec sheet?

It means the system rinses its membrane on its own, and the trigger differs by model: a fixed schedule, a period without use, or start-up. On a schedule, the Viomi V8 PRO and Viomi V6 PRO open the inlet valve every 12 hours and rinse for 70 seconds with the pump off, and they also rinse when they are new or after a filter change, so the membrane is looked after without anyone having to plan for it, even in weeks when the kitchen draws little water. Other designs rinse after a stretch without use, as iSpring's RO500-BN does, or run an automatic pre-flush, as on the iSpring RO800G.

Does the number in a model name match its rating?

Often it does: the iSpring RO800G, Waterdrop G3P800 and Viomi V8 PRO are all rated at 800 GPD, and the Viomi V6 PRO and Waterdrop G3P600 at 600 GPD. It is not a rule, though. APEC's manual for the ROTL-600 does not attach a GPD figure to the "600" in its name, which is why the grid sorts models by each brand's published rating rather than by model number.

Narrowing the Tankless Field for Your Kitchen

1. Within one output tier, the ratio decides how much water goes to the drain. In the 800 GPD row, the Viomi V8 PRO and Waterdrop G3P800 run at 3:1, the iSpring RO800G at up to 2.5:1 and the Frizzlife PD800-N at 2:1, so the same output comes with different amounts of drain water.
2. Longer-lasting cartridges mean fewer replacements over the years, as with the Viomi M1's PCB filter at up to 2 years and RO filter at up to 4 years, which lets the RO cartridge on that model stay in place through several years of daily use.
3. Mineral-added water changes how the water tastes compared with plain RO, and a live-display faucet keeps TDS and filter life in view at every pour, while a standard faucet keeps the countertop look simple.