Tankless vs. Tank-Based Reverse Osmosis: How the Leading Brands Compare

A tankless reverse osmosis system filters water in real time, the moment the faucet opens, with no storage tank taking up cabinet space. A tank-based system filters water ahead of time and holds it, already pressurized, in a tank until it's needed. That single design choice drives almost every other difference between the two categories: cabinet footprint, how fast water reaches the glass, whether the unit needs power, and what it costs to buy and run.

This comparison walks through both categories using specific brands and models, not just generic pros and cons, so the tradeoffs are grounded in real dimensions, flow rates, and verified specs rather than marketing language.

How the Two Designs Actually Work

A tank-based RO system runs feed water through its filter stages and RO membrane whenever the tank isn't full, storing the output in a pressurized bladder tank, usually a cylinder roughly 9 to 11 inches in diameter and a foot or more tall, until a draw on the faucet releases it. A built-in shutoff valve stops production once the tank reaches capacity; as water is drawn from the faucet, tank pressure drops enough to reopen that valve, so production resumes in the background rather than running nonstop. Many tank-based systems also run a brief auto-flush cycle across the membrane after each fill, which helps clear concentrated minerals off the membrane surface and extend its working life.

A tankless system skips that storage step. Water passes through the filter stages and membrane only when the faucet is opened, often with the help of a small booster pump to maintain pressure through the membrane, and the unit stops producing water the instant the faucet closes.

Viomi's own product page addresses this directly in its FAQ: "A traditional tank RO system makes water slowly. It takes one minute to produce two to three ounces of RO water. If you were to turn on your faucet for a glass of water at the actual membrane production rate, then you would have to wait at least 5 minutes for it to fill." That's the reason tank systems store water ahead of time instead of producing it live. It's also why a tank system's water sits for hours between draws, which is the specific tradeoff tankless designs are built to avoid.

Cabinet Space: Where Tankless Systems Have a Real Edge

This is the most consistent difference between the two categories, and it's easy to verify with actual dimensions rather than taking either side's word for it.

Every tank-based system on this list needs a second, separate cylinder sitting next to the filter housing, and that cylinder is non-negotiable: it's what makes the system work. In a cabinet that already holds a trash bin, cleaning supplies, or a garbage disposal, that tank is frequently the reason an RO system doesn't fit at all.

Viomi's product page for the Viomi M1 makes this explicit, stating its tankless design "take[s] up 65% less space than tank-based filters" and pointing to the unit's 17.1 x 6.4 x 15.9-inch footprint as the reason. Given that a typical pressurized tank alone adds a 9-to-11-inch-diameter cylinder standing over a foot tall, that comparison holds up against the competitor dimensions above, not just as a marketing line.

Water Flow: Instant Delivery vs. Stored Burst

Flow feels different between the two architectures, so the useful comparison is steady on-demand output versus a short stored burst.

A full tank delivers water at close to full household faucet pressure for as long as its stored volume lasts, because that water was already filtered and pressurized ahead of time. That flow does taper as the tank empties and the internal bladder pressure drops, but a full tank filling a large stockpot is still fast. The tradeoff shows up after the tank empties: a 50-to-75 GPD tank system (APEC's ROES-50 and iSpring's RCC7AK both fall in this range) then has to refill from a slow trickle, and heavy, back-to-back use can outpace that refill rate.

A tankless system has no such reservoir to draw down, but its instantaneous flow is capped by how fast the membrane can produce water in real time, and that cap does scale with a model's GPD rating rather than landing on one fixed number across the category. Viomi B6W and Viomi V6 PRO are rated at roughly 53 oz/min (about 1.57 L/min, 0.41 GPM) at the faucet, while the higher-output Viomi V8 PRO reaches roughly 71 oz/min (about 2.10 L/min, 0.56 GPM). In everyday terms, that's about 9 seconds to fill an 8-ounce glass on the Viomi B6W or Viomi V6 PRO, and roughly 7 seconds on the Viomi V8 PRO, enough for filling glasses, a coffee maker, or a kettle without a noticeable wait. Filling a large stockpot in one go will still take longer at the tap than drawing the same volume from a full tank.

In practice, this plays out as: tank-based systems win for occasional large, fast draws when the tank is full; tankless systems win for steady, repeated use throughout the day, since there's no tank to deplete and no refill period where flow slows down.

Installation and Power Requirements

Entry-level tank-based systems like the APEC ROES-50 and the base iSpring RCC7AK don't need electricity at all. Feed water pressure alone drives the membrane and fills the tank, which matters in a cabinet without an accessible, always-on outlet. That's a genuine installation advantage for tank-based systems in older kitchens.

Tankless systems, including Viomi's entire lineup, need a dedicated electrical outlet to run their booster pump. Viomi V6 PRO, Viomi V8 PRO, and Viomi M1 pair that pump with a smart display built into the faucet itself, showing live TDS and filter life at the tap; the Viomi B6W uses a standard faucet and shows the same data through its companion app instead. Either way, it has to be a standard always-on outlet rather than the switched half of a garbage disposal's outlet. That's a real constraint worth checking before ordering, not after.

Both categories typically drill a single hole for the dedicated faucet and tap into the same drain saddle connection, and both usually install with push-to-connect fittings rather than soldered pipe. The one detail worth confirming ahead of time is faucet hole diameter: standard RO faucets and air-gap faucet styles don't always share the same hole size, so a system that replaces an existing air-gap faucet, or a sink with a non-standard number of pre-drilled holes, may need an adapter regardless of category. The tank-based systems add one more step beyond that: mounting and plumbing in the separate tank, with its own shutoff valve and fittings, which is one more set of connections that can eventually need attention.

Maintenance and Filter Costs Over Time

Filter replacement schedules are broadly similar across most models in this comparison: a pre-filter or carbon stage replaced every 6 to 12 months, and an RO membrane replaced every 2 to 4 years. Viomi M1 is the outlier on the pre-filter side: its PCB (pre-carbon-block) filter is rated for 2 years, well beyond the typical 6-to-12-month cadence for that stage. Its RO filter, rated for 4 years, sits at the upper end of the typical 2-to-4-year range rather than beyond it. Annual filter costs land in a broadly comparable range for most of these systems, though Waterdrop's G3P800 is a notable outlier once its higher-priced RO membrane cartridge is factored in, so filter spend is worth checking model by model rather than assuming it's uniform across the category. Current filter pricing changes by brand and over time, so check water.viomi.com for Viomi's replacement filter costs and each competitor's current listing for theirs.

Where they differ is the tank itself. A pressurized bladder tank holds standing water between draws, and manufacturers generally recommend periodically sanitizing the tank as part of routine maintenance, an extra step tankless systems skip entirely since there's no stored water sitting between uses. For a tank-based buyer, that's worth treating as a routine item on the same schedule as filter changes rather than an afterthought.

Water Efficiency: Pure-to-Waste Ratios

Reverse osmosis always sends some feed water down the drain along with the purified water; the ratio between the two varies a lot by design.

Standard tank-based systems without a permeate pump are the least efficient designs here, often sending 3 to 5 gallons down the drain for every gallon of purified water produced, with the exact ratio shifting with incoming water pressure and how full the tank already is. Viomi's own product pages describe this as the baseline problem tankless design solves: "traditional RO systems... waste up to 3 gallons for every gallon purified."

Home Master's TMAFC-ERP is a partial exception among tank-based systems. Its built-in, non-electric permeate pump reduces back-pressure and reaches roughly a 1:1 ratio, or about 50% pure-to-waste efficiency. Viomi's tankless range reaches 2:1 to 3:1 ratios, or about 66.7% to 75% efficiency, while also adding app or faucet-based TDS and filter-life monitoring. Model-specific certification and SGS evidence are compared separately below.

Certification Comparison

Pricing across every brand on this list shifts with promotions and model updates, so it's not a stable reference point for a side-by-side table. Check water.viomi.com for Viomi's current pricing on the Viomi B6W, Viomi V6 PRO, Viomi V8 PRO, and Viomi M1, and check each competitor's own listing for theirs. The table below focuses on type, output, and certification instead, since those specs don't change with sales.

Within Viomi’s lineup, Viomi B6W is the value-focused 600 GPD entry at a regular price of $249.99, followed by Viomi V6 PRO at $379.99, Viomi V8 PRO at $479.99, and Viomi M1 at $899.99. The four tankless models share the same core space-saving design logic while scaling flow, monitoring, filtration features, and remineralization.

The verification packages differ by model. Viomi B6W, Viomi V6 PRO, and Viomi V8 PRO publish SGS testing against NSF/ANSI 42, 53, and 58 performance criteria together with NSF/ANSI 372-certified lead-free materials. Viomi M1 is certified to NSF/ANSI 42, 58, and 372 and adds model-specific SGS results for TDS, lead, PFOS, and PFOA.

Viomi M1’s differentiators are its 1,000 GPD output, nine-stage filtration, HD touchscreen smart faucet, and remineralization stage. Its model-specific SGS data records performance across the rated filter life, including alkaline mineral output and named reduction results. Buyers can therefore choose Viomi B6W for value, Viomi V6 PRO or Viomi V8 PRO for smart high-flow filtration, and Viomi M1 for maximum capacity plus remineralization.

Which Type, and Which Brand, Fits Your Kitchen

Tight or crowded cabinet, especially with a garbage disposal already installed: A tankless system is the practical choice, since there's no second tank competing for space. Viomi V6 PRO is the narrowest option in this comparison at 5 inches wide, with the Viomi B6W trading some width for a shorter depth.

No accessible, always-on electrical outlet under the sink: Viomi’s tankless models use a booster pump to deliver their high flow and therefore need a standard always-on outlet. Adding an outlet during installation preserves the cabinet-space and monitoring benefits; where electrical work is outside the project scope, a pressure-fed tank system fits the existing utility setup.

Household that regularly fills large pots or pitchers: Viomi M1’s 1,000 GPD output is designed for high-volume on-demand use without reserving cabinet space for a storage tank. Viomi V8 PRO provides an 800 GPD middle tier, while the Viomi V6 PRO and Viomi B6W provide 600 GPD for typical drinking and cooking routines.

Budget-conscious buyer who wants verified tankless performance: Viomi B6W provides 600 GPD at the regular price of $249.99, with SGS testing against NSF/ANSI 42/53/58 criteria and NSF/ANSI 372-certified materials.

Steady, all-day use across a larger household, with a higher-output model needed: Viomi V8 PRO (800 GPD) or Viomi M1 (1,000 GPD) scale output without adding a tank to the cabinet, and the Viomi M1 adds a remineralization stage for households whose complaint is flat-tasting water rather than chlorine taste.

FAQ

Do tankless RO systems need electricity? Yes. Tankless systems, including Viomi's full lineup, use an electric booster pump, so they need a dedicated, always-on outlet under the sink. On Viomi V6 PRO, Viomi V8 PRO, and Viomi M1, that same connection also powers a smart display built into the faucet; the Viomi B6W uses a standard faucet and shows TDS and filter life through its companion app instead. Entry-level tank-based systems like the APEC ROES-50 typically don't need power at all.

Is water from a tankless system safer than water stored in a tank? Tankless systems avoid the specific issue of water sitting in a tank for hours between draws, which is why Viomi's own product materials highlight it as a design advantage. That said, manufacturers of tank-based systems address standing water through routine tank sanitation, so a well-maintained tank-based system isn't inherently unsafe; it just requires that extra maintenance step.

Do tankless systems waste less water than tank-based systems? Generally yes, though the specific model still matters. Viomi's and Waterdrop's tankless systems run 2:1 to 3:1 pure-to-waste ratios, meaning 66.7% to 75% of the feed water drawn ends up as purified water. Home Master's tank-based TMAFC-ERP uses a non-electric permeate pump to reach roughly 1:1 (about 50% efficiency), which is a real improvement over standard tank-based designs but still behind every tankless system in this comparison. Standard tank-based systems without a permeate pump, like the APEC ROES-50, are usually the least efficient of either category.

How long does a tank-based system take to refill after it runs low? It depends on the system's GPD rating. A 50 GPD system like the APEC ROES-50 produces roughly one gallon every 30 minutes under good pressure, so refilling a drained 4-gallon tank can take a couple of hours, during which flow at the faucet slows to that production rate.

Can I switch from a tank-based system to a tankless one without redoing the plumbing? Mostly yes. Both types tap into the same cold-water shutoff valve and use a similar drain saddle connection, so the core plumbing carries over. The main changes are removing the tank (freeing up cabinet space) and adding an electrical connection for the tankless unit's pump and, on models with one, its faucet display.

Why do some tank-based systems not list an NSF/ANSI 58 performance certification? Some manufacturers, including Home Master on the TMAFC-ERP, certify individual components (like materials being lead-free) to NSF standards without putting the complete system through NSF/ANSI 58 performance testing for contaminant reduction. That's a meaningful distinction worth checking before assuming a brand's certification claims cover the whole system.

Bottom Line

Cabinet space is the factor that most often settles this decision. Viomi’s tankless layout removes the separate storage cylinder, preserves useful cabinet room, delivers steady all-day output, and reaches 2:1 to 3:1 pure-to-drain efficiency depending on the model.

For most kitchens with an accessible outlet and a cabinet that also holds cleaning supplies, a disposal, or a trash bin, a tankless system is the more practical fit. Viomi covers the range from Viomi B6W at 600 GPD through Viomi V6 PRO, Viomi V8 PRO, and the 1,000 GPD Viomi M1. A tank-based system remains useful when a nearby outlet is unavailable and cabinet space is not a constraint.