Tankless vs Tank Reverse Osmosis: Which Is Better?

The difference between tankless and tank reverse osmosis is where your filtered water comes from: a tank system makes water slowly and stores it in a pressurized tank, while a tankless system filters water on demand the moment you open the faucet. Both use the same basic reverse osmosis membrane. They differ in cost, cabinet space, flow, freshness and whether they need electricity.

How a reverse osmosis tank system works

A reverse osmosis membrane is slow on its own. With only household water pressure behind it, a membrane produces filtered water a little at a time. A traditional system solves that by storing water in a reverse osmosis tank under the sink so there is some ready when you want it.

Inside a typical reverse osmosis pressure tank is a flexible bladder or diaphragm with a pocket of compressed air on one side. Here is the cycle:

1. The tank fills slowly. Filtered water trickles from the membrane into the tank, squeezing the air pocket. Refilling an empty tank typically takes a few hours.
2. The air pushes water out. When you open the RO faucet, the compressed air pushes stored water up to the faucet.
3. Pressure drops as it empties. As water leaves, the air expands and pressure falls, so flow is strongest when the tank is full and weakens as it empties.
4. The system shuts off when full. Most tank systems use an automatic shutoff valve that stops production once the tank reaches pressure.

Between uses, the water sits in the tank and in the lines until someone drinks it. Most under-sink tanks hold a few gallons, and only part of that volume is usable because the air pocket takes up space.

How a tankless reverse osmosis system works

A tankless RO system skips storage. Instead of relying only on household pressure, tankless systems typically use an electric pump to push water through the membrane fast enough to fill a glass directly. Open the faucet and the system starts filtering; close it and it stops.

That design has two practical consequences. The unit needs electricity, so you need an outlet under or near the sink. And its flow depends on how much water the system can produce per day, which is why tankless systems are usually rated in gallons per day. The Sanavia Hydra, for example, is rated at up to 800 gallons per day.

Tankless vs tank reverse osmosis: pros and cons

	Tank RO	Tankless RO
Upfront price	Usually lower	Usually higher
Needs electricity	No, runs on water pressure	Yes, plugs into power
Cabinet space	Filter manifold plus a separate tank	One compact unit
Water freshness	Water sits in the tank between uses	Filtered on demand
Flow at the faucet	Strong when full, slows as the tank empties	Steady, set by production rate
Running out	Possible after heavy use until the tank refills	No stored supply to run out of
Extra maintenance	Tank can lose air charge over time and may need recharging, sanitizing or replacing	No tank to maintain
Works in a power outage	Yes, while tank water lasts and water pressure holds	No

Cabinet space

A tank system has two main parts: the filter assembly and a round tank that often takes up a large share of the cabinet floor. In a small kitchen, or a sink cabinet already holding a garbage disposal and cleaning supplies, that tank is often the deciding factor. A tankless unit puts everything in one box and usually leaves far more room.

Freshness

With a tank, you drink water that was filtered earlier and has been waiting in the tank. Most tank systems pass that stored water through a post-filter on its way to the faucet to polish the taste. With a tankless system there is no stored supply, so the water in your glass has just come through the filters.

Flow

Tank flow is at its best when the tank is full and fades as it empties. If you fill a large pot for cooking and then a few water bottles, the last bottle may come out slowly, and after that you wait for the tank to refill. A tankless system gives steadier flow because it is not draining a store. The trade-off is that its flow is capped by its production rate, which is why higher-capacity tankless systems matter.

Power

This is the main practical downside of tankless RO. A tankless unit needs an electrical outlet in or near the sink cabinet, and it will not make water during a power outage. Tank systems need no power at all, which also makes them simpler. If your sink cabinet has no outlet, factor in the cost of adding one.

Cost

Tank systems are generally cheaper to buy, and they have a long track record. If upfront price is your top concern and you have the cabinet space, a tank system is a reasonable choice. Tankless systems cost more upfront in exchange for space, freshness and flow.

Why the Hydra is tankless

We built the Hydra tankless to avoid the trade-offs a tank brings: a crowded cabinet, water that sits between uses, and flow that fades as the tank empties. The Hydra filters on demand at up to 800 gallons per day through five stages: sediment, carbon, carbon block, reverse osmosis membrane and post-filter. It has a real-time TDS readout so you can see what the system is doing, and it comes with its own faucet, tubing and fittings.

In independent accredited lab testing of an Arlington, VA home's tap water, total trihalomethanes fell from 58 ppb to 2.79 ppb after the Hydra, a 95% reduction. See the lab results.

The Hydra is $999 to self-install (about 45 minutes plus an automatic 30-minute rinse; see the installation guide) or $1,250 with professional installation in the DC, Maryland and Virginia area. It ships free in the U.S., with 60-day returns and a 3-year warranty. Just plan for an outlet under the sink.

Which should you choose?

• Choose a tank system if you want the lowest upfront price, have room under the sink, have no outlet nearby, or want water available during a power outage.
• Choose a tankless system if cabinet space is tight, you want water filtered on demand, or your household goes through a lot of drinking and cooking water.

Not sure you need RO at all? Start with your water: look up what your utility reports on our water quality page, or get a free TDS water test.