Does a tankless water heater work with an Ella walk-in tub?

Yes. A tankless water heater works with an Ella walk-in tub as long as it can keep pace with the fast-fill faucet, and the guidance our support team gives homeowners and installers is a tankless unit rated for at least 11 gallons per minute. The tank sizes for each model sit alongside that figure in Recommended Water Heater Size by Walk-In Tub Model, and this page explains why the flow rating is the whole question with a tankless heater, what to check before your tub arrives, and how to spot a unit that is falling behind.

Why the Flow Rating Matters More Than Anything Else

A tank water heater and a tankless heater fall short in different ways. A tank supplies stored hot water at any flow rate until the stored water is used up. A tankless heater has to heat the water as it passes through, so its limit is not how much hot water it holds but how many gallons per minute it can warm at once. An Ella fast-fill faucet is built to fill the tub quickly and flows far more than an ordinary fixture, which is exactly the situation that exposes an undersized tankless unit.

The support team's own example makes the point: a 27 kW tankless heater rated at 4.8 gallons per minute struggles to keep up with an Ella fast-fill faucet. The unit is not broken. It is simply being asked to warm water faster than it can, so the water leaving the spout arrives cooler than the temperature you set. That is why the recommendation is a unit rated for at least 11 gallons per minute, and why any other fixture running at the same time counts against that figure, because it draws from the same supply.

Tank Versus Tankless: The Figures Side by Side

The difference for you is in how the shortfall shows up. An undersized tank runs out of hot water partway through the fill, so the last part of the bath arrives cold. An undersized tankless unit never runs out, but the whole fill arrives cooler than you set because the heater cannot warm water at the rate the spout draws it. Both end the same way, with a bath that is not as warm as you wanted, and the in-line heater cannot correct either one.

What to Check Before Your Tub Arrives

1. Find the rated gallons per minute of the tankless unit you have or plan to install. Your plumber can confirm the rated output of the unit and whether it is enough for your home.
2. Compare that rating with the 11 gallons per minute guidance. A unit below it can still fill the tub, but expect a cooler fill, and plan to run nothing else during the bath.
3. Look up your tub's occupied capacity in the water capacity guide. Occupied capacity is what you actually fill, because your body displaces part of the volume. The Big4One, for example, holds 127 gallons unoccupied and 75 to 100 gallons with a bather seated.
4. Check the supply lines. Three quarter inch supply lines with accessible shut-off valves are recommended, and both Ella fast-fill faucet sets include two 5 foot braided supply lines.
5. Consider the rest of the home. Laundry, a dishwasher or a shower elsewhere shares the same heater, and with a tankless unit that sharing shows up immediately as a temperature drop at the tub.
6. Have a licensed plumber weigh house water pressure, pipe size, the distance from the heater to the tub and the heater's set temperature together. Ella's Bubbles does not send technicians, so this assessment is done by your plumber.

What the In-Line Heater Will and Will Not Do

Every Ella tub with hydro massage includes an in-line water heater, and it is easy to assume it will make up for a heater that is falling behind. It will not. The in-line heater maintains the bath temperature while the hydro pump runs and never raises the temperature of water that arrived cool, as Do Walk-In Tubs Have Heaters to Keep the Water Warm? explains. The short video on this page shows the routine that works: set a comfortable temperature at the fast-fill faucet, fill hot, then engage the hydro massage so the in-line heater and the pump hold that warmth for the rest of the bath. With a tankless heater, that first step is the one that depends on the gallons per minute rating.

Signs Your Tankless Heater Is Falling Behind

Two symptoms point straight at the heater's flow rating rather than at the tub.

• The hand sprayer runs hot but the tub spout is only lukewarm. The sprayer passes a small amount of water, so a tankless heater warms it easily, while the spout draws far more than the heater can warm. Hand Sprayer Runs Hot but the Tub Spout Is Only Lukewarm walks through the checks, including confirming the diverter is fully switched to the spout and looking for anything added between the hot valve and the diverter.
• The bath is fine when the house is quiet but cools when someone showers or runs laundry. That is the shared-supply effect, and the answer is either scheduling baths for quiet times or a higher-rated unit.

If the water is hot at first and then turns cold partway through, that pattern belongs to an undersized tank rather than a tankless unit, and Why Does My Hot Water Run Out Before the Tub Is Full? covers it. When a tankless unit cannot be brought up to the guidance, the options your plumber may raise are a higher-rated unit or a dedicated heater for the bathroom, which the largest tubs may call for in any case. For help matching a heater to your specific model and options, contact your Ella dealer or the team at (800) 480-6850, or open a ticket at ellasbubbles.com/support with your model and the rating of your heater.

Watch: Ella's Bubbles: In-line Water Heater Maintains Comfortable Water Temperature with chapters and a full transcript.

Related Guides

• What Size Water Heater Do I Need for My Walk-in Tub?

Common Questions