How long does a walk-in tub take to fill on a tankless water heater?

Under good conditions an Ella walk-in tub fills in about 10 minutes to a level that is safe for the hydro jets, roughly 1 to 2 inches above the highest jets, and takes about 6 minutes more to reach the overflow. With a tankless water heater you get that result only when the unit's gallons per minute rating can keep pace with the fast-fill faucet, which is why Ella recommends a tankless heater rated at least 11 GPM, and on well water only when the pump and pressure tank are adjusted correctly. The general fill picture is in How long does it take to fill a walk-in tub?, and this page covers what changes when the hot water comes from a tankless unit, a well, or both.

What a Tankless GPM Rating Actually Gives You

A tank heater stores a set number of gallons of hot water and hands them over as fast as your plumbing can carry them. A tankless heater stores nothing. Its GPM rating is how many gallons per minute it can warm as the water passes through, and that rating caps the hot side of the fill no matter how fast the faucet can flow. Ella's fast-fill faucets are built for 16 GPM on the 2-piece set and 18 GPM on the 5-piece set, so the faucet is usually capable of drawing water faster than a small tankless unit can warm it.

When the heater is the slower of the two you have two choices, and neither makes a good bath. You can hold the hot side back to the heater's pace and wait, or you can open the faucet fully and let unheated water dilute the hot, which is the lukewarm fill owners describe. A 27 kW unit rated at 4.8 GPM struggles to keep up with an Ella fast-fill faucet, and units in the 4 to 5 GPM range make the fill slow and lukewarm. An 11 GPM unit is the minimum Ella recommends because it can keep pace with the fast-fill faucet instead of holding it back. The sources do not give a separate minute count for an 11 GPM heater, so treat the 10 minute figure above as the good-condition target and expect your own result to move with pressure and pipe size.

The table tells you where the bottleneck sits. The faucet is rarely the slow part. A small tankless unit or an undersized tank is, and the symptom is different for each: a tank that is too small starts hot and runs cold partway through, while a tankless unit that is too small never gets properly hot in the first place, or fills at a trickle if you throttle it. If you are shopping for a tankless unit, the 11 GPM figure is the one to compare against the heater's label, not the kilowatt rating.

Well Water: The Pump and Pressure Tank Set the Pace

A heater's GPM rating only matters if the water behind it arrives fast enough to use it. On well water, the fill fixtures only reach their full potential when the pump and pressure tank are correctly adjusted, so the pump and tank are the first link in the chain and the heater is the second. A tankless unit rated at 11 GPM cannot warm 11 gallons a minute that the well system is not delivering. The same house factors that affect any Ella fill apply on a well as well:

• House water pressure, which on a well is set by the pump and the pressure tank.
• Pipe size. 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.
• The distance from the heater to the tub.
• Any hot water being used elsewhere at the same time, since laundry, a dishwasher or a shower shares the same supply.

Ask your plumber to assess the pump, the pressure tank, the pipe size and the heater rating together as one supply picture rather than replacing one part at a time. Ella does not send technicians, so this assessment is done by your local licensed plumber, and the support team can answer their questions by phone, email or video.

How Many Gallons You Are Actually Heating

Ella tubs fill after you are seated, so the gallons that matter are the occupied figure, not the printed unoccupied capacity. Most standard-size Ella walk-in tubs hold 65 to 107 gallons unoccupied, and you typically use 30 to 75 gallons per bath once seated. A Transfer60 takes 55 to 75 gallons depending on the bather, and the Big4One holds 127 gallons unoccupied and 75 to 100 gallons occupied. The largest models, the ShaK, Escape and Big4Two, hold up to 145 gallons unoccupied. Every model is listed in How many gallons does each Ella walk-in tub hold? Water capacity guide.

With a tank heater the gallon count tells you whether you will run out. With a tankless heater it tells you how much water has to pass through at the heater's rated pace, so a bigger tub on a borderline unit means a longer wait at the hot handle. The largest tubs may call for a dedicated heater in addition to the one serving the house, and the per-model recommendations are in Recommended Water Heater Size by Walk-In Tub Model.

What the In-Line Heater Does and Does Not Do

Owners with a slow tankless fill sometimes expect the tub's own in-line heater to make up the difference. It does not. The in-line heater works with the water pump while the hydro massage runs to hold the bath at the temperature you set at the faucet, and it never raises the temperature of water that arrived cool. The first video on this page shows that sequence: you set a comfortable temperature with the 5-piece or 2-piece fast-fill faucet, and once the hydro massage is engaged the in-line heater keeps that temperature steady. The second video answers the most common question Ella hears, whether you get cold while the tub fills and drains: the fast-fill faucets keep the wait short, the glossy acrylic interior acts as an insulator, and an optional heated seat warms up after a few minutes and switches itself off after 20 minutes. None of that speeds up the fill itself, which is why heater sizing has to be solved on the supply side.

Draining is not affected by any of this. The two 2-inch gravity drains empty an average tub in approximately 80 seconds under ideal conditions in the video, and typically in two minutes or less once connected to a proper 2-inch drain line, as Drainage and Fill Time explains.

What to Check Before You Replace Anything

1. Find your tub's occupied capacity in the capacity guide and compare it with the heater you have.
2. Fill once when nothing else in the house is using hot water. If the fill is fine alone, the fix is scheduling or a bigger heater, not the tub.
3. Read the tankless unit's label for its GPM rating. Anything well below 11 GPM is the likely cause of a slow or lukewarm fill.
4. On a well, have your plumber confirm the pump and pressure tank settings and the pipe size feeding the tub.
5. If the hot water starts fine and then goes cold partway through, read Why Does My Hot Water Run Out Before the Tub Is Full?, which works through tank sizing in the same order.

If you still cannot tell whether the heater or the well system is the limit, open a ticket at ellasbubbles.com/support or call (800) 480-6850 with your tub model, the heater's rating and a note that you are on a well, and the team can tell your plumber what to look at.

Related Guides

• How fast will my walk-in tub fill? Fast-fill faucet expectations and operation
• What Temperature Should I Set My Water Heater for a Walk-In Tub?
• Electrical requirements by model: dedicated GFCI circuits for your walk-in tub

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