Can a tankless booster let me keep my 40 gallon tank for a walk-in tub?

A tankless booster tied into the hot line can let you keep a 40 gallon tank, as long as the booster can warm water as fast as the Ella fast-fill faucet draws it, which is why Ella's Bubbles recommends a tankless unit rated at 11 gallons per minute or more. On its own, a 40 gallon tank works only if nothing else in the house draws hot water during the fill, and on most models it runs out about halfway through. What Size Water Heater Do I Need for My Walk-in Tub? gives the general sizing rule of a 75 gallon or larger tank. This page is for the homeowner weighing a booster, a bigger tank or a dedicated system, and it puts the flow and capacity figures side by side so your plumber can make the call.

Why the 40 Gallon Tank Falls Short on Its Own

The number that matters is your tub's occupied capacity, which is the water you actually fill once you are seated. Depending on the model, a walk-in tub takes roughly 55 to 75 gallons or more to fill once you are in it. The single-seat Flow, Standard, Zen and Petite use 30 to 50 gallons occupied, the Transfer60 uses 55 to 75, the Standard60 uses 70 to 90, and the Big4One holds 127 gallons unoccupied and 75 to 100 occupied. A 40 gallon tank cannot hold that much hot water, so the fill turns cool partway through, which is exactly the symptom owners describe in Why Does My Hot Water Run Out Before the Tub Is Full?. Both figures for every model are in How many gallons does each Ella walk-in tub hold? Water capacity guide.

One thing the tub itself cannot do is rescue a cool fill. The in-line heater included with hydro massage tubs maintains your bath temperature while the hydro pump runs, and it never raises the temperature of water that arrived cool. The short video on this page shows that relationship: you set a comfortable temperature at the faucet, and the in-line heater working with the water pump holds it there once the hydro massage is on. The house water heater, or the booster you add to it, does all the work of filling the tub hot.

What a Booster Has to Keep Up with

A tank heater stores hot water and hands it over as fast as your plumbing can carry it. A tankless unit stores nothing, so its gallons per minute 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 can almost always draw water faster than a tankless unit can warm it. When the heater is the slower of the two you either hold the hot side back and wait, or open the faucet fully and let unheated water dilute the hot, which is the lukewarm fill people complain about.

• 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. A unit rated just above that, such as an 11.2 GPM on-demand heater, meets the minimum.
• The guidance is written in gallons per minute, so give your plumber the unit's rated flow and let them compare it with your supply piping.

Tank, Tankless or Both: The Figures Side by Side

Read across the table and the booster question answers itself. Keeping the 40 gallon tank is reasonable when the tankless unit on the hot line is rated at 11 GPM or more, because the tank then covers the rest of the house while the booster carries the fill. Keeping it with a 4 to 5 GPM unit simply moves the bottleneck from the tank to the heater. If your model is a two-seat or bariatric tub, the recommendation climbs to 80 or 100 gallons, and the team suggests a dedicated heater in addition to the one serving the home for tubs such as the ShaK and Big4Two. Recommended Water Heater Size by Walk-In Tub Model lists the figure for each tub.

How the Combination Is Set Up

The hot water supply video on this page suggests a dedicated hot water supply system just for the walk-in tub, or a combination of two systems, and a separate in-line tankless heater can be installed to assist your primary water heater, which is the booster arrangement. Ella's Bubbles does not make or install water heaters and does not send technicians, so a licensed plumber sizes and fits the unit, and the team is happy to answer their questions about your model. Give your plumber these items before they choose a unit:

1. The occupied water capacity for your model, from the spec sheet or the capacity guide.
2. Your house water pressure, supply pipe size and the distance from the heater to the tub, since all three move the result.
3. The faucet set you ordered. Both Ella fast-fill sets include two 5 foot braided supply lines, and 3/4 inch supply lines with easily accessible shut-off valves are recommended.
4. What else in the house draws hot water at bath time, because any other fixture running at the same time shares the same supply.

What the Fill Looks Like When It Is Right

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. The sources do not give a separate minute count for an 11 GPM heater, so treat that as the good-condition target and expect your own result to move with pressure and pipe size. How long does a walk-in tub take to fill on a tankless water heater? covers what changes on a tankless unit or a well. Fill hot from the faucet, then run the hydro massage to hold the temperature. If the fill still goes cool after the booster is in, fill when nothing else is running as a test. A tub that fills fine alone points to scheduling or a bigger heater rather than to the tub.

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

Common Questions