Water heater sizing for a walk-in tub with shower
Direct answer: size the heater for the busiest hour in the house, not for the tub alone. In a combo household that hour often holds one full bath and one or two standing showers. With typical temperatures about three-quarters of every gallon comes from the heater, so a 60-gallon bath plus one 8-minute shower needs roughly 57 gallons of hot water. Compare that with the first-hour rating on your heater’s EnergyGuide label. Tankless heaters never run out, but they cap how fast the tub fills.
A soak-only walk-in tub asks your heater for one big fill now and then. A walk-in tub with shower is different, because it is usually bought for a home where somebody else showers in the same tub. The question is not “is my tank as big as my tub?” but “can the heater cover the bath and the showers that happen around it?”
The mixing math, in one line
A bath is hot water from the heater blended with cold water from the street. The part that must come from the heater is:
Hot share = (bath temperature − cold supply temperature) ÷ (heater setting − cold supply temperature)
At a 102°F bath, 50°F supply water and a 120°F heater, that is 52 ÷ 70, or about 74%. Showers run a little hotter than baths for most people; at 105°F the share is about 79%. Colder winter supply water pushes both numbers up, which is why a heater that keeps up in August can fall short in January.
Published tub capacities and the hot water they take
Use the capacity with a bather seated, because your body displaces water. Ella’s Bubbles lists an occupied range; Hydro Dimensions lists one operating capacity. The hot-water column applies the 74% share above.
Capacities from manufacturer spec sheets; hot water at a 102°F bath, 50°F supply, 120°F heater.
| Model (length) | Water with bather seated | Hot water per bath |
|---|---|---|
| Ella Petite (52 in.) | 30–50 gal | 22–37 gal |
| Ella Elite (52 in.) | 35–55 gal | 26–41 gal |
| Ella Royal (52 in.) | 45–65 gal | 33–48 gal |
| Ella Deluxe (55 in.) | 60 gal | 45 gal |
| Ella Ultimate (59.5 in.) | 55–85 gal | 41–63 gal |
| Hydro Dimensions 3060L | 50 gal | 37 gal |
| Hydro Dimensions 3052W | 60 gal | 45 gal |
| Hydro Dimensions 3048 | 65 gal | 48 gal |
| Hydro Dimensions 3252W | 70 gal | 52 gal |
| Hydro Dimensions 2851 | 75 gal | 56 gal |
| Hydro Dimensions 3555 | 90 gal | 67 gal |
The busiest hour in a combo household
Here is the same 60-gallon bath with zero, one and two standing showers in the same hour. Each shower is 8 minutes at 2.0 gallons per minute, the WaterSense limit, at 105°F.
Math: hot share at 102°F bath and 105°F shower, 50°F supply, 120°F heater. Dashed lines: ENERGY STAR minimum first-hour ratings (heat pump 45 gallons, gas storage 51 gallons). A heater that only meets the minimum covers the bath alone but not the bath plus a shower.
Show the numbers as a table
| Hour | Bath (hot gal) | Showers (hot gal) | Total |
|---|---|---|---|
| Bath only | 45 | 0 | 45 |
| Bath + 1 shower | 45 | 12.6 | 57 |
| Bath + 2 showers | 45 | 25.1 | 70 |
The first-hour rating is the right yardstick because it counts what a full tank can deliver plus what the burner or element reheats during that hour. Two tanks of the same size can have quite different ratings, so read the label rather than the gallon number on the side.
Check your own heater
Estimate only. Pipe length, a heater that has not fully recovered, and the real flow of your faucet all change the result. Your plumber confirms the final numbers.
Tankless heaters: the fill rate is the catch
A tankless heater is sized by flow at a temperature rise, the difference between the cold water coming in and the hot water going out. The faucet on a walk-in tub can ask for far more than most tankless units supply. Ella rates its five-piece fast-fill faucet at up to 18 gallons per minute at 60 psi; a tankless heater delivers only a fraction of that as hot water.
The tub still fills, just more slowly, because the faucet can only blend in as much cold water as the hot share allows. The fill rate works out to the heater’s hot flow divided by the hot share. Take a gas tankless unit that only meets the ENERGY STAR floor of 2.8 gallons per minute over a 67°F rise, with 53°F supply water and a 120°F setting. At a 102°F bath the hot share is 73%, so the tub fills at about 3.8 gallons per minute, and a 60-gallon bath takes about 16 minutes. You sit on the seat for every one of those minutes, so ask for the heater’s flow chart before you decide.
The same limit applies when two fixtures run at once. A shower started while the tub is filling takes its share of the heater’s flow, and the tub slows down further.
What to do if the numbers come up short
- Schedule around it. Showers in the morning, baths in the evening. A tank recovers between them, and this costs nothing.
- Pick a tub with a smaller occupied capacity. The spec table shows a wide spread between models of similar length.
- Use a heater with a higher first-hour rating. A bigger tank, or a gas unit with a faster recovery, raises the number that matters.
- Size a tankless unit to the fill. Use your winter supply temperature for the rise.
- Do not count on an in-line heater. Ella lists 650-watt and 1,000-watt in-line heaters on its jetted models. They hold the temperature of water already in the tub; they do not heat the fill.
- Keep the heater at 120°F unless a thermostatic mixing valve protects every outlet. Turning it up stretches the tank but raises the scald risk; the safety guide has the numbers.
The rest of the plumbing, from the drain end to the shower valve, is on the plumbing checklist. Aging Safely Baths also explains how it checks water heaters before an install.
Questions people ask
What size water heater does a walk-in tub with shower need?
Size it for the busiest hour, not the tub alone. Add the hot water for one full bath (about three-quarters of the tub's occupied capacity with typical temperatures) to the hot water for any showers taken in the same hour, then compare the total with the first-hour rating on the heater's EnergyGuide label.
How much hot water does one shower use?
Multiply the shower head's flow by the minutes, then by the hot share. An 8-minute shower at 2.0 gallons per minute uses 16 gallons, and with 50-degree supply water and a 120-degree heater about 12 to 13 of those gallons come from the heater.
Will a tankless water heater fill a walk-in tub?
It will never run out, but it limits how fast the tub fills. Divide the heater's flow at your temperature rise by the hot share of the bath to get the fill rate. A unit at the ENERGY STAR floor of 2.8 gallons per minute over a 67-degree rise fills a 60-gallon bath in roughly 16 minutes with 53-degree supply water.
Can someone shower while the walk-in tub is filling?
On a tank heater, yes, but both draw down the same stored hot water, so the end of the bath may run cool. On a tankless heater, the shower takes part of the heater's flow, so the tub fills more slowly. It is usually better to schedule the bath and the showers apart.
Should I turn up the water heater for a bigger tub?
Not without a thermostatic mixing valve. The CPSC urges a 120-degree setting because hotter water burns skin much faster, and older adults are among those most often scalded. A larger tank, a tankless unit sized for the fill, or spacing out baths and showers are safer fixes.
Sources
- Occupied capacities, faucet rating (18 gpm at 60 psi, ideal conditions) and in-line heater wattage: Ella’s Bubbles spec sheets for the Petite, Elite, Royal, Deluxe and Ultimate (PDF).
- Operating capacities: Hydro Dimensions HD Series schematics for the 3060L, 3052W, 3048, 3252W, 2851 and 3555 (PDF).
- First-hour rating definition and minimum ratings (heat pump 45 gal, gas storage 51 gal, gas tankless 2.8 gpm over a 67°F rise): ENERGY STAR, Water Heater Key Product Criteria.
- Sizing tankless units by temperature rise and simultaneous flow: U.S. Department of Energy Building America, Measure Guideline: Transitioning to a Tankless Water Heater (2012, PDF).
- Shower head flow of 2.5 gpm (standard) and 2.0 gpm (WaterSense): U.S. EPA, WaterSense showerheads.
- 120°F setting and burn times: U.S. Consumer Product Safety Commission, Avoiding Tap Water Scalds (PDF).
Not sure your heater is up to it?
Send a photo of the EnergyGuide label with your quote request. We will compare it with the tubs that fit your space and tell you plainly whether you need an upgrade.