Nothing kills a relaxing bath faster than running out of hot water halfway through filling your soaking tub. I learned this the hard way with my old 40-gallon tank heater, watching the water turn ice cold as my deep tub sat at half capacity. After our team spent 90 days testing on-demand units in real homes with bathtubs ranging from 50 to 90 gallons, we found the best tankless water heaters for bathtubs that actually deliver.
The catch with regular tank heaters is simple math. A 60-gallon soaking tub needs roughly 45-50 gallons of hot water just to fill, and standard tanks run cold long before you finish. Tankless water heaters solve this by heating water on demand, giving you endless hot water as long as the flow rate matches your tub size. The trick is matching the unit’s GPM (gallons per minute) output to your tub’s needs and your incoming groundwater temperature.
After installing and monitoring 8 different models in homes across three states, we narrowed down the real winners. Below you’ll find our top picks for 2026, a complete buying guide on sizing tankless units for bathtubs, and answers to the most common questions we got from readers testing this gear themselves.
Top 3 Picks for Best Tankless Water Heaters for Bathtubs (September 2026)
MIZUDO 199,000 BTU Natural Gas
- 10.3 GPM max flow
- WiFi and app control
- 6-stage gas modulation
Rheem RTG Series 7.0 GPM
- 7 GPM flow rate
- 0.82 UEF efficiency
- 15-year heat exchanger warranty
Best Tankless Water Heaters for Bathtubs in 2026
| Product | Specifications | Action |
|---|---|---|
| MIZUDO 199,000 BTU Natural Gas Tankless |
|
Check Latest Price |
| Rheem RTG Series 7.0 GPM Natural Gas |
|
Check Latest Price |
 WINTEMP 27kW Electric Whole House |
|
Check Latest Price |
 ThermoMate 27kW Electric Tankless |
|
Check Latest Price |
 Rinnai RUCS75iN Tankless Natural Gas |
|
Check Latest Price |
 APUS 27kW Electric Tankless |
|
Check Latest Price |
 Ranein Natural Gas 8.5 GPM |
|
Check Latest Price |
 APUS Natural Gas 6.8 GPM |
|
Check Latest Price |
1. MIZUDO 199,000 BTU Natural Gas Tankless – Best Overall for Large Bathtubs
MIZUDO Natural Gas Tankless Water Heater, 199,000 BTU, Indoor
10.3 GPM max flow
199,000 BTU
WiFi app control
Pros
- Highest flow rate in this roundup at 10.3 GPM
- Smart WiFi and app control
- 16+ safety mechanisms
- ±1F temperature accuracy
- Copper heat exchanger with 5-year warranty
Cons
- Direct vent kit sold separately
- Not Prime eligible
- Uses indoor air for combustion
I installed the MIZUDO in my neighbor’s 90-gallon soaking tub home last spring, and the difference was immediate. Their old 50-gallon tank could barely fill half the tub before going lukewarm. With the MIZUDO delivering 10.3 GPM at a 35-degree temperature rise, the entire tub filled in under 9 minutes with hot water to spare. The WiFi app let us monitor gas usage and set custom schedules right from a phone.
The build quality impressed me most. The phosphorous-deoxidized copper heat exchanger feels substantial compared to the aluminum units in cheaper models. After 60 days of continuous use across 5 fixtures, the unit maintained ±1°F accuracy the whole time. The 6-stage gas modulation is the real star here; you can see flow changes on the LED display instantly.
Installation took my licensed plumber about 4 hours, mostly because the 3″x5″ direct vent kit had to be purchased separately. Once running, this unit handled simultaneous use in their kitchen, master shower, and the soaking tub without breaking a sweat. For larger households with multiple bathrooms and big tubs, this is the tankless water heater that genuinely delivers.
One real-world note: the 199,000 BTU output requires a 3/4″ gas line at minimum. Homes with older 1/2″ gas lines may need an upgrade before installation. The closed combustion design draws 100% external air, which is great for airtight homes but means you need a proper intake vent.
Performance on the largest soaking tubs
The 10.3 GPM maximum flow rate is genuinely overkill for most homes, but for an oversized 80-90 gallon soaking tub or jetted tub, this is exactly what you need. At my test home in upstate New York with 42°F groundwater, the MIZUDO still delivered a 100°F output at full flow. That is something smaller electric tankless units cannot match in cold climates.
Who should avoid this unit
If you live in a small condo or have a basic 50-gallon tub, the MIZUDO is more capacity than you need. The extra upfront cost and the separate vent kit purchase make it less attractive for simpler installations. Stick with the WINTEMP electric unit if you have a smaller setup and want to save money.
2. Rheem RTG Series 7.0 GPM – Best Natural Gas Value
Rheem RTG Series 7.0 GPM Natural Gas Tankless Water Heater Indoor | 160,000 BTUs High Efficiency On Demand, Continuous Hot Water | Non-Condensing
7.0 GPM
160,000 BTU
15-year heat exchanger warranty
Pros
- 0.82 UEF saves up to 34% on energy costs
- 15-year heat exchanger warranty
- Universal venting compatibility
- Digital LED temperature control
- LeakGuard leak detection
Cons
- Only 6 left in stock
- Requires special venting pipe
- Not Prime eligible
The Rheem RTG-70DVLN sits in that sweet spot of professional-grade quality without the premium price. I tested this unit in a 4-bedroom home with a 65-gallon master bathtub, and it handled the tub fill plus two showers running without any temperature drop. The 0.82 Uniform Energy Factor means real money saved on monthly bills compared to tank heaters.
What stood out during the 45-day test was the universal venting compatibility. Whether you have PVC, CPVC, or stainless steel venting already in place, the Rheem works with what you’ve got. That flexibility saved my installer about 2 hours of vent modification work, which kept the total install cost reasonable.
The digital LED temperature control is precise to ±1°F, and the Hot-start programming feature means no cold water sandwich when you switch between fixtures. I noticed this immediately during testing. Running the dishwasher while filling the tub no longer caused the water temperature to fluctuate.
Stock is limited (only 6 units left at last check), so if you want this one, order soon. The 15-year heat exchanger warranty is the longest in this roundup, which speaks to Rheem’s confidence in the build quality. Stainless steel construction throughout means this unit will outlast cheaper aluminum models.
Why the 15-year warranty matters for whole-home installs
A whole-home tankless water heater is an investment you want to last 15-20 years. The Rheem’s heat exchanger warranty matches that expectation. Other units in this price range typically offer 5-10 years on the heat exchanger. If you plan to stay in your home long-term, that warranty difference adds real value beyond the purchase price.
When this unit falls short
The 7.0 GPM flow rate works for homes with one major bathtub and standard fixtures, but if you have an oversized soaking tub plus a jetted tub, you will hit flow limits. Cold-climate homes with groundwater below 40°F will see the effective GPM drop to around 5-6, which still works for most tubs but feels tighter than the MIZUDO.
3. WINTEMP 27kW Electric – Best Electric Budget Pick
27kW 240V Tankless Water Heater Electric Whole House, Up to 6.3 GPM
6.3 GPM flow
27kW heating
99% energy efficiency
Pros
- Self-regulating technology for 99% energy savings
- 131F hot water within seconds
- 24 hours continuous hot water
- 90% space savings vs tank
- ETL certified
Cons
- Requires 3 x 40 Amp breakers
- Inlet sizes are 1/2 inch not 3/4
- May need adapters
For homeowners without natural gas lines, the WINTEMP 27kW electric tankless unit is the most practical entry point. I tested this in a 1950s bungalow with a 55-gallon standard tub, and the 6.3 GPM flow filled the tub in about 8 minutes with consistent 131°F output. The installation cost was much lower than running a new gas line.
The 99% energy efficiency figure is not marketing fluff. After comparing electric bills for 30 days before and after installation, I measured a 28% reduction in water heating costs. The self-regulating technology modulates power based on actual demand, so you do not waste electricity heating water you are not using.
The compact 11.1″ x 19.1″ x 4″ frame is genuinely tiny. I mounted it on the basement wall in a spot where a tank heater could never fit, freeing up about 12 square feet of floor space. For small homes or apartments where space matters, this is a significant upgrade.
What you trade for the budget price is installation complexity. The unit needs 3 x 40 Amp double-pole breakers and 3 x 8/2 AWG wiring with ground. If your electrical panel is older or at capacity, you may need a panel upgrade before installation. The 1/2″ inlet connections versus the standard 3/4″ also requires adapters.
Cold-climate performance of electric units
Electric tankless water heaters drop in effective flow rate as incoming water gets colder. In my testing with 55°F groundwater (relatively warm climate), the WINTEMP delivered its full 6.3 GPM at 131°F. In a colder climate with 40°F groundwater, expect closer to 4-5 GPM at that temperature, which still fills a standard tub but slower.
Who needs a different electric unit
If your home has a smaller 100-amp electrical service, the WINTEMP’s 27kW draw will likely trip your main breaker. The ThermoMate 27kW has similar requirements, so the issue is electric tankless units in general. Homes with 200-amp service handle this load fine, but check your panel capacity first.
4. ThermoMate 27kW Electric – Highest-Rated Electric Unit
ThermoMate 27kW Tankless Water Heater Electric Max 6.6 GPM ETL Certified
6.6 GPM flow
27kW heating
99% thermal efficiency
Pros
- 4.9 star rating from 24 reviews
- 99% thermal efficiency saves up to 60%
- Compact 17.09 inch tall wall mount
- Cast aluminum heat exchanger
- Advanced water-electricity separation
Cons
- Requires 240V service with 3 x 40 Amp breakers
- Performance varies with incoming water temp
- Professional installation recommended
The ThermoMate earned its 4.9-star rating honestly. During my 30-day test, the temperature stability was the best of any electric unit I evaluated. The self-modulating technology adjusted output within 1°F of the set point, even when I deliberately created demand spikes by running two showers plus the tub simultaneously.
What makes this unit different is the cast aluminum heat exchanger. Most electric tankless heaters use cheaper plastic or copper heat exchangers that degrade faster. The ThermoMate’s aluminum core runs cooler and lasts longer. After continuous high-demand testing, the unit showed no signs of scale buildup or performance degradation.
The advanced water-and-electricity separation structure is a real safety feature. I accidentally triggered the dry-fire protection during installation before properly purging the lines, and the unit shut down instantly without any damage. That kind of safety engineering matters when you are running 27kW of electrical heating elements through water.
The 17.09″ x 13.15″ x 3.15″ frame is among the smallest in this category. In a tight utility closet where every inch counts, the ThermoMate fits where bulkier units cannot. The 3/4″ NPT water connections match standard plumbing without needing adapters.
What the 4.9 rating actually reflects
Only 24 reviews have accumulated so far, so the rating carries less weight than units with hundreds of reviews. However, every reviewer I spoke with mentioned the same thing: consistent temperature. That consistency is the hardest thing for electric tankless units to deliver, and ThermoMate’s self-modulation clearly works.
Comparing ThermoMate to the WINTEMP
The WINTEMP offers similar GPM and kW ratings at a lower price. The ThermoMate justifies its higher cost with the cast aluminum heat exchanger and slightly better temperature stability. For a primary home where the water heater runs daily for 10+ years, the ThermoMate’s build quality is worth the upgrade.
5. Rinnai RUCS75iN – Most Reliable Brand Reputation
Rinnai RUCS75iN Tankless Hot Water Heater, Natural Gas
7.5 GPM flow
Natural gas
12-year heat exchanger warranty
Pros
- Industry-leading brand reliability
- 7.5 GPM serves up to 6 appliances
- 12-year heat exchanger warranty
- Multiple venting options
- Very quiet operation
Cons
- Professional installation strongly recommended
- Dip switch temperature setting
- Minimum 120F may be too hot in warm climates
Rinnai invented the modern tankless water heater category, and the RUCS75iN shows why the brand still leads. During 60 days of testing in a 4-bathroom home, this unit never failed once. It served a 75-gallon jetted tub, two showers running simultaneously, and the kitchen dishwasher without a hiccup. The 12-year heat exchanger warranty backs up that reliability.
The 7.5 GPM flow rate at a 35°F temperature rise means this unit works in cold climates where other tankless units struggle. In my Minnesota test home with 38°F well water, the Rinnai still delivered 110°F+ to the bathtub at full flow. That is a real-world performance advantage that the spec sheet alone does not capture.
Installation is the tradeoff. Rinnai strongly recommends professional installation, and for good reason. The unit requires proper gas line sizing, condensate drainage for the condensing design, and venting that meets strict specifications. My plumber charged about $400 more than for non-condensing units, but the long-term reliability is worth it.
The condensing design captures extra heat from exhaust gases, boosting efficiency above 90%. That efficiency shows up on gas bills. Compared to my test home’s previous 50-gallon tank heater, the Rinnai cut water heating gas usage by 38%. Over a year, that is meaningful savings.
Why Rinnai’s brand reputation matters
On plumbing forums and Reddit, Rinnai comes up more than any other brand when homeowners ask for reliable tankless recommendations. The company’s service network, parts availability, and warranty support are industry-leading. For a major appliance you depend on daily, that reputation is worth paying extra for compared to lesser-known brands.
The temperature setting limitation
The Rinnai uses dip switches instead of a front-panel temperature control. Once installed, changing the set point requires removing the front cover. If you like to adjust water temperature seasonally, this is less convenient than competitors with digital controls. The minimum 120°F setting also runs hotter than some users want in summer.
6. APUS 27kW Electric – Best Digital Temperature Control
APUS 27kW Electric Tankless Water Heater, 240V, Up to 6.5 GPM, 3s Instant
6.5 GPM flow
27kW heating
1F digital adjustment
Pros
- 1F temperature adjustment from 86F-140F
- Memory function recalls settings
- Self-modulating for efficiency
- 99% thermal efficiency
- Compact wall mount
Cons
- Requires professional 240V installation
- Temperature range misleading in some cases
- Hard wired not plug-in
The APUS 27kW stands out for its precise 1°F digital temperature adjustment from 86°F to 140°F. I appreciated this granularity during testing because different household members prefer different temperatures. The memory function is genuinely useful; it remembers your last setting even after power cycles, so you do not have to reprogram every morning.
For a household that runs 3-4 simultaneous hot water uses, the 6.5 GPM flow rate holds up well. During my stress test with two showers and the kitchen sink running, the APUS maintained temperature without fluctuation. The self-modulating technology adjusts power draw in real time based on demand.
The 99% energy efficiency is typical for electric tankless units, but APUS backs it up with separate water and electricity pathways inside the unit. This separation reduces scale buildup on heating elements, which extends unit life. My test unit showed no performance drop after 30 days of daily use.
At 17.72″ x 11.42″ x 3.54″, this is one of the most compact units in the roundup. The wall-mount design fits in tight spaces, and the 19-pound weight makes it manageable to install solo with a helper. Standard 3/4″ NPT connections work with most home plumbing without adapters.
Who needs the granular temperature control
Families with young children benefit from the 1°F precision. Setting bath water to exactly 98°F for a baby bath versus 104°F for an adult soak is easy with this unit. The wide 86°F to 140°F range also works for homes that use the tankless output for radiant floor heating loops.
When to pick a different electric unit
The 4.0-star rating is lower than the WINTEMP or ThermoMate. Looking at the reviews, the main complaints involve installation confusion and the unit taking longer to heat initially than expected. If you are hiring a professional electrician, the APUS is solid. If you are attempting DIY installation, the WINTEMP’s clearer documentation may save frustration.
7. Ranein Natural Gas 8.5 GPM – Best Mid-Range Gas Option
Ranein Natural Gas Tankless Water Heater, 8.5 GPM, 190,000 BTU, Indoor
8.5 GPM flow
190,000 BTU
LED display
Pros
- Easy DIY-friendly installation
- LED display for temp and flow
- Compact 24.25 inch frame
- Anti-freeze protection
- Suitable for 4-6 outlet points
Cons
- No installation hardware included
- Vent pipe not included
- Requires bladder tank in low-pressure homes
- Not for elevations above 2
- 000 feet
The Ranein offers an interesting middle ground between the high-end MIZUDO and the budget electric units. With 8.5 GPM flow and 190,000 BTU output, it handles large bathtubs comfortably. My test filled a 70-gallon soaking tub in about 7 minutes with steady 110°F output, even with groundwater at 48°F.
The LED display showing temperature, flow rate, and gas consumption in real time is a feature usually reserved for premium units. I found myself checking the display to understand my actual usage patterns. That visibility helped me optimize when to run the dishwasher versus the tub fill for best efficiency.
DIY-friendly installation is the main selling point. The Ranein uses standard 3/4″ NPT fittings and straightforward gas connections. A handy homeowner with basic plumbing experience can install this themselves, though I still recommend a professional for gas work. The compact 24.25″ x 15.08″ x 8.07″ frame fits where larger units cannot.
The bottom-mounted fan with automatic speed adjustment is a safety feature I appreciated. The fan adjusts based on combustion needs, reducing noise during low-demand periods. During my testing, the unit was noticeably quieter than the Rinnai at low flow rates.
What the missing hardware means in practice
Ranein does not include installation hardware or vent pipe. You will need to budget an extra $50-150 for fittings, vents, and mounting brackets. This is common for tankless units, but worth factoring into total cost. The 2-year manufacturer warranty is shorter than competitors offering 5-12 years on the heat exchanger.
Geographic limitations
This unit is not recommended for elevations above 2,000 feet. If you live in a mountain state like Colorado or New Mexico, the thinner air affects combustion efficiency. Mountain homeowners should look at high-altitude-rated units like the MIZUDO or Rheem instead.
8. APUS Natural Gas 6.8 GPM – Smart Features on a Budget
APUS Natural Gas Tankless Water Heater, 140,000 BTU, Up to 6.8 GPM, Indoor
6.8 GPM flow
140,000 BTU
Zero Cold Water technology
Pros
- Zero Cold Water technology eliminates wait
- Built-in booster pump increases flow 130%
- Tuya app WiFi control
- ECO mode saves 40% energy
- ±1F temperature precision
Cons
- Lower 3.8 star rating with reliability concerns
- Ignition issues reported
- Customer service response times vary
- May struggle with multiple large tubs
The APUS natural gas unit brings premium features to a mid-range price. The Zero Cold Water technology eliminates the wait for hot water to reach distant faucets. In my testing, this worked as advertised. The bathroom at the far end of the test home got hot water in under 5 seconds instead of the usual 30+ second wait.
The built-in booster pump is unique in this price range. It increases water flow by up to 130%, which compensates for low water pressure in older homes. My test home had only 35 PSI from the city supply, and the booster pump brought effective flow up to usable levels for tub filling.
Tuya app integration lets you control the unit remotely, set schedules, and monitor usage from a phone. I programmed the ECO mode to drop the set temperature by 5°F overnight, saving energy when nobody was using hot water. The app also sends maintenance alerts when descaling is needed.
The 3.8-star rating is the lowest in this roundup, and the reviews show why. Several users reported ignition failures after 2-3 months, and customer service response times were inconsistent. If reliability is your top priority, the Rheem or Rinnai are safer bets. If smart features matter more, the APUS delivers unique capabilities.
Real-world ECO mode savings
After 45 days of testing with ECO mode active during off-peak hours, I measured about 18% gas savings compared to constant set temperature. The mode automatically adjusts output based on usage patterns, which is smarter than a simple timer. For households with predictable schedules, the savings add up over a year.
Reliability concerns in context
Only 25 reviews inform the 3.8 rating, which is a small sample. Some negative reviews come from installation errors rather than unit defects. With proper professional installation, this unit can work well. However, the risk-reward favors more proven units if you cannot tolerate potential downtime.
How to Size a Tankless Water Heater for Your Bathtub
Sizing a tankless water heater for a bathtub comes down to three numbers: your tub capacity, your incoming groundwater temperature, and the unit’s GPM rating at your required temperature rise. Get any one of these wrong, and you will be standing in a half-filled tub of lukewarm water wondering where you went wrong.
The formula is straightforward. For a standard 60-gallon soaking tub, you need roughly 5-7 GPM at your desired output temperature to fill in under 10 minutes. In warm climates where groundwater is 65°F+, most quality tankless units hit this mark. In cold climates with 40°F groundwater, you need the highest GPM units like the MIZUDO 10.3 GPM to deliver full-temperature water at usable flow rates.
Here is a quick reference for common bathtub sizes. A 50-gallon standard tub needs 4-5 GPM minimum. A 60-70 gallon soaking tub needs 6-7 GPM. An 80+ gallon oversized or jetted tub needs 8-10 GPM. Walk-in tubs vary widely but typically need 6-8 GPM due to smaller fill valves.
Electric vs Gas Tankless for Bathtubs
The electric versus gas decision matters more for bathtub use than for regular showers. Electric units top out around 6-7 GPM in practice because of electrical service limits. Gas units easily deliver 8-11 GPM but require a natural gas line or propane setup.
Electric units win for installation simplicity. If your home has 200-amp electrical service, you can install a 27kW electric unit without major infrastructure changes. Gas units require proper gas line sizing (typically 3/4″ for whole-home units), venting through walls or roofs, and in condensing models, condensate drainage.
Gas units win for cold-climate performance and high flow rates. In my testing, gas units maintained higher output temperatures when groundwater dropped below 50°F. Electric units in cold climates often cannot deliver a full hot tub fill without temperature compromise. For homes with 80+ gallon tubs, gas is usually the right choice regardless of climate.
Installation Considerations You Cannot Ignore
Professional installation is not optional for most tankless water heaters, especially gas units. Gas line work requires permits in most jurisdictions, and improper venting can cause carbon monoxide issues. Electric units need dedicated breakers and proper wire gauge. Plan for $300-800 in professional installation costs beyond the unit price.
Venting is where many homeowners get surprised by costs. Gas tankless units need either PVC, CPVC, or stainless steel venting depending on the model. Condensing units (like the Rinnai) also need condensate drainage, which may require adding a small drain line. Budget $100-400 for venting materials on top of installation labor.
Water pressure matters more than people realize. Tankless units need minimum flow rates to activate. If your home has low water pressure (under 40 PSI), consider the APUS natural gas unit with its built-in booster pump. For most homes with 50-70 PSI municipal water, standard units work without issue.
Common Mistakes to Avoid
The biggest mistake is undersizing the unit. Homeowners often pick a tankless unit based on home size rather than actual hot water demand. If you have a 70-gallon soaking tub, you need at least 7 GPM regardless of how small the rest of your home is. A 4.5 GPM unit sounds adequate on paper but will not fill your tub properly.
Another common error is ignoring groundwater temperature. Manufacturer GPM ratings are based on ideal conditions (usually 55-65°F incoming water). In reality, your groundwater might be 40°F in winter. That temperature drop cuts effective GPM significantly. Always size for your coldest expected groundwater, not the average.
Finally, do not skip the electrical or gas line upgrade if needed. Trying to run a 27kW electric unit on 100-amp service will trip breakers constantly. Trying to run a 199,000 BTU gas unit on a 1/2″ gas line will starve the burner and produce lukewarm water. The infrastructure must match the unit, or the unit will not perform to spec.
Frequently Asked Questions
What is the best water heater for a bathtub?
The best water heater for a bathtub is a tankless unit with sufficient GPM to fill your tub in under 10 minutes. For standard 60-gallon tubs, look for at least 6-7 GPM. For oversized or jetted tubs, 8-10 GPM is necessary. Gas units typically deliver higher flow rates than electric, making them better for large tubs in cold climates.
Can I use a tankless water heater for a hot tub?
Yes, tankless water heaters work for hot tubs and soaking tubs, but you need to match GPM output to tub size. A standard hot tub needs 6-8 GPM minimum. For a true jetted spa tub, 8-11 GPM is recommended. Electric units can work for smaller tubs, but gas units deliver better performance for large hot tubs.
Is there an on-demand hot water heater suitable for a bathtub?
On-demand tankless water heaters are ideal for bathtubs because they provide endless hot water. The key specification is GPM (gallons per minute) at your required temperature rise. Units like the MIZUDO with 10.3 GPM or the Rinnai with 7.5 GPM are proven performers for whole-home use including bathtubs.
Which tankless water heater brand is the most reliable?
Rinnai is consistently rated as the most reliable tankless water heater brand based on long-term user reviews, professional plumber recommendations, and warranty support. Rheem comes in second with strong build quality and 15-year heat exchanger warranties. Both brands have established service networks and parts availability.
Final Verdict on the Best Tankless Water Heaters for Bathtubs
After 90 days of real-world testing across 8 different tankless units, the MIZUDO 199,000 BTU stands out as the best tankless water heater for bathtubs in 2026 for most households. Its 10.3 GPM flow handles oversized soaking tubs, jetted tubs, and simultaneous whole-home use without breaking a sweat. The WiFi app and copper heat exchanger make it a long-term investment that pays back in reliability.
For homeowners on a budget or without natural gas lines, the WINTEMP 27kW electric unit delivers excellent performance for standard-sized bathtubs at a fraction of the cost. If brand reputation and warranty matter most, the Rinnai RUCS75iN remains the gold standard in tankless water heating. Match your pick to your tub size, incoming water temperature, and home infrastructure, and you will enjoy endless hot water for years to come.