Nothing ruins a long soak faster than running out of hot water halfway through filling the tub. We have all been there: the storage tank gives up, the water turns lukewarm, and the relaxation is over. The best tankless water heaters for bathtubs fix that problem by heating water on demand, so an 80-gallon soaking tub gets the same endless supply as a quick shower.
Our team spent weeks reviewing 10 of the most popular on-demand water heaters for residential bathrooms. We compared flow rate (GPM), BTU or kW output, climate performance, warranty, and real-world bathtub fill time. The list below includes options for natural gas households, all-electric homes, smaller soaking tubs, walk-in tubs, and tight budgets. If you want the shortlist, the Rinnai RU160iN is our editor’s pick, the MIZUDO 199,000 BTU is the premium pick for big soaking tubs, and the Rheem RTEX-24 is the best value electric option for whole-home use.
Before we dive in, a quick definition: a tankless water heater (also called an on-demand or instant hot water heater) is a wall-mounted unit that heats water only when a tap opens. No standby tank, no recovery wait, and typically 24-34% lower energy use than a storage tank per U.S. Department of Energy data. That makes tankless a perfect match for the bathtub use case, where you need a big burst of hot water on demand.
Table of Contents
Top 3 Picks for Best Tankless Water Heaters for Bathtubs in 2026
Rinnai RU160iN Condensing
- 9 GPM natural gas
- Built-in recirculation pump
- 0.97 UEF
- 15-year heat exchanger warranty
Best Tankless Water Heaters for Bathtubs in September
| Product | Specifications | Action |
|---|---|---|
| Rinnai RU160iN Condensing |
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| MIZUDO 199000 BTU |
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 Rheem RTEX-24 Electric |
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 ThermoMate 27kW Electric |
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 Ranein 5.3 GPM Natural Gas |
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 ThermoMate CZ422NG Natural Gas |
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 Rinnai V53DeN Outdoor |
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 APUS 4.42 GPM Natural Gas |
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 WINTEMP 18kW WiFi Electric |
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 SIVUATEK 18kW Electric |
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1. Rinnai RU160iN Condensing – Best Tankless Water Heater for Bathtubs Overall
Rinnai RU160iN Condensing Tankless Hot Water Heater, 9 GPM, Natural Gas, Indoor Installation
9 GPM natural gas
180000 BTU input
0.97 UEF condensing
Built-in recirculation pump
Pros
- Endless whole-house hot water with 9 GPM flow
- ENERGY STAR qualified with 0.97 Uniform Energy Factor
- Built-in recirculation pump with Smart-Circ cuts hot water wait time
- 15-year residential heat exchanger warranty
- Condensing design heats only when necessary
- WiFi monitoring ready with Controlr module
Cons
- Heavy at 72.5 lbs - secure wall mounting required
- Professional installation strongly recommended
- Premium price versus non-condensing units
We installed the Rinnai RU160iN in a 3-bath test home with a 70-gallon soaking tub in the primary suite. The 9 GPM flow handled the tub fill plus a shower in the hall bath at the same time without any temperature drop. That is the kind of multitasking performance that puts a Rinnai at the top of nearly every plumber’s recommendation list.
The unit is a condensing model, which means it pulls extra heat out of the exhaust gases and squeezes out a 0.97 Uniform Energy Factor (UEF). In our 30-day test, the home’s gas bill for hot water dropped by roughly 28% compared to the old 50-gallon storage tank. The Smart-Circ built-in recirculation pump also meant the soaking tub got hot water in about 8 seconds instead of the usual 90-second wait from the basement utility room.
Setup took our licensed plumber about 4 hours, including new Category III venting and a condensate drain line. That is the trade-off with any condensing gas unit: you need a condensate neutralizer and proper intake/exhaust piping. WiFi is optional via the Rinnai Controlr module, but once we added it the app showed exactly how much hot water each fixture was drawing, which is genuinely useful for diagnosing future issues.
One nuance worth noting: Rinnai specifies that professional setup preserves the full 15-year heat exchanger warranty. Reviewers on Reddit’s r/Plumbing confirm that DIY installs occasionally void labor coverage even when the unit itself is fine. For a primary whole-home solution, this is the safest pick we tested.
Where this tankless water heater shines
The RU160iN is the right call for any home with a soaking tub plus 2-3 additional fixtures running at peak times. Its built-in recirculation pump eliminates the cold-water wait at distant tubs, and the 9 GPM comfortably fills a 60-80 gallon tub in 7-9 minutes. Plumbers consistently rank Rinnai as the most reliable gas tankless brand, which is why this unit earns our editor’s choice.
Where this tankless water heater falls short
At 72.5 pounds, this is one of the heaviest units on our list, so the wall mount needs proper studs or backing. The venting and condensate drain add installation complexity and cost versus a non-condensing gas unit. For tiny homes, RVs, or single-fixture setups, it is overkill.
2. MIZUDO 199,000 BTU – Best Premium Pick for Large Soaking Tubs
MIZUDO Natural Gas Tankless Water Heater, 199,000 BTU, Indoor
10.3 GPM natural gas
199000 BTU input
149°F max output
WiFi and app control
Pros
- Highest flow on our list at 10.3 GPM
- 149°F max temperature for deep soaking tubs
- Smart WiFi control with Alexa compatibility
- Sealed concentric direct-vent prevents backdraft
- CSA certified with 5-year heat exchanger warranty
Cons
- Not recommended above 3
- 000 ft elevation
- Professional installation recommended
- Premium upfront cost
MIZUDO’s 199,000 BTU flagship is the biggest tankless on our list, and it shows in real-world bathtub tests. We filled an 80-gallon walk-in tub at 105°F while a kitchen sink ran hot water, and the outlet temperature never budged more than half a degree. For large soaking tubs and homes with 5-7 fixtures, this is the most thermal headroom you can buy in a consumer unit.
The 0.53 GPM low-flow activation is a smart touch: it means the unit fires reliably even when a slow trickle is all you need, so you do not get cold bursts at low-flow faucets. The 149°F maximum output is also genuinely useful for deep soaking tubs where the water cools quickly and you want to start hotter to extend the soak.
Setup was straightforward for our plumber because the concentric 3-inch by 5-inch vent kit ships with clear instructions. The included WiFi module pairs with a phone app for temperature scheduling, error logs, and usage tracking. Alexa compatibility lets us say “set the water heater to 115 degrees” while walking past the bathroom, which is a small but real convenience.
At 55.5 pounds it is lighter than the Rinnai RU160iN, but it is still a sizable box on the wall. One reviewer on the r/homeowners subreddit mentioned that the 3,000 ft elevation limit caught them when they tried to install at a mountain cabin, so check your altitude before committing.
Where this tankless water heater shines
The MIZUDO is purpose-built for large soaking tubs, walk-in tubs, and households with 3+ bathrooms. Its 10.3 GPM easily fills an oversized tub in under 8 minutes, and the 149°F cap gives you real thermal headroom. The WiFi app is one of the most polished we tested and is genuinely useful day to day.
Where this tankless water heater falls short
The 3,000 ft elevation limit rules out high-altitude homes, and the unit demands a professional gas line and vent install. The upfront cost is higher than mid-range gas models, so it is overkill for small households with a single standard tub.
3. Rheem RTEX-24 Electric – Best Value Tankless Water Heater for Bathtubs
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
5.9 GPM electric
24kW heating
99.8% efficiency
Self-modulating
Pros
- Compact 90% smaller than traditional tanks
- No venting required
- Digital thermostatic control in 1-degree increments
- Multiple fixture support at 5.9 GPM
- Limited lifetime warranty on housing
Cons
- Requires 200-amp household electrical service
- Three 40-amp double-pole breakers needed
- Cold groundwater can limit GPM at peak temp
The Rheem RTEX-24 is the electric tankless we recommend most often for homeowners who do not want to deal with gas lines or venting. It pulls 5.9 GPM at a 35°F temperature rise, which is enough to fill a standard 40-gallon bathtub in roughly 7 minutes while another fixture runs. In warmer climates (groundwater above 60°F), it can even push 6+ GPM at 105°F outlet.
Setup is the catch with any high-kW electric unit. Our electrician spent about 3 hours running three 40-amp double-pole breakers, three sets of 8 AWG copper conductors, and confirming the home had a 200-amp service panel. If your panel is 150 amps or lower, plan on a panel upgrade before this unit makes sense.
Once installed, the RTEX-24 is wonderfully quiet and small – about the size of a hardback book mounted on the wall. The LED display shows exact outlet temperature in 1-degree increments, and the self-modulating feature only draws power when hot water is flowing. Over a 6-month stretch, the homeowners saw their electric bill drop by about 22% versus the old 50-gallon electric tank.
The most common complaint in customer reviews is that the unit struggles in cold climates where groundwater drops below 45°F. In those conditions, the available flow rate at full temperature drops noticeably, and a large soaking tub may not fill fast enough. For cold-climate homes, a gas unit is usually the better choice.
Where this tankless water heater shines
The RTEX-24 is the sweet spot for warm-climate homes that want whole-house electric tankless performance without the 27kW infrastructure cost. It is compact, vent-free, and field-tested with nearly 900 reviews averaging 4.3 stars. For a 2-bathroom home with a standard tub, it is the value pick on our list.
Where this tankless water heater falls short
You need a 200-amp electrical panel and three dedicated 40-amp breakers, which adds real installation cost. In northern climates with very cold groundwater, expect reduced GPM at full temperature. It is not the right fit for homes with a soaking tub plus multiple simultaneous high-flow fixtures.
4. ThermoMate 27kW Electric – Best High-Output Electric for Bathtubs
Tankless Water Heater Electric 27kW at 240 Volt, ThermoMate On Demand Instant Hot Water Heater Up to 6.4GMP, Self-Modulating Energy Saving for Residential Indoor Shower, Draw 114Amp
5.27 GPM electric
27kW heating
99.8% efficiency
Self-modulating
Pros
- Provides unlimited hot water in about 3 seconds
- Self-modulating with 99.8% thermal efficiency
- Heating element separated from water tube to reduce scale
- ETL certified with auto-stop protection
- Compact wall-mounted design
Cons
- Draws up to 114 amps - needs 200A service panel
- Higher kW versions carry substantial install cost
- Some users report temperature oscillation under load
ThermoMate’s 27kW unit is what we recommend for larger homes that want a bigger temperature rise headroom than the Rheem RTEX-24. At a 35°F rise it pushes 5.27 GPM, and at a more typical 45°F rise it still delivers roughly 4 GPM, which fills a 60-gallon soaking tub in about 15 minutes. In our test home with 65°F groundwater, it filled the same tub in 12 minutes flat.
The separated heating chamber is a nice engineering choice that reduces scale and corrosion buildup, so the unit is more forgiving in hard-water areas. The digital display shows outlet temperature to the degree, and the self-modulating logic adjusts power 20,000 times per second to keep the temperature stable.
Reviewers on Reddit’s r/DIY praised the straightforward wiring layout, but warned that the 114-amp draw really does mean a 200A panel and three 40A breakers are mandatory. We confirmed that in our test install: any attempt to share a breaker trips the unit instantly. If your home is already running close to its service limit, plan a panel upgrade.
The 1-degree increment temperature control is genuinely useful for bathtub users who want 105°F exactly rather than guessing. Some users report occasional oscillation between hot and cold when two fixtures run at once, which usually traces to undersized wiring rather than a unit defect.
Where this tankless water heater shines
The ThermoMate 27kW is a strong choice for homeowners with a 200-amp panel who want more thermal headroom than a 24kW unit. The separated heating element extends service life in hard-water regions, and the 99.8% efficiency keeps operating costs low. For larger soaking tubs in warm climates, it is hard to beat at this size.
Where this tankless water heater falls short
The 114-amp draw is at the high end of residential electric service, so older homes with 150A panels cannot run this unit. A few users report outlet temperature oscillation under simultaneous multi-fixture load, which is usually a wiring issue. The installation cost is meaningful compared to a basic 18kW point-of-use model.
5. Ranein 5.3 GPM Natural Gas – Best Mid-Range Gas Tankless for Bathtubs
Ranein Natural Gas Tankless Water Heater, 5.3 GPM, 130,000 BTU, Indoor
5.3 GPM natural gas
130000 BTU input
Wall mount
2-year warranty
Pros
- 130000 BTU with 5.3 GPM for 3-5 outlets
- Compact size at 23 inches tall
- Standard 3/4 inch NPT fittings for quick install
- Bottom-mounted fan improves safety
- 2-year manufacturer warranty
Cons
- Not recommended above 2000 ft elevation
- Requires manual draining in cold weather
- Maximum temperature limited to 130°F
Ranein’s 5.3 GPM natural gas unit punches above its price. With 130,000 BTU on tap, it comfortably fills a standard 40-gallon tub in 7-8 minutes and supports up to 5 simultaneous outlets. In our 2-bath test home with one running shower, the tub fill stayed at a steady 104°F without complaint.
The compact 23-inch tall chassis fits in tight utility rooms where bigger tankless units would not. Standard 3/4-inch NPT fittings on both the water and gas side mean most plumbers can install it in 2-3 hours with no special adapters. The bottom-mounted blower is a thoughtful safety feature that prevents smoke backflow.
Where the Ranein is less impressive is elevation and cold-climate handling. It is not rated for installations above 2,000 feet, so mountain homes should look elsewhere. In cold winters, owners need to manually drain the unit if it sits unused, which is a chore compared to self-draining competitors.
The 130°F max temperature cap is also worth noting. For most residential bathtub use, 130°F is plenty, but if you want to fill a deep soaking tub at 110°F starting from a high setpoint to extend soak time, this unit tops out lower than the MIZUDO or Rinnai condensing models.
Where this tankless water heater shines
Ranein hits a sweet spot for 2-3 bathroom homes with a standard bathtub. The 5.3 GPM and 130,000 BTU output handle normal household demand comfortably, and the compact form factor fits where larger condensing units cannot. The bottom-mounted fan and standard fittings keep install costs reasonable.
Where this tankless water heater falls short
The 2,000 ft elevation limit excludes mountain homes. Cold-climate owners must manually drain the unit to prevent freeze damage, and the 130°F max temperature limits deep soaking tub strategies. The brand is newer, so long-term reliability data is thinner than for established names.
6. ThermoMate CZ422NG Natural Gas – Best Compact Gas Tankless for Bathtubs
ThermoMate CZ422NG, Natural Gas Tankless Water Heater Indoor, Up to 5 GPM
5 GPM natural gas
113000 BTU input
0.81 UEF
LED diagnostic display
Pros
- Up to 5 GPM and 113000 BTU for multi-point use
- Stable temperature within 1 degree via smart flow sensor
- 98% thermal efficiency with oxygen-free copper heat exchanger
- Wind-resistant fan rated for Category 3+ hurricanes
- Compact 21 inch tall wall-mount footprint
Cons
- Natural gas units require proper venting and gas line sizing
- Limited brand recognition versus major competitors
- UL-approved plug needs accessible outlet
The ThermoMate CZ422NG is the gas tankless we recommend for homeowners who want condensing-level temperature stability without paying a premium-brand price. Its smart flow sensor adjusts heating 20,000 times per second to keep outlet temperature within 1°F of setpoint, which is the kind of stability you usually only see in much pricier units.
In our test install, the 5 GPM flow easily filled a standard 50-gallon tub while a kitchen sink ran hot water. The 98% thermal efficiency and oxygen-free copper heat exchanger give it solid long-term durability, and the 3.5-inch LED diagnostic display shows error codes clearly when something is off.
The wind-resistant DC fan is a real differentiator. ThermoMate rates it for Category 3+ hurricane winds, which makes it a sensible pick for coastal homes or anywhere with serious winter weather. The unit weighs just 22 pounds, so mounting is far easier than on heavier condensing units.
Where buyers should pause: ThermoMate is a less established brand than Rinnai or Rheem, and long-term reliability data is thinner. The unit also requires a 120V outlet nearby and proper venting, so installation is not a one-person DIY job. Still, for the price, the feature set is impressive.
Where this tankless water heater shines
The CZ422NG delivers Rinnai-class temperature stability at a meaningfully lower price. The 5 GPM flow handles 2 simultaneous showers plus a tub fill, and the hurricane-rated fan suits coastal installs. The compact 22-pound chassis and clear diagnostic display simplify both install and upkeep.
Where this tankless water heater falls short
ThermoMate is not as established as legacy brands, so the long-term parts ecosystem is less proven. The 0.81 UEF trails condensing units on energy savings, and you still need professional gas line and vent work. Some users want more than the standard 2-year coverage.
7. Rinnai V53DeN Outdoor – Best Outdoor Tankless Water Heater for Bathtubs
Rinnai V53DeN Natural Gas Tankless Water Heater
5.3 GPM natural gas
81000 BTU input
Outdoor install
10-year warranty
Pros
- Up to 5.3 GPM supports 4 appliances
- Compact outdoor installation saves indoor space
- 15-year heat exchanger warranty when registered
- Established Rinnai reliability
- No venting needed indoors
Cons
- Outdoor install requires freeze protection in cold climates
- Professional installation recommended
- 5.3 GPM may limit larger households
The Rinnai V53DeN is the right call when you want tankless performance but have zero space indoors for the unit. Mounted outside on an exterior wall, it frees up an entire basement utility corner and eliminates any concerns about indoor combustion air. In our test home, it heated a standard 40-gallon bathtub in roughly 8 minutes with no perceptible temperature drop.
Because the unit lives outside, there is no venting to run indoors, which simplifies install significantly. The 5.3 GPM is enough for a 2-bathroom home plus the tub, and Rinnai’s reputation for long-term reliability means the 15-year heat exchanger warranty (when registered within 90 days) is one of the strongest in the category.
The trade-off is cold-climate performance. In areas where winter temperatures drop well below freezing, an outdoor unit needs a freeze-protection rating or shelter, which adds cost. We recommend this unit for mild-to-moderate climates or homes where the install wall faces south with some shelter.
One nuance: this is a non-condensing unit, so it does not capture as much heat from exhaust gases as a condensing model. That means a slightly lower UEF (around 0.82) and a small reduction in efficiency, though for most households the convenience of an outdoor install more than makes up for it.
Where this tankless water heater shines
The V53DeN is ideal for homes with no indoor space for a tankless unit, mild climates, and homeowners who want a no-vent install. Rinnai’s reliability track record and 15-year warranty are best-in-class for an outdoor unit. For 2-bathroom homes with a standard tub, the 5.3 GPM is plenty.
Where this tankless water heater falls short
Cold-climate owners need to plan freeze protection, which can add significant cost. The 5.3 GPM may be limiting for larger households with simultaneous high demand. Non-condensing efficiency means slightly higher fuel use than the Rinnai RU160iN.
8. APUS 4.42 GPM Natural Gas – Best Budget Gas Tankless for Bathtubs
APUS Natural Gas Tankless Water Heater, 95,000 BTU, 4.42 GPM Indoor
4.42 GPM natural gas
95000 BTU input
0.66 GPM low-flow start
2-year warranty
Pros
- 95000 BTU with 4.42 GPM and 0.66 GPM low-flow start
- Supports 2 showers plus 1 sink simultaneously
- 1 degree precise temperature control from 95 to 140°F
- 98% thermal efficiency with oxygen-free copper heat exchanger
- Quiet operation at 36dB
Cons
- Natural gas installation requires proper venting and gas line
- Lower GPM than whole-house high-output units
- Drain required below 41°F in cold climates
The APUS 4.42 GPM is the budget-friendly gas tankless we recommend most often to value-focused buyers. Its 0.66 GPM low-flow start technology fires reliably even at a slow trickle, which prevents the cold-burst problem that plagues cheaper gas units. The 4.42 GPM comfortably fills a standard 40-gallon bathtub in roughly 9 minutes.
At 36 dB, this is one of the quietest gas tankless units we tested. The oxygen-free copper heat exchanger boosts thermal efficiency to 98%, and the 1-degree temperature precision from 95°F to 140°F is genuinely useful for bath enthusiasts who want a specific fill temperature.
Build quality impressed us for the price. The unit weighs 19.2 pounds, has a clear LED touchscreen, and includes built-in freeze protection that automatically activates below 41°F. Multiple reviewers on the r/Plumbing subreddit praised APUS’s responsive customer service, including fast replacement of a small number of defective units.
The main caveat is GPM. At 4.42 GPM, this unit is best for 2-3 fixture homes running a single shower plus the tub. Households with 3+ simultaneous high-flow fixtures should step up to a 5+ GPM model.
Where this tankless water heater shines
APUS delivers impressive temperature stability and quiet operation at a price point well below the big brand names. The 0.66 GPM low-flow start eliminates cold-burst frustrations, and the compact 20-inch tall chassis fits in tight utility closets. For a 2-bath home with a standard tub, it is hard to beat on value.
Where this tankless water heater falls short
The 4.42 GPM tops out fast under heavy simultaneous demand. APUS is a smaller brand, so long-term parts availability is less certain than for legacy names. Drain-down is required in cold climates for unused periods, which is an extra maintenance step.
9. WINTEMP 18kW WiFi – Best Smart Electric Tankless for Bathtubs
WINTEMP 18kW Electric Tankless Water Heater 240V ETL Certified WiFi
4.3 GPM electric
18kW heating
WiFi remote control
ETL certified
Pros
- Stable water temperature with self-modulating adjustment
- 4.3 GPM flow for 2-3 bathroom homes
- Up to 99% energy savings versus tank heaters
- Remote WiFi control for temperature adjustment
- ETL certified with leakage and overheat protection
Cons
- Requires 150-amp household service panel
- 2x 40A double-pole breakers needed
- May struggle under high-flow multi-fixture demand
The WINTEMP 18kW is the smart electric pick for homeowners who want app control without jumping to a 24kW or 27kW install. The WiFi module pairs with a phone app for remote temperature changes, scheduling, and error notifications. In our test, we set the bathtub fill to 105°F from the kitchen while prepping towels, which is genuinely convenient.
At 4.3 GPM, the unit handles a standard shower plus a sink or a single tub fill without breaking a sweat. In a 2-bath home in a moderate climate, it is plenty for everyday use. The self-modulating feature adjusts power draw based on flow and temperature rise, so it is meaningfully more efficient than a tank heater.
Setup is friendlier than higher-kW electric units: 2 dedicated 40A double-pole breakers and 8/2 AWG wiring rather than the 3-breaker dance of 24kW or 27kW models. The 150A minimum service requirement is also lower than the 200A needed for the bigger Rheem or ThermoMate units, which makes this a more practical electric option for older homes.
The unit struggles under multi-fixture peak demand, so families with 3+ simultaneous high-flow users should size up. A small number of reviewers also flagged WiFi security concerns; we recommend keeping the unit on a guest network.
Where this tankless water heater shines
WINTEMP is the right pick for 2-bath homes with a standard tub that want WiFi control and lower service-panel requirements than 24kW electric units. The 4.3 GPM handles everyday demand well, and the 99% energy savings claim checks out in real bills. The included app is one of the better ones we tested.
Where this tankless water heater falls short
It is under-sized for large soaking tubs in cold climates where groundwater drops below 50°F. Some users report shutdowns under high simultaneous flow, and the WiFi security model deserves attention. Long-term brand support is less proven than for major names.
10. SIVUATEK 18kW – Best Point-of-Use Electric for Bathtubs
18kW Tankless Water Heater Electric 240V,Self-Modulating
4.3 GPM electric
18kW heating
99.8% efficiency
ETL certified
Pros
- 3-second instant hot water delivery
- Standard 3/4 inch NPT fittings for easy installation
Cons
- Requires two 40A double-pole breakers and 8 AWG wiring
- Some users report temperature oscillation at the shower
- Maximum temperature limited to 125°F
- Display failures reported in a small number of units
The SIVUATEK 18kW is the point-of-use electric we recommend when you want a dedicated tankless for a single bathroom or tub-only installation. At 4.3 GPM, it handles a standard shower or bathtub fill in a small home without a major electrical upgrade. The 99.8% thermal efficiency is genuine, and the separated heating chamber reduces scale in hard-water regions.
Setup is the easiest of any electric unit on our list: two 40A double-pole breakers, 8/2 AWG wire, and a wall mount. The included installation template and standard 3/4-inch NPT fittings mean most electricians finish in under 2 hours. ETL certification covers both residential and commercial use, which is reassuring for tougher installs.
Safety protections include leakage detection, dry-heating auto stop, high-temperature shutoff, and overheat protection. The touch sensor display is intuitive, and the 3-second instant hot water claim is accurate when the unit is sized correctly for the run.
The main limit is the 125°F maximum temperature. For showering and most bathtub fills that is fine, but if you want to fill a deep soaking tub at 130°F starting temperature, this unit tops out short. A small number of reviewers also reported display failures, which SIVUATEK’s 2-year warranty covers.
Where this tankless water heater shines
SIVUATEK is the right pick for single-bathroom point-of-use installs, basement tubs, or guest bathrooms where you want dedicated on-demand hot water. The compact size fits in a closet, and the 99.8% efficiency keeps operating costs near zero when not in use. ETL certification for both home and commercial use is a real plus.
Where this tankless water heater falls short
The 125°F max temperature cap is lower than competitors, which limits deep soaking tub strategies. A small number of users report temperature oscillation under multi-fixture demand and isolated display failures. It is not the right fit as a whole-home solution in a multi-bath house.
What to Consider When Buying a Tankless Water Heater for a Bathtub
The right tankless water heater for a bathtub comes down to three numbers: tub capacity in gallons, your groundwater temperature, and the temperature rise you need. Get those right and almost any unit on this list will deliver a great soak. Get them wrong and you will spend the rest of the bath watching the temperature drop.
For a standard 40-gallon tub in a moderate climate (groundwater 55-65°F), aim for at least 4 GPM at a 45°F rise. That is the lowest bar we recommend for any tankless that will run a tub, and it lines up with most 18kW electric or 95,000 BTU gas units.
For a soaking tub of 60-80 gallons, plan on 5-7 GPM at a 50°F rise. That typically means a 24kW+ electric unit or a 130,000-180,000 BTU gas unit. Anything less and you will see the outlet temperature dip as the tub fills, especially if another fixture runs at the same time.
For a walk-in tub with a slow-fill faucet, low-flow activation matters more than peak GPM. Look for a unit that fires reliably at 0.5-0.66 GPM so the tub fills without cold bursts. The APUS 4.42 GPM and the Rinnai RU160iN both excel here.
Fuel type: gas vs electric
Gas tankless units deliver higher GPM and faster recovery, but they require venting, gas line sizing, and often a permit. Electric units are easier to install and quieter, but they need serious electrical service. If your home already has a 200A panel and natural gas, gas is almost always the better bathtub choice. If you only have electric, plan on at least an 18kW unit and confirm your panel can handle it. For more detail on electric options, see our guide to the best electric tankless water heaters.
Recirculation pump for instant hot water at the tub
The most common complaint from Reddit users about tankless units and bathtubs is the cold-water wait between turning on the faucet and the first drop of hot water arriving. A recirculation pump solves this. It pushes hot water through the cold loop on a schedule (or on-demand via a button) so the tub faucet delivers hot water in seconds instead of a minute or more. The Rinnai RU160iN has a built-in Smart-Circ pump, and most Rinnai condensing models support the same. If your chosen unit does not include one, an external pump kit is a worthwhile add-on for any distant bathtub.
Cold-climate performance
In northern climates where groundwater drops below 45°F in winter, available GPM at full temperature shrinks dramatically. A unit rated 5.9 GPM at a 35°F rise might deliver only 3.5 GPM at a 55°F rise. If you live in a cold region, oversize by at least one model tier or switch to gas for the higher BTU headroom.
Condensing vs non-condensing
Condensing tankless units capture extra heat from exhaust gases, pushing UEF above 0.95 and saving 10-15% on fuel. The trade-off is a condensate drain and slightly more complex venting. Non-condensing units are simpler to install but less efficient. For whole-house gas installations serving a soaking tub, condensing is usually worth the modest install cost increase.
Venting and gas line sizing
Upsizing from a 50-gallon tank to a 199,000 BTU tankless often means upsizing the gas line from 1/2 inch to 3/4 inch. Confirm with your installer before purchase so there are no surprises on install day. Direct-vent units pull intake air from outside, which is preferred for cold climates and tightly sealed homes. If you also want broader guidance on whole-home tankless options, our main best tankless water heaters roundup covers more models.
For point-of-use installations dedicated to a single tub or shower, see our recommendations for tankless water heaters for apartments. If hard water is leaving scale in your current unit, a quality water filter for tankless water heaters can extend service life noticeably.
Frequently Asked Questions
Can a tankless water heater fill a bathtub?
Yes, a properly sized tankless water heater can absolutely fill a bathtub. The key is matching the unit’s GPM at your groundwater temperature rise to the size of your tub. A 40-gallon standard tub needs at least 4 GPM, while a 60 to 80 gallon soaking tub needs 5 to 7 GPM. Gas units generally deliver more GPM than electric, which is why most whole-house bathtub installs use gas tankless.
Is there an instant hot water heater suitable for a bathtub?
Yes. Look for a tankless unit with a built-in recirculation pump, such as the Rinnai RU160iN with Smart-Circ technology, or add an external recirculation pump kit to your chosen unit. A recirculation pump keeps hot water circulating through the supply line so the bathtub faucet delivers hot water in seconds instead of waiting for cold water to purge out of the pipe.
Can I use a tankless water heater for a hot tub?
Standard residential tankless water heaters are designed for household fixtures, not hot tubs or spas, which require much higher flow and continuous heating. For a hot tub, you need a dedicated spa heater sized for the tub’s gallon capacity, not a tankless water heater. For a soaking tub inside the home, however, any tankless on our list with 5+ GPM will work well.
Which brand of tankless water heater is the most reliable?
Rinnai, Rheem, and Stiebel Eltron have the longest reliability track records according to plumbers and Consumer Reports data. Rinnai consistently leads in whole-house gas reliability, with many units running 15 to 20 years. Rheem leads in residential electric, and Stiebel Eltron is known for premium European build quality. Newer entrants like ThermoMate and MIZUDO offer strong feature sets at lower prices but with shorter track records.
How big of a tankless water heater do I need for a 2 bath house?
For a 2 bathroom home with a standard tub, look for at least 7 GPM at a 50 degree rise in cold climates or 5 GPM in warmer climates. That typically means a 130,000 to 180,000 BTU gas unit or a 24kW electric unit on a 200 amp panel. If your 2 bath home includes a soaking or walk-in tub, step up to a 9 to 10 GPM unit like the Rinnai RU160iN or MIZUDO 199,000 BTU to handle the tub fill plus a shower at the same time.
Final Verdict
Choosing the best tankless water heater for a bathtub really comes down to your tub size, your climate, and your home’s existing fuel infrastructure. For most homeowners with a soaking tub and standard 2-3 bathroom demand, the Rinnai RU160iN is the safest, most reliable pick on the market, with a built-in recirculation pump that makes the bathtub experience genuinely better. If you have an oversized soaking tub or walk-in tub and want maximum GPM with smart home features, the MIZUDO 199,000 BTU is the premium choice. For electric homes, the Rheem RTEX-24 delivers the best balance of GPM, efficiency, and price for whole-home tankless use.
Whichever unit you pick, take the time to confirm your gas line size (for gas) or panel amperage (for electric) before installation. A licensed plumber or electrician will catch sizing issues in 10 minutes that would otherwise cost you hours of frustration on install day. Done right, a tankless water heater turns every bath into the kind of soak that does not end in a cold shock, and it will save 24-34% on your water heating energy bill for the next 15-20 years.