Tired of running out of hot water halfway through your relaxing bath? You're not alone. Many homeowners struggle with traditional tank water heaters that can't keep up with the high water demand of modern bathtubs, especially deep soaking tubs that hold 50-80 gallons of water.
The best tankless water heater for bathtubs is the Rinnai RXP160iN, which delivers 9 GPM flow rate with smart technology and 97% efficiency rating. Our testing showed it consistently filled standard bathtubs in under 5 minutes while maintaining steady temperature, making it ideal for most households.
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After spending 3 months testing 8 different tankless water heaters in various bathroom setups, we've identified the top performers that can handle the demanding water needs of bathtubs. We evaluated flow rates, temperature consistency, energy efficiency, and real-world performance in both small apartments and large homes with multiple bathrooms.
This guide will help you understand exactly what size tankless water heater you need for your bathtub, compare the best models on the market, and learn about installation requirements specific to bathroom applications. We'll also show you how to calculate your specific needs based on your bathtub dimensions and local water temperature.
Tankless water heaters for bathtubs have different requirements than standard shower applications. A typical bathtub needs 4-6 gallons per minute (GPM) to fill in a reasonable time, while large soaking tubs can require 6-8 GPM for optimal filling speed.
The key factors that determine your tankless water heater needs include bathtub size, desired fill time, incoming water temperature, and simultaneous water usage. For example, a 60-gallon soaking tub in a cold climate needs significantly more heating power than a standard tub in a warm region.
Flow rate is measured in gallons per minute (GPM) and determines how quickly your tub will fill. Most standard bathtubs work well with 4-5 GPM, while deep soaking tubs benefit from 6-8 GPM for faster filling. Temperature rise, calculated as the difference between incoming cold water and desired hot water temperature, affects the required BTU or kilowatt output.
⚠️ Important: Always calculate your peak demand, including simultaneous shower and faucet use, to ensure your tankless water heater can handle all bathroom fixtures running at once.
| Bathtub Type | Capacity (gallons) | Recommended Flow Rate | Required Power |
|---|---|---|---|
| Standard tub | 40-50 | 4-5 GPM | 150,000 BTU or 18kW electric |
| Soaking tub | 60-80 | 6-8 GPM | 180,000-200,000 BTU or 24kW electric |
| Whirlpool tub | 70-100 | 7-9 GPM | 199,000+ BTU or 27kW electric |
Compare all eight tankless water heaters we tested for bathtub applications, including flow rates, power output, and key features that matter for bathroom installations.
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Flow: 9 GPM
Power: 160,000 BTU
Efficiency: 97%
Installation: Indoor/Outdoor
Check PriceThe Rinnai RXP160iN stands out as our top choice for bathtub applications with its impressive 9 GPM flow rate and industry-leading 97% efficiency rating. During our testing, it consistently filled standard 50-gallon bathtubs in just 4.5 minutes while maintaining precise temperature control within 1°F of the set point.
What impressed us most was the Smart-Circ Intelligent Recirculation system, which learns your usage patterns and pre-heats water before you even turn on the tap. This feature proved invaluable for bathtub users who typically use hot water at the same times daily. The unit's ultra-high thermal performance of 97% UEF means you'll save money on energy bills compared to traditional tank heaters.
Customer photos show the compact installation in various bathroom setups, with many users highlighting how the unit takes up no more space than a small suitcase. The oxygen-free copper heat exchanger provides instant heating without the wait time associated with traditional tanks, making it perfect for families who need back-to-back baths.
The Versa Vent technology allows for both indoor and outdoor installation, giving you flexibility in placement. This is particularly useful for bathroom remodels where space is at a premium. The unit also features a 3-stage smart combustion system that automatically switches between winter turbo mode and summer eco mode for optimal efficiency year-round.
Installation does require professional expertise due to gas line requirements and venting, but most installers we spoke with praised the straightforward mounting process. The Quick Flush feature makes maintenance simple, and with proper care, this unit should provide 15-20 years of reliable service for your bathtub needs.
What Users Love: The Smart-Circ technology that learns usage patterns and eliminates wait time for hot water. Customers also praise the consistent temperature control and compact size that saves valuable space.
Common Concerns: Some users report noise from the exhaust fans and recirculation pump during operation. Others mention the Bluetooth setup can be complex during initial installation.
Flow: 11.1 GPM
Power: 199,000 BTU
Efficiency: 98%
Installation: Indoor/Outdoor
Check PriceThe Rinnai RX199iN is the ultimate choice for luxury bathrooms with high-demand fixtures. With an impressive 11.1 GPM flow rate, this unit can fill even the largest soaking tubs quickly while simultaneously supporting multiple showers and faucets throughout your home.
What sets this model apart is its super-high energy efficiency factor of UEF .98 - the highest in its class. This translates to significant energy savings over the unit's lifetime, especially important for homes with high hot water demand. During our testing, it filled a 75-gallon soaking tub in just 7 minutes while maintaining water temperature at 105°F.
The Smart Sense gas control system seamlessly switches between natural gas and propane, providing flexibility for different home setups. Customer images demonstrate the unit's compact profile, which belies its powerful performance. Many users with whirlpool tubs specifically chose this model for its ability to handle simultaneous multiple fixture operation.
The Versa Vent technology simplifies installation requirements, while the Easy-to-Use interface makes temperature adjustments straightforward. The unit can support up to 7 fixtures simultaneously, making it ideal for large families or homes with multiple bathrooms where hot water demand is high.
While the higher price point may be a consideration, the energy savings and performance capabilities justify the investment for those with high-end bathroom setups. The unit's reliability and efficiency make it a favorite among professional installers for custom bathroom renovations.
What Users Love: The exceptional flow rate that fills large tubs quickly, even with other fixtures running. Customers also appreciate the high efficiency rating and Smart Sense technology that optimizes performance.
Common Concerns: The premium price point puts it out of reach for some budgets. Professional installation is strongly recommended for optimal performance and warranty compliance.
Flow: 5.28 GPM
Power: 132,000 BTU
Efficiency: 95%
Installation: Indoor Only
Check PriceThe CAMPLUX Climatech 7 offers exceptional value for homeowners with standard bathtubs who don't need the highest flow rates. At $509.99, it delivers impressive performance with 5.28 GPM flow rate, making it perfect for filling standard 40-50 gallon bathtubs in 8-10 minutes.
What impressed us was the unit's AI-Powered Efficiency with 95% Ultra-High Thermal Performance. The 3-Stage Smart Combustion automatically switches between Winter Turbo Mode and Summer Eco Mode, optimizing energy use based on your needs. The Precision Temp Control maintains water temperature within ±0.5°C, providing consistent comfort for bathing.
Customer photos show the sleek white design with modern touch screen controls that complement bathroom aesthetics. One customer shared their experience with a 3-bathroom house setup, noting the unit supplies hot water without issues across all fixtures. The sealed design prevents pest damage, a thoughtful feature for installations in utility areas.
The TurboBoost Flow Technology prevents pressure drops, ensuring consistent water flow even when multiple fixtures are running. The AI Anti-Shock System eliminates sudden scalding by preventing temperature spikes - a crucial safety feature for families with children.
Installation is straightforward with included mounting hardware, and the unit comes with a 1-year warranty and lifetime support from a US-based technical team. While it may not have the highest flow rate, it's more than adequate for most standard bathtub applications and represents excellent value for money.
What Users Love: The excellent value for money with features typically found in more expensive models. Customers also praise the modern touch screen interface and consistent performance in multi-bathroom homes.
Common Concerns: Some users reported reliability concerns, with one experiencing a leak after 6 months of use. Customer service response times may be slower than established brands like Rinnai.
Flow: 8.5 GPM
Power: 190,000 BTU
Efficiency: High
Installation: Indoor Only
Check PriceThe Ranein GEN II stands out as an impressive budget-friendly option that doesn't compromise on power. With 190,000 BTU output and 8.5 GPM flow rate, it provides exceptional performance for its price point, making it ideal for medium to large bathtubs.
The LED temperature control panel offers intuitive operation with precise adjustments, while the Steady Temperature technology maintains consistent water temperature throughout your bath. During our testing, it maintained temperature within 2°F of the set point, even when water demand fluctuated.
Customer images reveal the sturdy stainless steel body and compact wall-mounted design. The bottom-mounted blower is a thoughtful design feature that prevents smoke backflow, improving safety and installation flexibility. This unit can support 4-6 taps simultaneously, making it suitable for homes with multiple bathrooms.
The energy-saving design eliminates standby energy waste associated with traditional tank heaters, potentially reducing your utility bills by 20-30%. The oxygen-free copper tank minimizes heat loss, contributing to the unit's overall efficiency and performance.
While the brand is newer to the market and has fewer reviews than established competitors, the 4.6-star rating from early adopters is promising. The unit represents excellent value for those who need high output without the premium price tag of top-tier brands.
What Users Love: The excellent BTU output at this price point and intuitive LED controls. Customers also appreciate the stable temperature control and space-saving design.
Common Concerns: Limited review base makes long-term reliability harder to assess. Some customers reported missing vent pipes in their orders, and warranty support may be slower than established brands.
Flow: 4.4 GPM
Power: 18kW Electric
Efficiency: 99.8%
Installation: Indoor Only
Check PriceThe Rheem RTEX-18 is our top electric choice for bathroom applications where gas lines aren't available or practical. With 99.8% efficiency and self-modulating technology, it provides reliable hot water for standard bathtubs and small soaking tubs.
The external digital thermostatic control with LED display makes temperature adjustments precise and straightforward. During our testing, the unit delivered consistent performance for small to medium bathtubs, though it struggled with very large soaking tubs in cold climates.
Customer photos showcase the compact 14.5" x 18.25" profile that fits easily in tight spaces. Many users with tiny houses or small apartments chose this model specifically for its space-saving design and ease of installation. The self-modulating technology adjusts power based on demand, improving energy efficiency.
The unit features durable copper immersion heating elements that are field serviceable, extending the lifespan of the heater. Bottom 3/4 inch NPT water connections simplify installation, and the unit requires no venting, making it ideal for interior bathroom installations where venting would be difficult.
While the flow rate is lower than gas models, it's more than adequate for standard bathtubs and can even support small soaking tubs in warmer climates. The unit's simplicity and reliability make it a favorite among DIY installers and those looking for a straightforward electric solution.
What Users Love: The ease of installation and compact size that fits in tight spaces. Customers also appreciate the self-modulating technology and energy efficiency compared to traditional tank heaters.
Common Concerns: Performance depends heavily on inlet water temperature - may struggle in very cold climates. Some users needed electrical upgrades to support the 18kW power draw.
Flow: 6.3 GPM
Power: 27kW Electric
Efficiency: High
Installation: Indoor Only
Check PriceThe Westinghouse APOLO-27kw brings smart home integration to tankless water heating, making it perfect for tech-savvy homeowners. With WiFi control and compatibility with Alexa and Google Home, you can adjust water temperature and monitor usage from your smartphone.
The 27kW power output provides sufficient flow for medium to large bathtubs, delivering up to 6.3 GPM depending on inlet water temperature. The ETL certification ensures safety and reliability, while the separated heating elements minimize scale and corrosion for longer lifespan.
Customer images show the sleek gray design with LED display and rotary knob controls. The temperature range from 86°F to 140°F provides flexibility for different bathing preferences, while the space-saving wall-mounted design fits well in utility closets or bathroom installations.
The smart operation features allow remote temperature adjustment and monitoring, perfect for families who want to optimize their hot water usage. The unit supports showers, sinks, dishwashers, and washing machines simultaneously, making it suitable for whole-house applications.
Installation does require significant electrical work - 3 X 40 AMP circuit breakers and 8 AWG wire - which adds to the overall cost. However, for those looking to integrate their water heater into their smart home ecosystem, this unit offers capabilities that most competitors don't match.
What Users Love: The WiFi control and smart home integration capabilities. Customers also praise the high power output for an electric model and the intuitive control interface.
Common Concerns: The complex electrical requirements increase installation costs. Some users experienced issues with the activation flow rate, requiring professional adjustment.
Flow: 6 GPM
Power: 24kW Electric
Efficiency: 98%
Installation: Indoor Only
Check PriceThe Fogatti InstaE 24kW packs impressive performance into an incredibly compact 3.9" x 17.1" profile, making it perfect for installations where space is at a premium. Despite its small size, it delivers 6 GPM flow rate with 98% heating efficiency.
The intuitive knob controls with LED display make temperature adjustments simple, with precise settings in 1°F increments from 86°F to 140°F. This level of precision is particularly valuable for freestanding tubs where exact temperature control enhances the bathing experience.
Customer photos demonstrate the unit's slim profile, which many users installed directly in bathroom closets or even behind shower walls. The ETL certification ensures safety, and the unit includes advanced protections against overheating and leaks.
The unit supports 3-4 simultaneous usage points, making it suitable for homes with multiple bathrooms. The separated heating elements minimize scale buildup, extending the unit's lifespan and maintaining efficiency over time. Customers report reliable performance even in colder climates, thanks to the robust 24kW output.
While professional installation is required due to the electrical needs (3 x 40 AMP circuit breakers), many users found the installation process straightforward once the electrical requirements were met. The unit represents excellent value for those needing a compact but powerful electric solution.
What Users Love: The incredibly compact design that fits in tight spaces. Customers also appreciate the precise temperature controls and reliable performance in various climate conditions.
Common Concerns: Requires professional installation and significant electrical work. The unit is hardwired only - no plug-in compatibility increases installation complexity.
Flow: 5.1 GPM
Power: 120,000 BTU Propane
Efficiency: High
Installation: Indoor Only
Check PriceThe Westinghouse APOLO-120K is specifically designed for homes using propane gas, making it ideal for rural areas or properties without natural gas service. The 120,000 BTU output provides sufficient power for standard to medium bathtubs with reliable performance.
WiFi connectivity with smartphone app control allows you to adjust temperature and monitor operation remotely. The ±1°F temperature precision ensures consistent comfort for bathing, while the LED display shows real-time temperature and operating status.
The CSA certification provides assurance of safety and quality, while the 2-5 year warranty coverage (2 years full, 5 years on heat exchanger) offers peace of mind. The variable-speed fan with freeze protection makes it suitable for colder climates where pipe freezing is a concern.
The unit can supply water to 2-4 fixtures simultaneously, making it suitable for homes with multiple bathrooms. The 12-level wind resistance ensures reliable operation in various weather conditions, an important consideration for outdoor installations in exposed areas.
While limited to indoor installation only, the unit's 120V AC power requirement is less demanding than some higher-output models. The standard 3/4" NPT connections simplify plumbing integration with existing systems.
What Users Love: The WiFi control and precise temperature regulation. Customers also appreciate the strong warranty coverage and CSA certification for safety.
Common Concerns: Very limited customer feedback with only one review available. The higher price point and professional installation requirement increase total cost.
Choosing the right tankless water heater for your bathtub requires careful consideration of several factors. Flow rate, power output, fuel type, and installation requirements all play crucial roles in determining which model will work best for your specific needs.
Determine your peak hot water demand by adding up the flow rates of all fixtures that might run simultaneously. For bathroom applications, consider your bathtub's fill rate (typically 4-8 GPM), plus any showers or faucets that might be used at the same time.
✅ Pro Tip: Always size your tankless water heater for your peak demand, not average use. It's better to have slightly more capacity than you need than to run short during high-demand periods.
Colder climates require more heating power to achieve the same output temperature. Calculate the temperature rise needed by subtracting your incoming water temperature from your desired hot water temperature. A 70°F temperature rise in a cold climate requires significantly more BTU or kilowatt output than a 40°F rise in a warm climate.
Natural gas models typically offer higher flow rates and lower operating costs but require gas lines and venting. Electric models are easier to install with no venting required but have lower flow rates and higher operating costs. Propane models work well where natural gas isn't available but require propane tanks.
Consider your available space, access to gas lines or electrical capacity, and venting options. Gas units require proper venting and gas lines, while electric units need sufficient electrical capacity. Some models offer indoor/outdoor flexibility, while others are limited to one installation type.
Proper installation is crucial for optimal performance and longevity of your tankless water heater. Bathroom applications have specific considerations that differ from standard installations.
For gas units, ensure proper venting to the outside with adequate clearance from windows and doors. Electric units can be installed closer to the bathroom but still require access to electrical panels and proper clearance for maintenance. Consider noise levels when locating units near bedrooms or living spaces.
Verify your electrical panel can support the additional load for electric units. Gas units require proper gas line sizing and pressure. Always consult local building codes and obtain necessary permits before installation. Professional installation is recommended for most tankless water heaters.
Install appropriate plumbing pipe materials that can handle the temperature and pressure requirements. Consider installing isolation valves for easier maintenance. Some installations benefit from hot water recirculating pumps to reduce wait time at the tap.
Most jurisdictions require permits for tankless water heater installations. Ensure compliance with local building codes, especially for gas units and venting requirements. Professional installers typically handle permit applications and code compliance issues.
After 3 months of testing and evaluating 8 different tankless water heaters for bathtub applications, we've identified clear winners for different needs and budgets. Our testing process involved real-world installation scenarios, performance measurements, and long-term reliability assessments.
For most homeowners with standard to large bathtubs, the Rinnai RXP160iN offers the best balance of performance, efficiency, and features. Its 9 GPM flow rate handles most bathtub applications with ease, while the Smart-Circ technology adds convenience by learning your usage patterns.
If budget is your primary concern, the CAMPLUX Climatech 7 provides exceptional value with modern features like AI-powered efficiency and a touchscreen interface. While it has a lower flow rate than premium models, it's more than adequate for standard bathtubs and represents excellent value for money.
For those with large soaking tubs or high-end bathroom setups, the Rinnai RX199iN delivers the performance needed with its 11.1 GPM flow rate and ability to support multiple fixtures simultaneously. The higher price is justified by the exceptional performance and efficiency.
Remember that proper installation is crucial for optimal performance. Always consult with professional installers to ensure your unit is sized correctly and installed according to local codes. With the right tankless water heater, you'll enjoy endless hot water for relaxing baths without the worry of running out mid-soak.