Few things feel more relaxing than soaking in a warm bathtub. But nothing ruins the moment faster than running out of hot water halfway through filling it. Traditional tank water heaters often fall short because they have a limited supply of pre-heated water. A tankless water heater solves this problem by heating water instantly, giving you all the hot water you need from start to finish.
This guide will show you the best tankless water heaters for bathtubs in 2025, explain how to choose the right one for your home, and offer simple installation and maintenance tips to keep it running for years. Whether you have a small soaking tub or a large whirlpool bath, there’s a perfect option for you.
Why Tankless Water Heaters Are Great for Bathtubs
- Unlimited hot water: No worrying about running out mid-fill
- Consistent temperature: Enjoy steady warmth from start to finish
- Space-saving: Wall-mounted units free up floor space in utility areas
- Energy savings: Only heats water when you turn on the tap
Quick Tips Before Choosing
Bathtubs require more hot water than showers. Most standard tubs hold 40–60 gallons, while larger soaker or whirlpool tubs can hold 70 gallons or more. The key factor is flow rate, measured in gallons per minute (GPM).
- Smaller tubs: 4–5 GPM capacity is usually enough
- Large or whirlpool tubs: Aim for 6–8+ GPM
- Colder climates: Increase capacity by 20–30% to offset incoming water temperature
Top Tankless Water Heaters for Bathtubs in 2025
1. Rinnai RU160iN (Natural Gas, Indoor)
This high-performance unit delivers up to 9 GPM, making it perfect for large bathtubs or multiple fixtures running at once. The Rinnai RU160iN features condensing technology for efficiency and a recirculation feature for faster hot water delivery.
- Best for: Large tubs or multiple bathroom homes
- Pros: High capacity, efficient, recirculation-ready
- Cons: Higher initial cost and professional installation required
2. Rheem RTGH-95DVLN (Natural Gas, Condensing)
Known for its high flow rate of up to 9.5 GPM, this unit can fill even the largest tubs without running cold. ENERGY STAR certified, it offers top efficiency and is built for long-term reliability.
- Best for: Households with frequent large tub use
- Pros: Powerful, energy-efficient, durable
- Cons: Requires venting and more space than smaller units
3. Stiebel Eltron Tempra 36 Plus (Electric)
For homes without gas lines, this powerful electric model delivers up to 7.5 GPM in warmer climates, which is enough for most standard bathtubs. It’s quiet, compact, and offers precise temperature control.
- Best for: Electric-only homes with medium to large tubs
- Pros: Sleek design, low maintenance, precise controls
- Cons: Lower GPM in cold regions
4. Noritz EZ111DV (Natural Gas)
This model is designed for high demand, delivering up to 11.1 GPM. It’s more than capable of filling a large bathtub while running other hot water fixtures at the same time.
- Best for: Luxury bathrooms and high-demand households
- Pros: Exceptional capacity, long warranty
- Cons: Premium price tag
5. EcoSmart ECO 27 (Electric)
A budget-friendly option for electric homes, this unit can supply up to 6.5 GPM in warm climates, which is suitable for standard tubs. It’s compact, easy to install, and energy-efficient.
- Best for: Smaller tubs and mild climates
- Pros: Affordable, compact, efficient
- Cons: May struggle with very large tubs or cold incoming water
How to Size for a Bathtub
To size correctly, calculate the total gallons your tub holds and how fast you want it filled. For example, a 60-gallon tub filled in 10 minutes requires at least 6 GPM. Factor in the incoming water temperature—colder water means the heater must work harder to reach your desired temperature.
Electric vs. Gas for Bathtub Use
Gas tankless heaters usually provide higher flow rates, making them ideal for large tubs or multiple simultaneous uses. They require venting and professional installation but excel in performance. Electric models are easier to install and maintain, but they may have lower flow rates in cold climates, making them better for standard or small tubs.
Installation Tips
- Install close to the bathroom for faster hot water delivery
- Upgrade gas lines or electrical panels if needed for high-capacity models
- Use a licensed plumber or electrician for safe installation
- Consider adding a recirculation pump for instant hot water
Maintenance Advice
To keep your tankless water heater performing at its best:
- Flush annually to remove mineral buildup
- Clean inlet screens regularly
- Inspect connections and vents for wear or leaks
Final Thoughts
If you want uninterrupted hot water for your bathtub, choose a tankless water heater that matches your tub size and climate. For large luxury tubs, the Rinnai RU160iN or Rheem RTGH-95DVLN offer high capacity and efficiency. For electric-only homes, the Stiebel Eltron Tempra 36 Plus is an excellent choice, while budget-conscious buyers may prefer the EcoSmart ECO 27.
With the right tankless water heater, you can enjoy long, relaxing soaks without worrying about running out of hot water. A well-chosen unit will keep your baths warm, your energy bills lower, and your home more comfortable year-round.