A tankless heater can provide a steady stream of hot water, but filling a bathtub places a bigger demand on it than a quick hand-washing or shower. The right choice depends on how fast you want the tub to fill, how cold your incoming water is, and whether your home can support a gas or high-power electric unit. Of these options, the Ranein and VancEpic gas models offer the strongest stated flow rates for bath filling. The small 110V electric units are better suited to modest, point-of-use demands than to quickly filling a large tub.
Use the comparison below as a starting point, then check the heater’s installation requirements and your home’s electrical or gas capacity before buying. Tankless flow can fall when incoming water is cold, so the listed maximum flow rates do not guarantee the same performance in every home.
Table of Contents
Quick Comparison
| Product | Fuel and power | Stated flow | Best fit |
|---|---|---|---|
| CNCEST 4,500W 110V Electric Tankless Heater | Electric, 110V | Not stated | Small point-of-use hot-water needs |
| Ranein 4.3 GPM Natural Gas Tankless Heater | Natural gas, 100,000 BTU | 4.3 GPM | Bath filling and multiple outlets, subject to conditions |
| Westinghouse 14kW Electric Tankless Heater | Electric, 240V | Up to 3.4 GPM under stated test conditions | Electric homes with suitable wiring and circuit capacity |
| Mahugde 4,500W 110V Electric Shower Heater | Electric, 110V | Not stated | Compact point-of-use installation |
| VancEpic 4.8 GPM Indoor Tankless Heater | Gas; fuel type needs confirmation | Up to 4.8 GPM | Higher-demand household use, if fuel and venting match |
Our Picks: Best Tankless Water Heaters For Bathtubs reviews
1. Ranein 4.3 GPM Natural Gas Tankless Heater: Best for Bath-Focused Controls
The Ranein is a 100,000-BTU indoor natural gas heater with a stated maximum flow of 4.3 gallons per minute. Its listed bathtub mode is a practical advantage for tub use: it supports a selectable fill volume from 50 to 800 liters. That feature may help you manage a fill, but it does not mean the unit can supply every tub at the same rate under all conditions.
The heater has four preset modes, a digital display, temperature memory, and a listed adjustable range of 95°F to 155°F. It also includes freeze protection and a vent pipe. The manufacturer recommends licensed installation and says the unit is not for elevations above 2,000 feet, so check both the installation instructions and local requirements before choosing it.
For bathtub use, its gas output and dedicated tub setting make it a more direct fit than the small 110V electric heaters below. Actual filling speed still depends on the incoming water temperature, desired bath temperature, and other fixtures drawing hot water at the same time.
Pros:
- 4.3 GPM stated flow and 100,000 BTU output
- Bathtub mode includes an adjustable fill-volume setting
- Temperature memory and a clear LED display
- Freeze protection and included vent pipe
Cons:
- Requires natural-gas compatibility and indoor venting
- Not intended for installations above 2,000 feet, according to the listing
Verdict: Choose the Ranein if you have natural gas, meet its installation limits, and want a heater with a specific bathtub mode.
2. VancEpic 4.8 GPM Indoor Gas Heater: Best Stated Flow for a Busy Home
The VancEpic is listed at up to 4.8 GPM and 110,000 BTU, the highest stated output in this group. The listing says it can serve three to five outlets, which may suit a home where a bathtub could be used while other hot-water fixtures are running. That capacity is not a promise of a particular tub-fill time: colder incoming water or several simultaneous draws can reduce available flow.
Its listed features include four preset temperature modes, a child lock, a copper heat exchanger, freeze protection, and a forced exhaust system. The product information is inconsistent about fuel: the title says propane, while the description calls it natural gas. Confirm the exact fuel version and approved installation setup with the seller and a qualified installer before purchase.
The stated 4.8 GPM makes this an appealing option on paper for a larger household, but the fuel ambiguity is a meaningful drawback. It also needs a suitable indoor exhaust arrangement, so it is not a simple plug-in replacement for an electric heater.
Pros:
- Highest stated flow rate in this comparison at up to 4.8 GPM
- 110,000 BTU output and stated support for multiple outlets
- Temperature presets and child lock
- Freeze protection and forced exhaust system
Cons:
- Listing conflicts on whether the unit is for propane or natural gas
- Requires professional attention to fuel, venting, and installation requirements
Verdict: Consider the VancEpic for higher hot-water demand only after confirming the fuel type and that its venting and installation requirements fit your home.
3. Westinghouse 14kW Electric Tankless Heater: Best Higher-Power Electric Option
This 14kW, 240V electric model is the most powerful electric option in this lineup. The listing states a maximum of 3.4 GPM when incoming water is 62°F and the outlet is set to 105°F, and says it can support two water points at those conditions. A tub can be one of those points, but the stated flow is not guaranteed if your inlet water is colder or another fixture is running.
The compact wall-mounted design has Wi-Fi controls, temperature memory, and a stated setting range of 86°F to 140°F. Its electrical requirements are significant: the listing calls for a dedicated 60A double-pole breaker and 6 AWG wire at 240V. Have an electrician confirm your panel and wiring can support it before buying; the heater should not be treated as a straightforward plug-in appliance.
Compared with the gas units, the Westinghouse avoids a gas connection but has a lower listed maximum flow than either gas model. Compared with the 110V heaters, it offers much higher stated output, though it also requires a substantial dedicated circuit.
Pros:
- Up to 3.4 GPM under the stated inlet and outlet temperatures
- 240V, 14kW design with compact wall mounting
- Wi-Fi controls and temperature memory
- ETL certification and listed safety protections
Cons:
- Requires a dedicated 60A double-pole breaker and 6 AWG wire
- Maximum flow depends on inlet temperature and simultaneous use
Verdict: This is the electric model to consider if your home can support its circuit requirements and its stated flow suits your tub and household use.
4. CNCEST 4,500W 110V Electric Heater: Best for Small Point-of-Use Needs
The CNCEST is a compact 4,500W, 110V electric heater with an LED temperature display and adjustable settings from 86°F to 131°F. The listing describes it as an on-demand heater for bath and shower use and says it has a thermostat intended to maintain the selected temperature.
For bathtub filling, the key limitation is its modest power rating compared with the 14kW electric and gas models in this guide. The listing does not state a flow rate, so there is no clear basis for estimating how quickly it can fill a tub or how much it can raise the water temperature at a given flow. Treat it as a possible small point-of-use option, not a proven fast tub-filling solution.
Its listed protections include over-temperature and leakage safeguards, water-electricity isolation, and an IPX4 rating. The product data specifies 40.9A rated current, so verify that the wiring, breaker, and installation location are appropriate with a qualified electrician. Do not assume that a standard outlet can safely power it.
Pros:
- Compact 110V design with an LED display
- Adjustable temperature range of 86°F to 131°F
- Thermostat and several listed protection features
Cons:
- No flow-rate figure is provided, making tub performance hard to judge
- Its 4,500W rating is far below the higher-output choices here
Verdict: Choose the CNCEST only for modest point-of-use needs after confirming electrical compatibility; it is not the strongest choice for quickly filling a bathtub.
5. Mahugde 4,500W 110V Electric Heater: Best Compact Basic Setup
The Mahugde is another 4,500W, 110V electric heater aimed at bathroom, bathtub, and RV use. It has an adjustable 86°F to 131°F range, a temperature-holding thermostat, and a compact design intended for vertical mounting. The listing also describes 1/2-inch connections and a low starting water-pressure range.
Its small format may suit a location where a compact point-of-use heater is preferred, but the listing gives no flow rate. As with the CNCEST, that makes it difficult to judge performance for a full bathtub. A 4,500W unit should not be assumed to match the tub-filling capacity of the larger gas heaters or the 14kW electric model.
The product description mentions tempered glass, stainless steel heating materials, and power isolation. It does not provide the same detailed output information as the higher-powered options, so confirm the electrical requirements, plumbing connections, and intended use with the seller or a qualified installer before relying on it for a bathtub.
Pros:
- Compact design intended for vertical installation
- Temperature adjustment from 86°F to 131°F
- Listed 1/2-inch water connections
- Power-off memory function
Cons:
- No stated flow rate or clear tub-fill performance figure
- Lower power than the 14kW electric and gas models
Verdict: The Mahugde may fit a compact point-of-use setup, but choose a higher-output model if your main goal is filling a bathtub quickly.
How to Choose a Tankless Water Heater for a Bathtub
Start with your tub and desired fill time
A large soaking tub needs more hot water than a small standard tub. Estimate the amount of water you typically use and how quickly you want the tub to fill. A heater’s maximum flow rate is only useful if it can deliver water at a suitable temperature, so compare the product’s stated flow conditions with your actual needs rather than relying on the highest number alone.
Account for incoming water temperature
Tankless heaters have to raise the temperature of incoming water as it flows through the unit. Colder inlet water can reduce the flow available at a comfortable bath temperature. If your water is cold for much of the year, a heater that appears to have enough capacity under warmer test conditions may deliver less flow in practice.
Choose gas or electric based on your home
Gas models can offer higher stated flow in this group, but they need the correct fuel supply and a suitable exhaust setup. Electric models avoid a gas connection, but higher-output units may need substantial dedicated wiring and breaker capacity. Ask a licensed plumber or electrician to check your home before you buy; installation requirements can affect the total project and whether a model is suitable at all.
Check the flow rate and the conditions behind it
Look for a stated flow rate and note the inlet and outlet temperatures used to calculate it. The Westinghouse listing, for example, states up to 3.4 GPM at a 62°F inlet and a 105°F outlet. The two gas options list higher maximum rates, but actual performance will still depend on water temperature, plumbing, and other fixtures in use. Listings without a flow figure are harder to assess for bathtub filling.
Consider simultaneous hot-water use
If someone may shower or use a sink while the tub is filling, the heater must serve those demands at the same time. A unit’s stated number of supported outlets can help with an initial comparison, but it does not mean every outlet will receive maximum flow simultaneously. Think about your household’s usual routine, then select enough capacity for that pattern.
Review controls and safety features
Temperature displays, presets, memory settings, child locks, and tub modes can make operation more convenient. Safety features such as overheat protection and freeze protection are useful, but they do not replace correct installation or regular care. Check which features are actually included with the model and follow its instructions, especially for indoor gas equipment and bathroom installations.
Confirm fuel, location, and installation limits
Before ordering, confirm the exact fuel type, venting needs, electrical rating, water connections, and any elevation or freeze-related limits. This is especially important for the VancEpic, whose product information conflicts about propane versus natural gas. Have a qualified professional confirm that the model is approved for your home and local code requirements.
Frequently Asked Questions
Can a tankless water heater fill a bathtub?
Yes, if its output can provide enough hot water at the flow and temperature your tub requires. Check the heater’s flow data and remember that colder incoming water and other active fixtures can reduce available flow.
Which option in this guide has the highest stated flow?
The VancEpic lists up to 4.8 GPM, followed by the Ranein at 4.3 GPM. These are stated maximums, not guaranteed flow rates for every home. Confirm the VancEpic’s fuel type before considering it.
Is a 110V, 4,500W heater a good choice for a large bathtub?
It may not be the best fit for quickly filling a large tub. The two 110V models in this guide do not provide a flow-rate figure, and their stated power is much lower than the other options. Ask the manufacturer or an installer to confirm whether a particular unit can meet your tub’s needs.
Will a tankless heater provide its maximum flow all year?
Not necessarily. Maximum flow depends in part on how much the heater must raise the incoming water temperature. When inlet water is colder, the available flow at the desired bath temperature may be lower.
Can I install one of these heaters myself?
Installation depends on the model and your home’s plumbing, electrical, gas, and venting setup. The Ranein listing recommends licensed installation, and the Westinghouse has specific breaker and wire requirements. Use qualified professionals and follow local codes.
What should I check before buying the VancEpic?
Confirm whether the exact model is designed for propane or natural gas. Its listing uses both fuel descriptions, so verify the product configuration, fuel supply, venting requirements, and installation approval before purchase.
Final Recommendation
For a bathtub-focused setup, the Ranein is the clearest match if your home has natural gas and meets its installation limits, thanks to its stated 4.3 GPM flow and dedicated bathtub mode. The VancEpic has the highest stated flow, but its conflicting fuel information makes confirmation essential. If you need an electric model, the Westinghouse offers substantially higher stated output than the 110V units, provided your home can support its dedicated circuit. The CNCEST and Mahugde are better treated as compact point-of-use choices than as proven fast-fill solutions for a large tub.
