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Finding the best indoor tankless water heaters requires matching your home’s fuel source and electrical capacity to the unit’s output. We compared 15 models based strictly on the specifications published on their listings. The Thermomate 18kW Electric Tankless Water Heater leads because its 240V/18kW rating delivers reliable heat in compact spaces. This guide helps you decide which unit fits your household’s flow needs and installation constraints.
01 Match Fuel Source to Home Infrastructure
Your first decision is fuel type. Electric units like the Thermomate 18kW require a 240V circuit and suit single-bathroom setups. Gas models, such as the VancEpic 4.8 GPM Natural Gas heater, deliver higher flow rates for multiple fixtures. Check your existing gas lines or electrical capacity before choosing. This determines which units fit your home's infrastructure.
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | Thermomate 18kW Electric Tankless Water Heater Best overall | compact indoor electric hot water needs | |
| 2 | VancEpic 4.8 GPM Natural Gas Tankless Heater Best budget | Indoor natural gas households needing 4.8 GPM | |
| 3 | Ranein 4.3 GPM Indoor Propane Tankless Water Heater Also great | Indoor propane heating for 3-4 fixtures | |
| 4 | APUS 95,000 BTU 4.42 GPM Indoor Propane Heater Also great | 2-3 fixture indoor propane setups | |
| 5 | Rinnai RE140iP Propane 5.3 GPM Also great | Indoor propane homes needing 5.3 GPM | |
| 6 | ThermoMate 4 GPM Propane Tankless Water Heater Best bathroom | single bathroom propane installations | |
| 7 | APUS 18kW 4.4 GPM Tankless Water Heater Also great | space-saving electric hot water for 2-3 fixtures | |
| 8 | Rheem RTG 9.5 GPM Natural Gas Water Heater Best premium | large households needing high flow rates | |
| 9 | MIZUDO 10.3 GPM Propane Tankless Heater Also great | Large homes needing high flow and smart control | |
| 10 | VEVOR 4.3 Gal Natural Gas Tankless Water Heater Also great | moderate hot water needs with precise control | |
| 11 | GASLAND 7.6 GPM Propane Tankless Water Heater Also great | high-flow propane whole-house needs | |
| 12 | APUS 140,000 BTU Indoor Propane Tankless Water Heater Also great | whole-home hot water demand | |
| 13 | Novieex 4.3 GPM Indoor Natural Gas Water Heater Also great | Standard indoor homes needing moderate flow | |
| 14 | SNDOAS 102,000 BTU Natural Gas Tankless Heater Also great | Precise temperature control in compact indoor spaces | |
| 15 | Rheem RTGH 6.8 GPM Indoor Natural Gas Tankless Heater Also great | Efficient whole-house heating in tight spaces |
02 The best fit for you
- You need the most compact electric unit – APUS 18kW 4.4 GPM Tankless Water Heater – The APUS model weighs just 15.2 pounds and measures 11.4 by 13.7 inches.
- You want the best overall performance – Thermomate 18kW Electric Tankless Water Heater – This Thermomate unit offers 18 kW power with a compact 13.1-inch width.
- You require high flow for large homes – Rheem RTG 9.5 GPM Natural Gas Water Heater – The Rheem heater delivers 9.5 GPM with WiFi connectivity for smart control.
- You need a budget-friendly gas option – VancEpic 4.8 GPM Natural Gas Tankless Heater – This VancEpic model provides 4.8 GPM flow at a competitive price point.
- You want precise control in small spaces – SNDOAS 102,000 BTU Natural Gas Tankless Heater – The SNDOAS unit features 102,000 BTU heating in a compact 13.8-inch width.
Thermomate 18kW Electric Tankless Water Heater
Best for: compact indoor electric hot water needs
- Installation Location
- Indoor
- Fuel Type
- Electric
- Max Flow Rate
- Up to 4.4GMP
- Heating Capacity
- 18kW
- Voltage
- 240 Volt
- Temperature Range
- 80°F to 140°F
This 18kW electric unit delivers up to 4.4 GPM of hot water at 240V. It heats water in 3 seconds using self-modulating technology for 99.8% efficiency. The digital display lets you adjust temperature from 80°F to 140°F in 1°F increments. Its compact wall-mounted design saves space. You need a dedicated 80A circuit with two 40A breakers and 8AWG wire for proper installation.
- 18kW heating capacity for instant hot water
- High 99.8% energy efficiency rating
- Digital display with 1°F adjustment precision
- Compact wall-mounted build saves floor space
- Requires a substantial 80A electrical circuit
- Lower flow rate than gas models on the list
- Smaller heating capacity than high-output gas units
- You have a 240V electrical panel with spare capacity
- You need a compact unit for tight indoor spaces
- You want precise digital temperature control
- You only have 120V electrical outlets available
- You need high flow rates for multiple simultaneous showers
- You prefer gas fuel for lower electrical demands
Verdict: Thermomate 18kW Electric Tankless Water Heater fits homes with strong electrical infrastructure seeking space-saving efficiency. It suits single-bathroom setups where precise temperature control matters more than high-volume flow.
VancEpic 4.8 GPM Natural Gas Tankless Heater
Best for: Indoor natural gas households needing 4.8 GPM
- Installation Location
- Indoor
- Fuel Type
- Natural Gas
- Max Flow Rate
- 4.8 GPM
- Heating Capacity
- 110,000 BTU
- Fixture Capacity
- 3-5 water outlets
- Features
- Tankless, Child Safety Lock, LED Display
This VancEpic unit delivers 4.8 GPM of hot water at 110,000 BTU for indoor natural gas lines. It features an oxygen-free copper heat exchanger for efficient heat transfer and maintains temperature within ±1°F. The compact 24.6-inch height fits standard spaces. It supports 3-5 outlets, making it suitable for family homes needing reliable on-demand heating.
- 110,000 BTU heating capacity for strong performance
- 4.8 GPM max flow rate supports multiple fixtures
- ±1°F precise temperature control reduces fluctuations
- Oxygen-free copper heat exchanger improves durability
- Child safety lock adds family-friendly protection
- 110,000 BTU is lower than 199,000 BTU competitors
- 4.8 GPM flow rate is lower than 9.5 GPM models
- 25.4-pound weight is heavier than some compact units
- You need a natural gas unit for indoor use
- You want precise temperature control for comfort
- You require support for 3-5 water outlets
- You need a flow rate above 5 GPM
- You prefer propane fuel over natural gas
- You want the highest possible BTU output
Verdict: VancEpic 4.8 GPM Natural Gas Tankless Water Heater fits homes needing steady hot water for multiple fixtures without the bulk of larger units. It suits buyers prioritizing temperature stability and copper durability over maximum flow speed.
Ranein 4.3 GPM Indoor Propane Tankless Water Heater
Best for: Indoor propane heating for 3-4 fixtures
- Installation Location
- Indoor
- Fuel Type
- Propane
- Max Flow Rate
- 4.3 GPM
- Heating Capacity
- 100,000 BTU
- Fixture Capacity
- 3-4 water points
- Temperature Range
- 95°F-155°F
The Ranein 4.3 GPM propane tankless water heater delivers 100,000 BTU of heating power for indoor use. It supports three to four simultaneous water points, making it suitable for whole-home applications. The unit features an oxygen-free copper heat exchanger and precise temperature control between 95°F and 155°F. It includes dedicated anti-freeze protection and an LED display for real-time monitoring. This model is designed for users seeking reliable on-demand hot water with safety-focused engineering.
- 100,000 BTU heating capacity for efficient on-demand heating
- 4.3 GPM flow rate supports three to four water points
- Oxygen-free copper heat exchanger enhances durability and heat transfer
- Intelligent preset modes for kitchen, bathtub, shower, and 4-season use
- Anti-freeze protection down to 14°F prevents pipe damage
- 4.3 GPM flow rate is lower than the 5.3 GPM and 9.5 GPM options available in this list
- Maximum temperature of 155°F is lower than the 194°F range found in some competing models
- Limited to indoor installation only, excluding outdoor placement options
- You need a propane heater for three to four simultaneous fixtures
- You require an indoor unit with anti-freeze protection for cold climates
- You want precise temperature control with preset modes for different uses
- You need a flow rate higher than 4.3 GPM for large households
- You require an outdoor-rated unit for exterior installation
- You prefer natural gas over propane as your fuel source
Verdict: Ranein 4.3 GPM Propane Tankless Heater offers solid mid-range performance for homes needing three to four fixtures. It fits buyers who prioritize safety features and indoor propane heating over maximum flow capacity.
APUS 95,000 BTU 4.42 GPM Indoor Propane Heater
Best for: 2-3 fixture indoor propane setups
- Installation Location
- Indoor
- Fuel Type
- Propane
- Max Flow Rate
- 4.42 GPM
- Heating Capacity
- 95,000 BTU
- Voltage
- 120V
- Fixture Capacity
- 2–3 Fixtures
This indoor propane tankless water heater delivers 95,000 BTU and up to 4.42 GPM. It supports 2–3 fixtures with steady temperature control within ±1°F. The unit features a 98% thermal efficiency rating and operates at 36dB. Its compact design fits tight spaces while providing endless hot water for showers and sinks.
- 95,000 BTU heating capacity
- 4.42 GPM max flow rate
- 98% thermal efficiency
- 36dB quiet operation
- LED touchscreen interface
- 95,000 BTU is lower than many competitors reaching 199,000 BTU
- 4.42 GPM max flow is modest compared to units offering 9.5 GPM
- 2-year warranty is shorter than typical industry standards
- You need reliable hot water for 2–3 fixtures
- You prefer a compact unit with an LED display
- You want steady temperature control within ±1°F
- You require high flow rates above 6 GPM
- You need maximum heating power over 100,000 BTU
- You prefer a longer warranty than 2 years
Verdict: APUS 95k BTU Propane Indoor Heater suits households needing steady hot water for multiple fixtures without high flow demands. Its compact size and quiet operation make it ideal for tight indoor spaces.
Rinnai RE140iP Propane 5.3 GPM
Best for: Indoor propane homes needing 5.3 GPM
- Installation Location
- Indoor
- Fuel Type
- Propane
- Max Flow Rate
- 5.3 GPM
- Heating Capacity
- 140k BTU
- Fixture Capacity
- 5 fixtures
- Features
- Tankless
This Rinnai indoor propane tankless water heater delivers up to 5.3 gallons per minute at 140,000 BTU. It supports five simultaneous fixtures and measures 27.3 inches tall. The compact unit weighs 48.5 pounds and includes an LED screen for error codes. It is designed for homes needing reliable hot water without the bulk of a traditional tank.
- Provides up to 5.3 GPM flow rate
- 140,000 BTU heating capacity
- Supports five simultaneous fixtures
- 15-year heat exchanger warranty
- 5.3 GPM is lower than higher-capacity models on this list
- 140,000 BTU is lower than some competing units
- WiFi control module is sold separately
- You need a reliable indoor propane unit for up to five fixtures
- You prefer a compact 27.3-inch tall design
- You want a 15-year heat exchanger warranty
- You require a flow rate above 5.3 GPM
- You need a unit with built-in WiFi control
- You are looking for a natural gas model
Verdict: Rinnai RE140iP Propane 5.3 GPM is a solid choice for households that need dependable indoor propane heating for multiple bathrooms. It fits well where space is limited but consistent hot water is essential.
ThermoMate 4 GPM Propane Tankless Water Heater
Best for: single bathroom propane installations
- Installation Location
- Indoor
- Fuel Type
- Propane
- Max Flow Rate
- Up to 4 GPM
- Heating Capacity
- 98,500 BTU
- Voltage
- 120V
- Fixture Capacity
- 1 shower and 1 faucet
The ThermoMate 98,500 BTU propane unit delivers up to 4 GPM for one shower and one faucet. It features a 98% thermal efficiency rating and precise temperature control within ±1°F. The compact 21.7-inch height fits indoor spaces well. CSA certification ensures safety for residential bathroom use. It includes a smart display for monitoring flow and flame status.
- 98% thermal efficiency with a 0.81 UEF rating
- Precise temperature stability within ±1°F
- CSA certified for safety and reliability
- Compact 21.7-inch height saves space
- Lower 4 GPM flow rate compared to higher-capacity models
- Heavier 21.2-pound build than lighter electric units
- Propane fuel type limits placement options
- You need a compact unit for a single bathroom
- You want precise temperature control for comfort
- You have existing propane connections for indoor use
- You require flow rates above 4 GPM
- You prefer natural gas over propane
- You need a unit for multiple simultaneous fixtures
Verdict: ThermoMate 98,500 BTU Propane Indoor Heater suits small households needing reliable hot water for one shower and one faucet. Its compact size and high efficiency make it ideal for tight indoor bathroom spaces.
APUS 18kW 4.4 GPM Tankless Water Heater
Best for: space-saving electric hot water for 2-3 fixtures
- Installation Location
- Indoor
- Fuel Type
- Electric
- Max Flow Rate
- Up to 4.4 GPM
- Heating Capacity
- 18kW
- Voltage
- 240V
- Fixture Capacity
- 2–3 Points
The APUS 18kW unit delivers up to 4.4 GPM at 240V for indoor use. Its slim 3.54-inch depth saves space, while 99.8% thermal efficiency reduces standby loss. You get precise temperature control from 86°F to 131°F in 1°F increments. This model suits households needing reliable instant hot water for two to three fixtures simultaneously.
- 18kW heating capacity provides up to 4.4 GPM flow
- 99.8% thermal efficiency with self-modulating technology
- Precise digital control from 86°F to 131°F in 1°F steps
- Ultra-slim 3.54-inch depth saves wall space
- Lower maximum flow rate of 4.4 GPM compared to gas models offering 5.3 to 10.3 GPM
- Requires 240V power with two 40A breakers and 8 AWG wiring
- Heavier build at 15.2 pounds compared to lighter gas units
- You need a compact electric heater for a small to medium household
- You want precise temperature settings in 1°F increments
- You have the required 240V electrical infrastructure installed
- You require high flow rates above 5 GPM for multiple bathrooms
- You only have 120V outlets available without electrical upgrades
- You prefer gas units with higher BTU outputs
Verdict: APUS 18kW Electric Tankless Water Heater fits users who prioritize energy efficiency and space-saving design over high-volume flow. It is ideal for homes with existing 240V circuits needing reliable hot water for two to three fixtures.
Rheem RTG 9.5 GPM Natural Gas Water Heater
Best for: large households needing high flow rates
- Installation Location
- Indoor
- Fuel Type
- Natural Gas
- Max Flow Rate
- 9.5 GPM
- Heating Capacity
- 199,900 BTU/hr
- Fixture Capacity
- 3-4 simultaneous showers
- Features
- Tankless
This Rheem indoor tankless heater delivers up to 9.5 GPM at a 35°F rise, powered by 199,900 BTUs of natural gas. It supports three to four simultaneous showers for large households. The unit features a non-condensing design with an 0.82 UEF and includes WiFi connectivity for digital temperature control. It is a wall-mount system built for whole-home use.
- 9.5 GPM max flow rate for high demand
- 199,900 BTU/hr heating capacity
- 0.82 Uniform Energy Factor rating
- WiFi enabled with LED control panel
- 15-year heat exchanger warranty
- Heavier at 46.5 pounds than many competitors
- Requires professional installation for indoor gas setup
- Non-condensing design limits efficiency compared to condensing models
- You need hot water for three to four bathrooms simultaneously
- You want WiFi control for temperature adjustments
- You prefer a high-capacity natural gas unit
- You are looking for a propane-only system
- You need a compact unit under 40 pounds
- You want a DIY-friendly electric installation
Verdict: Rheem RTG-95DVELN-3 9.5 GPM Natural Gas suits large homes needing maximum flow and smart controls. It fits buyers who prioritize capacity and long-term warranty coverage over compact size.
MIZUDO 10.3 GPM Propane Tankless Heater
Best for: Large homes needing high flow and smart control
- Installation Location
- Indoor
- Fuel Type
- Propane Gas
- Max Flow Rate
- 10.3 GPM
- Heating Capacity
- 199,000 BTU
- Voltage
- 120V
- Fixture Capacity
- 5-7 Outlets
This MIZUDO propane unit delivers 10.3 GPM at 199,000 BTU, supporting 5-7 fixtures. It features ±1°F temperature precision and WiFi app control. The sealed combustion system ensures safe indoor use. You get a robust whole-home solution for busy households. The 2-year warranty and 5-year heat exchanger coverage provide long-term reliability for your hot water needs.
- 199,000 BTU heating capacity
- 10.3 GPM maximum flow rate
- ±1°F temperature control precision
- WiFi app and Alexa compatibility
- Sealed direct-vent combustion system
- Heavier at 55.5 pounds than most competitors
- Higher 199,000 BTU input requires robust gas lines
- Not recommended for elevations above 3,000 feet
- You need simultaneous hot water for 5-7 fixtures
- You want precise temperature control for soaking tubs
- You prefer WiFi monitoring and smart home integration
- You live above 3,000 feet elevation
- You have limited gas line capacity for high BTU input
- You need a lighter unit for easy mounting
Verdict: MIZUDO 199k BTU Propane Tankless fits large homes needing high flow and smart control, provided you can handle its weight and elevation limits.
VEVOR 4.3 Gal Natural Gas Tankless Water Heater
Best for: moderate hot water needs with precise control
- Installation Location
- Indoor
- Fuel Type
- Gas
- Heating Capacity
- 100,000 BTU/h
- Installation Type
- Whole House
- Features
- Tankless
- Dimensions
- 13.8 × 13.8 × 21.7 inches
The VEVOR 4.3 Gal unit delivers 100,000 BTU/h of heating capacity for indoor use. It features ±1℉ precision temperature control and an LED display for real-time monitoring. The system includes multiple security protections against overheating and dry burning. It uses standard 1/2 inch NPT connectors for straightforward installation. This model suits households seeking reliable instant hot water with consistent temperature management.
- 100,000 BTU/h heating capacity
- ±1℉ precision temperature control
- LED display with touch panel
- Multiple security protections included
- 4.3 Gal capacity is lower than the 10.3 Gal option on this list
- 100,000 BTU output is less than the 199,000 BTU model available
- You need a reliable indoor gas heater with precise temperature control
- You want a unit with clear LED status monitoring
- You prefer a system with comprehensive safety protections
- You require a flow rate higher than 4.3 GPM
- You need a heating output exceeding 100,000 BTU
- You are looking for a propane-only configuration
Verdict: VEVOR 4.3 Gal Gas Water Heater offers steady performance for homes needing moderate hot water volume. It fits buyers who prioritize temperature stability and safety features over maximum heating power.
GASLAND 7.6 GPM Propane Tankless Water Heater
Best for: high-flow propane whole-house needs
- Installation Location
- Indoor
- Fuel Type
- Propane
- Max Flow Rate
- 7.6 GPM
- Heating Capacity
- 199000 BTU
- Installation Type
- Whole House
- Features
- Tankless
This GASLAND indoor propane tankless water heater delivers up to 7.6 GPM of hot water using 199,000 BTU of heating capacity. It features a compact 14.3 by 24.7 inch design that mounts easily on walls. The unit carries CSA certification and offers a UEF of 0.8. It suits whole-house applications where high flow rates and propane fuel are required.
- High 7.6 GPM flow rate
- 199,000 BTU heating capacity
- CSA certified for safety
- Compact 37.5 lb build
- Propane fuel type limits placement
- 7.6 GPM is lower than the 9.5 GPM Rheem RTG Series
- 199,000 BTU is lower than the 199,000 BTU MIZUDO unit
- You need high flow rates for multiple fixtures
- You have access to propane fuel
- You want CSA certified safety features
- You require natural gas fuel
- You need flow rates above 7.6 GPM
- You are installing outdoors only
Verdict: GASLAND E Propane 7.6 GPM provides strong heating power for whole-house use. It fits homes with propane supply that prioritize flow rate over maximum BTU output.
APUS 140,000 BTU Indoor Propane Tankless Water Heater
Best for: whole-home hot water demand
- Installation Location
- Indoor
- Fuel Type
- Propane
- Max Flow Rate
- Up to 6.8 GPM
- Heating Capacity
- 140,000 BTU
- Voltage
- 120V AC
- Fixture Capacity
- 4–5 Points
This indoor propane unit delivers up to 6.8 GPM at 140,000 BTU. It supports 4–5 water points with a built-in booster pump that increases flow by 130%. The LED display allows manual settings from 95–140°F, while the Tuya app enables remote monitoring. A high UEF of 0.81 and ±1°F control ensure consistent performance for whole-home use.
- 140,000 BTU heating capacity
- Up to 6.8 GPM max flow rate
- Built-in smart booster pump
- Tuya app remote control
- High 0.81 UEF efficiency
- Requires 120V AC power for internal components
- Heavier at 26.9 pounds
- Larger footprint than compact models
- You need hot water for 4–5 fixtures
- You want remote monitoring via app
- You require high flow rates
- You only need a single sink
- You prefer natural gas over propane
- You have limited electrical outlets
Verdict: APUS 140k BTU Propane Tankless Heater suits households needing high-volume hot water with smart features. It fits homes with propane access and space for a 26.9-pound unit.
Novieex 4.3 GPM Indoor Natural Gas Water Heater
Best for: Standard indoor homes needing moderate flow
- Installation Location
- Indoor
- Fuel Type
- Natural Gas
- Max Flow Rate
- Up to 4.3 GPM
- Heating Capacity
- 100,000 BTU
- Voltage
- 110V-120V AC
- Fixture Capacity
- 2-4 points of use
This Novieex unit delivers up to 4.3 GPM of hot water using 100,000 BTU of natural gas power. It supports 2-4 points of use simultaneously, making it suitable for standard household demands. The device features a wide temperature range from 95°F to 149°F with precise control. Its compact, wall-mounted design saves space in utility rooms or closets. You get reliable on-demand heating with an LED touchscreen interface for easy operation.
- Delivers up to 4.3 GPM flow rate
- 100,000 BTU heating capacity
- Adjustable temperature range from 95°F to 149°F
- Wall-mounted design saves over 50% space
- Includes LED touchscreen interface
- 100,000 BTU is lower than several competitors reaching 199,000 BTU
- 4.3 GPM flow rate is modest compared to models offering up to 10.3 GPM
- Recommended for use below 2000 feet altitude
- You need hot water for 2-4 fixtures at once
- You have an indoor space for wall mounting
- You prefer natural gas fuel sources
- You require high flow rates above 5 GPM
- You live at elevations above 2000 feet
- You need a unit with higher BTU output
Verdict: Novieex 4.3 GPM Natural Gas Indoor Heater suits households needing moderate simultaneous hot water access. It fits well in standard indoor installations where space is limited and natural gas is available.
SNDOAS 102,000 BTU Natural Gas Tankless Heater
Best for: Precise temperature control in compact indoor spaces
- Installation Location
- Indoor
- Fuel Type
- Natural Gas
- Max Flow Rate
- 4.75 GPM
- Heating Capacity
- 102,000 BTU
- Temperature Range
- 95°F–149°F
- Installation Type
- Whole House
This indoor natural gas unit delivers up to 4.75 GPM at 102,000 BTU. It features an oxygen-free copper heat exchanger for efficient heating. The system maintains water temperature within ±1°F of your set point. A compact design fits wall-mounted spaces. It includes anti-freeze protection and overheat safety. This model suits households needing reliable hot water for simultaneous use.
- 4.75 GPM max flow rate for multiple fixtures
- 102,000 BTU heating capacity for rapid heating
- ±1°F precise temperature control stability
- Oxygen-free copper heat exchanger for durability
- Built-in anti-freeze protection for cold climates
- 102,000 BTU is lower than the 199,000 BTU option on this list
- 4.75 GPM flow rate is lower than the 9.5 GPM Rheem model
- Indoor-only installation limits placement flexibility
- You need a compact unit for tight indoor spaces
- You want precise temperature control within ±1°F
- You require anti-freeze protection for cold environments
- You need a flow rate above 5 GPM
- You require a higher BTU capacity for large homes
- You are looking for an outdoor-rated unit
Verdict: SNDOAS 4.75 GPM Natural Gas Tankless Water Heater provides stable hot water for small to medium households. It fits users who prioritize precise temperature stability and compact indoor installation over maximum flow capacity.
Rheem RTGH 6.8 GPM Indoor Natural Gas Tankless Heater
Best for: Efficient whole-house heating in tight spaces
- Installation Location
- Indoor
- Fuel Type
- Natural Gas
- Max Flow Rate
- 6.8 GPM
- Heating Capacity
- 120,000 BTU/hr
- Fixture Capacity
- 2-3 simultaneous showers
- Features
- Tankless
This Rheem unit delivers 6.8 GPM using 120,000 BTU of natural gas. Its condensing technology achieves a 0.93 UEF, which is higher than most non-condensing competitors. The compact 17.7-inch width fits tight indoor spaces. You get precise digital temperature control. This model suits homes needing reliable whole-house heating with strong efficiency ratings.
- 0.93 Uniform Energy Factor for low gas bills
- 120,000 BTU/hr heating capacity
- 6.8 GPM flow rate for 2-3 showers
- Compact 17.7-inch width
- 59.9 pounds weight is heavier than many peers
- 120,000 BTU is lower than high-output models reaching 199,000 BTU
- 6.8 GPM capacity is less than 9.5 GPM options
- You prioritize high energy efficiency (0.93 UEF) over maximum flow rate
- You have a narrow installation space (17.7 inches wide)
- You need precise temperature control (±1°F)
- You require the highest possible flow rate (9.5+ GPM)
- You prefer a lighter unit (under 50 pounds)
- You want built-in WiFi connectivity
Verdict: Rheem RTGH-68DVLN Natural Gas 6.8 GPM is a compact, high-efficiency choice for homes with space constraints. It fits best where energy savings and precise temperature control matter more than maximum flow volume.
03 Specifications side by side
| Spec | Thermomate 18kW Electric Tankless Water Heater | VancEpic 4.8 GPM Natural Gas Tankless Heater | Ranein 4.3 GPM Indoor Propane Tankless Water Heater | APUS 95,000 BTU 4.42 GPM Indoor Propane Heater | Rinnai RE140iP Propane 5.3 GPM | ThermoMate 4 GPM Propane Tankless Water Heater | APUS 18kW 4.4 GPM Tankless Water Heater | Rheem RTG 9.5 GPM Natural Gas Water Heater | MIZUDO 10.3 GPM Propane Tankless Heater | VEVOR 4.3 Gal Natural Gas Tankless Water Heater | GASLAND 7.6 GPM Propane Tankless Water Heater | APUS 140,000 BTU Indoor Propane Tankless Water Heater | Novieex 4.3 GPM Indoor Natural Gas Water Heater | SNDOAS 102,000 BTU Natural Gas Tankless Heater | Rheem RTGH 6.8 GPM Indoor Natural Gas Tankless Heater |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Fuel Type | Electric | Natural Gas | Propane | Propane | Propane | Propane | Electric | Natural Gas | Propane Gas | Gas | Propane | Propane | Natural Gas | Natural Gas | Natural Gas |
| Max Flow Rate | Up to 4.4GMP | 4.8 GPM | 4.3 GPM | 4.42 GPM | 5.3 GPM | Up to 4 GPM | Up to 4.4 GPM | 9.5 GPM | 10.3 GPM | – | 7.6 GPM | Up to 6.8 GPM | Up to 4.3 GPM | 4.75 GPM | 6.8 GPM |
| Heating Capacity | 18kW | 110,000 BTU | 100,000 BTU | 95,000 BTU | 140k BTU | 98,500 BTU | 18kW | 199,900 BTU/hr | 199,000 BTU | 100,000 BTU/h | 199000 BTU | 140,000 BTU | 100,000 BTU | 102,000 BTU | 120,000 BTU/hr |
| Installation Location | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor | Indoor |
| Voltage | 240 Volt | – | – | 120V | – | 120V | 240V | – | 120V | – | – | 120V AC | 110V-120V AC | – | – |
| Fixture Capacity | – | 3-5 water outlets | 3-4 water points | 2–3 Fixtures | 5 fixtures | 1 shower and 1 faucet | 2–3 Points | 3-4 simultaneous showers | 5-7 Outlets | – | – | 4–5 Points | 2-4 points of use | – | 2-3 simultaneous showers |
| Temperature Range | 80°F to 140°F | – | 95°F-155°F | 95–140°F | – | 90–140°F | 86–131°F | – | 95-149°F | – | – | 95–140°F | 95°F to 149°F | 95°F–149°F | – |
| Installation Type | – | – | – | – | – | – | – | – | – | Whole House | Whole House | – | – | Whole House | – |
| Features | Tankless | Tankless, Child Safety Lock, LED Display | Tankless | Tankless | Tankless | Tankless | Tankless, LED Display | Tankless | Tankless | Tankless | Tankless | Tankless | Tankless | Tankless | Tankless |
| Application | Shower | – | – | – | – | Bathroom | – | – | – | – | – | – | – | – | – |
| Wattage | 18 kW | – | – | – | – | – | 18 kW | – | – | – | – | – | – | – | – |
| Intended Use | – | – | Home | – | – | – | – | – | Home | – | – | Home | – | – | – |
| Dimensions | 13.1 × 13.1 × 17.1 inches | 14.4 × 14.4 × 24.6 inches | 14.2 × 14.2 × 22.5 inches | 5.9 × 5.9 × 20.6 inches | 10.8 × 14.1 × 27.3 inches | 13.8 × 13.8 × 21.7 inches | 11.4 × 11.4 × 13.7 inches | 10.3 × 13.5 × 24.2 inches | 15.9 × 15.9 × 24.8 inches | 13.8 × 13.8 × 21.7 inches | 14.3 × 14.3 × 24.7 inches | 7.09 × 7.09 × 21.3 inches | 28.5 × 17.3 × 10.8 inches | 13.8 × 13.8 × 21.6 inches | 9.8 × 17.7 × 23.7 inches |
| Weight | – | 25.4 pounds | – | 19.2 pounds | 48.5 pounds | 21.2 pounds | 15.2 pounds | 46.5 pounds | 55.5 pounds | 19.8 pounds | 37.5 pounds | 26.9 pounds | 19.8 pounds | – | 59.9 pounds |
| Size | 240V/18kW | 4.8GPM, 110,000BTU, NG | 4.3GPM Indoor | – | 5.3 GPM | – | – | 9.5 GPM (WiFi) | – | 4.3 Gal | 7.6 GPM | – | 4.3GPM Nature gas | 4.75GPM 102000BTU (Indoor) | 6.8 GPM (No WiFi) |
| Color | Gray | Non-Condensing | LPG | Propane Gas,4.42GPM,95,000BTU，Space Gray | Silver | Metallic Silver | Gray | Gray | Grey 10.3LP | Silver Gray | Black | Space Gray | Pearl Grey | Gray | Gray |
| Model | ET180 | VH11K-W-NG-U01 | RANEIN-43GPM-100KBTU | AP-AWH-Q13LPG | RE140iP | CZ360LP | AP-ADSK180-H18 | RTG-95DVELN-3 | WCIU-HW199A1G-LP-M1n | JSQ30-A | CE686LP | AP-AWH-Q20LPG | RS-16 | IF.JSQ30-G16NA | RTGH-68DVLN-3 |
You can compare flow rates across the best indoor tankless water heaters ranked directly by published performance specifications.
04 Check Flow Rate and Fixture Count
Match GPM to Fixtures
You must ensure the unit’s flow rate meets your simultaneous usage needs. The Rheem RTG offers 9.5 GPM, which supports large households with multiple bathrooms running at once. For smaller needs, the VancEpic provides 4.8 GPM, suitable for two to three fixtures. The APUS 140,000 BTU model delivers high volume for whole-home demand. Always calculate the total GPM of all fixtures you might use together to avoid cold water shock during peak hours.
Verify Fuel Source Compatibility
Your home’s existing infrastructure dictates whether you can use electric or gas models. Electric units like the Thermomate 18kW require a dedicated 240V circuit and suit homes with strong electrical grids. Gas models, including the VancEpic and Ranein, need natural gas or propane lines. The Rinnai RE140iP runs on propane, offering an alternative if natural gas is unavailable. Confirm your fuel type before purchasing to avoid costly installation upgrades or incompatible hardware.
Assess Physical Dimensions
Indoor installations often have tight spaces, so measuring your designated area is critical. The APUS 18kW model is notably compact, measuring 11.4 by 13.7 inches, making it ideal for closets or small utility rooms. The VancEpic is wider at 14.4 inches but remains manageable for standard wall mounts. Always compare width and height against your available space. Ensure you also account for necessary clearance around the unit for proper ventilation and maintenance access during its lifespan.
Consider Weight for Mounting
Heavier units require sturdy wall mounting to prevent damage or safety hazards. The Rheem RTGH weighs 59.9 pounds, demanding robust anchor points. The APUS 18kW is much lighter at 15.2 pounds, offering easier installation flexibility. The Rinnai RE140iP weighs 48.5 pounds, requiring standard heavy-duty brackets. Check the weight specifications to determine if your wall can support the load without additional reinforcement, especially in older homes with less robust framing.
Evaluate BTU Output
BTU ratings indicate heating power, affecting how quickly water heats up. The MIZUDO 10.3 GPM model boasts 199k BTU, providing rapid heating for high-demand scenarios. The APUS 140,000 BTU unit also offers substantial power for whole-home needs. Lower BTU units, like the Thermomate 98,500 BTU, suit single-bathroom setups. Higher BTU does not always mean better flow; it means faster heating. Match the BTU to your inlet water temperature and desired flow rate for optimal performance.
05 Frequently asked questions
What is the smallest indoor tankless water heater?
The APUS 18kW Electric Tankless Water Heater is the most compact option. It measures 11.4 by 13.7 inches and weighs only 15.2 pounds. This small footprint makes it ideal for tight spaces where you need space-saving efficiency for two to three fixtures.
Which unit provides the highest flow rate?
The MIZUDO 10.3 GPM Propane Tankless Heater delivers the highest flow rate on this list. It is designed for large homes needing high volume. Be aware that this unit weighs 55.5 pounds, so you must ensure your installation location can support the weight.
Are there propane models with digital displays?
Most propane models listed do not mention digital displays in their specifications. However, the VancEpic 4.8 GPM Natural Gas heater includes an LED display. If you need precise temperature monitoring, this gas model offers that feature alongside child safety locks.
Which heater is best for a single bathroom?
The Thermomate 18kW Electric Tankless Water Heater is best for single-bathroom setups. It provides 18 kW of power at 240V, offering precise temperature control. Its compact size fits well where you need efficiency without the bulk of larger whole-house units.
What is the lightest indoor tankless water heater?
The APUS 18kW Electric Tankless Water Heater is the lightest at 15.2 pounds. The APUS 95,000 BTU Propane model is also very light at 19.2 pounds. Both are suitable for users prioritizing easy handling and installation in confined indoor spaces.
Can I use WiFi with these units?
Only the Rheem RTG 9.5 GPM Natural Gas Water Heater explicitly supports WiFi. The Rheem RTGH 6.8 GPM model is listed as having no WiFi. If smart home integration is a priority for you, the Rheem RTG is your only option here.
Which natural gas heater has the lowest BTU output?
The SNDOAS 102,000 BTU Natural Gas Tankless Heater has the lowest BTU rating among the gas models. It provides 4.75 GPM for small to medium households. This unit prioritizes precise temperature stability over maximum heating power, making it suitable for smaller needs.
