Choosing an efficient gas water heater means matching its fuel type, hot-water output, and installation needs to your home—not just picking the unit with the biggest flow-rate number. The options below are tankless models, which heat water on demand. Two list a Uniform Energy Factor (UEF), a useful efficiency measure, while other models highlight features such as condensing heat recovery or reduced standby losses. Use the comparison and buying guide to find a model that fits your household and installation plan.
Table of Contents
Quick Comparison
| Water heater | Fuel and installation | Listed output | Efficiency details | Best fit |
|---|---|---|---|---|
| ThermoMate Condensing Tankless Water Heater | Natural gas; installation location not specified | Up to 6 GPM; 114,000 BTU | 0.86 UEF; condensing heat exchangers | Homes seeking a compact natural-gas unit with a displayed UEF |
| Rheem RTGH Series Natural Gas | Natural gas; indoor, direct vent | 9.0 GPM at a 35°F rise; 180,000 BTU | 0.93 UEF; condensing technology | Large households with higher simultaneous hot-water demand |
| Rheem RTGH Series Liquid Propane | Liquid propane; indoor, direct vent | 9.5 GPM at a 35°F rise; 199,900 BTU | 0.93 UEF; condensing technology | High-demand homes that use liquid propane |
| Ranein Condensing Natural Gas | Natural gas; installation location not specified | 5.1 GPM; 120,000 BTU | Condensing design; claims up to 20% gas-bill savings | Homes looking for preset modes and included installation accessories |
| Westinghouse Outdoor Natural Gas | Natural gas; outdoor installation | Up to 5.1 GPM; 120,000 BTU | On-demand heating; UEF not provided | Homes with a suitable outdoor installation location |
Our Picks: Best Most Efficient Gas Water Heaters reviews
1. ThermoMate Condensing Tankless Heater: Efficient Natural-Gas Pick
This natural-gas tankless model combines a listed 0.86 UEF with a condensing design. Its two heat exchangers—one stainless steel and one oxygen-free copper—are designed to recover some heat from exhaust that a conventional non-condensing design would lose. The manufacturer says this approach may save 15% compared with non-condensing units; actual energy use depends on your household and installation.
The heater is rated for up to 6 GPM and 114,000 BTU. The product information gives one example: two showers running at 2 GPM each with 105°F water when incoming water is 65°F. That example is useful, but colder incoming water or a greater hot-water demand can change the flow the unit can deliver. The touchscreen displays temperature, flow, fan operation, and flame status, with an adjustable output range of 90°F to 140°F.
Its listed protections include freeze, overheating, flame-out, and other operating safeguards. The unit also produces mildly acidic condensate, so you need to plan for a drain; the manufacturer recommends a separate neutralizer kit. Compared with the Rheem models below, it has lower listed output and a lower UEF, but may suit a smaller household that wants a natural-gas condensing model with clear on-unit controls.
Pros:
- Listed 0.86 UEF and condensing heat recovery
- Up to 6 GPM and 114,000 BTU
- Touchscreen shows operating information and error codes
- Multiple listed safety protections, including freeze protection
Cons:
- Condensate drainage is required, and a neutralizer kit is not included
- Lower listed capacity and UEF than the Rheem options
Verdict: Choose the ThermoMate if you want a natural-gas condensing heater with a stated UEF and your household’s peak demand fits its capacity.
2. Rheem RTGH Natural Gas: High-Capacity Indoor Choice
This indoor natural-gas model is rated at 180,000 BTU and 9.0 GPM at a 35°F temperature rise. Its listed 0.93 UEF is the highest stated UEF among the models here. Rheem describes it as a condensing unit and says it is designed for whole-house hot water, with capacity intended for homes that may run several showers at once.
The unit uses direct venting and has a digital temperature control with listed precision of ±1°F. Built-in freeze protection and a multi-point safety system are also specified. Rheem says it is CSA certified and describes the model as ultra-low NOx; professional installation is recommended. The listed warranty is 15 years for the heat exchanger and five years for parts.
The 9.0 GPM figure is tied to a 35°F rise, so it should not be read as a guaranteed flow rate in every home or climate. Your incoming water temperature, desired output temperature, and simultaneous use all matter. This model is a strong natural-gas option for higher demand, but its indoor direct-vent setup and installation requirements need to match your home.
Pros:
- Listed 0.93 UEF and condensing technology
- 9.0 GPM at a stated 35°F rise and 180,000 BTU
- Indoor direct-vent design with digital temperature control
- Listed heat-exchanger and parts warranty terms
Cons:
- Requires a suitable indoor direct-vent installation
- Actual flow depends on temperature rise and household demand
Verdict: This is a compelling pick for a high-demand home with natural-gas service and an appropriate indoor venting setup.
3. Rheem RTGH Liquid Propane: Highest Listed Output
This Rheem model is the liquid-propane version of the high-capacity RTGH series. It lists 199,900 BTU and 9.5 GPM at a 35°F rise, along with a 0.93 UEF and condensing technology. Its rated output makes it the highest-capacity option in this group, but only households with compatible propane service should consider it.
The unit is designed for indoor installation with direct venting. It includes digital LED temperature control, built-in freeze protection, and a multi-point safety system. The product information lists CSA certification and the same warranty terms as the natural-gas RTGH: 15 years for the heat exchanger and five years for parts. Professional installation is recommended.
Like the natural-gas model, its flow rating depends on a specific temperature rise, so the number alone does not guarantee how many fixtures it can supply in your home. It also cannot be treated as a natural-gas substitute: fuel type must match your home and the appliance. If your household uses natural gas, the 9.0 GPM Rheem version is the relevant comparison; if you use propane and need strong output, this model is the better fit.
Pros:
- Listed 0.93 UEF with condensing technology
- Highest listed capacity here at 199,900 BTU
- 9.5 GPM rating at a stated 35°F rise
- Indoor direct-vent design with freeze protection
Cons:
- Suitable only for a compatible liquid-propane installation
- Output changes with temperature rise and actual hot-water demand
Verdict: Pick this model if you use liquid propane, need substantial hot-water capacity, and can meet its indoor direct-vent requirements.
4. Ranein Condensing Natural Gas: Controls and Accessories
The Ranein is a natural-gas condensing tankless heater rated at 5.1 GPM and 120,000 BTU. Its four preset modes include Eco, Kitchen, 4-Season Auto, and Bathtub, while the touchscreen can show water flow, gas use, and fault codes. The temperature range is listed as 95°F to 149°F, with stated precision of ±1°F.
The manufacturer says its heat-recovery design can save up to 20% on gas bills compared with standard non-condensing units. Treat that as a product claim rather than a guaranteed household result. The listed package includes a vent pipe, condensate drain line, mounting hardware, and an installation template. The product information also describes freeze protection and safety features such as overheat shutoff and exhaust blockage detection.
At 5.1 GPM, it has less listed capacity than either Rheem model, so check your likely simultaneous demand rather than relying on the claim that it can serve multiple bathrooms. The product information says it is not recommended for elevations above 2,000 feet. Confirm that the included venting and other parts suit your home and local requirements before purchasing, and plan for professional installation.
Pros:
- Condensing design with several temperature and operating modes
- Touchscreen displays flow, gas use, and fault codes
- Listed package includes venting and installation accessories
- Freeze protection and multiple listed safety features
Cons:
- No UEF is provided in the product details
- Listed capacity is below the Rheem options, and the model is not recommended above 2,000 feet
Verdict: Consider the Ranein if its 5.1 GPM capacity fits your needs and you value preset controls and a package that lists installation accessories.
5. Westinghouse Outdoor Natural Gas: Space-Saving Exterior Setup
This Westinghouse tankless heater is designed for outdoor installation and uses natural gas. Its listed output is up to 5.1 GPM with a 120,000-BTU burner. The product information describes self-modulating operation that adjusts gas and water flow to help keep temperature within ±1°F, as well as a wall-mounted design that avoids taking up indoor floor space.
The unit is listed as CSA certified and includes freeze protection. It requires a 120V AC power source and uses standard 3/4-inch NPT residential plumbing connections. The manufacturer advises fully draining the unit if temperatures fall below 5°F or it will sit unused for an extended period. That cold-weather instruction matters when deciding whether an outdoor installation is suitable for your property.
No UEF is provided, so there is not enough listed information to compare its rated efficiency directly with the Rheem or ThermoMate models. Its 5.1 GPM rating is also not a promise of the same flow in every climate or at every temperature rise. Compared with the indoor Rheem models, its main distinction is the outdoor setup, not higher capacity.
Pros:
- Outdoor design can free up indoor space
- Up to 5.1 GPM and 120,000 BTU listed
- Self-modulating temperature control and freeze protection
- CSA certification is specified
Cons:
- No UEF is listed for direct efficiency comparison
- Outdoor placement requires attention to freezing conditions and the stated drain-down guidance
Verdict: Choose this Westinghouse model if you want an outdoor natural-gas unit and your location and hot-water demand are a match for its listed capacity.
How to Choose an Efficient Gas Water Heater
Match the fuel to your home
Start by checking whether your home is set up for natural gas or liquid propane. The Rheem models are sold for different fuels, and the natural-gas and propane versions are not interchangeable based on the information provided. Confirm your fuel supply and appliance requirements before comparing other features.
Size for peak hot-water demand
Think about which fixtures may run at the same time, such as showers, a kitchen faucet, or a dishwasher. Compare that demand with the heater’s flow rating, but read the conditions behind the number. For example, the Rheem flow ratings are stated at a 35°F rise, while flow from any unit can vary with incoming water temperature and desired output temperature.
Compare efficiency figures carefully
UEF gives you a standardized efficiency figure when the manufacturer provides one. In this group, the Rheem models list a 0.93 UEF and the ThermoMate lists 0.86 UEF. The Ranein describes potential savings compared with non-condensing models, but it does not provide a UEF in the supplied details; the Westinghouse also has no listed UEF. Avoid treating different kinds of efficiency claims as directly equal.
Check venting and installation location
Some models are designed for indoor installation with direct venting, while the Westinghouse is specifically an outdoor unit. Check available space, venting paths, and the product’s installation instructions before you buy. A licensed professional can confirm whether the unit and its venting plan suit your home and local requirements.
Plan for condensate and drainage
Condensing heaters create condensate that must be handled as part of the installation. The ThermoMate description identifies the condensate as mildly acidic and recommends a separate neutralizer kit. Ask what drainage and treatment your chosen model requires, and verify the plan against the manufacturer’s instructions and local rules.
Consider climate and freeze protection
Freeze protection is listed for several models, but that does not remove the need to follow each unit’s cold-weather instructions. The Westinghouse product information calls for draining the system in temperatures below 5°F or during extended periods without use. If the installation location may freeze, check the exact protections and operating limits before choosing a unit.
Review electrical needs, controls, and warranty
Tankless gas models may still need electricity for controls or other functions. The ThermoMate and Westinghouse descriptions specify a 120V power connection, so account for that at the installation location. Compare display features and safety information as well. The Rheem listings provide warranty terms, while the supplied details for the other models do not provide comparable warranty information.
Frequently Asked Questions
Which model has the highest listed UEF?
The two Rheem models list a 0.93 UEF, higher than the ThermoMate’s listed 0.86 UEF. The Ranein and Westinghouse product details do not provide a UEF, so their efficiency cannot be ranked by that measure using the information here.
Which option has the highest listed capacity?
The liquid-propane Rheem lists the highest output: 199,900 BTU and 9.5 GPM at a 35°F rise. It is intended for compatible propane service, so natural-gas homes should compare the 180,000-BTU Rheem natural-gas model instead.
Can a 5.1 GPM heater run two showers?
Possibly, but the answer depends on the incoming water temperature, shower flow, and desired temperature rise. A listed maximum GPM is not a guarantee that the heater will deliver that flow under every condition. Have your expected simultaneous demand checked before choosing.
Do condensing tankless heaters need a drain?
Condensing models produce condensate that needs a suitable drain plan. The ThermoMate listing also recommends a separate condensate neutralizer. Requirements can vary by model and installation, so check the manual and local rules.
Is an outdoor water heater always a better fit?
No. An outdoor unit may save indoor space, but it must be suitable for the location and climate. The Westinghouse listing includes specific freeze-related drain-down guidance, so compare that with the conditions where you plan to install it.
Can I install one of these heaters myself?
Installation involves gas or propane connections, venting or outdoor placement, plumbing, and sometimes condensate drainage and electrical power. The Rheem and Ranein descriptions recommend professional installation. Consult a qualified installer and follow the manufacturer’s requirements for your specific model.
Final Recommendation
For a high-demand natural-gas home, the Rheem RTGH natural-gas model stands out for its listed 0.93 UEF and 9.0 GPM rating at a 35°F rise. If your home uses propane, the matching Rheem version has the highest listed capacity in this group. For a smaller natural-gas household, the ThermoMate provides a stated 0.86 UEF and condensing design, while the Ranein and Westinghouse offer different control and installation approaches. Before choosing, confirm fuel type, peak flow needs, venting or outdoor suitability, and condensate and freeze-protection requirements.
