The best efficient gas water heater depends on household demand, installation location, and the efficiency measure listed for the model. For larger homes, the Rheem outdoor 8.4 GPM model offers higher stated flow than the indoor Rheem 7.0 GPM unit. The MIZUDO lists the highest maximum flow, while ThermoMate is a lower-capacity option with a stated thermal-efficiency figure. Compare the figures carefully: UEF and thermal efficiency are different measures and should not be treated as interchangeable.
| Product | Best for | Key listed figure |
|---|---|---|
| Rheem RTG 7.0 GPM Indoor | Indoor installation and moderate simultaneous demand | 0.82 UEF; 7.0 GPM at 35°F rise |
| MIZUDO 10.3 GPM Indoor | High stated flow and app control | Up to 10.3 GPM; 199,000 BTU |
| Rheem RTG 8.4 GPM Outdoor | Outdoor installation and higher flow | 0.82 UEF; 8.4 GPM at 35°F rise |
| ThermoMate CZ422NG | Smaller demand and stated thermal efficiency | Up to 98% thermal efficiency; up to 5 GPM |
| GASLAND Gasmart E | Indoor whole-home use and compact placement | 0.8 UEF; up to 7.6 GPM |
Rheem RTG 7.0 GPM Indoor
This indoor, non-condensing Rheem tankless model is best for households that want a wall-mounted unit and have moderate concurrent hot-water demand. Its listed capacity is 7.0 GPM at a 35°F temperature rise, with 160,000 BTU per hour of input. The product information describes it as suitable for two to three simultaneous showers or water points, though actual flow at a home depends on inlet-water temperature and demand.
The listed Uniform Energy Factor is 0.82. The manufacturer’s product description says this model can save up to 34% on energy costs compared with a 50-gallon electric tank heater; that comparison is not a guarantee of an individual household’s savings. Choose this model if indoor installation is needed and its capacity aligns with the home’s peak usage.
- Best for: Homes seeking an indoor, space-saving tankless replacement.
- Why it stands out: It combines a stated 0.82 UEF with a 7.0 GPM rating at a specified temperature rise.
- Consider: It requires an indoor direct-vent installation, and its listed flow is below the two higher-capacity options in this roundup.
MIZUDO 10.3 GPM Indoor
The MIZUDO is best for buyers prioritizing a high stated maximum flow rate and electronic controls. It lists 199,000 BTU of heating input and up to 10.3 GPM. The product description says it is intended to serve demand across two to three bathrooms along with fixtures such as kitchen faucets, laundry equipment, and dishwashers. These are manufacturer-provided capacity claims, not a guarantee that every combination of fixtures will receive that flow in every climate.
Its controls include Wi-Fi and app control, and the stated temperature range extends from 95°F to 149°F. The manufacturer also lists ±1°F temperature control and a 0.53 GPM low-flow activation threshold. Those specifications may appeal to households that want fine temperature adjustment or hot water at lower fixture flows.
- Best for: Larger households comparing high maximum flow and app-based controls.
- Why it stands out: It has the highest listed maximum GPM among these five products and a broad stated temperature range.
- Consider: The supplied product details do not list a UEF or thermal-efficiency percentage, so efficiency cannot be directly compared with models that provide those figures.
Rheem RTG 8.4 GPM Outdoor
This Rheem model is best for homeowners who can install a water heater outdoors and want a higher stated flow rating than the indoor Rheem in this list. It is rated at 8.4 GPM at a 35°F temperature rise and 180,000 BTU per hour. The product description says this output is designed for three to four simultaneous showers, but local inlet-water temperature and the actual fixture mix affect delivered flow.
The unit is non-condensing and lists a UEF of 0.82. Its outdoor installation eliminates the need for indoor venting, according to the supplied product information. A durable metal cabinet and wall-mount design are also listed. Choose it if outdoor placement suits the property and the household needs more stated capacity than the 7.0 GPM indoor Rheem provides.
- Best for: Homes with suitable outdoor placement and higher simultaneous demand.
- Why it stands out: It pairs an 8.4 GPM rating at a stated temperature rise with a listed 0.82 UEF.
- Consider: It is designed for outdoor installation, so it is not the choice for a buyer who needs an indoor unit.
ThermoMate CZ422NG Up to 5 GPM
The ThermoMate is best for households with lower peak demand that want an indoor natural-gas tankless unit with a listed thermal-efficiency figure. It offers up to 5 GPM and 113,000 BTU, and its product description says it can supply two showers at the same time while maintaining 105°F when inlet water is 65°F. That condition matters: colder incoming water requires a greater temperature rise and may affect available flow.
The product lists up to 98% thermal efficiency and a 0.81 UEF. These are separate measures and should not be used as equivalent values. Its features include an oxygen-free copper heat exchanger, a smart flow sensor, and multiple listed safety protections. The stated freeze protection activates at 41°F (5°C).
- Best for: Smaller households or homes with a more modest simultaneous hot-water load.
- Why it stands out: It supplies both a thermal-efficiency figure and a UEF, along with a specific two-shower performance condition.
- Consider: Its maximum listed flow is lower than the other four products, making it less suited to high concurrent demand.
GASLAND Gasmart E 7.6 GPM
The GASLAND Gasmart E is best for buyers seeking an indoor whole-home tankless model with a compact, wall-mounted design. It lists 199,000 BTU and up to 7.6 GPM. Its product information describes it as a natural-gas unit for residential indoor installation and says it provides hot water on demand for uses such as showers, laundry, and dishwashing.
The listed UEF is 0.8. The product description also claims it is 34% more energy-efficient than a 50-gallon tank water heater and reports a 65% reduction in NOx emissions compared with a 50-gallon gas tank. Treat these as manufacturer-stated comparisons; household results depend on the equipment being replaced and patterns of use. The listing states CSA certification and multiple safety features.
- Best for: Homes wanting indoor installation, a compact form, and a mid-to-high stated flow rate.
- Why it stands out: It provides a 7.6 GPM maximum rating and a stated 0.8 UEF in a wall-mounted design.
- Consider: Its listed UEF is lower than the two Rheem models’ 0.82, although UEF alone does not determine actual energy use in a specific home.
Buying Guide: How to Choose an Efficient Gas Water Heater
A gas water heater labeled “efficient” can refer to different performance measures. Check the exact metric on the product listing and compare like with like. UEF is a standardized measure used to compare water-heater energy performance; thermal efficiency is another measure. A 98% thermal-efficiency claim should not be read as a 0.98 UEF unless the manufacturer explicitly provides that UEF value.
How much hot-water flow does the household need?
Estimate the fixtures likely to run at the same time, rather than counting every fixture in the home. Two showers and a dishwasher operating together create a different peak load from one shower at a time. Use the model’s flow rating at its stated temperature rise when available. A maximum GPM without a specified temperature rise is harder to compare directly with a rating that includes one.
- For lower simultaneous demand, the ThermoMate lists up to 5 GPM.
- The Rheem indoor model lists 7.0 GPM at a 35°F rise.
- The GASLAND lists up to 7.6 GPM, while the Rheem outdoor model lists 8.4 GPM at a 35°F rise.
- The MIZUDO lists up to 10.3 GPM, but its supplied details do not state the temperature rise for that maximum.
Does installation location favor an indoor or outdoor model?
Choose based on the available installation space and local requirements, not just the product’s flow rating. The two Rheem products here have distinct installation types: one is an indoor direct-vent model and the other is designed for outdoor installation. The GASLAND, MIZUDO, and ThermoMate are described as indoor models. A qualified professional can assess venting, gas supply, clearances, and applicable local code requirements before purchase.
What does “90% efficient” mean for this purchase?
Do not assume that every product marketed as highly efficient uses the same rating system. These listings provide UEF values for some models and a thermal-efficiency figure for the ThermoMate. The MIZUDO details provided here do not include either figure. If a specific minimum efficiency is required, verify the exact model’s current specification sheet and compare the same metric across candidates.
How should gas input and water temperature affect the choice?
BTU input describes the heater’s input capacity, but it does not independently tell you how much hot water will be available under your conditions. Incoming water temperature matters: colder water needs more heating to reach the target temperature, which can reduce available flow. Use local winter inlet-water conditions and the heater’s capacity information when sizing a unit.
Which controls and protections are useful?
Consider whether you need app control, a particular temperature range, low-flow activation, or listed freeze and fault protections. The MIZUDO lists Wi-Fi and app control, while the ThermoMate lists multiple protective functions and freeze protection that activates at 41°F. Confirm that the features match the installation environment and the household’s needs rather than selecting by feature count alone.
What should be checked before ordering?
- Confirm the fuel type, indoor or outdoor installation requirement, and available gas supply with a qualified installer.
- Check the product’s complete installation instructions, venting requirements, dimensions, and local code requirements.
- Compare flow ratings at the same or clearly stated temperature rise whenever possible.
- Verify current efficiency specifications and whether the listed value is UEF or thermal efficiency.
- Ask whether the home’s existing gas line and installation location are suitable; do not assume that a tank replacement is a direct swap.
Frequently Asked Questions
Is a tankless gas water heater always more efficient than a tank model?
Not necessarily in every home. Tankless units heat water on demand and avoid maintaining a stored tank of hot water, but actual energy use depends on the model, installation, household demand, and the tank heater used for comparison.
Can a 5 GPM water heater run two showers?
The ThermoMate product description says it can supply two showers at once under a stated condition of 105°F output with 65°F inlet water. Colder inlet water or additional hot-water use can change the available flow.
Is 98% thermal efficiency the same as 0.98 UEF?
No. Thermal efficiency and Uniform Energy Factor are different measures. Compare thermal-efficiency figures with the same measure and UEF values with UEF values.
Which product has the highest stated maximum flow?
The MIZUDO lists up to 10.3 GPM, the highest maximum in this selection. Its supplied listing details do not specify the temperature rise for that maximum, so compare it cautiously with models whose GPM ratings include a stated temperature rise.
Should I choose indoor or outdoor installation?
Choose the type supported by the home’s layout and local installation requirements. Indoor units need an appropriate venting arrangement, while outdoor units require a suitable exterior location. A qualified installer can determine which option fits the property.
Does a higher BTU rating guarantee more hot water?
No. BTU input is one factor, but delivered flow also depends on incoming-water temperature, target temperature, and the unit’s design. Compare the flow rating and its test conditions along with BTU input.