The best natural gas water boiler depends on whether you need domestic hot water, whole-home space heating, or both. The Rinnai I-Series Plus is the only listed combi boiler designed for space heating and hot water. For hot water alone, the Rheem indoor 7.0 GPM model suits indoor installations, while the Rheem outdoor 8.4 GPM and Ranein outdoor 8.5 GPM models offer higher listed flow capacities. The ThermoMate is a lower-capacity indoor option with detailed temperature-control and safety features.
| Product | Best suited for | Listed capacity or feature |
|---|---|---|
| ThermoMate CZ422NG | Smaller hot-water demands | Up to 5 GPM; indoor |
| Ranein outdoor tankless | Higher-flow outdoor installation | 8.5 GPM; 190,000 BTU |
| Rheem RTG indoor | Indoor tankless hot water | 7.0 GPM; 160,000 BTU |
| Rheem RTG outdoor | Higher-flow outdoor tankless hot water | 8.4 GPM; 180,000 BTU |
| Rinnai I-Series Plus | Hot water and home heating | Combi boiler; 150,000 BTU |
ThermoMate CZ422NG Indoor Tankless Heater
Best for: Households seeking an indoor natural gas heater with a listed capacity of up to 5 GPM and detailed temperature-control features. The product information says it can supply two showers at once under stated conditions: a 65°F inlet-water temperature and a 105°F target.
Its features include a 113,000 BTU rating, a copper heat exchanger, freeze protection that activates at 41°F, and multiple safety protections. The listed efficiency figures are up to 98% thermal efficiency and a UEF of 0.81; these are different measures and should not be treated as interchangeable.
Limitation: The two-shower claim depends on the stated inlet and target temperatures. Actual available flow can vary with incoming water temperature and household demand.
Ranein 8.5 GPM Outdoor Tankless Heater
Best for: Buyers comparing outdoor tankless units with higher stated flow and burner capacity. The listing identifies this Ranein model as an outdoor natural gas heater rated at 8.5 GPM and 190,000 BTU.
Those figures make it a higher-capacity listed option than the ThermoMate and the indoor Rheem model in this selection. Choose it if your installation plan calls for an outdoor unit and its specifications match your home’s hot-water demand.
Limitation: The supplied product information does not include efficiency, safety, or temperature-control details. Confirm installation requirements and suitability for your climate with the manufacturer’s documentation or a qualified installer.
Rheem RTG 7.0 GPM Indoor Heater
Best for: Homeowners who want an indoor non-condensing tankless water heater with a 7.0 GPM listing and a 160,000 BTU rating. It is a middle option between the lower-flow ThermoMate and the higher-capacity outdoor models listed here.
The product description specifies on-demand, continuous hot water and identifies the unit as a high-efficiency Rheem RTG Series model. Its indoor design is the key comparison point for buyers who prefer not to install the heater outdoors.
Limitation: This is a non-condensing model. Compare venting and installation needs with the product documentation and local requirements before choosing it.
Rheem RTG 8.4 GPM Outdoor Heater
Best for: Buyers who need an outdoor tankless unit and want a higher listed flow rate than the Rheem 7.0 GPM indoor model. This RTG Series heater is rated at 8.4 GPM and 180,000 BTU.
It is identified as a high-efficiency, non-condensing model that supplies on-demand hot water. Choose it if the outdoor configuration suits the property and the listed capacity aligns with expected simultaneous water use.
Limitation: The product data does not give a specific efficiency percentage or temperature-rise conditions for its 8.4 GPM figure. Actual usable flow depends on the installation and incoming water temperature.
Rinnai I-Series Plus Combi Boiler
Best for: Homeowners who need one appliance for both domestic hot water and space heating. Unlike the tankless water heaters above, this Rinnai is a combi boiler intended to serve both functions.
The listed product features include a 150,000 BTU rating, Smart Sense Gas Valve technology, and a 96% AFUE rating. Its description says it can operate with natural gas or propane; confirm the exact fuel configuration required for your installation.
Limitation: The supplied listing does not specify hot-water flow rate or space-heating output details. Check sizing information with a qualified heating professional before selecting it for a particular home.
Buying Guide: How To Choose A Natural Gas Water Boiler
Do you need hot water, home heating, or both?
A tankless water heater heats domestic water on demand. A combi boiler can provide domestic hot water and space heating. If you are replacing a heating boiler, compare combi-boiler specifications rather than assuming a water heater can serve radiators or hydronic heating.
How much flow capacity should you compare?
Use household peak demand to compare models, including showers and appliances that may run at the same time. GPM figures alone do not guarantee a particular flow at every temperature. Incoming water temperature and the required temperature rise affect available hot-water output.
Should you choose an indoor or outdoor unit?
Match the heater’s listed location type to the planned installation. Indoor and outdoor models have different placement and installation requirements. Review manufacturer instructions, local codes, venting needs, and climate considerations with a qualified installer.
What do efficiency ratings tell you?
Compare like-for-like ratings. AFUE describes seasonal space-heating efficiency, while UEF is a water-heater efficiency measure. A listed thermal-efficiency figure is another measure; do not compare these percentages as if they were the same rating.
What should you verify before buying?
- Whether the appliance supports natural gas, and whether the listing also describes propane operation.
- Required gas supply, venting or outdoor placement, and installation clearances.
- Capacity at the temperature rise expected in your area.
- Whether your home needs domestic hot water only or space heating as well.
- Local permitting and installation requirements.
Frequently Asked Questions
Is a tankless water heater the same as a boiler?
No. A tankless water heater supplies domestic hot water. A boiler heats water for space heating; a combi boiler can also provide domestic hot water.
Can a 5 GPM heater run two showers?
It may under the conditions stated for a specific product. The ThermoMate listing describes two showers at a 65°F inlet temperature and 105°F output, but real performance varies with temperature rise and simultaneous demand.
Is a higher GPM rating always better?
No. A higher listed GPM can indicate greater potential flow, but the appropriate model depends on incoming water temperature, usage patterns, and installation conditions.
Which listed product is designed for space heating?
The Rinnai I-Series Plus is described as a combi boiler for both home heating and hot water. The other listed products are tankless water heaters for domestic hot water.
Can I install a natural gas boiler or water heater myself?
Gas appliances require careful attention to fuel supply, venting, and local codes. Consult the product instructions and a qualified installer to determine the appropriate installation.