Best Gas Tankless Water Heaters (2026)

The RTG is a non-condensing gas tankless with 160,000 BTU of input and a stated UEF of 0.82. Non-condensing means the burner exhaust leaves at a high temperature, so the unit needs stainless or other approved vent material but produces no acidic condensate to drain.

Using the standard formula, 160,000 BTU at 0.82 gives about 131,000 BTU of useful heat, which is about 3.7 GPM at a 70F rise and 6.5 GPM at a 40F rise. That is why the 7.0 GPM headline suits mild climates. Against the 199,000 BTU units it has about 80 percent of the input, and against the 102,000 BTU ThermoMate it has about 57 percent more.

Ownership is routine: descale yearly in hard water, keep the inlet screen clear and have a technician check combustion. Rheem's service network means parts and technicians are easy to find. If your existing tank heater vents through a masonry chimney, budget for a new dedicated vent, because a chimney liner rarely meets the requirements for a tankless exhaust.

GASLAND's Gasmart E puts 199,000 BTU of input into a whole-house natural gas unit with a stated UEF of 0.8, CSA certification and a rated flow up to 7.6 GPM. The listing claims about 34 percent more efficient than a standard tank, which is a manufacturer comparison.

The formula shows what 199,000 BTU can do: at 0.8 efficiency that is about 159,000 BTU of useful heat, roughly 4.5 GPM at a 70F rise and 8 GPM at a 40F rise. The 7.6 GPM headline therefore assumes a rise near 42F. Compared with the Rheem RTG it has more input and a slightly lower efficiency figure, and compared with the Rinnai it is non-condensing and less costly.

Ownership requires a gas line sized for 199,000 BTU, proper venting and a yearly descale. As a smaller brand, confirm the warranty and keep your receipt. Ask for the gas pressure and line size requirements in writing before you buy, and confirm that the unit is certified for your region.

MIZUDO lists 199,000 BTU and a 10.3 GPM rating, with gas modulation that holds the temperature within one degree and low-flow response. The listing says it supports 5 to 7 water points.

The 10.3 GPM number deserves a hard look. At 199,000 BTU, only a condensing unit near 95 percent efficiency can deliver that, at a rise near 36F. In a typical 70F rise, even a condensing 199,000 BTU unit makes closer to 5.3 GPM. Against the GASLAND at 7.6 GPM on the same input it claims much more flow, so treat the number as a best case.

Ownership needs the same gas line, venting and descaling routine. Ask the seller whether the unit is condensing or non-condensing, since that decides the venting and the condensate drain. A listing that gives a very high GPM without saying whether the unit is condensing is worth a question to the seller before you order.

APUS's 120,000 BTU unit lists 5.43 GPM, a stated UEF of 0.81, copper heat transfer and temperature control within one degree between 95F and 149F. It is aimed at three to four water points.

Its position is the middle of the lineup. At 120,000 BTU and 0.81 efficiency, the useful heat is about 97,000 BTU, so roughly 2.8 GPM at a 70F rise and 4.9 GPM at a 40F rise. Compared with the Rheem RTG it has about three-quarters of the input, and compared with the ThermoMate it adds about 18 percent.

Ownership is routine. Descale yearly, keep the inlet screen clean and check the vent. The temperature range is a useful control, because setting 120F rather than 140F stretches the flow. A power-vent fan means the unit needs an electrical outlet nearby, so a power cut stops it even when gas is flowing.

ThermoMate's CZ422NG has 102,000 BTU of input, a rated flow of up to 5 GPM, CSA certification and an oxygen-free copper heat exchanger. The listing highlights boil-dry, freeze, flame-out and overheat protections and says it can run two showers at once.

Two showers at once is a mild-climate claim. At 102,000 BTU the useful heat is about 80,000 BTU, so roughly 2.3 GPM at a 70F rise and 4 GPM at a 40F rise. That serves one shower at a time in a cold climate. Against the APUS 120,000 BTU it has less input, and against the Rheem RTG it costs less.

Ownership centers on yearly descaling, an inlet screen check and an inspection of the vent. The protections are useful, but they do not replace a properly sized gas line. If you keep the setpoint near 120F instead of 140F, the same burner stretches to a larger flow, which is the cheapest way to add capacity.

The Rinnai is the only condensing unit here, which means a second heat exchanger extracts extra heat from the exhaust, so the exhaust is cool enough to vent in PVC. That raises efficiency, and the 199,000 BTU input with 11.1 GPM makes it the capacity leader.

Its Versa Vent system lets the same model install indoors or outdoors, and it runs on natural gas or propane. The tradeoff is a condensate drain and a neutralizer to handle the acidic water. Against the MIZUDO at 10.3 GPM it states a condensing design that makes its high number more plausible, and against the Rheem RTG it has far more flow.

Ownership is yearly descaling, a clean inlet screen and a professional check. Rinnai's service network is the deepest here, which counts for a lot in year eight. The built-in protections are only as good as the installation, so use a licensed installer and keep a record of the gas pressure test.