Best Tankless Water Heaters (2026)

The RX199iN is a condensing gas unit, which means a second heat exchanger pulls extra heat from the exhaust, so the flue gas is cool enough for PVC venting. That is the mechanism behind its efficiency and its simpler venting than a non-condensing unit. The 199,000 BTU input and 11.1 GPM rating make it the highest-capacity pick here.

It installs indoors or outdoors with the same model through its Versa Vent system, and it runs on natural gas or propane. The tradeoff is cost and condensate: you need a drain for the acidic condensate and usually a neutralizer. Against the Rheem RTG at 7.0 GPM it moves far more water at a given rise, and against the electric units it avoids panel upgrades but needs a properly sized gas line.

In ownership terms, plan on yearly descaling in hard water, a clean inlet screen and a professional combustion check. Gas tankless units are only as reliable as their venting and water quality. Ask your installer to confirm the vent length and the condensate route on paper before they start, since those two details decide most of the cost of a gas swap.

Rheem's RTG is a non-condensing gas unit with a 0.82 Uniform Energy Factor and 160,000 BTU of input, rated at 7.0 GPM. Non-condensing means the exhaust leaves hot, so it needs stainless or other approved vent material, but it avoids the condensate drain.

The listing claims savings up to 34 percent compared with a standard tank, which is a manufacturer comparison, not a guarantee. At 7.0 GPM it handles two showers at once in a temperate climate but will struggle in a cold one. Against the Rinnai it has about 60 percent of the flow and a less expensive venting profile only if you already have a metal vent.

Ownership centers on yearly descaling, a clean inlet filter and a clear vent path. In hard water areas, a scale inhibitor or a softener protects the heat exchanger. If you replace a tank with it, expect the gas line, vent and a new exhaust penetration to be part of the quote, not just the heater.

Rheem's 18kW model modulates its power to match the demand, with an external digital thermostat accurate to one degree and two copper immersion heating elements that are field serviceable. It is rated up to 4.4 GPM, which is the maximum under the best conditions.

The electrical load is the main fact. 18 kW at 240V draws about 75 amps, so you need a double-pole breaker in that range and usually a 150 amp service. In a cold climate the 4.4 GPM figure drops sharply, because the same kilowatts must raise colder water. Against the 24kW Rheem it has about three quarters of the capacity, and against the gas units it avoids venting at the cost of panel capacity.

Ownership is simpler than gas: no vent, no combustion check. Flush the heater for scale yearly in hard water and keep the inlet screen clean.

The RTEX-24 is the largest electric Rheem in the group, at 24 kW and up to 5.9 GPM. It is meant for multiple fixtures, and the listing tells you to verify the electrical requirements before you buy.

The number to check is current: 24 kW at 240V is about 100 amps, which in practice means a 200 amp service panel. Compared with the 18 kW it adds about a third more capacity, but the panel demand is the real cost. In a cold climate it still will not deliver 5.9 GPM, so treat the number as a warm-water figure.

Ownership is similar to the 18 kW model: yearly descaling, a clean inlet screen and attention to the electrical connections. A licensed electrician should size the wire and breaker. Have the electrician check the main breaker and the service cable gauge, since the panel label alone does not prove the service can carry the load.

The 11 kW Rheem is the smallest whole-house-style electric here, rated up to 2.68 GPM. That is enough for one shower or a kitchen sink plus a bathroom tap in a warm climate, not two showers at once.

At 240V it draws about 46 amps, which fits within the panel capacity of many apartments and small homes without an upgrade. Compared with the 18 kW unit it has a bit more than half the flow, but it demands far less from your panel. It is a sensible choice for a cabin, an addition or a condo.

Ownership is routine: descaling, a clean inlet screen and secure electrical connections. Do not buy it for a family home with two bathrooms.

This ThermoMate is a point-of-use heater for a single sink, drawing 29.2 amps at 120V and delivering 0.4 to 0.66 GPM. The listing is clear that it must be hard-wired and has no plug, which means a dedicated 40 amp circuit.

The low flow is the point: it gives a sink hot water in seconds and removes the wait for a distant tank. It cannot run a shower. Compared with the whole-house units it costs a fraction and needs only a short wire run, but it is not a substitute for them.

Ownership is simple, with a periodic check of the aerator and a flush if your water is hard. Treat it as a convenience heater for a far bathroom or workshop sink. Place the heater close to the sink so the pipe run stays short and the benefit of instant hot water is not lost to a long cold line.