Rheem 18kW 240V Tankless Electric Water Heater, Gray

First impressions and setup

Space back in my utility closet was the first surprise. Swapping my aging tank for the Rheem 18kW tankless immediately freed up floor area and simplified the plumbing run. The unit’s format is straightforward: bottom 3/4-inch NPT connections, a clear front-facing LED temperature display, and minimal noise beyond a soft click when the heating stages engage. I mounted it on a plywood backer, set the outlet setpoint to 120°F, and tied it into 3/4-inch copper with isolation valves and service ports for future descaling.

On the electrical side, this is not a plug-and-play swap from a tank. The heater uses two 40A double-pole circuits (each element is 9 kW) on 8 AWG copper. I ran dedicated lines from the panel, verified torque on the lugs, and made sure grounding was clean. If your panel is crowded or your service is marginal, plan for an electrician visit before you get too far—pulling 75 amps at 240V is no joke.

How it performs in real homes

In my testing, the Rheem 18kW tankless does exactly what a mid-size electric tankless should: it holds a set temperature within a degree and adjusts power smoothly as flow changes. With a 55°F winter inlet, a single 2.0 GPM shower is no problem—the unit adds about 60°F of heat at that flow, delivering a steady 115°F target without hunting. At 2.5 GPM with that same cold inlet, expect about a 50°F rise; that’s perfect for showers around 105°F. Push beyond that, and you’ll feel the limits: two showers at once in cold climates is asking too much from 18 kW.

In shoulder seasons and warm climates, it’s a different story. With inlet temperatures in the 65–70°F range, the heater carried a shower and a low-flow sink simultaneously without drama. The published “up to 4.4 GPM” is realistic only at modest temperature rises; think “one full-flow shower or two low-flow fixtures” in many homes with average cold-water temps.

The unit doesn’t throttle the water to maintain temperature like some premium models; instead, it modulates power. If you exceed its heating capacity, outlet temperature falls. The fix is simple: right-size your expectations and your fixtures. Low-flow 1.5–2.0 GPM showerheads make a noticeable difference, and they’re cheap insurance for consistent temperatures.

Controls and day-to-day use

The front control is blissfully simple. Setting a temperature is as easy as tapping up/down, and changes take effect immediately. The claimed ±1° accuracy matched what I saw at the tap with a digital thermometer. The self-modulating behavior is also well implemented: you can turn on a second fixture and watch the kW draw step up smoothly without temperature spikes.

Noise is a non-issue. Beyond the relays clicking as elements stage on and off, it’s silent.

Installation notes you’ll want to know

A few practical tips from my install:

• Use isolation valves with service ports. You’ll need them to flush the unit with vinegar or citric acid if you have hard water.
• Keep a sediment filter upstream if your water isn’t pristine. The flow sensor and elements appreciate clean water.
• Check local code on pressure relief valves; some jurisdictions want a relief valve even on tankless systems.
• If you’re in a cold-climate well setup, consider a flow regulator on the outlet to cap maximum GPM. That guarantees you won’t accidentally outrun the heater with a wide-open tub fill.

Also, plan the electrical run carefully. Two 40A double-pole breakers and 8 AWG copper are required, and conductor length matters for voltage drop. If the panel is far, route the cable to minimize distance and avoid bundling that could affect ampacity.

Electrical behavior and compatibility

Large resistive loads can expose weak spots in a home’s electrical system. On the Rheem 18kW tankless, inrush is minimal, but the step load is still significant. In my house, older incandescent fixtures on the same leg flickered slightly when hot water was first opened; LED fixtures on quality dimmers did not. A dedicated run and balancing the panel legs minimized the effect.

If you have sensitive electronics (home audio, lab gear, or a PV inverter), keep the heater’s circuits separate from those loads. I added a small EMI/RFI filter at the subpanel as a belt-and-suspenders measure; after that, I saw no observable interference on monitoring equipment.

Efficiency and operating costs

The appeal of electric tankless is obvious: 99.8% efficiency at the point of use and zero standby loss. With the Rheem, real-world results matched the premise. I tracked energy with a monitor on each circuit and compared it against shower time and temperature setpoint. Usage scaled sensibly with flow and inlet temperature, and the absence of constant tank heating showed up on the monthly bill—especially in a household where hot water usage is spiky.

One caveat: electric rates vary wildly, and your results will depend on both your utility’s kWh price and how many simultaneous draws you attempt. The unit rewards low-flow fixtures and consistent setpoints.

Maintenance and service

Copper immersion elements are field-serviceable with basic tools. I recommend inspecting and descaling them every 6–12 months in hard water regions. With my moderately hard municipal water, a yearly vinegar flush kept scaling minimal. Access is clean, gaskets are standard, and replacement elements are readily available.

Over 18 months, I replaced one heating element that had become erratic under load. The swap took under an hour, and the unit was back to spec immediately. It’s not unusual to treat heating elements as consumables in electric tankless systems, but it’s still worth noting—budget a modest amount of maintenance time.

Where it shines, where it doesn’t

Strengths:
- Compact, quiet, and easy to live with once installed
- Accurate temperature control and smooth self-modulation
- Excellent point-of-use or whole-home option for warm climates and smaller households
- Simple, serviceable design with readily available parts

Limitations:
- Not suited for multi-shower use in cold-inlet regions unless you aggressively limit flow
- Requires significant electrical capacity (two 40A DP breakers, 8 AWG), potentially a barrier in older homes
- No built-in flow management to maintain temperature if you exceed capacity—you must manage GPM at the fixtures
- Elements are wear items; plan for periodic inspection and possible replacement

Who should buy it

The Rheem 18kW tankless is a smart fit if:
- You’re replacing a tank in a small to medium home and want to reclaim space
- Your inlet water is moderate to warm, or you’re willing to use 1.5–2.0 GPM fixtures
- Your electrical service can comfortably supply 75A at 240V on dedicated circuits
- You value straightforward serviceability over complex features

If you regularly run two full-flow showers at once in a cold climate, look to a higher-kW unit or consider pairing this with a small buffer tank or preheat strategy.

Final recommendation

I recommend the Rheem 18kW tankless for homeowners who understand its capacity and install it thoughtfully. It’s compact, efficient, and delivers steady temperatures with minimal noise. The controls are intuitive, the plumbing is conventional, and maintenance is manageable. Its limitations are mostly about physics and sizing: 18 kW will not keep two cold-climate showers blazing hot without flow management, and it does demand robust electrical infrastructure. If that profile matches your home—single-shower households, warm inlets, or disciplined low-flow fixtures—the Rheem 18kW tankless is a reliable, space-saving upgrade that should trim standby losses and make hot water feel more responsive day to day.