How to Turn On Your Rheem Water Heater: Complete Guide

The water’s been cold for two days. You’ve checked the breaker, maybe fiddled with the dial, and now you’re staring at the Rheem logo wondering if you need to call a plumber. Most of the time, you don’t. Turning on a Rheem water heater is a straightforward process once you know which type you own and how its ignition system works.

This guide walks you through the exact steps for every common Rheem configuration. You’ll learn how to safely start a gas unit with a standing pilot, what to do with an electronic ignition model, and why purging air from an electric tank matters more than you think. I’ll also cover Vacation Mode, temperature settings, and what those blinking lights actually mean.

One thing before we start: safety is non-negotiable. If you smell gas near the unit, do not touch anything. Leave the house, call your gas company from a neighbor’s phone, and let professionals handle it. Everything below assumes no gas odor is present.

Before You Begin: Safety and Tools Required

You don’t need much for this job. A flathead screwdriver, a flashlight, and maybe a thermometer that can clip to a faucet. For gas models, keep a long-reach lighter or a barbecue igniter handy. The igniter is better than a match because your hand stays farther from the flame.

Look around the base of the heater. Check for standing water, rust streaks, or a damp floor. If you see water, stop right there. A leaking tank or a failing pressure relief valve requires repair before you power anything up. The pressure relief valve sits on the side or top of the tank. It should never be dripping. If it is, that’s a separate problem.

Also, locate the cold water inlet valve, usually on top of the unit with a blue handle. You’ll need to open this to fill the tank. Confirm the gas shut-off valve is in the open position for gas units. For electric units, find the dedicated circuit breaker and confirm it’s in the off position before you begin.

Identifying Your Rheem Model (Tank vs. Tankless, Gas vs. Electric)

Rheem makes several lines, but the startup procedure splits into four basic categories. Get this right first, because the steps differ significantly.

• Gas tank models (e.g., Performance Plus, Fury): Have a gas line, a flue pipe on top, and a visible burner access panel near the bottom.
• Electric tank models (e.g., Marathon, Performance Plus Electric): No flue, no gas line. Two access panels on the side hide the thermostats and elements.
• Gas tankless models (e.g., Condensing Tankless, Performance Tankless): Wall-mounted, compact, with a digital control panel on the front.
• Electric tankless (e.g., Performance Plus Tankless): Wall-mounted, high amperage draw, usually 120-amp or higher circuits.

The nameplate on the side of the tank or the front of a tankless unit lists the model number. If the model starts with a ‘G’, it’s gas. If it starts with an ‘E’, it’s electric. That’s the quickest way to tell.

This guide focuses on tank models, which are the most common. Tankless units follow a different startup path that I’ll touch on in the troubleshooting section.

How to Turn On an Electric Rheem Water Heater

Electric heaters are simpler mechanically, but they have a critical failure point: dry-firing. If the heating elements run without water, they burn out in seconds. The number one cause of premature element failure is turning on the breaker before the tank is full.

Step-by-Step: Filling the Tank and Purging Air

1. Close the drain valve at the bottom of the tank. Make sure it’s snug, but don’t wrench it down hard.
2. Open a hot water faucet in the house, preferably the one closest to the heater. This lets air escape as the tank fills.
3. Open the cold water inlet valve on top of the heater. You’ll hear water running.
4. Wait. The tank takes a few minutes to fill. When the hot water faucet flows steadily with no sputtering, the tank is full.
5. Leave the faucet open for another minute to ensure all trapped air is gone.
6. Check the drain valve for drips. If it leaks, tighten it slightly.

That’s it. The tank is now full and ready for power. Do not skip the faucet step. A full tank with a closed faucet still has air pockets at the top.

Step-by-Step: Powering the Unit and Setting the Thermostat

1. Flip the dedicated circuit breaker to the on position.
2. Wait about 15 minutes. The elements will heat the water gradually.
3. Locate the upper thermostat access panel. It’s the top panel on the side of the tank. Remove the screws and pull the insulation back.
4. You’ll see the thermostat dial, usually set to ‘Hot’ or 120°F. The dial has a slot for a flathead screwdriver.
5. Turn the dial to your desired temperature. 120°F is the standard recommendation for safety and efficiency.
6. Replace the panel and insulation. Do the same for the lower thermostat if you want to adjust it, but the upper thermostat is the primary control.

After 30 minutes, check the hot water faucet. If it’s hot, you’re done. If not, the unit might have tripped a high-limit switch, which I cover later.

How to Turn On a Gas Rheem Water Heater

Gas units are more involved because you’re dealing with combustion. The process depends on whether your model has a standing pilot light or an electronic ignition system. Check the gas valve for a knob with ‘Pilot’, ‘On’, and ‘Off’ positions. If you see a red or black button labeled ‘Pilot’, you have a standing pilot. If there’s no knob, just a small digital display or a simple on/off switch, you have electronic ignition.

Step-by-Step: Lighting the Standing Pilot Light

1. Turn the gas valve knob to ‘Off’. Wait five minutes. This lets any gas in the combustion chamber dissipate.
2. Remove the access panel covering the burner. You’ll see the pilot tube and the thermocouple.
3. Turn the knob to ‘Pilot’. Press and hold the knob down. This opens the gas valve to the pilot.
4. While holding the knob, use your long-reach lighter to ignite the pilot. The flame should be a small, blue cone wrapping around the thermocouple.
5. Keep holding the knob for 30 to 60 seconds. This heats the thermocouple, which generates a small voltage that keeps the gas valve open.
6. Release the knob. The pilot flame should stay lit. If it goes out, wait five minutes and repeat.
7. Turn the knob to ‘On’.
8. Replace the access panel.

The burner should ignite shortly after you turn the knob to ‘On’. You’ll hear a soft whoosh. Give it a minute, then check the draft hood on top of the tank. You should feel warm air rising.

Step-by-Step: Electronic Ignition Models (Direct Spark)

Newer Rheem gas models use a spark igniter and a flame sensor instead of a standing pilot. There’s no knob to hold. The startup is much simpler.

1. Ensure the gas shut-off valve on the supply line is open. The handle should be parallel to the pipe.
2. Plug the unit in or ensure the wiring is connected to a standard outlet.
3. Turn on the power switch, usually located on the side of the unit or behind a small access door.
4. Set the temperature on the digital display or dial.
5. The unit will run an ignition sequence. You’ll hear a clicking sound (the spark) followed by the burner firing.

If the ignition sequence fails, the unit will flash an error code on the display. That’s your cue to check the troubleshooting section. Do not keep cycling the power. Repeated failed ignitions can flood the combustion chamber with gas.

Adjusting the Temperature Dial for Efficiency and Safety

The temperature dial is a trade-off. Higher settings deliver hotter water but increase standby heat loss and the risk of scalding. Lower settings save energy but might not be hot enough for a dishwasher or a large family.

120°F is the sweet spot for most households. It’s hot enough for showers and cleaning, and it slows sediment buildup in the tank. If you have a dishwasher without a built-in heater, you might need 130°F. For households with young children or elderly residents, consider 115°F.

Set the dial at the midpoint and test the water temperature at the faucet after an hour. Use a simple cooking thermometer. If it’s too hot, dial it down a notch. If it’s too cool, dial it up. Small adjustments make a noticeable difference.

Remember, the dial on the thermostat is not a precise gauge. It’s a reference point. The actual water temperature depends on the tank’s location, the ambient air temperature, and the amount of sediment in the tank.

How to Use Vacation Mode and the ‘Low’ Setting

Most Rheem gas valves have a ‘Vacation’ or ‘Low’ setting. This is not the same as turning the unit off. Vacation Mode keeps the pilot lit but prevents the burner from firing. Your water stays at room temperature, but you avoid the hassle of relighting the pilot when you return.

To set it, turn the gas valve knob to ‘Vacation’. For electric models, you can use the ‘Low’ setting on the thermostat, or simply turn the breaker off. On electric models, Vacation Mode isn’t a standard feature, so the breaker is your best bet.

Why use Vacation Mode instead of shutting the gas off completely? Two reasons. First, relighting a pilot is a pain. Second, turning the gas off can allow moisture to accumulate in the combustion chamber, leading to rust over time. Vacation Mode avoids both issues.

For tankless models, check the control panel. Many have a ‘Standby’ or ‘Power Save’ mode that serves the same purpose. It keeps the electronics warm but stops the burner.

Troubleshooting: What to Do If It Won’t Start

So you’ve followed the steps, but there’s no hot water. Here’s where the decision tree gets interesting. The fix depends on the symptom.

Decoding Blinking Lights and Error Codes

Newer Rheem models have a status light on the gas valve or a digital display. Blinking patterns correspond to specific faults. Here’s a quick reference for common codes on gas models:

Blink Code / Display	Meaning	What to Do
1 blink	Ignition failure	Check gas supply, ensure shut-off valve is open, inspect the igniter.
2 blinks	Flame sense fault	Clean the flame sensor with fine sandpaper. The sensor is a small rod near the burner.
3 blinks	Overheat	Check the thermal cutoff switch. May require professional service.
4 blinks	Blower failure (condensing models)	Check the vent for blockages. If clear, the blower motor may need replacement.
E1 / E2 on display	Thermistor failure	Thermistor is a temperature sensor. Likely needs replacement.

For electric models, the diagnostic is simpler. If you have power but no hot water, the high-limit switch likely tripped. This switch sits behind the upper thermostat access panel. It’s a small red button.

Press the red button firmly. If it clicks, the switch was tripped. Wait a few minutes and see if the heater starts. If it trips again immediately, you have a deeper issue, likely a shorted element or a thermostat problem.

The Thermostat Reset Button (High-Limit Switch)

The high-limit switch is a safety device. It cuts power if the water temperature exceeds about 170°F. A tripped switch means something went wrong, so don’t just reset it blindly.

First, check for a leaking tank or a stuck thermostat. If the water is scalding hot, the thermostat failed and the element ran continuously. Reset the switch only after you’ve verified the tank is full and the thermostat is set to a reasonable temperature.

To reset, remove the upper access panel, press the red button, and replace the panel. If the breaker trips again or the button pops out again, call a professional. This is not a DIY repair.

When to Call a Professional vs. DIY

Relighting a pilot or resetting a breaker is DIY territory. Anything beyond that gets risky. Replacing a gas valve, a thermocouple, or a heating element requires specialized tools and knowledge. A mistake can cause a gas leak or an electrical short.

Call a professional if you smell gas, if the unit makes popping or rumbling noises (that’s sediment boiling, which can eventually crack the tank), or if the error codes persist after basic troubleshooting. Also call if the pressure relief valve is leaking. That valve is a critical safety component.

One more thing: if your tank is more than 10 years old and needs a major repair, consider replacement. The repair bill might be half the cost of a new Rheem water heater, and the new unit will be more efficient.

Before you call, check if the unit is still under warranty. Rheem tanks typically have a 6- or 10-year warranty on the tank and parts. The labor is on you, but the parts might be free.

Frequently Asked Questions (FAQ)

How long should I wait for hot water after turning on my Rheem?

For a gas unit, expect hot water in 20 to 30 minutes. Electric units take longer, about 45 to 60 minutes for a full tank. The recovery rate depends on the BTU rating for gas or the wattage of the elements for electric. A 40,000 BTU gas burner heats faster than two 4,500-watt electric elements.

Why does my Rheem pilot light keep going out?

The thermocouple is the usual culprit. It’s a safety device that senses the pilot flame. If it’s dirty or worn out, it won’t generate enough voltage to keep the gas valve open. Clean it with fine sandpaper first. If that doesn’t work, replace it. It’s a cheap part and a common repair. A bad thermocouple is one of the most frequent reasons for a Rheem water heater restart.

Is it okay to turn off my water heater when I go on vacation?

For a gas unit, use the Vacation setting on the gas valve. It keeps the pilot lit, which prevents moisture buildup, but stops heating. For an electric unit, just flip the breaker off. That’s the equivalent of Vacation Mode. Turning off the gas supply entirely is not recommended for long periods.

What does the ‘A’ and ‘B’ mean on my Rheem thermostat?

Those letters refer to the temperature calibration. ‘A’ is the lowest setting, ‘B’ is the highest. The dial has a range between them. The exact temperature varies by model, but ‘A’ is usually around 90°F and ‘B’ is around 150°F. The midpoint is typically 120°F.

Can I turn on my Rheem water heater without filling it with water first?

Never do this. For electric models, turning on the power without water will burn out the heating elements instantly. For gas models, running the burner without water can crack the tank from thermal stress. Always fill the tank and purge the air before applying power or gas.

What to Remember When You’re Done

• Always confirm the tank is full before flipping the breaker or lighting the pilot.
• Set the thermostat to 120°F for the best balance of safety, comfort, and energy use.
• Use Vacation Mode on gas units instead of shutting the gas off completely.
• Learn what your model’s blinking light codes mean; they’ll save you a service call.
• If the high-limit switch trips more than once, stop resetting it and call a pro.
• Check the pressure relief valve annually by lifting the lever briefly to ensure it’s not stuck.
• Keep the area around the base of the heater clear of clutter and flammable materials.

Getting your Rheem running is usually a 15-minute job. The hardest part is knowing which of the four configurations you own. Once you’ve got that figured out, the steps above will get you hot water. If you hit a snag, the pilot light guide covers the gas models in more detail.