How to Install an Electric Tankless Water Heater

Electrical Panel Check: Make Sure Your Home Has the Power

Before you remove your old water heater or buy a new unit, you must check your home electrical capacity. Traditional water heaters hold 40 to 50 gallons of water in a large tank and warm it slowly using a single 30-amp circuit. An electric tankless unit works differently. It heats ice-cold water instantly as it flows past electric elements. To heat water that fast, a whole-house tankless unit requires a huge amount of electrical power all at once.

Most whole-house electric tankless units require between 18 kilowatts and 36 kilowatts of power. That equals a total draw of 80 to 150 amps. Because of this massive demand, your home must have an adequate electrical setup before you can safely install one. Take a flashlight, walk over to your service panel (the main electrical box feeding power to the house), and run through this quick checklist:

• Check your main service rating: Look at the large main breaker (the master switch at the very top or bottom of your electrical box). It should have a number stamped on the handle, usually 100, 150, or 200. Most whole-house tankless heaters require a 200-amp service panel. If your home has a 100-amp or 150-amp panel, you cannot safely run a whole-house unit without overloading your system.
• Count your open breaker slots: Whole-house units do not run on a single switch. They typically require two, three, or four separate double-pole breakers (thick double switches used for heavy 240-volt appliances). Each double-pole breaker takes up two vertical slots in your panel. That means you need between four and eight completely empty slots in your box.
• Check your wire thickness: Tankless units require heavy-gauge copper wiring, usually 8-gauge or 6-gauge wire. Standard household outlets use thinner 14-gauge or 12-gauge wire. Thinner wire cannot handle the heavy current and will melt or cause a fire. You will need to run new, dedicated heavy cables directly from the panel to the heater location.
• Consider a single-point unit for small spaces: If your panel cannot support a whole-house model, you might only need a small point-of-use unit for an isolated sink. These small units often run on a single 30-amp breaker and fit under a counter, making them much easier to power without upgrading your entire panel.

Step-by-Step Guide to Installing an Electric Tankless Water Heater

Once you confirm your electrical panel has enough room and capacity, you can begin the physical installation. Gather your tools before you start. You will need an adjustable wrench, a tubing cutter, a drill, a level, wire strippers, and a non-contact voltage sensor (a small wand that lights up or beeps when live electricity is nearby). Follow these steps in order to get your unit installed safely.

1. Shut off the electrical power and the main water supply.

   Go to your service panel and switch off the breaker connected to your current water heater. If you are adding new breakers, turn off the main service disconnect if you plan to access the panel. Then, find your home main water shutoff valve and turn it clockwise until the water stops flowing. Open the lowest hot water faucet in your house to drain the residual water from the pipes.

   Why this step matters: Turning off both utilities protects you from high-voltage shocks and prevents pressurized water from flooding your work area while you disconnect fittings.

2. Disconnect and remove your old water heater.

   Attach a garden hose to the drain valve at the bottom of your old tank and empty it completely into a floor drain or outside. Use an adjustable wrench or pipe wrench to disconnect the cold water inlet and hot water outlet pipes. If your old unit was electric, disconnect the electrical wiring from the junction box on top of the tank. Remove the tank from the room to give yourself plenty of open space to work.

   Why this step matters: Clearing the old tank out of the way lets you inspect the wall studs, measure straight pipe runs, and mount your new compact unit at a comfortable working height.

3. Mount the tankless unit securely to the wall.

   Remove the front cover of your new tankless water heater according to the manual. Hold the unit or its mounting bracket against the wall at eye level and check it with a level. Drive heavy wood screws directly into wall studs. If you cannot align the unit with two studs, screw a solid three-quarter-inch piece of plywood across the studs first, and then screw the heater bracket directly into the plywood.

   Why this step matters: A full water heater vibrates slightly whenever water valves open and close throughout the home. Anchoring the box firmly to solid wood prevents the plumbing joints from loosening over time.

4. Install isolation valves on both water connections.

   Screw a set of tankless isolation valves (special shutoff valves with side ports that let you flush cleaning solution through the unit) directly onto the cold and hot water nipples at the base of the heater. Wrap the male threads with plumbing tape and apply a thin layer of pipe thread sealant before tightening the valves with two wrenches. Point the side service ports forward so you can reach them easily in the future.

   Why this step matters: Mineral scale builds up inside compact heating chambers over time. Isolation valves allow you to flush the heater clean every year without shutting off running water to the rest of your house.

5. Connect the household plumbing lines.

   Run your cold water supply line to the cold water inlet valve, which is usually marked with blue plastic or an arrow pointing inward. Connect the hot water distribution pipe to the hot outlet valve, which is marked with red plastic. You can use copper pipe, stainless steel flexible water heater connectors, or heat-rated PEX tubing. Check your local plumbing code, as some areas require at least 18 inches of copper or braided metal pipe before transitioning to plastic PEX.

   Why this step matters: Using proper fittings and heat-rated connectors prevents leaks under pressure and protects plastic water pipes from hot water back-siphoning.

6. Install a temperature and pressure relief valve if required.

   Check your local plumbing codes to see if a temperature and pressure relief valve (a safety valve that pops open if the water gets dangerously hot or builds excessive pressure) is required on your unit. Many tankless heaters have a dedicated port for this valve on the hot water side. Screw the valve into place and run a copper discharge pipe straight down toward the floor, ending roughly six inches above the ground.

   Why this step matters: This valve acts as an emergency pressure release, preventing the water lines from bursting if the electronic controls ever fail.

7. Run dedicated electrical cables from the panel to the heater.

   Pull the required number of heavy electrical cables from your main service panel through your wall studs or inside protective metal conduit to the water heater. Strip about eight inches of the outer cable jacket at each end. Feed the wires through knockout holes in the heater cabinet, securing them with approved cable clamps so the sharp metal edges cannot cut into the wire insulation.

   Why this step matters: Each individual heating element inside the box pulls heavy electrical current and needs its own direct path back to the panel to prevent wires from overheating inside your walls.

8. Wire the double-pole breakers and unit terminals.

   Inside the water heater, attach the black and red hot wires to the corresponding terminal screws for each heating element. Fasten the bare copper ground wires to the green grounding bar inside the chassis. At the service panel, snap the correct double-pole breakers into empty slots. Connect each pair of colored wires to the breaker lugs and attach the ground wires to the panel ground bus bar. Tighten all screws firmly using the torque specs listed in the manual.

   Why this step matters: Loose electrical connections create electrical resistance, which causes wires to spark, melt terminal blocks, or trip breakers unexpectedly.

9. Purge all air from the plumbing lines before turning on the power.

   Keep the electrical breakers switched off. Slowly turn your main water supply back on. Open the cold and hot isolation valves on your water heater. Walk through your house and open every hot water faucet completely. Let the water run for two to three minutes until all sputtering, hissing, and air bubbles stop, leaving a solid, quiet stream of cold water flowing through the system.

   Why this step matters: This step completely floods the heating chambers. If you turn on the power while an air bubble is trapped inside, the dry element will melt in seconds.

10. Turn on the circuit breakers and verify operation.

    Inspect all pipe joints and isolation valves one more time to confirm there are no water drips. Replace the front cover on the water heater and screw it down. Go to the service panel and switch on all the double-pole breakers feeding the unit. The digital screen on the front of the heater should light up. Turn on a hot water faucet, set your desired temperature on the display, and confirm that hot water begins flowing steadily.

    Why this step matters: Putting the system under full electrical load while water is running lets you confirm that the control board reads the flow rate and warms the water without tripping breakers.

Why You Must Purge Water Lines Before Turning on the Power

Skipping the water purging step is the single most common mistake people make when installing an electric tankless water heater. It results in an immediate failure known as dry fire (turning power on with air in the chamber, melting the heating element). Understanding what happens inside the heater helps explain why this step cannot be rushed.

Inside the heater, water passes through small copper or stainless steel chambers containing electric heating elements. These elements are designed to transfer an intense amount of heat directly into cold, moving water. Water absorbs this energy instantly, which keeps the metal elements from overheating. When you open a brand new heater, those heating chambers are filled with nothing but dry air.

Air cannot absorb heat like water does. If you flip the electrical breakers while air pockets remain trapped around the elements, the metal coils will reach temperatures over 1,000 degrees Fahrenheit in less than two seconds. The element will glow bright red, blister, and melt in half. Once an element suffers dry fire damage, it is permanently ruined. The unit will not produce hot water, and you will have to buy and install replacement heating elements before you can use the heater.

To purge the lines properly, leave every electrical breaker off. Open the cold water supply valve to the heater, then open the hot water isolation valve. Walk to every sink, shower, and tub in your house and turn the hot water tap fully open. You will hear loud knocking, hissing, and sputtering as trapped air is pushed out of the pipes. Let the water run continuously until it flows in a smooth, quiet stream from every tap for at least two full minutes. Only then is it safe to touch the electrical breakers.

Permits and When to Call a Licensed Professional

Installing an electric tankless water heater is a rewarding project, but it involves serious electrical work. Mounting the cabinet on the wall, cutting copper pipes, and installing isolation valves are straightforward jobs for someone with basic plumbing experience. Running multiple 240-volt circuits into a main service panel is an entirely different matter.

Working inside a service panel carries a real risk of severe electrical shock or electrocution. Even with the main breaker turned off, the thick service entrance cables coming from the street into the top of the box remain energized and live at all times. If you do not have experience working inside a live panel, sizing heavy-gauge wire, or torqueing breaker terminals properly, hire a licensed electrician to handle the electrical hookup.

Building permits are another reason to involve professionals. Most cities and counties legally require both a plumbing permit and an electrical permit when replacing a storage tank with a tankless water heater. A local building inspector will visit your home to check that the wire size matches the breaker capacity, the grounding is correct, and the unit complies with local safety codes. If you ever sell your house or file an insurance claim after an electrical fire, having permitted, inspected work protects you legally and financially.

Special living situations also bring unique electrical limits. Manufactured homes have strict electrical limits and specific space constraints; if you are upgrading systems in a manufactured property, checking our guide on the best heater for mobile home spaces can help you understand how electrical layouts differ. Furthermore, if you are installing your water heater in an unheated space like a garage or basement workshop, you must protect exposed water lines from freezing temperatures during winter. Adding the best electric heater for garage with thermostat control can keep the ambient air well above freezing so your pipes never burst.

Call a licensed electrician if you notice any of the following conditions in your home:

• Your main service panel is rated for 100 amps or 150 amps and needs an upgrade to 200 amps.
• Your panel has no remaining empty slots for double-pole breakers.
• You have an older fuse box or a panel with aluminum branch wiring.
• You feel uncertain about how to pull heavy 6-gauge or 8-gauge cables through walls safely.

Frequently Asked Questions

Can I plug an electric tankless water heater into a standard wall outlet?

No, you cannot plug a whole-house or point-of-use water heater into a standard 120-volt household wall outlet. Standard wall outlets only provide 15 or 20 amps at 120 volts, which is far too weak to heat flowing water instantly. Electric tankless heaters require hardwired 240-volt connections running on dedicated heavy-duty circuits directly from your main electrical panel.

Can I connect PEX pipe directly to my electric tankless water heater?

In many areas, local plumbing codes require you to install at least 18 inches of copper or braided stainless steel pipe directly out of the heater before connecting to plastic PEX lines. While electric units do not have hot exhaust vents like gas models, the water immediately leaving the heating chamber can still get hot enough to soften plastic fittings over time. Always check your unit manual and local building codes before attaching PEX directly to the isolation valves.

How far can the water heater be from the main electrical panel?

You can install the water heater almost anywhere in your home, but longer wire runs require thicker electrical cables. If the distance between your service panel and the heater exceeds 50 feet, electrical resistance causes a drop in voltage that can reduce the heater output and cause the wires to run hotter. If you must mount the unit far from the panel, you may need to increase the wire thickness by one size to prevent voltage loss.

How often does an electric tankless water heater need maintenance?

You should flush your electric tankless water heater once every 12 months using ordinary food-grade white vinegar. Water contains dissolved minerals like calcium and magnesium that bake onto the surface of the heating elements, forming a hard crust called scale. Running vinegar through the isolation valves with a small pump for 45 minutes dissolves this scale, restores heating speed, and keeps the elements from burning out prematurely.