How to Install a Water Heater: A Step-by-Step Guide

Step 1: Turn Off the Power and Water Supply

Start by shutting off the energy and water supply to the existing water heater. For an electric water heater, switch off the dedicated circuit breaker at the electrical panel, remove the access cover if needed, and use a non-contact voltage tester to confirm the unit is de-energized. For a gas water heater, turn the gas control valve to the OFF position and close the manual gas shutoff valve on the supply line.

Next, close the cold-water inlet shutoff valve above or near the heater. If the unit does not have a working dedicated shutoff valve, close the home’s main water supply. Open a nearby hot-water faucet and let the water run until it cools, then leave the faucet open to relieve pressure, prevent a vacuum, and help the tank drain more freely in the next step.

Step 2: Remove the Old Water Heater (if replacing)

After shutting off the utilities and allowing the water to cool, drain the tank completely, disconnect the water and energy connections, detach any venting or discharge piping, and remove the old unit from the installation area. Follow the manufacturer’s removal instructions and use lifting assistance because tank-style water heaters can remain heavy even after draining.

Drain the Old Water Heater

Attach a garden hose to the drain valve near the bottom of the heater and direct the other end to a floor drain, bucket, sink, or suitable outdoor drainage area. Open the drain valve and keep a nearby hot-water faucet open to relieve pressure and help the tank drain more freely. If the heater was recently operating, let the water cool first or use a hose rated for hot water to reduce the risk of burns. Sediment buildup may also slow or block drainage.

To speed up draining, open a hot water faucet in another part of the house. This relieves pressure and lets the water drain more freely. Once the water drains, move on to disconnecting the lines.

Disconnect the Old Heater

Once the tank is empty, disconnect the hot and cold water lines using the appropriate wrenches or a tubing cutter, as needed, and keep towels or a bucket nearby for residual water. For a gas heater, confirm the gas is off before disconnecting the supply line at the union or flare fitting and detaching the vent connector. For an electric heater, verify the power is off before disconnecting the supply wires, ground wire, cable, and strain relief. Disconnect the T&P discharge pipe as required.

Remove the Old Heater

After you detach all water, gas, and electrical venting and discharge connections, remove any wall-mounting brackets, securing bolts, or other hardware holding the unit in place. Carefully lift or roll the heater out of the installation area using a hand truck or appliance dolly, and have a second person assist if needed. Check the floor and surrounding area for water, rust, corrosion, or damaged connections before positioning the new heater, then dispose of or recycle the old unit according to local requirements.

Step 3: Position the New Water Heater

After removing the old unit, place the new water heater in its final installation area before making any plumbing, gas, electrical, or vent connections. Confirm the floor is level and can support the filled unit, provide the manufacturer-required clearance around the heater, and keep the controls, drain valve, T&P valve, and service areas accessible.

Move the New Water Heater into Position

Carefully move the heater into place using an appliance dolly or a second person if needed, then set it on a stable, level surface. Leave enough room for the hot and cold water lines, gas or electrical connections, maintenance access, and any required drain pan or seismic bracing. Water heater installation in San Jose requires a watertight pan with at least a 3/4-inch drain where leakage could cause damage. Water heaters up to 52 gallons require two seismic straps, while 75-gallon units require three and 100-gallon units require four. The upper strap must be within the upper third of the tank, and the lower strap must be within the lower third and at least 4 inches above the controls.

Ensure Proper Ventilation (for Gas Models)

For a gas water heater, position the unit so the vent connector or direct-vent system can route combustion gases outdoors without obstruction. Keep combustion-air openings clear and maintain the manufacturer-specified clearance from combustible materials. The U.S. Department of Energy notes that atmospherically vented water heaters require adequate combustion and dilution air to operate properly and reduce the risk of combustion gases entering the living space. Where seismic restraint is required, secure the tank with approved straps after positioning it.

Step 4: Connect the Water Supply Lines

Connect the cold-water supply to the heater inlet and the hot-water line to the outlet using fittings approved for the pipe material and water heater model. Keep connections accessible, follow the manufacturer’s instructions, and separate dissimilar metals where required to reduce corrosion.

Attach Cold and Hot Water Supply Pipes

Connect the cold-water line to the inlet and the hot-water line to the outlet, using threaded, press, push-fit, or sweat fittings as permitted for the installation. Install a shutoff valve on the cold-water line and use dielectric fittings when joining dissimilar metals such as copper and steel. For example, San Jose requires insulating the first 5 feet of both hot and cold water lines in certain water heater installations, and its basement installation requirements show a 3/4-inch union within 12 inches.

Inspect for Leaks

Open the cold-water shutoff valve slowly and allow the tank to fill while keeping a nearby hot-water faucet open to release trapped air. Once a steady stream of water flows, close the faucet and inspect the hot and cold connections, valves, unions, and fittings for leaks. San Jose requires potable water systems to be tested with actual water pressure or 50 psi of air for at least 15 minutes before approving concealed work.

Step 5: Connect the Power or Gas Supply

After connecting the water lines, connect the heater to its required energy source. Electric and gas water heaters use different connections, so follow the manufacturer’s wiring, gas piping, grounding, venting, and startup instructions for the specific model.

For Electric Water Heaters:

1. Keep the dedicated circuit breaker OFF while making electrical connections.
2. Confirm the circuit voltage and amperage match the water heater nameplate.
3. Route the supply cable through the approved junction box or wiring compartment.
4. Connect the supply conductors and equipment grounding conductor according to the manufacturer’s wiring diagram.
5. Tighten terminal connections and reinstall all electrical covers before restoring power.
6. Do not energize the heater until the tank is completely full and you have purged air from the hot-water piping.
7. If the installation requires a new circuit or electrical upgrade, check local permit requirements before proceeding.

For Gas Water Heaters:

1. Confirm the manual gas shutoff valve remains OFF while connecting the appliance.
2. Connect the gas supply using approved piping, connectors, fittings, and gas-rated thread sealant where applicable.
3. Install any required sediment trap or drip leg according to the manufacturer and local code.
4. Confirm the gas pipe is adequately sized for the heater’s BTU input, especially when replacing the unit with a higher-capacity model.
5. Connect the venting system and keep required combustion-air openings clear before startup.
6. Open the gas valve only after all connections are complete, then test every new gas joint for leaks using an approved leak-detection solution.
7. Do not ignite or operate the heater if you detect a gas leak.

Step 6: Install the Temperature and Pressure Relief Valve (T&P Valve)

Install an approved temperature and pressure relief valve in the designated opening on the top or side of the water heater. The T&P valve automatically releases water if the tank reaches excessive temperature or pressure, so its relief rating must meet or exceed the water heater’s heating input, and it must not be altered or blocked.

1. Apply the manufacturer-approved thread sealant to the valve threads and tighten the T&P valve into the designated opening.
2. Attach a discharge pipe no smaller than the relief valve outlet and one that does not reduce the internal diameter.
3. Do not install a shutoff valve, cap, plug, or other restriction in the discharge line.
4. Route the discharge pipe independently rather than connecting it to a drain pan, A/C condensate line, or sanitary sewer.
5. Keep the terminal end unthreaded and point it downward at an approved discharge location.
6. Follow local termination requirements. For example, San Jose generally requires an exterior T&P discharge to terminate 6 to 24 inches above the ground. Garage installations may terminate 6 to 12 inches above the garage floor.
7. Install an expansion tank where required in a closed plumbing system, such as a home with a pressure-reducing valve on the water service.

Step 7: Fill the Tank with Water

Before restoring power or gas, close the drain valve, then fully open the cold-water supply valve to begin filling the tank. Open the nearest hot-water faucet to let trapped air escape and keep it open until water flows in a steady stream without sputtering.

Continue running the hot-water faucet for about 3 minutes after a steady flow begins to help purge remaining air from the tank and piping. Then close the faucet and inspect the cold-water inlet, hot-water outlet, valves, unions, and other plumbing connections for leaks. Do not energize an electric water heater until the tank is completely full because operating it dry can damage the heating elements.

Step 8: Power or Gas Up the Heater

Once the tank is full, you have purged all trapped air, and you have checked the water and energy connections for leaks, restore the heater’s power or gas supply according to the manufacturer’s startup instructions.

For Electric Water Heaters

Turn the dedicated circuit breaker back ON only after confirming the tank is full of water. Check the control panel or status display, if equipped, and listen for normal startup operation. Applying power before the tank is full can dry-fire and damage the heating elements.

For Gas Water Heaters

Open the manual gas shutoff valve and follow the heater’s lighting instructions to start the pilot or electronic ignition system. Confirm the burner ignites properly, check the gas connections again for leaks, and verify that the status light or control display shows normal operation. Do not continue startup if you smell gas, detect a leak, or the burner repeatedly fails to ignite.

Step 9: Set the Desired Temperature

Set the water heater thermostat to the temperature recommended for your household and specific model. For most residential applications, the U.S. Department of Energy recommends 120°F (49°C), which can reduce energy use while lowering the risk of tap-water scalding. The CPSC also recommends 120°F to help reduce burn injuries.

For a gas water heater, adjust the temperature using the gas control or thermostat dial. Electric models may have one or more thermostats behind access panels, so turn off power before making adjustments. After changing the setting, let the heater reach the selected temperature, then verify the actual hot-water temperature at a faucet with a thermometer before regular use.

Step 10: Test the Heater

After the water heater reaches its set temperature, test the system before regular use. Confirm that hot water flows at the expected temperature, all connections remain dry, and the heater starts and operates without fault codes, repeated shutdowns, or unusual sounds.

Check the following before considering the installation complete:

• Hot-water flow: Open a hot-water faucet and confirm a steady flow at the expected temperature.
• Water connections: Inspect the hot and cold lines, valves, unions, drain valve, and surrounding area for leaks.
• T&P system: Check the T&P valve and discharge piping for unintended dripping or leakage.
• Electric operation: Confirm the breaker remains on and the heater operates without tripping or displaying an error.
• Gas operation: Verify normal ignition and burner operation, and recheck all gas connections for leaks with an approved leak-detection solution.
• Venting: For gas models, confirm the vent and combustion-air connections are secure and operating as intended.
• Controls: Check the thermostat, indicator lights, or digital display for normal operating status.
• Unusual operation: Stop and recheck the installation if you notice persistent hissing, popping, rumbling, gas odor, leaks, or repeated shutdowns.

Manufacturer installation checklists from Rheem and A.O. Smith similarly call for confirming the tank is full, air is purged, water connections are leak-free, gas connections pass a leak test, and venting is correctly connected before placing the heater into normal service.

Step 11: Clean Up

After confirming the water heater operates correctly and has no leaks, clean the installation area and remove all leftover materials. Before leaving the unit in regular operation, make sure nothing blocks the heater, venting, combustion-air openings, drain pan, shutoff valves, or access panels.

Complete these final cleanup steps:

• Remove all packaging and debris from around the water heater.
• Wipe up spilled water and check the floor and nearby connections for moisture.
• Collect old fittings, valves, piping, and wiring for proper disposal.
• Clear tools and materials away from access panels, valves, venting, and burner areas.
• Remove combustible items from around gas water heaters and ignition sources.
• Dispose of the old water heater properly according to local recycling or bulky-item rules.
• Drain the old tank completely before disposal or pickup.
• Make a final leak and operation check before leaving the heater in service.

For San Jose, the City lists water heaters as accepted Junk Pickup items and requires the tank to be fully drained before collection. Residents can schedule no-cost curbside Junk Pickup for up to 12 bulky items per appointment through San José 311.