Most homeowners have never watched a water heater installation happen. Knowing what to expect — the sequence of steps, how long each takes, and what to watch for — helps you evaluate whether your installation was done correctly and what questions to ask when it's complete. Here is a complete, honest walkthrough of a professional water heater replacement.
Clear the area around your water heater — move stored items at least 3 feet away on all sides. Locate your cold water inlet valve (at the top of the unit on the cold supply pipe) and your gas shutoff valve or electrical breaker, and make sure you can access both. Make sure an adult 18 or older will be home for the entire installation window.
The technician arrives, introduces themselves, and confirms the work order — the unit being installed, the address, and what was booked. Before starting any work, they should do a brief assessment of the installation space: confirming the new unit will physically fit, checking the existing venting configuration, verifying the gas pressure or electrical supply, and noting any existing code deficiencies that need to be addressed as part of the installation.
This assessment is where a thorough technician earns their fee. Discovering that the gas line needs to be upsized for a tankless conversion, that the vent pipe has corroded sections that need to be replaced, or that an expansion tank is required — all of this should be identified and communicated to you before work begins, not after.
The technician shuts off the gas supply at the unit's gas valve and confirms the pilot is out (for older standing pilot units). For electric units, the circuit breaker is turned off. The cold water inlet valve at the top of the unit is closed. A hot water tap in the house is opened to relieve pressure from the system and allow air in for drainage.
A hose is connected to the drain valve at the base of the unit and run to a floor drain or outside. The drain valve is opened and the tank is emptied. This is often the most time-variable step — a 50-gallon tank full of sediment can take 35–40 minutes to drain fully versus 20 minutes for a clean tank. Heavily sediment-laden water may also clog the drain valve, requiring the technician to work around it.
Once drained, the gas connection is disconnected at the union fitting. Water supply lines are disconnected at the inlet and outlet. The flue connector is detached from the draft hood. Seismic straps (in California) are removed. The old unit is now free-standing and ready to remove.
The old unit is moved out of the installation space and to the technician's vehicle for haul-away. Water heaters are heavy — a drained 50-gallon tank weighs approximately 120–150 pounds — so moving it through tight spaces takes care and sometimes a dolly. The installation space is cleared and inspected.
The new unit is moved into position. Its location is adjusted to provide the required clearances from walls and combustibles per the manufacturer's installation instructions. For installations on elevated surfaces (required for garage installations), the stand or platform is positioned first.
Water supply lines are connected to the inlet and outlet. Dielectric unions are installed if copper pipe is connecting to a steel unit (prevents galvanic corrosion). Gas connection is made using a flexible connector and union fitting. For electric units, the wiring is connected per the manufacturer's wiring diagram and the appropriate breaker amperage is confirmed. The flue connector is attached to the draft hood and all vent joints are properly secured.
If an expansion tank is required (and it usually is in modern plumbing systems), it's installed on the cold water inlet pipe with the port facing down or horizontal, pre-charged to the incoming water pressure. The T&P relief valve is installed in the designated port on the unit (or confirmed to be the correct rating if reusing the existing valve) and the discharge pipe is configured per code.
In California, two seismic straps are installed — one in the upper third of the unit and one in the lower third — anchored through the unit jacket to structural wall studs or a backing board.
The cold water supply is restored. Hot water taps throughout the house are opened and left open until water flows steadily from them — this purges air from the system and confirms the tank is full. The taps are then closed. For gas units, the pilot is lit (if applicable) and the burner is fired. All gas connections are tested with an approved leak detection solution — every single joint. The unit is allowed to heat to operating temperature and the delivery temperature is confirmed at a tap. For electric units, the breaker is restored once the tank is confirmed full.
A thorough technician finishes by walking you through the new unit: where the thermostat is and how to adjust it, where the gas shutoff valve is and how to close it, how to relight the pilot if it goes out (for units with a standing pilot), what the T&P valve is and how to test it annually, and what basic maintenance is recommended. You receive documentation of the installation including the unit model and serial number, the technician's license number, and the installation date — everything you need to register the warranty and support a future claim.
A standard like-for-like gas tank replacement takes 2 to 3.5 hours from arrival to departure for an experienced technician in a straightforward installation. Add 30–60 minutes for complications: heavy sediment, corroded connections, venting work, seismic strapping replacement, or a tankless conversion. Budget a full day for a tankless conversion — typically 4 to 7 hours.
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