Start with the tag on the old tank, if there is one. The install year and the model number tell you how far into its service life the unit already sits. That year is your planning input for the next install, so write it down before anything comes off the wall.
Before anything comes off the wall
Check whether your area requires a permit for a water heater swap. Many do. Ask who pulls it and whether the inspection happens before the connections are closed up. The permit paper is your record of who did the job.
Confirm that the new unit matches what is already there. Fuel type, venting type, and the electrical supply all have to match the nameplate. If the existing gas tank vents through a chimney, the new vent has to fit that path. If anything is off, stop and ask the installer before you cut a line.
Set the pan and the drain
Put the pan under the tank before the tank goes in. Where a leak would reach living space, a pan with its own drain line is the usual approach. Route the pan drain to a floor drain or to a spot you can see from the doorway. A drain that empties into a hidden cavity hides the first sign of a leak.
Set the tank level. Leave room in front for the valves, the drain, and future service. Keep combustibles back by the distance the manual lists. Read the manual, not your memory of the last job.
Water connections
Connect cold to the inlet and hot to the outlet, and check the labels on both. Put a shutoff valve on each line so the tank can be isolated later. Where copper meets steel, use the dielectric fittings the manual calls for. Use only the thread sealant the manual names. Check each joint with a dry paper towel before you move on, and check again after the tank has heated.
The relief valve line
The temperature-and-pressure relief valve has its own discharge line. This is the part most often done wrong. The line should run from the valve to a safe discharge point, pitched so it drains, with nothing in it that can trap water. It should not be capped, reduced in a way the manual forbids, or routed upward. Ask the installer to show you where it ends.
Gas and electric connections
On a gas unit, check the gas line for a sediment trap, a short leg below the appliance inlet that catches debris. Confirm that the vent or flue is sized and sloped to the manual and that combustion air is available. Test every joint for leaks with the leak solution the trade uses, not with a flame.
On an electric unit, the circuit should be dedicated, the breaker matched to the nameplate, and the wire size matched to the breaker. If you are unsure what the panel can carry, the electrical panel page on this site lists what to check on the panel side. Electrical work is a licensed job in many places, so confirm that before anyone starts.
Seismic straps and final fit
Where local rules call for seismic straps, the strap goes around the upper part of the tank and anchors to wall studs, not to drywall alone. Push on the tank once it is set. It should not tip or walk.
Fill, purge, and test
Open the hot tap at the nearest sink and let the air clear until the water runs steady. Then close it. Check every joint for seepage. Set the thermostat to the position the manual gives. On a gas unit, light the pilot or start the unit as the label directs, and wait for the burner to cycle. Let the tank recover, then check the hot water at the tap.
Write it down before you close the room
Record the install date, model, and serial on the card you keep. Ask the installer to note the relief valve test and the pan drain route on the invoice or permit card. Keep the manual with the permit paper. When you plan the next swap, those papers will answer the questions the tag cannot.
Questions to ask before you sign
Ask who pulls the permit and when the inspection happens. Ask where the relief line ends and how the pan drains. Ask whether the installer checks the relief valve and the gas or electrical connections before leaving. Ask what the manufacturer warranty covers, how long the parts portion runs, and who needs to register the unit. Ask whether a service visit is included and what happens at that visit.
If a service plan is offered
A service plan or extended warranty is a separate contract from the install. Read it before you agree. Look for the waiting period, which is the time after the start date before a covered repair is accepted. Look for the start date, the list of covered parts, the exclusions, the cancellation and transfer rules, and what you receive if you cancel early. Those lines decide what you hold, so read them against your own tank's age and install year. For the published figures on how plans compare, use the tables in the index on this site rather than a sales page.