Water Heater Installation, Step by Step

Water heater installation is one of those jobs where every step matters. Get the sizing wrong and the household is taking cold showers within minutes. Get the venting wrong on a gas unit and the safety stakes are real. Here's a plumber's step-by-step look at what a proper water heater installation involves, what gets decided before the truck pulls up, and where things go sideways.

Step 1: Sizing the Tank

Before any wrenches come out, the new unit has to be sized right. The two main numbers to look at:

• Tank capacity (gallons). A typical 2-bathroom Nashville home runs on a 40 or 50-gallon tank. A larger home with multiple showers, a tub, and a teenage household leans toward 50 to 75 gallons.
• First-hour rating. How much hot water the unit can deliver in the first hour of heavy use, accounting for both the tank capacity and the recovery rate.

Under-sizing leads to running out during back-to-back showers. Over-sizing wastes energy heating water nobody uses. A plumber sizes based on the number of bathrooms, the household's hot water habits, and whether there's a soaking tub in play.

Step 2: Gas vs. Electric (and Tank vs. Tankless)

The decision tree:

• Existing gas tank. Cheapest swap. Replace with a new gas tank of the same fuel type.
• Existing electric tank. Cheapest swap. Replace with electric.
• Gas tank to gas tankless. A bigger project — different venting, different gas line sizing, and a wall-mount location instead of a closet floor.
• Electric tank to electric tankless. Almost always requires a major electrical upgrade. Electric tankless units pull enormous current.
• Fuel switch. Going from gas to electric, or electric to gas, is a major project with significant utility work involved.

Step 3: Connections That Need to Be Right

Every water heater install has the same set of connections that have to be done correctly:

1. Cold water inlet — the "in" side. A shutoff valve goes here so the heater can be isolated for future service.
2. Hot water outlet — the "out" side. Goes to the rest of the house.
3. Temperature and pressure relief (T&P) valve — the safety valve that opens if temperature or pressure climb dangerously high. The discharge line must run to a safe location, usually within six inches of the floor, never plugged or threaded.
4. Drain line / pan — most local codes require a drip pan with a drain line under any water heater installed where a leak could damage the structure.
5. Expansion tank — required in any system with a check valve or pressure-reducing valve on the incoming water. It gives heated, expanding water somewhere to go.
6. Power or gas — wired correctly with a disconnect for electric, properly sized gas line and shutoff for gas.
7. Vent — for gas units, the flue connection has to draft correctly with no negative pressure issues.

Step 4: The Actual Install Sequence

Once the new unit and parts are on site, the work goes like this:

1. Shut off water at the cold inlet of the old unit. Shut off gas or kill the breaker.
2. Connect a hose to the drain valve and run it to a floor drain or outside. Open a hot tap upstairs to let air in.
3. Drain the old tank. (This can take 20 minutes on a 50-gallon unit. Older tanks sometimes drain slowly because the dip tube and drain valve have scale buildup.)
4. Disconnect the supply lines, the T&P discharge, the vent (gas), the wiring or gas line.
5. Move the old tank out. Place the new tank in its spot, perfectly level.
6. Drop in a new drain pan if needed.
7. Install fresh dielectric unions or flex connectors at the hot and cold ports.
8. Install a new T&P valve and discharge line.
9. Install (or carry over) the expansion tank, sized and pre-charged to match the household's static water pressure.
10. Connect gas with new piping and check for leaks with soap solution, or wire electrical to the new heater.
11. Restore venting (gas) to confirm proper draft.
12. Open the cold supply slowly to fill the tank. Open the hot tap upstairs and run it until water flows steadily — that confirms the tank is full and air is purged.
13. Fire the unit (gas) or energize the breaker (electric).
14. Set the thermostat — typically 120°F is the recommended starting point.
15. Let the unit run a full heat cycle and verify hot water at multiple fixtures.

What Plumbers Catch That DIYers Miss

The shortcuts that come back to haunt installs:

• Skipping the expansion tank in a house with a PRV or check valve. The result is a T&P that weeps water every heat cycle until you eventually have a flood.
• Threading or capping the T&P discharge line. Never. Do. This. The T&P has to be able to relieve pressure freely.
• Leaving the dielectric union off and connecting copper directly to the steel tank fittings. Sets you up for accelerated corrosion at the connection.
• Not strapping the tank in earthquake regions (less of an issue here, but worth knowing).
• Sizing a gas line that's adequate for the old unit but too small for a higher-BTU new one.

Permits and Code

Water heater installation in Nashville requires a permit, and the install must be inspected. A licensed plumber pulls the permit, builds to current code, and meets the inspector. DIY installs that don't get permitted can become a problem at resale.