How to Replace a 20 Gallon Water Heater? | Swap It The Right Way

Replacing a 20-gallon water heater starts with a like-for-like match: same fuel type, voltage, and physical size, before a licensed pro confirms otherwise.

Swapping a 20-gallon tank sounds like a major plumbing project, but for a careful DIYer the sequence is straightforward: shut off power or fuel, drain the old tank, disconnect it, connect the new one, refill it, and only then restore power. The details in the middle are where mistakes happen, and most of them come down to buying the wrong unit before the work even starts.

Replace Like-For-Like: The Rule That Guides Everything

The safest path is to replace the old unit with one that matches its fuel type, voltage, venting (if gas), connection size, and physical dimensions. If you want a different configuration, a licensed professional must confirm the new setup is code-compliant.

Capacity labels also vary slightly. Current listings show 19.5-, 19.9-, and 20-nominal-gallon tanks, so the rounded “20 gallon” label matters less than the actual dimensions and where the inlet and outlet sit. When you are ready to buy, our tested roundup of the best 20-gallon water heaters can point you to models that fit the measurements you take from your old unit.

Step-By-Step Tank Swap

These eight steps work for any standard replacement. Take them in order, and do not skip the safety ones.

1. Turn off the power or fuel. For electric models, switch off the breaker at the panel. For gas models, shut the gas valve completely before anything else.
2. Close the cold-water supply valve feeding the tank. This is usually a gate or ball valve directly above the heater.
3. Drain the tank through the drain valve into a floor drain, bucket, or hose routed outdoors. Open a hot-water faucet in the house to let air in and speed the flow.
4. Disconnect the plumbing. Unthread the inlet and outlet connections using wrenches. For electric units only, disconnect the wiring after you have verified the power is off with a tester; for gas, disconnect the vent and gas line.
5. Remove the old tank and set the new one in place. Position it so the plumbing and wiring reach without straining, and confirm it clears any cabinet, closet, or lowboy space. Most current 20-gallon units range from about 22-1/4 to 30-1/2 inches tall, so measure before you commit.
6. Install the T&P relief valve if it does not come pre-fitted. Many commercial 20-gallon units include a CSA/ASME temperature-and-pressure valve in the box; residential models often need one added. This valve is a critical safety device, so never skip it.
7. Reconnect plumbing, open the cold-water supply, and purge air. Open a hot-water faucet somewhere in the house and let the tank fill completely until water flows steadily with no sputtering. Leave the element circuit off until this is done — energizing an empty tank dry-fires and destroys the elements.
8. Restore power only after the tank is full. Check every connection for leaks, confirm the water heats, and verify the thermostat holds its set temperature.

Compatibility Checks That Prevent The Biggest Mistakes

Twenty-gallon units exist in several electrical and fuel configurations, and mismatching them is the most common failure. A marine or indirect model occupies a different category entirely. Match the new heater to the circuit and pipe size that already exist in your home.

Beyond voltage, confirm the inlet and outlet are both 3/4-inch, since that is the common standard across these units. Finally, check local permitting rules before you start. Water-heater replacement commonly triggers permit and code requirements in US jurisdictions, covering gas venting, seismic strapping, and drainage. No national standard covers every town, so a quick call to your local building office prevents complications later.