Can I replace my water heater with a smaller tank?

Tank size is the amount of water the unit holds. Rheem's EnergyGuide explainer describes it as "the actual amount of water the unit contains, also known as the rated capacity." That is a storage number, and storage is only half of what you get.

The number to compare is the First Hour Rating. ENERGY STAR defines it as "an estimate of the maximum volume of hot water in gallons that a storage water heater can supply within an hour that begins with the water heater fully heated." It counts the tank plus what the burner or element makes while you are using it. Rheem makes the same point for gas models: two units with the same tank size can deliver different amounts of hot water, depending on burner size and efficiency.

That is why two heaters with the same gallons on the label can deliver different hot water, and why a newer, smaller model can sometimes keep up with an older, larger one. Look at the yellow EnergyGuide label or the spec sheet for the FHR, not the front-of-box gallons.

Here is the test:

1. 1Estimate your busiest hour. Rheem's example: two showers, a load of laundry and some kitchen use might add up to 55 gallons, so that household should look for a first hour rating of at least 55 gallons. Use your own mornings, not the number of people in the house. Rheem says the most important consideration is "your household’s peak hot water demand, not just the number of people living in the home."
2. 2Read the FHR on the candidate heater. If it clears your busiest hour with room to spare, the smaller tank works.
3. 3Check what has changed. ENERGY STAR advises treating your current tank's gallon capacity as a starting point, and says you may want a larger unit if you have added or plan to add occupants or a new shower, faucet, dishwasher or washer in frequent use. Downsizing runs the other way, so ask whether anything has been removed, not just added.

• Fewer people, same schedule. Two adults who shower at different times rarely need what a family of five did.
• A short busy hour. If your peak is one shower and a dishwasher, a smaller tank with a healthy FHR is plenty.
• A big old heater that was never needed. Rheem notes that a unit that is too large "may cost more upfront than necessary and consume more energy than your household needs." Treat that as a manufacturer's general statement, not a promise of savings for your home.
• A tight closet or garage. A shorter or narrower tank can be easier to fit. Check the clearance rules in our page on water heater access and clearance before you commit.

• You run out now. If the current heater is already short on hot water, a smaller tank makes it worse. Read why you run out of hot water fast in the shower first.
• A soaking tub or a multi-head shower. Rheem points out that deep soaking tubs can hold up to 140 gallons and that small tubs are usually about 40 gallons. Fill one while someone showers and a small tank is gone.
• You are switching to a heat pump water heater. That is a different kind of heater, so do not assume a smaller tank behaves the same way. Read what a hybrid water heater is and compare the manufacturer's FHR for that exact model.
• Guests, an addition or a growing family on the way. Size for the house you will have in a few years, not this month.

Incoming water temperature sets how hard the heater has to work. Rheem's rule of thumb for sizing puts inlet water at "60°F year-round in Southern California, the Southwest and Gulf states," against 40°F for the northern tier of states. A smaller rise to reach 120°F means a Phoenix household often gets more from a given heater than the same heater would deliver in Minnesota. It is a real edge, but it is a reason to shave a size, not to guess. Our page on water heater recovery time shows how that plays out, and what size water heater you need walks through the full sizing method.

1. 1Write down your busiest hour (who showers, which appliances run) and total it in gallons.
2. 2Pull the FHR for the current heater from its EnergyGuide label and the new model from its spec sheet.
3. 3Confirm the connections still fit. A different height or diameter changes where the vent, gas line, T&P discharge and pan sit, which can make a "simple swap" a bigger job.
4. 4Ask whether your city treats the swap as an in-kind replacement. Our guide to the water heater permit in Phoenix lists what needs a permit, and it can change when the fuel type, venting or heater type changes.
5. 5Keep the old unit's nameplate photo handy. The age and capacity on it help your installer size the new one.

A smaller tank is a fine decision when the First Hour Rating beats your busiest hour. It is a bad one when you pick it by gallons alone. If you are not sure, our team gives free estimates and upfront pricing before work starts, and we will size the heater to how your house really uses hot water. See our water heater services when you are ready.