🚿 Water Heater Sizing: How to Size Tank and Tankless Systems for Demand

The Goal: Hot Water When It Is Needed

Sizing a water heater means matching its output to the building's peak hot-water demand without grossly oversizing, which wastes energy and money, or undersizing, which leaves occupants with cold showers. The two families of equipment — storage (tank) and instantaneous (tankless) — are sized by completely different logic, so the first step is understanding how each delivers hot water.

How Tank Heaters Deliver Hot Water

A storage water heater keeps a reservoir of hot water ready and reheats it as it is drawn. Its capacity to meet demand is described by two numbers.

• Storage volume — the gallons of preheated water held in the tank, available immediately for a burst of demand.
• Recovery rate — how many gallons per hour the burner or element can reheat to the setpoint, governed by the input energy and the temperature rise. A high-input gas heater recovers far faster than a standard electric one, so a smaller gas tank can serve the same load as a larger electric tank.

The recovery rate follows directly from energy balance: a higher heating input or a smaller temperature rise produces more gallons per hour. Approximately 8.33 BTU heats one gallon by one degree Fahrenheit, so the deliverable recovery equals the usable input (input times efficiency) divided by 8.33 times the temperature rise.

First-Hour Rating: The Real Tank Metric

The single most useful number for sizing a residential tank is the first-hour rating (FHR). It states how many gallons of hot water the heater can supply in one hour starting with a full tank of hot water. Crucially, it combines both the stored volume and the recovery during that hour, so it reflects what a household actually experiences during a busy morning.

To size by first-hour rating, estimate the peak one-hour demand — the worst hour of the day when showers, laundry, and the dishwasher might overlap — and select a heater whose FHR meets or slightly exceeds it. A practical approach counts the hot-water use of the simultaneous activities likely during that peak hour: a shower uses on the order of 10 to 20 gallons of hot water, a tub fill considerably more, and so on. Matching FHR to that peak-hour total is more reliable than choosing a tank by nominal gallon size alone, because two 50-gallon heaters with different recovery rates have very different first-hour ratings.

How Tankless Heaters Are Sized

A tankless (instantaneous) heater has no stored reservoir; it heats water on demand as it flows through a heat exchanger. It therefore cannot be sized by volume at all — it is sized by the flow rate it can heat across the required temperature rise, expressed in gallons per minute.

The governing relationship is temperature rise versus flow. The temperature rise is the difference between the desired hot-water temperature and the incoming cold-water temperature. A tankless unit has a fixed heating capacity, so as you demand a larger temperature rise, the maximum flow it can deliver at that temperature falls. The same unit that delivers a generous flow in a warm climate, where incoming water is mild, delivers far less in a cold climate where incoming water might be near freezing.

• Determine the design incoming cold-water temperature for the location — this is the make-or-break input, and winter is the worst case.
• Set the desired output temperature, commonly around 120 degrees Fahrenheit at fixtures.
• Compute the required temperature rise as the difference.
• Add up the flow of the fixtures expected to run simultaneously at peak, and select a unit whose published flow at that temperature rise meets the total.

For example, two showers running together might demand roughly 4 to 5 gallons per minute; a tankless unit must be rated to deliver that flow at the local winter temperature rise, not at an optimistic mild-climate rating. Undersizing shows up as a drop in temperature when a second fixture opens.

Estimating Peak Demand

Both methods hinge on a realistic peak demand. Two complementary approaches are used.

• Fixture-based — list the fixtures and the hot-water flow or volume each contributes, then estimate which run simultaneously during the worst period. This is direct and works well for both tank (peak-hour gallons) and tankless (simultaneous gpm) sizing. Because not every fixture runs at once, a simultaneity (diversity) factor is applied for larger buildings so the system is sized for realistic concurrent use rather than the theoretical maximum.
• Occupancy-based — estimate demand from the number of people or units and a per-person hot-water allowance. A common residential planning figure is roughly 12 to 20 gallons of hot water per person per day, with the peak hour being a fraction of that daily total. For hotels, dormitories, and apartments, published guidelines give demand per unit and a recovery plus storage combination that meets the peak.

Tank vs. Tankless: Choosing the Approach

Tank heaters absorb short, intense bursts of demand from their stored volume and tolerate brief over-demand by drawing down the tank, recovering afterward. They are simpler and inexpensive but lose standby heat and occupy floor space. Tankless heaters deliver endless hot water at their rated flow, save standby energy, and are compact, but they impose a hard flow ceiling at the design temperature rise — exceed it and the temperature sags. For high simultaneous demand, tankless units are often installed in multiples or banks, or a hybrid storage-plus-tankless arrangement is used.

Sizing Checklist

• Establish the peak demand by fixtures or occupancy, including realistic simultaneity.
• For tanks, match the first-hour rating to the peak-hour gallons, remembering that recovery rate and storage together set the FHR.
• For tankless, compute the winter temperature rise and select a unit that delivers the required simultaneous gpm at that rise.
• Do not size a tankless unit on its headline gpm; that figure assumes a small temperature rise and collapses in cold climates.