Commercial buildings use five main types: storage tank (gas or electric), tankless or on-demand units, tankless racks for high demand, heat pump units, and a boiler with an indirect storage tank. Point-of-use heaters serve remote sinks. The right choice depends on peak demand and the temperature you need.
The five main commercial water heater types
A storage tank heater holds heated water in an insulated tank. Commercial sizes run from 50 to 100-plus gallons. It can be fired by gas or heated by electric elements. Its strength is meeting spiky demand. When a rush hits, the stored hot water is already there. The trade-off is standby loss, the heat that escapes the tank between uses. Gas units recover faster and cost less to run where gas is available. Electric units are simpler to install and vent.
A tankless or on-demand unit heats water only as it flows through, so it has no standby loss. The U.S. Department of Energy says that for homes using up to 41 gallons of hot water a day, tankless units are "24%-34% more energy efficient than conventional storage tank water heaters." The catch is flow. One unit can only raise the heat of so many gallons per minute. A single commercial unit usually handles 2 to 5 GPM. When several fixtures run at once, one tankless heater gets overwhelmed.
That is why high-demand buildings use a tankless rack, a manifold of several units wired to fire together. The rack adds capacity by stacking units. A hotel or busy restaurant gets endless hot water without a giant tank. Racks scale up as the building grows. They keep running if one unit is down for service.
A heat pump water heater (HPWH) moves heat from the surrounding air into the water instead of making it directly. That makes it the most efficient electric option where the space suits it. It needs a warm, vented area with enough air volume. A large mechanical room or rooftop works, while a small closet does not. A boiler with an indirect storage tank is common in larger buildings. The building boiler heats a coil inside a separate tank, so one heat source covers both space heating and hot water.
Finally, a point-of-use heater is a small unit mounted right at a remote sink. Running a long hot-water line to one far restroom wastes water and energy. A point-of-use heater at that spot delivers hot water fast, with no wait.
How the types compare
| Type | Best for | Standby loss | Sizing basis | Watch-outs |
|---|---|---|---|---|
| Storage tank (gas) | Spiky, simultaneous demand | Yes | First-hour rating | Venting, floor space, recovery rate |
| Storage tank (electric) | Simpler installs, no gas | Yes | First-hour rating | Slower recovery, electric load |
| Tankless / on-demand | Steady low-to-moderate use | No | GPM + temperature rise | One unit limited by flow |
| Tankless rack | High, continuous demand | No | Total GPM + temperature rise | Higher upfront cost, more units |
| Heat pump (HPWH) | Efficiency where space allows | Yes | First-hour rating | Needs warm, ventilated space |
| Boiler + indirect tank | Larger buildings with a boiler | Some | First-hour rating | Ties hot water to boiler uptime |
| Point-of-use | A remote single sink | Small | Fixture flow | Not for whole-building supply |
The pattern in the table is the heart of the choice. Storage-based types (tank, heat pump, boiler with indirect tank) buffer demand with stored hot water, so they shine when use comes in bursts. Flow-based types (tankless and racks) waste no heat standing by. They shine when use is steady, or when endless supply matters more than instant volume.
How to size each type by demand
Sizing is where the type and the building meet. The method differs by category, and DOE publishes the rules for each.
For storage tank and heat pump units, you size by first-hour rating (FHR). That is the gallons of hot water the unit can deliver in a busy hour, starting with a full tank. DOE's guidance is direct: "When using the worksheet to find a water heater's first hour rating, look for water heaters with a first hour rating that matches within 1 or 2 gallons of your peak hour demand." So you estimate the building's peak-hour demand, then pick a unit whose FHR meets it.
For tankless units and racks, you size by total gallons per minute (GPM) at once, plus the temperature rise needed. Temperature rise is the gap between the incoming cold-water temperature and your target. To reach a common 120 degrees from cold city water, many regions need about a 70-degree rise. You add up the flow of every fixture set to run at once. Then you choose enough tankless capacity to deliver that GPM at that rise. A rack just adds units until the combined GPM clears the peak.
A useful Phoenix note: warm inlet water means a smaller temperature rise is needed than in cold climates. That is a real sizing advantage for tankless equipment here. The flip side is that Phoenix has hard water. Hard water scales heat exchangers and tank bottoms, and it drags down efficiency. Plan on regular flushing and descaling to protect any commercial unit. See how-hard-water-damages-plumbing for the maintenance side.
The restaurant angle versus a light-use office
The same building size can need very different equipment based on use, and food service is the clearest example. A restaurant has high, spiky demand all at once. Dish lines, prep sinks, and handwashing all run during a rush. On top of volume, sanitizing dish machines and three-compartment sinks usually call for water near 140 degrees at the point of use. High heat plus high flow at once favors high-recovery gas storage or a tankless rack that can hold the GPM and the heat together. Storing water hot also has a safety angle. The CDC notes Legionella bacteria "grow best" in water between 77 and 113 degrees Fahrenheit, so keeping stored water hot matters in any building with at-risk occupants.
Hot delivery temperatures raise a scald concern at sinks meant for hand use. The common fix is to store water hot for sanitizing, then use a thermostatic mixing valve to temper it down at handwashing fixtures. That lets one system serve both the 140-degree dish line and the cooler handwash sink.
A light-use office sits at the other end. The only hot water may be a break-room sink and a few low-use restrooms. A small storage unit, or even point-of-use heaters at each sink, may cover it. There is no rush of demand all at once to design around. Paying for a rack or a large tank wastes money and space.
Choosing the right type for your building
Start with two numbers and one fact: your peak-hour demand, your target temperature, and whether you have gas service. Spiky demand that needs instant volume points toward storage or a tankless rack. Steady, moderate use points toward tankless. A far-off single sink points toward point-of-use. Efficiency goals with the right mechanical space point toward a heat pump, and a building already running a boiler often makes the most of an indirect storage tank.
ENERGY STAR can narrow the field once the type is set, since certified models cut energy use within each category. Whatever you install, the Phoenix hard-water reality means a maintenance plan, flushing and descaling on a schedule, is part of the cost of ownership, not an afterthought.
Two related pages go deeper on the questions that come up next. If your current system runs out of hot water during a rush, see commercial-water-heater-not-enough-hot-water for diagnosis and fixes. If you are weighing energy sources for a tank, see gas-vs-electric-water-heater. And for the underlying sizing math that applies to tanks of any kind, see what-size-water-heater-do-i-need. HQ Plumbing & Air sizes and installs commercial water heating across metro Phoenix and can match the type to your building's real peak-hour load.