What are the different types of commercial water heaters?

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's Building America fact sheet notes that storage tanks carry higher standby energy losses than some other types, and that demand (tankless) heaters produce a limited amount of hot water, best suited to low-demand or remote applications. 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 properly designed rack can meet the specified simultaneous demand. Capacity, temperature rise, controls and the hydraulic layout determine performance. If one unit is offline, the remaining arrangement may provide reduced capacity; redundancy and future expansion need to be designed, not assumed.

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.

Buildings that already run a boiler for heating can also use an indirect water heater, a storage tank heated by boiler water through a coil.

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.

Sizing is where the type and the building meet. The method depends on the commercial system and its actual demand. DOE's Building America fact sheet discusses household heaters; it does not establish a complete sizing method for every commercial installation. Use the manufacturer’s commercial sizing information and applicable design requirements.

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 fact sheet says to use the FHR, not tank capacity, and to choose a model with an FHR within about 2 gallons of your estimated peak-hour demand. That household guidance should not be used as the complete design method for a commercial kitchen, hotel or process load. Have the designer evaluate simultaneous demand, storage, recovery and distribution using the actual commercial equipment data.

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, the figure DOE's fact sheet uses when it notes a tankless gas unit can deliver about 5 GPM and an electric unit about 2 GPM. 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 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.

Start with two numbers and one fact: your peak-hour demand, your target temperature, and whether you have gas service. If you're moving to a bigger gas unit, check whether the gas line needs upsizing too. 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. If you're deciding whether a business needs commercial equipment at all, start with commercial vs residential water heaters.

For how the county sizes a heater for a food business, see how to size a restaurant water heater in Maricopa County, and for dish machines see whether a restaurant needs a booster heater.

Once you have a heater, the upkeep schedule is in how often to flush and service a commercial water heater.

Where the relief valve discharge pipe goes and when a pan is required are covered in commercial water heater relief valve discharge and pan rules.