December 12, 2017
By Jason Fleming
Hot water is the lifeblood for many commercial applications. A restaurant lacking hot water will be forced to shut down for lack of hygiene. Hotel room showers that cannot reach the set-point temperature leave clients dirty and disappointed. A laundromat without the ability to wash clothes loses its purpose.
Selecting a water-heating system that efficiently and reliably meets demand is crucial. For many, the gut reaction might be to follow convention and choose a storage-tank water-heater. However, the North American market is gradually learning what the rest of the world has known for 50 years—tankless water-heaters, whether operating individually or banked together, will always meet hot-water demand at a fraction of the gas input of the storage-tank variety.
Advantages of tankless
Tankless water-heating technology relies on heating incoming water to reach the set-point temperature in seconds. The heaters carry with them the advantages of efficiency, redundancy, versatility, and serviceability.
Tankless water-heater performance relies on modulating boiler technology, which tracks and meets hot-water demand with pinpoint accuracy, matching energy consumption to the present requirements. Unlike with storage tank heaters, there are no standby losses resulting from the burner firing during periods of inactivity. When combined for larger commercial applications, the entire system adjusts usage to match demand.
In terms of thermal efficiency, tankless water-heaters can be up to 20 per cent more efficient than a minimally compliant storage-tank water-heater, based on thermal efficiency standards reported by the U.S. Department of Energy (DOE). This can translate to significant energy savings over time that offset—and eventually pay back—any upfront capital investment, which will typically be higher for tankless. In certain large commercial applications, though, the upfront capital investment may actually be less for tankless, especially when savings from pre-engineered (i.e. pre-racked) tankless systems are factored in. These savings, combined with reduced annual maintenance and further supplemented by provincial energy rebates, can drive lower cost of ownership for the water-heating system.
When banked (i.e. combined together into one interconnected system), tankless water-heaters offer much-needed redundancy of operation. If one unit happens to go offline and needs to be serviced, the remaining units will split the heat demand to maintain the set-point temperature. This capability keeps hot-water-reliant operations running, even if one or more units need servicing.
Aside from being smaller and able to be wall-hung or rack-mounted, tankless water-heaters can work in concert with other heating units in an application. A large hotel may wish to combine a storage tank with a tankless heater to cut down on the required number of units, while continuing to reap the benefits of efficiency and redundancy. Tankless units can even supplement solar-heated applications, serving as a fallback on cloudy days. Further, units can be installed indoors or outdoors and be individually or common-vented.
Tankless heaters are built with modular construction, meaning they are fully ‘parts-replaceable.’ Thus, in the case of a malfunction, only the faulty component needs to be replaced, instead of the entire system. This not only reduces cost, but also eliminates the downtime that would otherwise be needed to install an entirely new unit.
Principles of tankless
In a tankless setup, an open hot-water fixture creates a pressure differential in the plumbing system, causing water to flow in its direction. The water-heater detects this pressure differential and a flow sensor measures the gallons per minute (gpm). The internal computer then automatically ignites the burner, modulating British thermal unit (Btu) usage to overcome the temperature differential between the incoming and set-point temperatures. Finally, the primary heat exchanger brings the water up to the set-point temperature.
In the following water-heating equation, the heat input (i.e. Btu/hour) is represented by Q. The variable ‘F’ represents the flow rate, and the temperature differential between incoming and set-point water temperatures is represented by (T2−T1):
Efficiency x Q = 500 x F x (T2−T1)
Btu input has a direct relationship to the flow rate and temperature differential. As a result, energy will increase with larger temperature differentials.
Condensing tankless technology
Condensing water-heaters will utilize the high-temperature exhaust in standard-efficiency units to preheat the incoming water temperature 8 to 11 C (15 to 20 F) higher through a secondary heat-exchanger. As a result, when the preheated water enters the primary heat-exchanger, less heat is needed to raise the water temperature to the set point. In fact, a condensing unit can have a thermal efficiency up to 10 percentage points higher than a conventional tankless unit and 20 points higher than a standard storage tank heater.
The extraction of heat from the exhaust gases to the incoming water provides an additional benefit—it cools the exhaust gases below the dewpoint. For example, the gas temperature at the top of the flue for conventional tankless units is roughly 200 C (400 F). In condensing units, the secondary heat exchanger will lower this temperature to around 50 C (120 F)—a major difference—without sacrificing hot-water output or water pressure. The cooler exhaust temperatures allow the use of venting materials that are less expensive, such as polyvinyl chloride (PVC).
A natural byproduct of the cooler gases escaping out the flue is condensate, which should be drained per local code requirements. Some codes may specify a neutralizer to lower the pH of the condensate entering the plumbing system.
Both condensing and noncondensing types of water-heater are significantly more efficient than a standard storage tank system. These efficiencies can be multiplied when banking heaters in larger commercial applications.
Benefits of banking and modulation
Some applications, such as hotels or large office buildings, can require significant quantities of hot water at any given time. Specifiers may be hesitant to install tankless systems in such situations, fearing either the demand will be too large or too many units will need to be installed.
To clarify, no hot-water demand is too large. In a multi-unit commercial system, the tankless heaters will communicate with one another to generate enough output to meet demand and to ‘smoothen’ that load among the various operating units in a way that extends system life. The typical firing sequence is as follows:
1. The first tankless water-heater activates as soon as there is a demand for hot water within the system—for example, when someone turns on a faucet or a shower.
2. That first unit continues to provide all the needed hot water until demand exceeds 80 per cent of the unit’s capacity.
3. Once demand surpasses the first unit’s preset capacity, it is equalized among two or more units while still meeting high-volume demand.
Modulating burner technology can track and meet any hot-water demand with pinpoint accuracy, matching energy consumption to present requirements. If energy from only one water-heater is needed, then only one heater activates. In contrast, a conventional heater or boiler without a modulating burner will immediately ramp up its full capacity to deliver hot water to that one hand sink or shower; a traditional system consumes far more energy than needed.
Some manufacturers offer prefabricated rack systems of up to 24 units controlled by one remote thermostat. However, for even larger applications, multiple systems can be linked together to meet demand. The ratio of the maximum to minimum Btu output of a system is called the turndown ratio. The higher the ratio, the greater the system’s flexibility in efficiently meeting hot-water demand. For example, a bank of 24 heaters with a Btu range of 11,000 to 6 million would have a turndown ratio of 545:1 to meet fluctuating hot-water demand.
Peace of mind through redundancy
As mentioned, linking multiple units means never having to worry about one faulty heater shutting down all hot-water production. The system is designed to always meet demand. In the extreme case all remaining heaters have reached maximum demand, the flow rate will slightly be reduced to lower the BTU input and maintain the set point temperature. In a conventional storage-tank system, failure of this single unit can put the entire domestic hot-water operation in jeopardy.
Specifiers, installers, and building owners may have concerns about the costs associated with installing multiple tankless units. However, tankless may not be more expensive, even in terms of upfront costs. It is also possible to supplement the system with a storage tank. This will allow fewer tankless units to be installed without losing the ability to modulate Btus.
Another option is to order a prefabricated rack system that streamlines installation costs by preassembling and preprogramming multiple units in a standard manifold for a particular application. The rack can come pre-engineered with isolation valves, system, and pump controllers. If condensing heaters are chosen, their cooler exhaust gases will allow for PVC venting instead of stainless steel, further reducing costs.
For those accustomed to using storage tank heaters, another point to keep in mind is tanks are often oversized as a safety precaution against running out of hot water. Tankless units avoid this potential waste with their modulating capabilities.
Specifiers should also consider durability, usability, and lifespan when selecting commercial water-heaters. Typical storage tank units contain:
It is here the modular components also come into play. The ability to replace faulty components instead of scrapping the entire unit increases lifespan. Full diagnostics alert owners to errors within the heater, as opposed to storage tank units, which give limited warning of system failures and must usually be replaced in their entirety when a part fails. This speaks to the overall longevity benefits of tankless water-heaters. In commercial applications, tankless units can last almost twice as long as storage tank heaters, based on comparisons of manufacturer warranties.
Of course, this presumes the tankless unit will be properly maintained. One must always consider the application. Consistent use of extremely hot water will hasten scale buildup and essentially mandate more frequent flushing of the tankless unit.
Nonetheless, tankless units are more resistant to scale buildup than tank-type units would be under similar conditions. This is because the absence of stored water helps prevent sediments from accumulating. It is also advised to filter incoming water to maximize quality and reduce the toll on the tankless system. Although this is true for any water-heating system, it is more important for a tankless assembly because it is smaller and, therefore, less clogging is needed to impact performance, relative to a storage tank.
One additional factor to consider is the gas-line requirement for tankless heaters, which may be larger, depending on the size of the temperature rise. One can ensure a proper gas supply by increasing the pipe diameter or gas meter size, as necessary.
Although switching to tankless brings with it many of the benefits mentioned in this article, it is not always a simple process, depending on the application. The gas line, venting, and power must all potentially be reconfigured for a tankless system, thereby increasing the installation cost. Specifiers must consider each application on a case-by-case basis to make the best decision.
Making a crucial decision
There are many factors to consider when selecting a commercial water-heating system. A storage tank might seem like the accepted, most straightforward option. It can be less expensive—at least initially—and likely more familiar. However, it is also important to look beyond the present moment.
Any system needs to meet hot-water demand consistently and without exception. Anything can go wrong in a commercial operation, but hot-water supply must never be compromised in a way that results in costly shutdowns. Tankless water-heaters provide the most reliable, energy-efficient solution; they are generally less expensive over the long run, after energy savings are considered. They are a tested and true option forward-thinking specifiers would be wise to consider.
|COMMERCIAL TANKLESS VS COMMERCIAL TANK|
| Commercial tankless:
Jason Fleming serves as vice-president of sales and marketing for Noritz America. He is a 20-year plumbing industry veteran. Fleming can be reached via e-mail at firstname.lastname@example.org.
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