Water storage heats up
CSE: When considering tankless, when might a decentralized—point-of-use—approach be more ideal than a centralized—one tankless heater—approach? MELODY LUTZ: When a water-heating system is decentralized, it may eliminate the necessity for long or multiple hot water piping runs.
CSE: When considering tankless, when might a decentralized—point-of-use—approach be more ideal than a centralized—one tankless heater—approach?
MELODY LUTZ : When a water-heating system is decentralized, it may eliminate the necessity for long or multiple hot water piping runs. This provides thermal efficiency by reducing temperature drop common in long pluming runs, reduces water consumption by decreasing time of delivery, and simplifies plumbing installation. Only cold water lines must be run near the point of hot water consumption.
It is necessary to consider the needs at each point of use for sizing. It may necessitate a higher amperage electrical circuit if care is given to achieve higher temperatures at maximum fixture draw.
JOHN ALFORD : A small project with just a few plumbing fixtures requiring hot water would qualify. Here in our area, the code doesn't require hot water at public restroom lavatories. Sometimes when the building owner decides to forgo hot water at the restroom lavatories, small point-of-use heaters are provided at the janitor closets. Occasionally, a project may have a breakroom sink located remotely from the remainder of the project fixtures. It is possible to supply cold water to the sink and a small point-of-use water heater under the counter.
WILLIAM HOOVER : While point-of-use heaters appear to be a good idea, the cost of the equipment and installation usually makes this approach cost prohibitive. Unless the distances are exceptionally long, the most cost-effective solution usually would be a whole house water heater and an on-demand recirculation system, which will provide hot water at all fixtures without water waste.
CSE: What are ways to reduce hot water loads to make systems smaller? For example, is providing only cold water to restrooms an option?
HOOVER : I don't believe that most health code officials would approve providing just cold water in restrooms. The hot water load can be reduced by using an on-demand recirculation system, by washing clothes in cold water, by the use of low-flow fixtures, and by limiting the duration of hot water usages.
ALFORD : As I mentioned before, our code doesn't require hot water at hand washing lavatories in the public restrooms. We limit this application to the public restrooms. Eliminating hot water to most other fixtures would be an inconvenience to the building occupants.
LUTZ : On-demand systems with accurate temperature control permit lower settings of water temperatures for lavatory applications. Temperatures of 90 to 100 F are more than adequate for hand washing. On-demand systems that activate as low as 0.15 gpm or 0.25 gpm allow the use of 0.5 gpm aerators for lavatory faucets, which reduces total consumption. Modulating controls to lower temperatures also permits service of multiple lavatory sinks with a smaller single unit, reducing utility requirements. Anytime systems modulate to desired consumption temperatures energy is saved by eliminating the need to cool incoming water for usage, thereby wasting heat energy.
CSE: Other than utility costs, what other design considerations are there for choosing gas versus electric water heaters?
ALFORD : Site location and utility availability. On several projects the owner must determine if the cost to bring natural gas to the project site was worth the first cost involved. On the other hand, some projects went with storage tanks with propane gas instead of electric to have the hot water make up capabilities over the electric power demand charges.
HOOVER : Gas water heaters require adequate venting, which can limit or complicate their locations. On the other hand, electric water heaters require substantial electrical service.
CSE: What are retrofit tradeoffs for tankless heaters swapping out a storage heater and vice versa?
HOOVER : When replacing a storage water heater with a tankless water heater, the installation can is expensive due to the needed modifications. First, the gas line usually will be replaced with a larger line—and a larger meter—to ensure that the water heater has an adequate gas supply. Secondly, because the burners on a tankless heater are much larger than those on storage water heaters, arrangements must be made to provide more make-up air. Thirdly, the vent system from the storage unit will not be adequate for a tankless heater, because it requires a double-walled stainless steel vent.
Finally, a tankless water heater requires electric power, while most storage water heaters do not. Hence, in a properly installed retrofit situation, the installation costs of a tankless water heater can far exceed the cost of the tankless hardware itself.
ALFORD : The project gains back some floor space when switching to tankless water heaters. The need for drip pans and floor drains is eliminated. But in most cases, tankless water heaters are limited to small projects that don't require hot water recirculation.
But with a storage water heater, the first cost is more economical than tankless types. The repairs later are easier with tank type heaters as more service persons can replace a tank type heater.
LUTZ : When retrofitting from a tank to a tankless system one can gain space and save energy. If a central system is retained one may have to adjust consumption habits for hot water usage based on output capacity. A tankless system supplies an unlimited amount of hot water, but one is limited to the instantaneous flow versus temperature rise capabilities of the tankless system. A tank system supplies hot water to multiple fixtures for a brief period of time before running out of hot water. Then you must wait the recovery period before having hot water again. Storage heaters take up a great deal of space that can be used for other types of storage.
CSE: What specifications maximize the advantages of storage heaters, such as controls or piping?
LUTZ : Timers on the water heater help with energy consumption. Gas models with electronic ignitions also save on utilities.
CSE: Under what circumstances are instantaneous—tankless—water heaters the right choice?
LUTZ : Anywhere space, energy efficiency, and accurate temperature controls are required. Also, situations where long draws are required. Tankless heaters are wonderful in process applications where either intermittent usage or continuous hot water supplies are required.
HOOVER : Tankless water heaters have little standby energy loss so applications where very little hot water is used and there are long periods of standby play to their advantage. Because they are not as efficient when their usage consists of many small draws as opposed to long, continuous draws, they will perform best in applications with long, continuous draws.
ALFORD : Small projects with minimal needs for hot water, hand washing and occasional clean up chores. But on multi-story residential projects, venting tankless water heaters can become an issue. Locating tankless water heaters on the exterior of the building could be less than desirable to the architect.
There are some projects where the demand for hot water is a constant load, such as industrial processes or hospitals. Hospitals have a constant demand for steam, so using a semi-instantaneous steam water heater in a hospital a good choice.
CSE: Under what circumstances are storage water heaters the right choice?
LUTZ : If space is unlimited, or of little concern. If larger utility supplies for instantaneous units are not available or cost prohibitive. If especially large volume, short duration consumption habits are preferred.
HOOVER : Storage water heaters are the most cost-effective alternative for most water heating applications. While tankless water heaters are more efficient than conventional storage water heaters, the cost savings do not justify the installed costs. For commercial applications with substantial hot water usage, condensing storage water heaters are more cost effective because their thermal efficiencies exceed 90%, while most tankless water heaters have efficiencies in the 82% range. In high hot water usage applications, the standby loss advantage of tankless heaters is swamped by the thermal efficiency of condensing storage water heaters.
ALFORD : Projects with high demands for hot water during short periods of time are best suited for storage type water heaters. Any type of project with large numbers of showers such as a jail, prison, or school will need hot water quickly for short periods. In some cases a project with large kitchen hot water demands will work best with a storage type water heater.
CSE: Assuming that they are not created equal, what are differentiating factors to consider when looking at tankless water heaters?
ALFORD : Specifiers should consider the energy source the heater uses to make hot water, the material used in the heat exchanger, the recovery rates and efficiencies of each of the heaters, the physical size of the water heaters, and if service is available in the local area.
HOOVER : The overall efficiency of the tankless water heater is an important consideration. In addition, the costs of installation and maintenance need to be considered. Availability of trained technicians to service the water heater should also be a consideration.
LUTZ : Pay particular mind to construction in regard to material and durability of heat exchangers. High-quality sheathed elements with low to moderate Watt-density for longer element life. Accurate and adjustable temperature control with custom tuning to your application. Proper voltage and phase availability for the installation location.
CSE: Similarly, what are differentiating factors among storage water heaters?
ALFORD : Selecting a tank water heater is like buying a car, there are many choices. Each water heater manufacturer knows the pros for his water heater over the other brands. The number of tank linings to choose from is endless. The venting options and efficiencies of the various types is another consideration. The first costs are an important consideration in the commercial design field as building developers look for an economical design.
One consideration is the manufacturer representative. Will this person stand behind his product if it develops problems on the project site? Will this person assist you in resolving problems after the project has been finished? If the representative is willing to help you in times of trouble, then his product is probably worth considering again.
HOOVER : The overall efficiency of a storage water heater will dictate the operating costs. In today's market, high-efficiency condensing storage water heaters readily are available from a number of suppliers. When a lifecycle cost analysis is completed, these high-efficiency storage water heaters usually are very attractive.
CSE: What other ways are there to create domestic hot water than using tankless or storage water heaters?
LUTZ : Solar and geothermal are ways to produce a great deal of free hot water, but usually require some kind of backup system to ensure adequate supply of hot water in peak consumption periods. Some tankless systems are perfect for that type of backup application, and temperature controlled units only add the energy required to boost to usage temperatures.
HOOVER : Domestic hot water is generated in an indirect hot water tank using a boiler loop and using solar energy. In the case of solar water heating, a conventional water heating backup system is needed to ensure adequate hot water when there is inadequate solar heating.
ALFORD : Today when talk is about green, solar is another option, but solar does involve a storage tank. Another option could be heat recovery, in most cases off an HVAC process or industrial process.
CSE: Are solar heaters becoming a specifying option? What are the considerations for this system?
HOOVER : Solar water heating is certainly a viable option. The biggest issue in my mind is one of economics. It is important to know that the money saved by using solar energy can offset the substantial investment in solar water heating equipment including a conventional backup system.
ALFORD : Solar is becoming a popular option these days as more developers are trying to build “green” buildings. The main limitation of solar water heating, is the ability to make large quantities of hot water in very short periods of time. Location of a project far north may limit the use solar water heaters. Roof space for the solar panels or even space on the project site might be a limitation. Again, first costs may be a concern as large numbers of solar panels increase the cost of the system.
John Alford, PE, LEED AP
Senior Plumbing and Fire Protection Engineer, TLC Engineering, Orlando, Fla.
William R. Hoover
Director-at-Large A. O. Smith Corp. Milwaukee
Vice President for Technical Sales
Ask the experts: hot water tanks
Every month, Consulting-Specifying Engineer editors ask a distinguished panel of experts for information about how to best solve your problems, challenges, and new engineering issues. At CSE gives its readers and Web visitors the opportunity to pose questions directly to the panelists. Below is a question for January's topic, specifically about hot water storage tanks.
“ What are the pros and cons of using tanks without heating coils—using a circulation pump through a heat exchanger to heat multiple tanks—as compared to using multiple tanks with heating coils?” — Ron Scanlan , chief engineer, Interior Health, British Columbia
JOHN ALFORD : Generally speaking, the fewer the heat exchangers, the lower the first cost of the system. The designer should question the owner if redundancy is required and if the building can't be without hot water then two or more heat exchangers will be required. This design approach usually requires more floor space on the project.
If only one heat exchanger is required, then providing separate storage tanks will provide a long lasting system. When the energy source is remote from the tanks, the tanks will normally last for a long period of time without needing repairs. If the heat exchange is designed properly, these too can provide very long service.
If space is limited, then providing the heat exchanger inside the tank may be the only option. Several manufactures provide new gas tank water heaters with small storage tanks and high recoveries and high efficiencies. These models are very economical when considering first cost. But with such high Btu inputs, and inexpensive tank construction, don't expect these water heaters to last 20 years.
MELODY LUTZ : In residential application, there is no real advantage to using recirc pumps or heat exchangers to heat tank systems. These systems will use more energy than tankless system. Whether heating tanks with heating coils or a heat exchanger, the same Btu/hr are required to raise the water temperature to the desired setting.
In a commercial application, while the same is true for Btu/hr requirements, one can have more hot water available for peak periods in school applications or other commercial applications. You can also use this type of system it to heat water above the 135 F flash point for Legionella bacteria to cleanse the lines. Long piping runs may be necessary for these applications to avoid the high cost of redesign of existing plumbing systems. A tankless system that can modulate to zero-energy consumption is the most efficient way to meet these design requirements.