Consulting-Specifying Engineer - January 2008 - (Page 18) M/E Roundtable 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 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 www.csemag.com/asktheexperts, 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. 18 Consulting-Specifying Engineer • JANUARY 2008 http://www.csemag.com/asktheexperts
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