Wath is the optim unitary flow in gal/min/sf/BTU in the tankless water heater?
Carlos Navarro Sarmiento
Energetica y Agroindustria scrl
carlosn@speedy.com.pe
5113310521 and FAX: 5113368910

WILLIAM HOOVER RESPONSE TO SEAN ELLIS: A tankless water heater does not supply hot water instantly. While they are often incorrectly referred to as instantaneous water heaters, they are more properly called tankless or on-demand water heaters.

Consider what happens when a hot water fixture is opened. Once the tankless heater senses water flow in the hot water line, it can begin the ignition sequence which involves purging the combustion chamber, igniting the burner which then heats the heat exchanger which eventually heats the water passing thru the heater. All of this takes from 10 to 15 seconds. So, not only is a tankless heater not instantaneous, it increases the delay (and water waste) after the hot water fixture is opened.

It is difficult to list the pros and cons of a 40 gallon storage water heater vs. a tankless heater concisely. Let me try. A storage water heater is inexpensive, reliable and proven. It provides a given volume of hot water independent of the flow rate desired. A tankless water heater is much more expensive to both purchase and install, it is relatively new technology to the American market, it is somewhat more energy efficient, it provides endless hot water if the flow rate required is within the capacity of the water heater, it consumes more water both by the ignition sequence mentioned above and from folks taking longer showers because they can not run out of hot water. Of course, the tankless heaters are smaller but not all the space gained us useful since the manufacturers warn to not use the space under them for anything which would be damaged by a water leak.

In a retrofit situation, the installation costs can be substantial. In most instances, a tankless heater will require a new, larger gas line, a new stainless steel (expensive) vent system, electrical power and additional make-up air. These modifications can add substantially to the total installation cost. In fact, the U.S. Department of Energy estimates that the installed cost to be in the range of $1470 to $2500 vs. an installed cost for a storage water heater of $865. This additional cost of $600 to $1635 is offset by an estimated operating savings (by the U.S. Department of Energy) of 74 therms of gas per year. At a national average gas cost of $1.218/Therm, this is an expected annual savings of $90.13. The homeowner must determine if this is a good return on their investment.

What are the pros and cons of a 40 gallon storage water heater to a tankless water heater.
how does a tankless water heater heat water instantly? if you have 30-35 degree water and turn a shower on how does that water get hot right away? doesnt make sense to me. seems you would still have a lag time for water to heat up. If you put water on a electic stove turn stove to high it still takes time for water to reach desired temp.

RESPONSES TO RON SCANLAN: 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.

Regarding Hot Water Storage Tanks:
What are the Pros and Cons of using tanks without heating coils, (using a circ pump thru a heat exchanger to heat multiple tanks). As compared to using multiple tanks with heating coils.

Thanks
Ron