Tank-type
water heaters, also known as storage units, are the most common type
of water heater used in homes in this country. While most are fueled by natural
gas or electricity, some use propane, fuel oil or the sun as an energy source.
Storage units come in a range of heights and diameters from tall, narrow ones
designed to fit in small closet spaces, to short, squatty ones that fit under
cabinets and in areas with little headroom. The most common sizes range from
20 to 80 gallon capacity. The higher gallon-capacity tanks are recommended for
customers with electric units that are on the utility's off-peak usage rates
because they sometimes need the higher capacity to hold enough hot water during
times when electricity is unavailable or more expensive.
While
they vary in energy source and size, these tanks share many common characteristics.
For one, the outer jacket of tank water heaters is made of sheet metal. Inside
the sheet metal, there are one or more inches of insulation wrapped around a
water containment tank. Typically, a cold water pipe is connected to the top
of the tank. Water enters the tank through this “dip tube,” which
delivers it to the lower portion of the tank. Hot water for the home is drawn
from the top of the tank and replaced with cold water entering through the dip
tube at the bottom. In combustion-fired water heaters, the heat source is at
the bottom of the tank.
Another
common feature is the drain valve located near the bottom of the tank. It is
used to drain off sediment that may accumulate at the bottom of the tank or
to drain the tank completely. At the top of the tank, there is a high-pressure
relief valve for safety. Should the temperature in the tank ever get too high,
pressure will build up in the tank and the pressure relief valve will open.
To prevent that from being needed, most tanks also have a high temperature thermostatic-switch,
which stops the heating if temperatures reach a dangerous level.
In
considering the energy efficiency of storage water heaters, keep in mind that
they are essentially hot water-filled tanks radiating heat through their shells
to the environment 24 hours a day. This heat loss is estimated to be 10 to 15
percent of the cost to operate the water heater. The rate of heat loss is directly
proportional to the surface area of the tank and the thickness of insulation.
Therefore, a smaller tank (e.g., 20 gallon vs. 80 gallon) has less surface area
from which to lose heat and has lower heat loss. In addition, a tank with more
insulation has lower heat loss. This is the reason people sometimes add an insulating
blanket around the outside of a tank. The primary difference between new water
heaters rated as highly energy efficient is they have more insulation around
the tank. They cost more, but are worth considering since the payback can be
rapid depending on your situation and energy cost.
If the water heater is located inside the home's conditioned space, in the winter, standby heat is not truly wasted since it contributes to heating the house. However, the reverse is true in the summer when, if you air condition, you are paying to remove the heat you already paid once for to heat your water. Storage water heaters are quite often located in unconditioned spaces like in garages, basements or crawl spaces. In these situations, the tank is just losing heat to the outside air.
