 |
The typical water heater is
usually a cylindrical tank with a heat source that heats water from the
incoming water supply and also maintains and stores the heated water until
it is used. Most water heaters have a 40 to 50 gallon
glass-lined steel tank. The
fuel types used to power the heat source can be either
gas
(natural or propane),
oil
or
electricity. The life expectancy of the average water heater
is 8 to 12 years. Other types of hot water systems used in the United
States are: tank-less (coil) systems that uses a boiler to heat the water on
demand; instant hot water heaters that are small (2 gallon), located near
the tap design that provides hot water for only one tap; high efficiency gas
systems; and combination systems that are tied into a gas warm air furnace
or boiler to take advantage of the additional heat source. |
Basic Components:
Whether the water heater uses gas, oil or
electric to fuel the heat source, all water heaters share the following
basic components:
| Cold Water Inlet Pipe |
| Cold Water Control Valve |
| Cold Water Dip Tube |
| Hot Water Outlet Pipe |
| Anti-Corrosion or Sacrificial Anode
|
| Drain Valve |
| Temperature/Pressure Relief Valve (TPR)
|
| TPR Discharge Pipe |
Gas-Fired Water Heaters:
| Gas-fired water heaters have a gas burner located below the tank, a
thermostatic switch, a gas valve and a pilot light. The thermostat will
open the gas valve when it senses the water temperature at the bottom of
the tank has dropped below the present limit. Gas flows to the burner and
is ignited by the pilot light, heating the tank from below. In addition,
the hot exhaust gases are vented through a hollow core in the center of
the tank to help heat the water. The exhaust is then passed into the draft
hood and to the vent connector where it is discharged to a chimney flue. |
| Once the water temperature reaches the set limit of 120° F to 140° F,
the gas valve is closed and the burner is turned off.
|
| Note: Fuel-fired systems heat the water by heating the tank. As a
result, the tank experiences more wear and tear than with an internal
electric heat source. In addition, proper venting of the exhaust is always
important to prevent carbon monoxide poisoning. Be sure that the
vent/exhaust system is properly installed at a positive angle (upward
slope). |
Electric Water Heaters:
Introduction:
| The electric water heater uses two immersion heating elements, one
near the top of the tank and one near the bottom to heat the water. The
heating elements are electrical-resistant elements that operate in a
similar fashion to an electric stove-heating element. The unit is powered
by a 220-volt circuit.
|
Heating Elements and Controls:
| Each heating element has a thermostatic switch that turns the
individual heating element on and off. Both thermostatic switches are
usually set to the same temperature. When the water temperature (at the
top or bottom) drops below the preset limit, a control switch closes to
create an electrical circuit through the heating element. The electric
current heats the element, thereby heating the water. When the water
temperature reaches its preset limit again, the heating element will turn
off. Thermostats are adjustable and are set between 120°F for increased
energy savings and scald protection.
|
Lower and Upper Heating Elements:
| The lower element maintains the water temperature in the tank (when
idle) because the water at the bottom of the tank will cool first (heat
rises). In addition, the lower element heats the cold water entering the
tank through the dip tube (when hot water is being drawn out of the top of
the tank). |
| When the lower heating element cannot keep the water being drawn of
the top of the tank hot (e.g. high hot water demand), the upper element
activates and will try to heat the warmer water at the top tank as it is
drawn. Note: most electric water heaters are designed so that both
elements will NOT work at the same time. The lower element will shut off
when the upper element is active.
|
Oil-Fired Water Heater:
| Oil-fired water heaters use an oil burner and a combustion chamber
(similar to an oil-fired furnace), which is located below the tank, as the
heat source to heat the water. When the thermostat (at the bottom of the
tank) senses that the water temperature has fallen below the preset limit,
the oil burner will ignite. A jet-like flame is created in the combustion
chamber to heat the bottom of the tank. In addition, the hot exhaust gases
from the combustion chamber are vented either through a hollow core at the
center of the tank (like a gas-fired system) or around the tank to help
heat the water. The exhaust is then passed into the vent connector and
directed to a chimney flue.
|
| Once the water temperature reaches the set limit of 120° F to 140° F,
the oil burner is turned off.
|
| Note: Fuel-fired systems heat the water by heating the tank. As a
result, the tank experiences more wear and tear than with an internal
electric heat source. In addition, proper venting of the exhaust is always
important to prevent carbon monoxide poisoning. Be sure that the
vent/exhaust system is properly installed at a positive angle (upward
slope).
|
Temperature Pressure Relief Valve (TPR) :
Introduction:
The temperature pressure relief (TPR) valve is a safety device that is
designed to:
| prevent the water in the tank from exceeding 212° F, and |
| prevent the water pressure in the tank from exceeding 150
pounds/square inch (psi). |
Basic Physics (Temperature and Pressure of Water):
| Water boils at 212° F when at sea level or under 14.7 psi of pressure,
however, the boiling point increases when under pressure, which allows the
water to superheat beyond 212° F. Typical residential water pressure
ranges between 40 and 80 psi and at 50 psi, the water can reach 300° F.
|
Potential Problem:
If the water heater's thermostat,
which controls the heat source, malfunctions, the pressurized water in the
tank could continue to heat and superheat (beyond 212°F). This will cause
two problems:
| First, since water expands when heated,
the water pressure in the tank will increase as the water is superheated.
If the pressure exceeds the maximum pressure threshold (approximately 300
psi) the tank could rupture or even explode. |
| Second, the release of superheated water
(now significantly above 212°F) would immediately return to atmospheric
pressure (the boiling point of water is 212°F) and flash into steam,
causing a sudden increase in volume and release of energy. The force of
the flash steam would exceed the explosion of one pound of trinitrotoluene
(TNT). |
| How it Works:
| If the water reaches a temperature
of 210°F (before superheating occurs) or the water pressure exceeds
150 psi, the valve will open and release the overheated water. This
will allow cold water to enter, lowering the temperature of the
water in the tank and consequently the water pressure. The valve
will remain open as long as the temperature or pressure exceeds the
preset limit. The valve must be located on the tank at the top or
on the side near the top, where the water is generally the hottest.
TPR valves are not included with the water heater, so be sure that
the TPR valve matches the BTU (heat rating) rating of the water
heater. A Discharge Tube must be attached to all TPR valves. |
TPR Discharge Tube:
| A discharge tube is a tube or pipe that is attached to the TPR
valve that directs the superheated water down to the floor and away
from anyone in the discharge area to prevent scalding or burning. |
| The pipe itself must be made of a material that is rated for
both high temperature and pressure, which includes most rigid wall
copper or iron. Also, the size of the pipe must match the opening
size of the TPR valve discharge (usually ¾ inch).
|
| The tube must terminate no more than 6 inches from the floor or
be directed to the exterior of the home.
|
| If the discharge tube is routed to the exterior, the pipe must
discharge 6 to 24 inches from grade, with a downward slope to
prevent the pipe from clogging or forming a trap. Blocked discharge
tubes will prevent the superheated water from discharging and will
burst. Though counterintuitive, it is often recommended that the
discharge tube terminate next to the water heater so that any
malfunction of the water heater will be more readily noticed.
|
|
 |
Safety and Efficiency:
Safety:
The following are general safety and installation requirements for a
standard water heater.
| All water heaters placed in the garage must be 18 inches off the floor
to prevent accidental combustion of gasoline fumes in the garage. |
| All water heaters in the garage must have a physical barrier to
prevent impact damage from automobiles. |
| In some locations, water heaters must be mechanically secured (i.e.
earthquake straps) to prevent the system from being dislodged. |
| Gas or oil-fired water heaters should not be located in sleeping
areas. |
| Heaters are generally set to 120 ° F to help prevent accidental
scalding. However, with some dishwashers that do not have a heating
element to raise the temperature of the water, spotting of glassware and
dishes may occur. Check the dishwasher's manufacturer information for the
recommended temperature setting. |
| Require properly installed temperature and pressure relief valve and
discharge tube.
|
Recovery Rate:
| Recovery rate is a measure of the number of gallons water that can be
heated (from approximately 50° F) to 100° F in one hour. The faster the
recovery rate, the more water can be used without running out of hot
water. Generally, oiled-fired systems create the most heat and have the
fastest recovery rate followed by gas-fired systems and then electric
systems. Since fuel-fired heaters have a higher recovery rate, a large
tank may not always be needed. Conversely, larger tanks that hold more
usable hot water, do not require a faster recovery rate and can be
sufficient for intermittent use. Either way, when choosing a water heater,
select one with an appropriate capacity and recovery rate to match the
daily hot water demands of the home.
|
Insulation:
| An external insulation blanket wraps around the sides of the water
heater. Insulation blankets prevent heat from radiating from the tank and
help reduce the fuel cost associated with marinating the 120° F to 140° F
storage temperature. To determine if one is needed, feel the water heater
jacket. If it is warm, heat is escaping and the tank should be insulated.
If it is cool, no insulation is needed. |
| Newer heaters do not require and actually have warnings posted not to
add an insulation blanket. With gas or oil-fired systems, the insulation
should be kept away from the burner area. No insulation should be
installed on the top of the heater
|
| Pipe insulation of the hot water line will also help increase
efficiency.
|
Peak Timers:
| Peak timers are installed on electric heaters and allow the water
heater to turn on during specific times of the day. However, this requires
that all activities using hot water (e.g. bathing, laundry, dishwashing,
etc.) be accomplished at generally the same time of the day.
|
|
