Note: Descriptions are shown in the official language in which they were submitted.
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CONTROL SYSTEM AND METHOD FOR OPERATING A LOWER RESISTIVE
HEATING ELEMENT OF AN ELECTRIC WATER HEATER TO KILL BACTERIA
TECHNICAL FIELD
The present invention relates to a system and method for operating a lower
resistive heating element of an electric water heater for a continuous
predetermined
sanitizing time period at a temperature sufficient to kill bacteria in a lower
portion of the
water holding tank.
BACKGROUND OF THE INVENTION
Some species of Legionella bacteria can be found in the soil, most species
live in
water that is stagnant and wherein such bacteria survive under a wide range of
temperatures, typically 65 to 115 degrees F. According to the Centers for
Disease
Control and Prevention, USA, between 8,000 and 18,000 people are hospitalized
with
Legionnaires disease each year. It is of great public concern as its fatality
rate during an
outbreak ranges from 5% to 30% in those who contract the disease. Actively
managing
the risk of Legionella in water systems is more cost effective than responding
to an
outbreak. Outbreaks of Legionella pneumophila can stem from showers and
potable
water systems. As water from such sources aerosolized, individuals can inhale
the
Legionella containing droplets and the organism is aspirated into the lungs.
The formation and multiplication of such Legionella bacteria is not only
promoted
by the temperature in the customary hot water systems, but also by the fact
that dead
spaces are present in such water distribution systems in which deposits and
sediment
formation can arise, and typically in the bottom zone of water heater tanks.
Deposits
therein can represent a culture medium for bacteria proliferation.
Most electric water heaters for domestic use have its water tank constructed
with
a dome shaped bottom wall. Such dome-shaped bottom walls form a surrounding
cavitated zone about the dome-shaped wall where sediments deposit can gather
and
where water is less agitated. This cavitated zone is spaced from the bottom
heating
element and thus water therein is less hot creating an ideal location for
bacterial
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proliferation. Should the bottom element fail, then the water temperature at
the bottom
of the tank will drop. It has also been determined by research that the
Legionella
bacteria does not survive at temperatures above 140 degrees F. When hot water
is not
drawn from a water heater, the water inside the tank becomes stagnant and the
water
temperature stratifies with the cooler temperature being at the bottom region
of the tank.
Water within the cavitated zone below the bottom element of the tank can fall
to about
85 to 105 degrees F which is favorable to bacteria growth. Lowering the bottom
element to place it close to the bottom wall of the tank has not proven to be
a viable
solution.
(0005) Reference is made to US Patents 4,940,024; 5,168,546 and 5,808,277
which
disclose various methods and apparatus to prevent bacteria proliferation in
electric
water heaters. One method teaches adding a heating element in the form of a
belt or
patch on the outside of the tank against the bottom end of the outer sidewall
of the tank
to heat the water at the bottom end of the tank to a temperature preferably
above 130
degrees F. Accordingly, this proposed solution provides an extra heating
element in the
form of a patch heater located in an area which is usually filled with
insulating foam
material and not practical to access should it fail and require replacement or
repair. It is
also costly and consumes more electricity. In US Patent 5,808,277 a third
element is
added into the tank to periodically raise the water temperature at the bottom
of the tank
beyond the pre-set consumption temperature, to a sanitizing temperature to
destroy
bacteria. This is also a costly proposition. US Patent 4,940,024 discloses a
method of
directing the cold water flow of all consumed drinking or domestically used
water
through the lower region of the tank wherein there is no stagnant water and
wherein no
deposits can be formed for bacteria growth. Accordingly, the lower region of
the tank is
continuously flushed with fresh water. This is a costly solution requiring a
new tank
design and cold water conduit network and therefore not a viable
SUMMARY OF THE INVENTION
It is a feature of the present invention to provide a system and a method for
operating a lower resistive heating element of an electric water heater during
a
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continuous predetermined sanitizing time period to kill bacteria in the bottom
portion of
the water holding tank of the water heater.
Another feature of the present invention is to provide a system and a method
provided with a controller having a computer which is programmed to operate
the
lowermost resistive heating element of an electric water heater during a
continuous
predetermined sanitizing time period at a temperature sufficiently high to
kill the
Legionella bacteria in the lower portion of the water holding tank while
monitoring the
temperature of the water in the lower portion of the tank.
A still further feature of the present inventiOon is to provide switching
means
operated by a controller to bypass the thermostats of an electric water heater
to
continuously energize the lowermost resistive heating element for a
predetermined
sanitizing time period without interruption by a load shedding signal to shut
down the
lower resistive heating element. .
According to the above features, from a broad aspect, the present invention
provides a safety system for operating a lowermost resistive heating element
of an
electric water heater to kill bacteria in a lower portion of a water holding
tank of the
electric water heater. The system comprises a controller having a computer
with a
memory for receiving instructions to perform a programmed function. A timer
circuit is
associated with the programmed function. A temperature sensor is connected to
the
tank lower portion feeding temperature signals to the controller. The
lowermost
resistive heating element has a pair of terminals to which is secured two
leads from a
supply voltage to energize the lowermost resistive heating element. The
controller
operates switch means to energize the lowermost resistive heating element
independently of thermostats associated with an uppermost resistive heating
element
and the lowermost resistive heating element during a continuous predetermined
time
period as programmed in its memory sufficient to maintain water temperature in
the
lower portion of the tank to a predetermined temperature sufficiently high to
kill bacteria
to sanitize the lower portion of the water holding tank.
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According to another broad aspect of the present invention there is provided a
method of operating a lowermost resistive heating element of an electric water
heater to
kill bacteria in a lower portion of a water holding tank of the electric water
heater. The
electric water heater has an uppermost resistive heating element. The
lowermost and
uppermost resistive heating elements are controlled by a respective thermostat
control
having a temperature sensor. The method comprising the steps of:
i) providing a controller having a computer with a memory for receiving
instructions to
perform a programmed function and having a timer circuit associated therewith;
ii) controlling a supply voltage to a pair of terminals of the lowermost
resistive heating
element to energize the lowermost resistive heating element;
iii) sensing the temperature of water in the lower portion of the water
holding tank, and
iv) energizing the lowermost resistive heating element by said controller
independently
of the thermostat controls and during a continuous predetermined sanitizing
time period,
as programmed in the memory of the controller, sufficiently to maintain water
temperature in the tank lower portion to kill bacteria to sanitize the lower
portion of the
water holding tank.
BRIEF DESCRIPTION OF THE DRAWINGS
A preferred embodiment of the present invention will now be described with
reference to the example thereof as illustrated in the accompanying drawing is
which:
Figure 1 is schematic diagram illustrating the system and method of operating
the
lowermost resistive heating element of a dual element electric water heater by
the use
of a controller having a computer with a programmed controlled function to
control a
supply voltage to the lowermost resistive heating element independently of the
thermostats associated with the resistive heating elements.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to Figure 1 there is illustrated at 10 a two wire circuit for a
dual
resistive heating element electric water heater 11. The water holding tank 12
of the
water heater 11 has an upper resistive heating element 13 and a lower
resistive heating
element 14. A thermostat 15 is associated with the upper resistive heating
element 13
and a thermostat 16 is associated with the lowermost resistive heating element
14.
Thermostat 15 has a temperature sensor 17 for sensing water temperature in the
upper
portion of the water holding tank 12 and a control, not shown but obvious to a
person
skilled in the art, to trip the normally closed contact 18 and the normally
open contact 19
upon the temperature in the upper region of the tank reaching a preset
temperature of
its control, usually 140 degrees F. By doing so the upper resistive heating
element 13 is
shut down and the lower resistive heating element 14 is energized to heat
water in the
lower portion of the water holding tank 12. The lower thermostat 16 also has a
temperature sensor 20 and a control to open the normally closed contact 21 to
shut
down the lowermost resistive heating element 14 upon reaching a preset
temperature in
its control, usually also140 degrees F in the lower portion of the water
holding tank 12.
As mentioned above, it is known that at this temperature the Legionella
bacteria does
not survive.
In order to ensure that bacteria does not develop in water holding tanks of
electric
water heaters it is desirable that the water temperature in the lower portion
of the tank
be raised to 140 degrees F during for an uninterrupted predetermined time
period to
sanitize the bottom portion of the tank, preferably during non-peak hours when
there is
likely very little need for hot water and when electricity on the grid is
plentiful and at its
lowest cost to ensure that the Legionella bacteria cannot proliferate in the
bottom
portion of the tank 12, particularly in the lower cavity regions of the bottom
wall of the
tank where sediments usually collect. To accomplish this, the present
invention
provides a controller 25 which has a computer inputed by a keyboard 26 and
equipped
with a memory 27 in which a programmed function is inputted to perform a task.
A timer
28 is also associated with the controller. A temperature sensor 29 is secured
to the
outer surface 30 of the water holding tank 12 in the lower portion of the tank
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suitable attaching means such as a strap 31. The temperatur sensor 29 feeds
temperature signals to the controller to monitor the water temperature in the
lower
portion of the tank. The temperature of the tank outer wall is substantially
the same as
the water temperature in the tank on the other side of the wall. Although not
illustrated ,
the tank is insulated from the outside by a foam insulation, as is well known.
Switch means is provided and operated by the controller 25 to connect a
voltage
supply directly to the lowermost resistive heating element 14. The connection
to switch
means can be accomplished by closing supply contacts of thermostats or a
switch of
the controller (not shown) to connect a power lead to terminal 39 of the
lowermost
resistive heating element 14. As illustrated, the other connector 40 of the
lower resistive
heating element 14 is connected to the other arm of the supply voltage 45 at
the high
limit control switch 41 located at the input end of the upper thermostat 15,
herein
connected to terminal 42. Thus, by controlling power directly to the bottom
element 14
the upper thermostat 15 and the bottom thermostat 16 are bypassed and the
lower
resistive heating element 14 is energized by the controller to heat the water
in the tank
bottom portion to maintain a predetermined temperature during a programmed
time
period., for example four hours, as directed by health regulations whereby to
kill any
bacteria that may be present in the bottom portion of the tank where the water
temperature is not has hot and sediments deposit..
As shown in the drawings, the high limit control switch 41 at the power input
of
the upper thermostat 15 is there to prevent water in the upper region of the
tank from
reaching an unsafe temperature of usually 180 degrees F. It has a pair of
normally
closed contacts 43 and 44 connected to the incoming power leads 45 and these
contacts are tripped to an open condition if the temperature exceed 180
degrees F.
As previously mentioned, in order to ensure that the Leg ionella bacteria is
not
present in the lower region of the water holding tank it is important that the
temperature
be raised to at least 140 degrees F and maintained at that temperature for a
predetermined period of time as prescribed by health safety regulations. In
the event
that the controller received a load shedding signal from the utility 35
wanting to shut
down the lower resistive heating element 14 during a sanitizing period, the
controller will
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advise the utility that it is in a sanitizing mode and that the lower
resistive heating
element will be controlled thereby until the end of the predetermined
sanitizing time
period. Thereafter, the controller can release the lower resistive heating
element to the
control of the utility.
As shown, a further temperature sensor 50 is secured to the tank wall in the
upper region of the tank to feed temperature signals to the controller 25
wherein the
controller can be programmed to monitor the temperature in the tank upper
portion in
the event that the water in the lower portion cannot be heated to the desired
programmed temperature for sanitization. Such temperature monitoring of the
upper
region of the tank could indicate that the bottom element is defective or its
power
connection faulty. On the other hand, this monitoring can also serve as a
detection of a
continuous draw of water from the upper region which needs to be attended to.
In such
situations, the controller would generate a signal fault condition requesting
servicing of
the water heater and provide an indication as to the defective area of the
water heater.
It is pointed out that although the safety system of the present invention is
herein
disclosed in association with dual resistive heating elements wired for non-
simultaneous operation, it is also intended to cover a simultaneous wiring
configuration
where the power switch 35 is connected to the incoming supply to energize the
lowermost resistive heating element when the switch is actuated by the
controller.
Briefly, the method of operation of the present invention comprises operating
the
lowermost resistive heating element 14 of the electric water heater 11 to kill
bacteria in
a lower portion of its water holding tank 12. The method comprises the use of
a
controller having a computer with a memory for receiving instructions to
perform a
programmed function and having a timer circuit associated therewith. A
controllable
switching means is associated with the controller and actuated to connect a
supply
voltage between the terminals 39, 40 of the lowermost resistive heating
element to
energize the lowermost resistive heating element 14. The temperature of water
in the
lower portion of the water holding tank 12 is sensed by a sensor 29 which
feeds
temperature signals to the controller 25 to monitor water temperature in the
lower
portion of the tank. The controller 25 takes full control of the bottom
resistive heating
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element and energizes the lowermost resistive heating element 14 independently
of the
thermostats 15, 16 and for a continuous predetermined sanitizing time period
as
programmed in the memory of the controller sufficiently to raise water
temperature in
the tank lower portion to kill bacteria but not exceeding a high temperature
limit of
heated water in an upper portion of the tank.
It is within the ambit of the present invention to cover any obvious
modifications of
the preferred embodiment described herein provided such modifications fall
within the
scope of the appended claims.
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