Recreational vehicle water heater having centrally controlled gas and electric power sources

A water heater for a recreational vehicle which employs both gas and electric powered system for heating the water within the water heater, wherein both the gas powered and electric powered systems are controlled through a single user interface control. The present invention provides a water heater for a recreational vehicle wherein a hot surface ignition system is used in conjunction with the gas burner and the electric resistance coil is immersed within the water such that either gas stored on board the recreational vehicle, or land-based electrical power can be used to heat the water either independently or in combination. Through the use of a single temperature sensor, and a single operator control, the water can be most efficiently brought up to the appropriate temperature in the most convenient manner for the user. Moreover, the present invention includes a built-in safety system, which does not require a separate energy cut-off switch, to insure that the water within the tank will not rise above an unacceptably safe temperature.

FIELD OF THE INVENTION 
The present invention generally relates to water heaters, and more 
particularly relates to means for controlling the source of powering the 
water heater. 
BACKGROUND OF THE INVENTION 
Water heaters, of the type adapted to heat water for the purposes of 
cooking, bathing, washing, and the like, are typically powered either 
through the use of a gas flame, or through the use of an electric 
resistance coil immersed within the water. If gas is used to heat the 
water, a flame is typically provided outside of the water tank, and the 
water itself is actually heated by convection as the tank itself is heated 
and dissipates energy into the water. With electric sources on the other 
hand, an electric coil having a relatively high resistance factor is 
immersed within the water tank and by providing electric current through 
the coil, the resistance causes the coil to become heated to a level 
sufficient to heat the water. 
With respect to gas powered water heaters, such devices have typically 
required a spark to serve as the ignition means to initially ignite the 
gas which can then be sustained through a continuous supply of gas or fuel 
thereto. The spark is typically provided through the use of a pair of 
conductors which are slightly spaced apart, and by providing electric 
current through one of the conductors, to a sufficiently high level, the 
current is caused to "arc" across the gap to the second conductor and 
thereby provide the spark necessary for ignition of the gas. Once the gas 
is ignited, the current flow through the conductors can be stopped in that 
the combustion can be sustained through continuously supplying gas. 
With respect to electric immersion coils, a relatively high level of 
electric power is necessary to cause the resistor to become heated to the 
level necessary for heating the water within the water heater tank. 
Typically the water heater requires at least a 120 volt AC power source, 
which is not a difficulty when the water heater is provided within a 
land-based home which can be connected to the community electrical grid. 
However, with mobile homes, or recreational vehicles, the ability to 
provide sufficient power to heat the water tank, is confronted with a new 
level of difficulty. The twelve volt DC power typically provided on a 
recreational vehicle is not sufficient to enable the electric resistance 
coil to be used. 
Various means have therefore been provided as a source of power for the hot 
water heater on board such recreational vehicles. For example, U.S. Pat. 
No. 5,632,614, assigned to the present assignee, discloses a gas fired 
appliance in the form of either a furnace or a hot water heater wherein 
the ignition means for initially igniting the gas is provided in the form 
of a "hot surface ignition system" wherein the electric power of the 
recreational vehicle is sufficient to heat a "hot surface", in the form of 
a resistor, to a level sufficient to ignite the gas, typically propane. 
Once ignition is reached, current flow to the hot surface can be stopped 
in that combustion of the gas can be sustained simply by providing a 
continuous flow of gas to the hot water heater. 
Alternatively, the '614 Patent also discloses the use of an electric coil 
which can be immersed within the water tank and powered by a land-based 
power source which is able to deliver the 120 volt AC power which is 
necessary to heat the electric coil sufficient to heat the water. 
While the system disclosed in the '614 patent does provide a means by which 
two different power sources can be used to heat the water within the water 
heater, it also requires two entirely separate means of sensing and 
control. For example, with regard to the gas system, a temperature probe 
is required to be provided within the hot water heater which in turn sends 
a signal to a processor wherein the processor then compares the actual 
temperature to the desired temperature, typically entered into the system 
through a thermostat. Depending on the difference between the actual water 
temperature and the desired water temperature, the processor can then send 
a signal to a valve to turn the flow of gas to the gas burner on or off. 
If in fact the recreational vehicle is parked and has access to a 
land-based power system, the electrical coil immersion system can be used 
either instead of, or in conjunction with, the gas burner. However, the 
electric coil immersion system also requires a temperature probe provided 
within the hot water heater, and a control mechanism for providing 
electrical power to the coil. 
As a result of these duel systems, the actual control of the water heater 
can be a rather cumbersome system resulting in an inefficient manner for 
heating the water within the hot water heater. The user is forced to set 
and monitor both the gas and the electric power systems. Moreover, the 
manufacturer of such recreational vehicles, are forced to make 
expenditures for the dual means of control which necessarily results in a 
higher cost for the recreational vehicle. 
SUMMARY OF THE INVENTION 
It is therefore a primary aim of the present invention to provide a water 
heater for a recreational vehicle having both a gas powered heater and an 
electric powered heater, and with a single control by which the user can 
set the desired water temperature, and then rely on the processor to 
adequately control the flow of gas and electric to bring the actual water 
temperature up to the desired water temperature level. 
It is an objective of the present invention to provide a water heater which 
uses both gas and electric as a source of power and which uses a single 
temperature sensing device as a means of feedback to the processor 
controlling the gas and electric supplies. 
It is another objective of the present invention to provide a water heater 
for use on recreational vehicles which enables the user to choose the 
source of power, i.e., gas, electric, or combination thereof, in addition 
to selecting the desired temperature. 
It is another objective of the present invention to provide a water heater 
for a recreational vehicle which employs a hot surface ignition system for 
use in conjunction with the gas powered water heater. 
In accordance with these aims and objectives, a feature of the present 
invention is a recreational vehicle water heater comprising a water tank, 
an electric heating means, a gas heating means, user interface means, a 
temperature measurement means, and a processor. The tank is adapted to 
hold water to be heated and includes an inlet for receipt of cold water 
and an outlet for exhaust of hot water. The electric heating means and the 
gas heating means are both adapted to heat the water within the tank 
either independently, or in combination. The user interface means enables 
a user to input the desired water temperature and the desired means for 
heating the water and is adapted to generate a signal proportional to the 
desired water temperature and a signal proportional to the desired heating 
means. The temperature measurement means is adapted to measure the 
temperature of the water within a tank and generate a signal proportional 
thereto which is then fed to the processor. The processor is adapted to 
receive the desired temperature signal, the desired heating means signal, 
and the measured temperature signal, and is further adapted to compare the 
measured temperature signal to the desired temperature signal to thereby 
generate signals to the electric heating means and the gas heating means 
or both, depending on the desired heating means signal. 
It is another feature of the present invention to provide a recreational 
vehicle water heater wherein the temperature measurement means is a 
thermistor disposed within the water, or on the outside surface of the 
tank. 
It is another feature of the present invention to provide a recreational 
vehicle water heater wherein the user interface means includes a desired 
heating means switch adapted to allow a user to select a gas heat mode, an 
electric heat mode, or a combination mode. 
It is yet another feature of the present invention to provide a 
recreational vehicle water heater wherein the inherent characteristics of 
the thermistor prevent the water temperature from exceeding approximately 
140.degree. F. This provides a safety feature against the water reaching a 
predetermined and unacceptably high level without the need for an energy 
cut-off switch. More specifically, if the thermistor fails in either an 
open or short condition, the processor recognizes these conditions and 
shuts down the water heater. Similarly, if the adjustable potentiometer 
fails in a short condition the water temperature will not exceed 
approximately 120.degree. F. If the adjustable potentiometer fails in an 
open condition, the water temperture will not exceed 140.degree. F. In 
both conditions, it is the processor that recognizes the failure mode and 
causes the unit to default at either 120.degree. F. or 140.degree. F. 
It is still another feature of the present invention to provide a water 
heater adapted to be powered either by a gas fuel source, an electric 
source, or a combination thereof and which includes a single temperature 
sensing device for monitoring the actual temperature of the water, and a 
single controller for allowing the user to select a desired water 
temperature. The controller is adapted to direct the gas fuel source, the 
electric source, or a combination thereof to cause the actual water 
temperature to match the desired water temperature. 
It is a still further feature of the present invention to provide a method 
for heating a body of water on board a recreational vehicle using both 
electric resistance and gas flame, the method employing a single 
temperature sensor and a single controller in communication with the 
electric resistance and gas flame, and including the steps of entering the 
desired water temperature and desired source of heat, sensing the actual 
water temperature, comparing the actual water temperature to the desired 
water temperature, and activating the heat output by the electric 
resistance and/or the gas flame until the desired water temperature is 
equal to the measured water temperature. 
These and other aims, objectives and features of the invention will become 
more apparent from the following detailed description when taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, and with specific reference to FIG. 1, the 
preferred embodiment of the present invention is depicted as water heater 
system 20. In accordance with the aims and objectives of the present 
invention, it can be seen that system 20 includes a water tank 22 which 
can be heated either through gas power or electric power. Moreover, 
through the use of a single temperature sensor, i.e., thermistor 24 and a 
single operator interface 26, a user of system 20 can simply select the 
desired choice of fuel and the desired temperature and system 20 then 
automatically regulates the heating of water tank 22 to the appropriate 
temperature using the appropriate fuel. Such a system is in stark contrast 
to prior art devices, which may be adapted to use either a gas powered or 
an electric powered water heater, but which require dual systems of 
temperature monitoring, and dual systems of user interface. 
The preferred embodiment of the present invention is able to accomplish the 
foregoing through the use of a computer processor 28. Processor 28 
receives signals from operator interface 26 as well as thermistor 24 and 
based on a series of calculations and choices, determines the most 
appropriate manner in which to bring water tank 22 to the desired 
temperature. While the specific software code by which the processor 28 is 
able to accomplish these objectives is not included herewith as a variety 
of software code programs could be written to accomplish same, a flow 
chart showing the basic sequence of decisions is depicted in FIG. 3, and 
will be described with further detail herein. However, it is important to 
note, that through the use of processor 28, a central location is provided 
by which all signals can be received and which can then calculate the best 
manner for bringing tank 22 up to the appropriate temperature while being 
relatively transparent to the user in that the user only needs to select 
the appropriate fuel source and the desired temperature. 
As shown in FIG. 1, operator interface 26, in the preferred embodiment, 
includes means for entering two variables. The two variables to be 
selected include the desired fuel source and the desired temperature, 
which as shown in FIG. 1, can be entered into the system through fuel 
source selection switch 30, and thermostat or potentiometer 32, 
respectively. Fuel source selection switch 30 actually includes three 
settings which allow the user to select gas power, electric power, or a 
combination thereof. 
Depending on the location and situation of the user, he or she can select 
the most appropriate mode of power to bring the temperature of the water 
tank 22 to the desired level most efficiently and quickly. For example, if 
water heater system 20 is disposed on a recreational vehicle 21 and the 
vehicle is in a remote location, access to electric power sufficient to 
power the water tank, i.e., at least a 120 volt AC power, may not be 
available. Gas power will then be needed to heat the water tank in which 
case the user can use fuel source selection switch 30 to select the use of 
the stored gas 34 on board the vehicle. Conversely, if the vehicle is 
located next to a land-based power source 36 the user can take advantage 
of the electric power to thereby heat the water tank 22 either 
independently, or through the use of a third mode on fuel source selection 
switch 30, in combination with the gas provided by gas fuel source 34. It 
can therefore be seen that water heater system 20 actually has three modes 
of operation which can most efficiently and quickly bring the water within 
tank 22 to the desired temperature depending on the location and available 
power sources. 
Turning now to the specific system by which gas power is able to heat water 
tank 22, gas system 38 is shown symbolically in FIG. 1, in its preferred 
embodiment. As will be readily appreciated by one of ordinary skill in the 
art, the preferred gas system 38 of the present invention uses the 
technology disclosed in U.S. Pat. No. 5,632,614, assigned to the present 
assignee, which disclosure is expressly incorporated by reference herein. 
By way of summarization, the preferred gas system 38 of the present 
invention employs a "hot surface" ignition system which is adapted to heat 
a surface 40 to a temperature sufficient to ignite gas 34 with which it 
comes into contact. Such a system is in contrast to "spark" ignition 
systems which require an electric arc to jump between conductors in order 
to ignite the gas. As best shown in FIG. 2, hot surface 40 in the 
preferred embodiment, is provided in the form of an electric resistor 
which is electrically connected between a first conductor 42 and a second 
conductor 44. By providing electric power through a first conductor 42 and 
second conductor 44, hot surface 40 is heated as a result of the electric 
resistance therein. The source of power to heat hot surface 40 can be 
provided either through a battery provided on the recreational vehicle, or 
through land-based power system 36. 
Once hot surface 40 is brought up to a temperature sufficient to ignite gas 
34, gas 34 is directed to flow through conduit 46 and through gas spreader 
48 such that increased gas atomization occurs to increase the likelihood 
of combustion once gas 34 comes into contact with hot surface 40. 
In order to regulate when hot surface 40 arrives at a sufficient 
temperature for ignition, and when to begin gas flow through gas conduit 
46, processor 28 receives a signal from infrared sensor 50 which is 
positioned to constantly monitor hot surface 40 and by reading the 
infrared radiation produced thereby, will send a signal to processor 28. 
Once hot surface 40 reaches the ignition temperature as read by infrared 
sensor 50, processor 28 sends a signal to gas valve 52 to allow gas 34 to 
flow through conduit 46, through gas spreader 48, and into contact with 
hot surface 40. 
Once ignition of gas 34 is reached, hot surface 40 will increase in 
temperature, which will therefore increase the thermal radiation emitted 
therefrom, which in turn will be read by infrared sensor 50 and 
communicated to processor 28. Processor 28 will time the duration hot 
surface 40 is supplied power to ensure combustion, and then current flow 
through first conductor 42 and 44 will be stopped. However, infrared 
sensor 50 will then monitor the thermal radiation emitted from emission 
zone 58 to insure that processor 28 is constantly aware of whether 
combustion is occurring. In the preferred embodiment, emission zone 58 is 
made of a thinly machined, or coined, piece of metal, which when heated 
emits infrared radiation. In an alternative embodiment to increase the 
thermal radiation provided by emission zone 58 and thus the ability of 
infrared sensor 40 to detect the actual temperature within emission zone 
58, a radiation coil can be provided in emission zone 58. In such an 
alternative embodiment of the present invention, the radiation coil can be 
provided in the form of a material referred to as Kanthal.TM. which is 
comprised of iron, chromium, and aluminum. While the foregoing represents 
a preferred material for emitting infrared radiation from within emission 
zone 58, those of skill in the art will readily understand that other 
materials are capable of emitting infrared radiation adequately for the 
present inventive system. 
The actual heating of water tank 22 by gas system 38 occurs through 
convection as hot combustion gases 89 are passed through heating tube 91 
provided within water tank 22. As the hot combustion gases 89 are passed 
through tube 91, the heat therein is dissipated into water tank 22. Cool 
combustion gases are then exhausted from heating tube 91 as shown in FIG. 
1. Hot water is withdrawn from tank 22 as needed through outtake tube 92, 
while cold water is supplied to replenish tank 22 through supply tube 90. 
Turning now to the electric system 62, it can be seen, as best depicted in 
FIG. 1, that system 62 includes an electric coil 64 adapted to be immersed 
into water tank 22 and by way of electric resistance heating is able to 
elevate the temperature of water within tank 22. It can also be seen that 
processor 28 is in communication with potentiometer 32 such that depending 
on the desired temperature of the water, as compared to the actual 
temperature of the water as measured by thermistor 24, processor 28 can 
determine whether to close electric relay 60 to thus provide power to 
electric coil 64 to most efficiently and quickly bring the water within 
tank 22 to the desired temperature. 
As stated above, thermistor 24 is provided within water tank 22 or on the 
outside surface of water tank 22 to monitor the temperature of the water 
therein and transmit a signal corresponding thereto to processor 28. As is 
readily known by those of skill in the art, thermistor 24 has a variable 
resistance which varies depending on the ambient temperature within which 
the thermistor is disposed, with the ambient temperature in this case 
being the temperature of the water. The use of thermistor 24 provides the 
invention with an important safety feature in preventing the water from 
reaching an unacceptably high temperature. In the preferred embodiment, if 
the thermistor 24 fails in either an open or short condition, the failure 
will be monitored by processor 28 which in turn will cut power to relay 60 
and thus power to coil 64. Relay 60 is a normally open contact such that 
when power is eliminated the relay opens. Similarly, processor 28 will 
cause valve 52 to stop the flow of gas. In so doing, the preferred 
embodiment of the present invention provides a safety feature to avoid 
scalding hot water, but without the need for an energy cut-off switch or 
the like. 
However, as a redundant safety feature, an energy cut-off (ECO) switch can 
be provided in alternative embodiments such that should the water 
temperature within tank 22 rise above a predetermined unsafe level, energy 
will be shut off to processor 28 and thus in turn to the gas system 38 and 
electric system 62. Such a feature is commonly required by manufacturers 
code, so as to avoid potential scalding if such extremely hot water were 
to come in contact with human skin. 
Referring now to FIG. 3, a flow chart is shown to depict the typical 
sequence of instructions and queries processed by processor 28 during the 
course of operation. As shown therein, the operation of system 20 is 
typically commenced by a user selecting the desired type of fuel as shown 
by step 72. As discussed above, such a selection will primarily be based 
upon the actual location of the vehicle and its proximity to a land based 
electric power system. Through step 72, a user can select either gas 
power, electric power, or a combination thereof. In addition to selecting 
the desired type of fuel source, a user can also select the desired 
temperature of the water through step 74. Step 74 allows the user to 
select the desired temperature of the water within a predetermined range, 
but for safety reasons prevents the desired temperature from exceeding 
that permitted by manufacturing code. 
Once a user selects the desired fuel source and desired temperature, 
signals corresponding thereto are provided to processor 28 which stores 
the signals in appropriate registers. In addition, processor 28 receives a 
signal from thermistor 24 to indicate the temperature within the water 
tank 22. This step is represented by step 76 shown in FIG. 3. Based on 
these three variables, processor 28 can then determine the best manner in 
which to bring the temperature of the water within tank 22 up to the 
desired temperature. In the preferred embodiment, once the user selects 
the desired fuel source, processor 28 will send signals either to electric 
relay 60, gas valve 52, or both to bring water within tank 22 up to the 
desired level. Simultaneously, the processor 22 will compare the actual 
temperature as measured by thermistor 24 to the desired temperature as 
measured by potentiometer 32 and perform the analysis as shown in FIG. 3, 
represented by step 78. For example, if the desired temperature is equal 
to the desired temperature, processor 28 will cut power to electric relay 
60 to disconnect coil 64 and close gas valve 52. Such calculation is shown 
by step 80 in FIG. 3. However, if the actual temperature as measured by 
thermistor 24 is less than the desired temperature, processor 28 will 
direct gas valve 52 to open and/or electric relay 60 to close to thereby 
begin heating of water tank 22 as shown by step 82. Such analysis will 
continuously be performed while water heater system 20 is activated. In an 
alternative embodiment electric system 62 and gas system 38 could be 
controlled by processor 28 in modulated form, wherein gas valve 52 or 
electric coil 64 are gradually increased or decreased, as opposed to the 
"on/off" system of the preferred embodiment. 
It can therefore be seen that the present invention brings to the art a new 
and improved water heater system which allows both a gas based and 
electric based heating system to heat the water within a water tank, and 
to do so with a single temperature sensing means and a single means of 
temperature control. The user of such a system therefore only needs to 
select the desired mode of heating, and the desired water temperature, and 
the processor based system of the present invention then performs the 
appropriate calculation and control to provide an essentially transparent 
system to the user. Not only is the system more user friendly, but also 
results in a more cost effective system in that dual systems of 
temperature monitoring and temperature control need not be employed as 
have heretofore been demanded by prior art systems. Moreover, through the 
use of a thermistor monitored by a processor, failure of the thermistor in 
an open or short condition will cause the processor to shut down the water 
heater. This will prevent the water temperature from reaching a 
predetermined high level, and eliminate the need for a separate energy 
cut-off switch, as currently required by manufacturer's code. Furthermore, 
alternative embodiments can modulate the flow of gas or electric current 
to thereby vary or regulate the rate at which the water with the water 
tank is brought up to the desired temperature.