Abstrict
An electric heater comprises a housing with an air inlet area,
an air outlet area, an electric heating element disposed between
the inlet area and outlet area, a fan that moves air through the
heater, a first high-temperature limit switch mounted on the housing
in the air inlet area, and a second high-temperature limit switch
mounted on housing in the air outlet area, the first and second
switches being electrically connected in series with each other
and to the heating element so as to disconnect power to the heating
element when either switch is tripped.
Claims
What is claimed is:
1. An electric heater having a housing made of metal with a back
plate, an air inlet area, an air outlet area, an electric heating
element disposed between the inlet area and outlet area, and a fan
that draws air into the inlet area, moves it across the heating
element, and discharges it from the outlet area, an improved overheating
system, comprising: a first high-temperature limit switch mounted
on the back plate of the housing in the air inlet area outside the
air stream passing through said heater, whereby the first high-temperature
switch detects the temperature of the metal at that location, the
switch being electrically connected to the heating element so as
to disconnect power to the heating element when the switch is tripped.
2. The improvement of claim 1, further comprising a second high-temperature
limit switch mounted on the back plate of the housing in the air
outlet area outside the air stream passing through said heater,
whereby the second high-temperature switch detects the temperature
of the metal at that location, the first and second switches being
electrically connected in series with each other and to the heating
element so as to disconnect power to the heating element when either
switch is tripped.
3. An electric heater, comprising: a metal housing with a back
plate an air inlet area, and an air outlet area; an electric heating
element disposed between the inlet area and outlet area; a fan that
draws air into the inlet area, moves it across the heating element,
and discharges it from the outlet area; and a first high-temperature
limit switch mounted on the backplate of the housing in the air
inlet area outside the air stream passing through said heater, whereby
the first high-temperature switch detects the temperature of the
metal at that location, the switch being electrically connected
to the heating element so as to disconnect power to the heating
element when the switch is tripped.
4. The heater of claim 3, further comprising a second high-temperature
limit switch mounted on the back plate of the housing in the air
outlet area outside the air stream passing through said heater,
whereby the second high-temperature switch detects the temperature
of the metal at that location, the first and second switches being
electrically connected in series with each other and to the heating
element so as to disconnect power to the heating element when either
switch is tripped.
5. The heater of claim 4, wherein the housing has a back, and wherein
both the first and second limit switches are mounted on the back
outside the air stream passing through said heater.
6. The heater of claim 4, wherein the housing is made of metal.
7. The heater of claim 3, wherein housing has a back, and wherein
the first limit switch is mounted on the back outside the air stream
passing through said heater.
8. The heater of claim 3, wherein the housing is made of metal.
9. An electric heater, comprising: a metal housing with a back
plate, an air inlet area, and an air outlet area; an electric heating
element disposed between the inlet area and outlet area; a fan that
draws air into the inlet area, moves it across the heating element,
and discharges it from the outlet area; a first high-temperature
limit switch mounted on the back plate of the housing in the air
inlet area outside the air stream passing through said heater, whereby
the first high-temperature switch detects the temperature of the
metal at that location; and a second high-temperature limit switch
mounted on the back plate of the housing in the air outlet area
outside the air stream passing through said heater, whereby the
second high-temperature switch detects the temperature of the metal
at that location, the first and second switches being electrically
connected in series with each other and to the heating element so
as to disconnect power to the heating element when either switch
is tripped.
10. The heater of claim 9, wherein the first and second limit switches
are electrically connected to the fan and the heating element such
that power is disconnected from both the fan and the heating element
when either switch is tripped.
11. An electric heater, comprising: a metal housing with a back
plate, an air inlet area, and an air outlet area; an electric heating
element disposed between the inlet area and outlet area; a fan that
draws air into the inlet area, moves it across the heating element,
and discharges it from the outlet area; a first high-temperature
limit switch mounted on the back plate of the housing in the air
inlet area outside the air stream passing through said heater, whereby
the first high-temperature switch detects the temperature of the
metal at that location; and a second high-temperature limit switch
mounted on the back plate of the housing in the air outlet area
outside the air stream passing through said heater, whereby the
second high-temperature switch detects the temperature of the metal
at that location, the first and second switches being electrically
connected in series with each other and to the heating element so
as to disconnect power to the heating element when either switch
is tripped.
12. The heater of claim 11, wherein the housing has a back, and
wherein both the first and the second limit switches are mounted
on the back of the housing.
13. An electric heater, comprising: a metal housing with a back
plate, an air inlet area, and an air outlet area; an electric heating
element disposed between the inlet area and outlet area; a fan that
draws air into the inlet area, moves it across the heating element,
and discharges it from the outlet area; a first high-temperature
limit switch for sensing the temperature of the inlet area having
a temperature trip range is between 130 and 170 degrees Fahrenheit,
the first high-temperature switch mounted on the back plate of the
housing in the air inlet area outside the air stream passing through
said heater, whereby the first high-temperature switch detects the
temperature of the metal at that location; and a second high-temperature
limit switch for sensing the temperature of the outlet area having
a temperature trip range is between 190 and 225 degrees Fahrenheit,
the second high-temperature switch mounted on the back plate of
the housing in the air outlet area outside the air stream passing
through said heater, whereby the second high-temperature switch
detects the temperature of the metal at that location, the first
and second switches being electrically connected in series with
each other and to the heating element so as to disconnect power
to the heating element when either switch is tripped.
14. The first high-temperature and the second high-temperature
switches of claim 13, having a self-holding feature such that when
the switch is opened due to a temperature rise, it will remain open
until power is removed either by turning down the thermostat or
disconnecting the power supply.
Description FIELD OF THE INVENTION
The present invention relates, generally, to electric heating devices.
More particularly, the invention relates to overheat protection
for an electric space heater. The invention has particular utility
in wall-mounted electric room heaters.
BACKGROUND OF THE INVENTION
The basic operation of all electric room heaters is the same. A
fan takes in room air in an inlet area and moves it across electrical
heating elements, which warm the air. The heated air is then discharged
from a discharge area back into the room. Typically there is no
variable power condition for the heating elements; they are either
off or on. When the heating elements are on, a minimum amount of
heat must be transferred from the heating elements to the air flowing
across them to keep the temperature of the heating elements below
a desired level. If there is too little heat transfer, the heating
elements can overheat and potentially burn out or cause a fire.
Heat transfer from the heating elements to the air flowing over
them depends on the initial temperature of the air flowing across
the heating elements and the amount of the airflow. If the airflow
amount is reduced, the air will be in contact with the heating elements
longer than desired, the outlet temperature will be the significantly
higher than desired, and the temperature of the heating elements
will also be significantly higher than desired. Likewise, if the
initial inlet air temperature is significantly higher than it should
be, even with the proper airflow amount, the outlet temperature
and the temperature of the heating elements can be significantly
higher than desired.
The state of the art includes various devices and methods for detecting
an abnormal heating condition, particularly overheating, in an electric
heater used to heat room air. Many heaters have a temperature limit
switch located near the heating element or output area to detect
an over-temperature condition.
U.S. Pat. Nos. 4,755,653 and 5,007,103 disclose heaters with circuits
that interrupt power to the heating elements when a sensor detects
an internal temperature greater than a predetermined maximum. However,
those patents do not disclose a particular location for sensing
the internal temperature.
In U.S. Pat. No. 5,083,011 to Munroe, two heater control signals
are produced. One signal is a minimum pressure differential between
static air pressure outside a hollow housing and the air velocity
pressure within the housing. The other signal is a temperature of
a portion of the housing lower than a predetermined maximum. The
heater is actuated only when both of those signals are present.
U.S. Pat. No. 5,245,691 to Wu discloses an electric circuit for
a heater which interrupts power to the heater when either an internal
temperature exceeds a limit or the heater is tilted.
A heater incorporating more safety devices is disclosed in U.S.
Pat. No. 5,805,767 to Jouas. The heater element is deenergized if
any of the following occur: the internal temperature of the heater
exceeds a set maximum, the rate at which the internal temperature
is increasing exceeds a given rate, the internal temperature exceeds
the ambient air temperature by a given amount, or when motion is
detected near the front of the heater. The features disclosed in
Jounas are more proactive than the purely reactive features of the
other patents in that they can shut off the heating element before
it has become too hot, but the features are relatively complex and
expensive.
The present invention provides an improved electrical heater which
overcomes the limitations and shortcomings of the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view of an electric wall heater
that uses the present invention.
FIG. 2 is a perspective view of the back side of the housing of
the heater of FIG. 1 with the limit switches of the present invention
installed. Wiring for the other electrical components of the heater
has been omitted for clarity. A portion of the housing is shown
cut away to show the location of the heating element.
FIG. 3 is a preferred schematic wiring diagram for the electrical
elements of the heater of FIG. 2.
FIG. 4 is an alternate schematic wiring diagram for the electrical
elements of the heater of FIG. 2.
FIG. 5 is a figure similar to FIG. 2 showing the wiring and sensors
in more detail.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a typical wall-mounted electric room heater
is generally indicated by the reference numeral 10. Heater 10 includes
a wall can 12 that receives housing 14 which has an inlet area 16
and an outlet area 18 that is curved to direct air outward from
housing 14. A fan motor 20 is mounted on the back of housing 14
and drives fan 22 to move air across heating element 24 located
in housing 14 between inlet area 16 and outlet area 18. Housing
cover 26 closes housing 14 and has an aperture 28 which receives
fan 22 and provides an air inlet. Housing 14 and housing cover 26
are preferably made of sheet metal. A thermostat 30 is mounted on
housing cover 26. Grill 32 attaches to wall can 12 and covers the
assembled housing 14 and housing cover 26.
The present invention installs two different temperature limit
switches on the housing 14, one for the inlet area 16 and another
for the outlet area 18. With housing 14 made of sheet metal, these
limit switches are mounted on the back of housing 14 and detect
the temperature of the sheet metal at those locations.
Referring to FIG. 2, temperature limit switches 40 and 42 detect
the temperature of the inlet area 16 and outlet area 18 respectively
of housing 14. Switches 40 and 42 are preferably held in place by
Z-shaped clips 44 and 46 respectively attached to the back of housing
14 at the desired locations by fastening means such as mechanical
fasteners or welding. Switches 40 and 42 are generally flat elongated
metal devices sheathed in a protective film. Such switches are readily
available from Thermtrol Corp. of North Canton, Ohio. The preferred
switches have a self-holding feature such that when the switch is
opened due to a temperature rise, it will remain open until power
is removed either by turning down the thermostat 30 or disconnecting
the power supply. For switch 40, sensing the temperature of the
inlet area, the preferred temperature trip range is between 130
and 170 degrees Fahrenheit. A switch suitable for that application
is Thermtrol part number SH7AM022A5 which trips at approximately
75.degree. C., (167.degree. F.) or SH7AM020A5 which trips at approximately
65.degree. C., (148.degree. F.). For switch 42, sensing the temperature
of the outlet area, the preferred temperature trip range is between
190 and 225 degrees Fahrenheit. A Switch suitable for that application
is Thermtrol part number SH7AM026A5 which trips at 95.degree. C.,
(203.degree. F.). The switches 40 and 42 easily slide into and fit
snugly in clips 44 and 46 respectively and contact the back side
of housing 14 to detect the temperature thereof. By sensing the
sheet metal temperatures, the switches are not susceptible to transient
conditions, such as the temporary disturbance of the airflow if
someone walks close to the heater outlet. Also, since the switches
are mounted on the back of housing 14, they are not in the airflow
stream and are, therefore, less susceptible to contaminants such
as dust, lint and the like which can reduce reliability.
Referring also to FIG. 3, switches 40 and 42 are wired in series
with each other and so that power to the heating element, and preferably
also to the fan, is disconnected if either of limit switches trip.
Alternatively, as shown in FIG. 4, the limit switches 40 and 42
can be wired such that only the heating element is disconnected
if either switch trips and the fan continues to run to circulate
air and cool the overheated portions.
In operation, if the air outlet area 18 is completely blocked,
the outlet area temperature will increase and limit switch 42 will
trip and disconnect power to the heating element. If an item, such
as a sofa or cardboard box, is placed a few inches in front of the
air outlet area 18, the heated air will be deflected by the object
and much of the heated air will be pulled into the air inlet area
16 by fan 22, thereby rapidly increasing the temperature of the
inlet area and tripping limit switch 40. The addition of the temperature
limit switch to sense the temperature of the inlet area of the heater
detects the abnormal operation condition of the heater where the
outlet is partially blocked to deflect a portion of the heated air
back into the inlet area of the heater and thereby provides additional
safety over heaters that only detect the outlet temperature.
The descriptions above and the accompanying drawings should be
interpreted in the illustrative and not the limited sense. While
the invention has been disclosed in connection with the preferred
embodiment or embodiments thereof, it should be understood that
there may be other embodiments which fall within the scope of the
invention as defined by the following claims.
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