CONTROL SYSTEM FOR HEATING ARRANGEMENT

INTRODUCTION AND BACKGROUND

This invention relates to a control system, interface and method for an electrically operable heating arrangement.

Floor heating systems such as under carpet, under floor or under tile heating systems are known in the art. One known system comprises an electrically operable elongate heating element which in use is connectable to mains electricity supply. A thermostat is connected between the element and the mains supply, to control electricity supply and average temperature in the region of the heating element. The average temperature is sensed by a probe mounted in the region of the element. The probe typically has a negative temperature coefficient (NTC) and an output thereof is connected to a control input of the thermostat. The thermostat utilizes this input to control the electricity supply by making and breaking the supply, so that the average temperature does not exceed a user selectable temperature.

A problem with such a system is that the system is not sensitive to so-called hot spots along the element. A hot spot typically is a region of the element where the local temperature may exceed a safe temperature limit, yet the aforementioned average temperature may

still be in the region of the selected temperature, it wiit be appreciated that such an undetected hot-spot could give rise to damage and a fire hazard. Still furthermore, many such known systems are also not sensitive and/or responsive to ambient temperature in the room.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide an alternative control system, an interface for use in a control system and a method of controlling a heating arrangement with which the applicant believes the aforementioned disadvantages may at least be alleviated.

SUIVIMARY OF THE INVENTION According to the invention there is provided a control system for a heating arrangement, the control system comprising: a thermostat connectable in a circuit with the heating arrangement and a power supply; the thermostat having a control input; - the thermostat operating in accordance with a first resistance and temperature characteristic to make the circuit while a resistance value at the control input is in a first range of values which is one of a range of values smaller than a first threshold

level and a range of values larger than the first threshold level

and to break the circuit when the resistance is in a second range of values which is the other of the smaller and larger

range of values;

a resistor arrangement comprising an output and which output is connectable to the control input;

a first temperature dependent parameter determining

arrangement connectable to the heating arrangement;

a controller connected to the first temperature dependent parameter determining arrangement and the resistor

arrangement; and the controller being operative while the first temperature

dependent parameter has a value in a third range of values

which is one of a range of values smaller than a second

threshold level and a range of values larger than the second

threshold level to provide at the output of the resistor

arrangement a resistance value in the first range of values and

while the first temperature dependent parameter has a value in

a fourth range of values which is the other of the smaller range

of values and the larger range of values, to provide at the output of the resistor arrangement a resistance value in the

second range of values.

The heating arrangement may comprise a heating element, a sense wire and an insulating melt-down iayer between the heating element and the sense wire, the first temperature dependent parameter determining arrangement may be connected to at least one of the heating element and the sense wire and the first temperature dependent parameter may be average resistance of the at least one of the heating element and the sense wire, having a second temperature characteristic. The heating arrangement may be similar to that being sold under the mark SMART™ wire. Such an arrangement is disclosed in US 6,310,332.

A control system may comprise a second temperature dependent parameter determining arrangement connected to the heating arrangement and the controller, the controller being operative whϋe a second temperature dependent parameter has a value in a fifth range of values which is one of a range of values smaller than a third threshold level and a range of values larger than the third threshold level to provide at the output of the resistor arrangement a resistance value in the first range of values and while the second temperature dependent parameter has a value in a sixth range of values which is the other of the smaller range of values and the larger range of values

to provide at the output of the resistor arrangement a resistance in the second range of values.

The second temperature dependent parameter may be resistance between the sense wire and the heating element and the second temperature dependent parameter determining arrangement may be connected between the sense wire and the heating element.

The first characteristic may have a negative temperature coefficient (NTC), so that the first range of values are larger than the second range of values, the second characteristic may have a positive temperature coefficient (PTC), so that the third range of values are smaller than the fourth range of values, and the third characteristic may have a NTC, so that the fifth range of values are larger than the fourth range of values.

The resistor arrangement may comprise first and second discrete resistors, the first resistor having a resistance value in the first range of values and the second resistor having a resistance value in the second range of values. in another embodiment, the controller may be configured, in response to first temperature dependent parameter values determined by the

first temperature dependent parameter determining arrangement, to simulate at the output of the resistor arrangement a resistance value in accordance with the first characteristic and which value is a mapping of the first temperature dependent parameter values onto the first characteristic for the same temperature.

Also included within the scope of the present invention, an interface for connection between a thermostat and a heating arrangement, the thermostat having a control input and being configured to operate in accordance with a first resistance and temperature characteristic to make a circuit between the heating arrangement and a power supply while a resistance value at the control input is in a first range of values which is one of a range of values smaller than a first threshold level and a range of values larger than the first threshold level and to break the circuit when the resistance is in a second range of values which is the other of the smaller and larger range of values; the interface comprising: a resistor arrangement comprising an output and which output is connectable to the control input; - a first temperature dependent parameter determining arrangement connectable to the heating element;

a controller connected to the first temperature dependent parameter determining arrangement and the resistor arrangement; and the controller being operative white the first temperature dependent parameter has a value in a third range of values which is one of a range of vaiues smaller than a second threshold level and a range of values larger than the second threshold level to provide at the output of the resistor arrangement a resistance value in the first range of values and while the first temperature parameter has a value in a fourth range of values which is the other of the smaller range of values and the larger range of values, to provide at the output of the resistor arrangement a resistance value in the second range of values.

Still further included within the scope of the present invention is a method of controlling a heating element by means of a thermostat having a control input which is configured to operate in accordance with a first resistance and temperature characteristic to make a circuit between the heating element and a power supply while a resistance value at the control input is in a first range of values which is one of a range of values smaller than a first threshold level and a range of

vatues larger than the first threshold level and to break the circuit when the resistance is in a second range of values which is the other of the smaller and larger range of values; the method comprising the steps of: - determining values of a first temperature dependent parameter; and providing at the control input of the thermostat a resistance value in the first range of values while the first temperature dependent parameter has a value in a third range of values which is one of a range of values smaller than a second threshold level and a range of values larger than the second threshold level; and to provide at the control input a resistance value in a second range of values, while the first temperature dependent parameter has a value in a fourth range of values which is the other of the smaller range of values and the larger range of values.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS

The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein figure 1 is a block diagram of a prior art floor heating system;

figure 2 is a block diagram of a floor heating system comprising a first embodiment of a control system according to the invention; figure 3(a) is a graph illustrating an output resistance of a resistor arrangement forming part of the control system as a function of a first temperature dependent parameter; figure 3(b) is a similar graph, but with the output resistance as a function of a second temperature dependent parameter; figure 4 illustrates graphs of the first and second temperature dependent parameters and a first resistance and temperature characteristic at a control input of a known thermostat, to enable same to control connection of the power supply to the heating element; figure 5 is a block diagram of the heating system comprising a second embodiment of the control system.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In figure 1 there is shown a block diagram of a known floor heating system designated 10.

The system comprises an elongate heating element 12 connected via a known thermostat 14, which is currently available on the market, to

mains power 16. An average temperature sensing probe 18 is connected to a control input 20 of the thermostat. The probe has a resistance with a negative temperature coefficient (NTC) and the characteristic thereof is illustrated at 22 in figure 4. The thermostat is operative to connect the power supply to the heating element while the resistance is in a first range of values 24 larger than a first threshold level Ti . When the resistance value is in a second range 28 of values smaller than Ti, the power supply is disconnected from the heating element. In this manner the average temperature in a region of the heating element is controlled to remain in the region of a user selectable temperature 30 and which temperature is typically selectable via a user operable dial 32 provided on a front face of the thermostat. The disadvantages, shortcomings and problems of this known system 10 are summarized in the introduction of this specification.

A floor heating system comprising a first exemplary embodiment of a control system 54 according to the invention is generally designated by the reference numeral 40 in figure 2. The system 40 comprises a heating arrangement 42 comprising an elongate heating element 44. A sensing wire 46 extends lengthwise along the heating element and the heating element and sensing wire are separated by an insulating melt-

down layer or sleeve 48. Such an arrangement is commercially available under the trade mark SMART™ wire and is described in US 6,310,332, the disclosure of which is incorporated herein by reference.

The resistance of the sensing wire has a second temperature characteristic having a conventional positive temperature coefficient (PTC), as shown at 50 in figure 4. The resistance of the melt-down sleeve has a third temperature characteristic having a NTC as shown at 52 in figure 4.

The control system 54 according to the invention for the heating arrangement 42 comprises the known thermostat 14 and an interface 56 according to the invention which is connected between the heating arrangement 42 and the thermostat 14.

The interface 56 comprises a first temperature dependent parameter determining arrangement (PDA) 60 and a second PDA 62. The first PDA 60 and the second PDA 62 are connected to a controller 64. The controller 64 controls an output resistance of a resistor arrangement

66. The output of the arrangement 66 is connected to the aforementioned control input 20 of the thermostat 14, In a first

embodiment of the control system 54 shown in figure 2, the resistance arrangement comprises first and second discrete resistors. The first resistor 68 has a relative large resistance value Ri failing in the first range of values 24, such as 10 Kohm, The second resistor 70 has a smaller resistance value n falling in the second range of values

28, such as 5 Kohm, for example. The interface 56 comprises a switch 72 which is operable by the controller as will hereinafter be described to connect one of Ri and π to the output 74 of the interface 56 which output in turn is connected to the control input 20 of the thermostat.

The first PDA 60 continually monitors the value of a first temperature dependent parameter, namely the average value of the resistance Rsw of the sensing wire 46, to determine whether the value is lower or higher than a second threshold value T2 of about 1200ohm shown in figures 4 and 3(a). This threshold value may be factory, installer or user selectable and settable. While the value of the second temperature dependent parameter is in a third range 76 of values below the second threshold value, the controller causes the switch 72 to connect to the output 74, resistor 68 having a resistance value failing in the first range of values. The thermostat 14 sees a resistance in the first range of values 24 and keeps the power supply 16

connected to the heating element 44, However, once the value of the first temperature dependent parameter exceeds the second threshold value T2, and falls in a fourth range of values 78, the controller 64 causes switch 72 to connect to the output 74, resistor π having a resistance value failing in the second range 28 of values, The thermostat now sees a resistance falling in the second range of values which indicates that the average temperature is exceeding the selected temperature 30 and automatically interrupts power to the heating element, thereby causing the average temperature naturaity to drop below the selected temperature.

Similarly the second PDA 62 continually monitors the value of the second temperature dependent parameter namely the resistance Ri between the heating element and the sensing wire 46 (which is indicative of whether a hot spot is developing along the length of the heating wire) to determine whether it is higher or lower than a third threshold value T3 θf about 65Kohm, shown in figures 4 and 3(b). This threshold value may be factory selectable and settable. While the value of the second temperature dependent parameter is in a fifth range of values 80 above the third threshold value, the controiler causes the switch 72 to connect resistor 68 to the output 74, The thermostat 14 sees a resistance value in the first range of values 24

and keeps the power supply 16 connected to the heating element. However, once the value of the second temperature dependent parameter falls below the third threshold value Tz ₁ and fails in a sixth range of values 82, the controller 64 causes switch 72 to connect to output 74, resistor 70 falling in the second range 28 of values. This is indicative that the resistance of the melt-down sleeve is failing and that a hot-spot may be developing. The thermostat 14 now sees a resistance value falling within the second range of values and automatically interrupts power to the heating element, thereby avoiding damage and a fire hazard.

The controller incorporates an OR functionality (not shown) to ensure that resistor 70 is connected to the output 74, when a) the first temperature parameter exceeds T2 or b) the second temperature dependent parameter falls below T3.

it will be appreciated that a third and further temperature dependant parameter may similarly be monitored by the control system 54. The heating element 44 may similarly be controlled via the interface 56 and thermostat 14 in accordance with this monitoring. Such a parameter may be indicative of ambient temperature in a room which is being heated by the floor heating system 10 and which temperature

may be monitored with a suitable sensor (not shown) mounted in spaced relation relative to the floor. A PDA {aiso not shown) connected to this sensor may similarly be connected to the controller 64 for operating the resistor arrangement 66 as hereinbefore described, to ensure that the ambient temperature remains in the region of a selected value.

In figure 5 there is shown a second embodiment of the controller and interface. In this embodiment the controller comprises a processor (not shown) configured to scale and map the resistance values of characteristic 50 onto that of characteristic 22 for the same temperature, with values in ranges 76 and 78 mapped onto vaiues in ranges 22 and 28 respectively and with the first and second threshold values coinciding. The controiter is configured to present at output 74 a continuously variable resistance value 90 (as opposed to the only two discrete resistance values in the first embodiment) corresponding to the mapped vaiues on characteristic 22. A similar arrangement may be provided for mapping characteristic 52 onto characteristic 22.