GB2447748A

GB2447748A - An underfloor heating element

Info

Publication number: GB2447748A
Application number: GB0804730A
Authority: GB
Inventors: George Frederick Warner
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-03-16
Filing date: 2008-03-14
Publication date: 2008-09-24
Also published as: GB0804730D0

Abstract

An underfloor heating element in the form of an elongate cable with an insulated conductive core comprising a resistive element 12 and insulation 14 which is surrounded by a return conductor16 and an insulating layer 18, to reduce electromagnetic emissions, the supply being applied though a controller at one end of the cable. Both conductors may be resistance heating conductors and a screen layer 20 and a protective sheath 22 may be included. The conductors may be single or multi-strand construction, conductor 16 alternatively comprising sheet material. Each insulating layer may resist temperatures up to 170{C.

Description

AN UNDERFLOOR HEATING ELEMENT

BACKGROUND OF THE INVENTION

1] This invention relates to a heating element in the form of a cable which is suitable for heating under carpet, under tile or under wooden flooring, referred to herein as "underfloor heating".

2] An underfloor heating installation typically makes use of a conductive wire i.e. heater element which is connected to an alternating electrical power supply. The heater element is made from a resistive material which generates heat which is radiated through the floor (i.e. the carpet, tile or wood as the case may be) into the room space above the floor.

3] The heater wire can be lengthy, particularly if the floor area is large, and can therefore act as an antenna which radiates an electromagnetic field at the frequency of the power supply. This type of radiation is often perceived to be undesirable in various aspects.

4] The invention is concerned with an underfloor heating element which emits less electromagnetic radiation.

SUMMARY OF THE INVENTION

5] The invention provides, in the first instance, a cable for use in underfloor heating which includes an electrically conductive core, a first electrically insulating sheath around the core, a second electrical conductor around the first sheath, and a second electrically insulating sheath around the second conductor.

6] The core may be selected from a single strand wire and a multi-strand wire.

7] The second conductor may be selected from the following: a single strand wire, a multi-strand wire, and sheet material. In the last mentioned case the sheet material may be in the form of a tube or it may be wrapped in helical fashion around the first insulating sheath.

8] In order to produce heat a resistive element must be included in the core, or in the second conductor, or in both components.

9] The resistive element (i.e. the heater element) can be either the core wire or the second conductor. The current flowing in the core wire is the same as the current flowing in the second conductor for these components are electrically connected in series. The core wire and the second conductor are connected together at a far end of the cable and power is connected to the core wire and the second conductor at an opposing near end of the cable.

0] The resistive element can be introduced, according to requirement, into the core wire or the second conductor, or both components, by fabricating the selected component or both components from suitable alloys of copper, iron, nickel, chrome or any other material which offers resistance to the passage of an electrical current.

1] The electrical resistance of the cable is calculated to give the desired wattage output per metre over the length of the cable. The electrical resistance characteristic, as indicated, may be displayed by the core wire or the second conductor or by both components.

2] The invention is based on the fact that the current in the core wire flows in a first direction and is the same as the current in the second conductor which flows in a second direction which opposes the first direction. The electromagnetic field generated by the current in the core wire is thus the same in strength as the electromagnetic field generated by the current in the second conductor which surrounds the core wire but as these fields arise from oppositely directed equal currents the electromagnetic fields oppose and cancel each other.

This effect is significantly more effective than if use is made of adjacent elongate conductors e.g. side by side wires, where one conductor does not enclose the other conductor. The arrangement of the invention (one conductor enclosing the other) substantially eliminates the radiation of an electromagnetic field from the cable.

3] The electrical resistance of the core wire is calculated to give an output wattage which is dependent on the installation requirement. Similarly the electrical resistance of the second conductor is calculated to give an output wattage according to installation requirements.

4] Each insulation sheath may be formed of any appropriate material capable of retaining insulation requirements at temperatures in excess of 100 C.

Preferably each sheath provides adequate electrical insulation capability at least in the temperature range 100 C to 170 C.

5] To enhance the electromagnetic radiation reduction characteristic of the cable an electrical screen may be provided around the second sheath. The electrical screen, typically in the form of a fine metallic gauze or mesh or formed by helically wrapped conductive strip material around the second sheath can, in turn, be protected by an outer sheath.

6] The invention also extends to an underfloor electrical heating installation which includes at least one cable of the aforementioned kind embedded in the floor, a controller and an electrical alternating supply which is connected by the controller to the cable. The alternating supply may include a live line which is connected to the core and a neutral line which is connected to the second conductor. The electrical screen referred to is preferably connected to earth.

BRIEF DESCRIPTION OF THE DRAWINGS

7] The invention is further described by way of example with reference to the accompanying drawings which: Figure 1 is a side view of a stripped end of a heating cable according to the invention, and Figure 2 is a schematic representation of an underfloor heating installation using the principles of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

8] Figure 1 of the accompanying drawings illustrates an end of a cable 10 according to the invention.

9] The cable includes an elongate flexible metallic core 12, a first electrically insulating sheath 14 around the core, a second electrical conductor 16 around the first sheath 14, a second electrically insulating sheath 18 around the conductor 16, an earth screen 20 and a protective outer sheath 22.

0] The core wire 12 can be in any suitable form e.g. a single strand resistance wire or a multi-strand resistance wire. The resistance of the wire is calculated in accordance with the required power consumption. The heat which is required in a particular installation determines the power consumption of the cable and this in turn determines the resistance per metre in the resistive components of the cable. The core wire, as noted, is made from any appropriate material e.g. copper, tin, nickel or any suitable resistive alloy.

1] It is possible to form the core wire as a helically wound strand from a plurality of strands around a suitable non-conductive former.

2] The sheaths 14, 18 and 22 are flexible and are made from any appropriate plastics material with a high heat resistance. The sheath 14 separates the second conductor 16 from the core 12 while the sheath 18 separates the second conductor from the screen 20. The outer sheath 22 is primarily a protective shield. The inner sheaths may be made from ethylene tetrafluoroethylene and the outer sheath from any appropriate flexible plastics material such as polyvinylchloride.

3] Preferably each sheath can withstand a temperature in excess of 100 C.

Depending on the application the heat resistance capability of the sheath may be as high as 200 C. A suitable heat resistance capability is in excess of 100 C and uptol7o C.

4] The second conductor 16 is made from an appropriate metal and can be in the form of a single strand which is wound around the first sheath 14 or in the form of conductive strip material or multi-stranded material wound around the sheath. The second conductor effectively provides a magnetic path, which surrounds the core wire, for the field generated by the current in the core wire. cc,

5] The earth screen 20 can be in the form of a woven mesh or can be strip material which is helically wound around the sheath 18. The invention is not limited in this regard. This screen further helps to restrict electromagnetic radiation from the cable.

6] Figure 2 illustrates a typical underfloor heating installation 30 using an elongate cable 10 of the kind shown in Figure 1. In one sense the core 12 can be regarded as an outgoing conductor and the second conductor 16 can be regarded as a return conductor. The two conductors are connected to each other at one end of the cable 10 in any appropriate way e.g. by directly bonding the core to the conductor 16 or by using a bridging wire 24 which is soldered to the second conductor 16 and the core 12. As is indicated schematically in Figure 2 the cable 10 is laid, in a known manner, on a floor and is then covered by a screed or other material. This aspect is known in the art and therefore is not further described herein. The cable is connected to a mains supply 32 via a controller 34. The controller includes an on/off switch and a thermostat. These aspects are also known in the art. A live line 36 passes through the controller and is connected to the core 12 and a neutral line, which also passes through the controller, is connected to the second conductor 1 6.

7] The current in the core wire flows in a first direction and is the same as the current in the second conductor which flows in a second direction which opposes the first direction. The electromagnetic field generated by the current in the core wire is thus the same in strength as the electromagnetic field generated by the current in the second conductor which surrounds the core wire but as these fields arise from oppositely directed equal currents the electromagnetic fields oppose and cancel each other. This effect is significantly more effective than if use is made of adjacent elongate conductors e.g. side by side wires, where one conductor does not enclose the other conductor. The arrangement of the invention (one conductor enclosing the other) substantially eliminates the radiation of an electromagnetic field from the cable.

8] The use of the cable 10 in an underfloor heating installation significantly reduces electromagnetic radiation at the frequency of the mains supply.

Claims

1. A cable for use in underfloor heating which includes an electrically conductive core, a first electrically insulating sheath around the core, a second electrical conductor around the first sheath and a second electrically insulating sheath around the second conductor.

2. A cable according to claim 1 wherein the core is selected from a single strand wire and a multi-strand wire.

3. A cable according to claim 1 or 2 wherein the second conductor is selected from the following: a single strand wire, a multi-strand wire and sheet material.

4. A cable according to any one of claims 1 to 3 wherein each sheath can resist temperatures in excess of 100 C.

5. A cable according to claim 4 wherein each sheath can resist temperatures upto 170 C.

6. A cable according to any one of claims ito 5 which includes an electrical screen around the second sheath.

7. A cable according to any one of claims 1 to 6 which includes an outer protective sheath.

8. An underfloor electrical heating installation which includes at least one cable according to any one of claims 1 to 7 embedded in the floor, a controller and an electrical alternating supply which is connected by the controller to the cable, and wherein the core is electrically connected to the second conductor at an end of the cable which is remote from the electrical supply.

9. An installation according to claim 8 wherein the supply includes a live line which is connected to the core and a neutral line which is connected to the second conductor.

10. An underfloor heating installation which includes at least one cable according to claim 6, a controller and an electrical alternating supply which is connected by the controller to the cable and wherein the core is electrically connected to the second conductor at an end of the cable which is remote from the electrical supply and the electrical screen is connected to earth.