GB2290415A - Thermostatic switches for electric storage heaters - Google Patents

Abstract

A thermostatic switch for an electric storage heater has contacts 18, 19 by means of which electrical heating elements of the storage heater are supplied, which are made to open when the temperature sensed by a first temperature sensor 10, eg a bulb, a bimetal or thermistor, which is responsive to room temperature, exceeds a predetermined value. The thermostatic switch also includes a second temperature sensor in the form of a bimetallic disc 24, the function of which is to open the contacts 18, 19 in the event of the temperature inside the storage heater becoming excessive. Being live, the bimetallic disc must be mounted with a sufficient clearance above a metal panel 7 of the storage heater but, in order to provide a high enough switching temperature of the bimetallic disc and a sufficient switching travel from its concave 24a to its convex shape 24, an unperforated shield 26, closed at the top and open at the bottom, surrounds the disc 24 to shield the bimetallic element 24 from stray air currents and confine the heat emitted by the heater. <IMAGE>

Description

THERMOSTATIC SWITCHES FOR ELECTRIC STORAGE HEATERS This invention relates to thermostatic switches for electric storage heaters.

Electric storage heaters are designed to utilise off-peak electricity for heating electrical heating elements to charge a heat storage medium. The heat is then progressively released, and the heat storage medium recharged during a subsequent off-peak period.

Such thermostatic switches comprise a pair of contacts having means for connection to the electrical heating element of a storage heater so that charging of the heater is controlled by opening and closing of the contacts, in response to temperature.

In one known electrical storage heater (GB-A-2 097 912), a first temperature sensor responsive to room temperature is arranged to control the opening and closing of the contacts.

As a safety feature, a second temperature sensor in the form of a bimetallic element has been proposed (GB-A-2 154 313). The second temperature sensor is mounted in the heater and responds when the temperature of the heat store rises above a predetermined value (representing a safe operating value). The bimetallic element opens the contacts in the event of the safe operating temperature being exceeded. This could happen if for example the user did not operate the storage heater according to instructions, possibly by partially obscuring the heater outlet.

Bimetallic elements in the form of discs which snap from a concave to convex shape at the operating temperatures have been used, a rod operated by the disc pushing the contacts open.

The thermostatic switch can be mounted in the storage heater above the heat store, conveniently on a metal panel to improve the sensitivity of the switch. This is permissible if the rod is electrically insulating or if the rod is electrically insulated from both contacts.

However, in some arrangements, the rod is conducting and is connected to one pole of the contacts. In these circumstances, a certain minimum clearance is necessary between the bimetallic element and/or rod, and the metal casing.

The result of this is that the point of switching representing unsafe store temperature now corresponds to a lower temperature than when the thermostatic switch could be mounted close to the metal panel. Now, the snap movement of a bimetallic switch depends mostly on its radius of curvature and as the required snap temperature is lowered, the manufactured curvature of the disc becomes reduced, as does the snap movement. The snap movement can be reduced to the point where adjustment ofthe linkage to the contacts is very difficult to manufacture due to the fine tolerances to be met and/or the bimetallic disc action is erratic allowing the appliance controlled to be unsafe or to break down.

The invention provides a thermostatic switch for an electric storage heater, comprising a pair of contacts having means for connection to electrical heating elements of the storage heater so that charging of the heater can be controlled by opening and closing of the contacts, a first temperature sensor responsive to room temperature and arranged to control the opening and closing of the contacts, a second temperature sensor comprising a bimetallic element responsive to heater temperature and arranged to open the contacts in response to a predetermined heater temperature, and an unperforated shield surrounding the bimetallic element which shield is open at the bottom and closed at the top, in order to shield the bimetallic element from stray air currents.

The shield increases the amount of heat received by the bimetallic element since the cooling effect of stray air currents is reduced and the closed region tends to confine the heat emitted by the store.

A thermostatic switch for an electric storage heater, constructed in accordance with the invention, will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a simplified schematic view of an electrical storage heater; Figure 2 is a view, partly in section, of the thermostatic switch; Figure 3 is a plan view of the lower moulding of the thermostatic switch, to a different scale to that of Figure 2; and Figure 4 is a perspective view from the underside ofthe lower moulding ofthe thermostatic switch, to the same scale as Figure 3.

Referring to Figure 1 the storage heater is contained in a casing indicated generally by the reference numeral 1 which is wall mounted above legs 2. The central region of the casing contains heat storage medium 3 surrounded by a layer of insulation 4, and an upper chamber 5 contains the thermostatic switch 6 mounted on a metal panel 7, and the knob of the switch 8 protrudes through the upper surface of the casing. A lower chamber 9 houses afrrsttemperature sensorinthe form ofa hydraulic bulb 10 which controls the opening and closing of contacts in the thermostatic switch. The sensor 10 is mainly influenced by room temperature, but is also partly influenced by heat transmitted from the store 3 through the insulation 4.

The storage heater is charged by off-peak electricity, via a pair of contacts in the thermostatic switch, and these contacts become opened when the bulb reaches a predetermined temperature.

Referring to Figures 2 to 4, the thermostatic switch 6 consists basically of an upper sheet metal housing 11 separated by an electrical insulation barrier 12 from a lower housing 13, which may be moulded from plastics or ceramic material, and the parts are held on the metal panel 7 by means of a mounting bracket 14. The hydraulic bulb 10, also shown in Figure 2 for the purposes of explanation, is connected by a capillary tube 15 to a bellows 16, the lower side of which carries a push rod 17 for operating the switch contacts 18 (fixed), 19 (movable). The bellows 16 is mounted on a leaf spring 20 and a bimetallic compensating means 21 is interposed between the leaf spring and the push rod in order that the effect oftemperature on the thermostatic switch due to its siting above the heat storage medium may be compensated for as far as operation of the contacts is concerned.A spindle 22, which carries the knob 8 shown in Figure 1, may be turned to alter the position of the leaf spring and thus the switching point of the contacts.

The moving contact 19 is mounted on a switch blade 23 of over-centre action, and normal operation of the heater consists in the contacts 18, 19 being closed at the start of an off-peak charging period, and being caused to open by the action of the push rod 17 acting on a ridge 23a in the switch blade and causing it to go over-centre to open the contacts 18, 19 when the temperature of the bulb reaches a predetermined value representative of the ambient temperature.

The contacts 18, 19 have terminal means 18a, 19a for connection to electrical heating elements of the storage heater, and it will be noted that the switch blade 23 forms part of the current carrying path.

As a safety cut-out feature the thermostatic switch includes a second temperature sensor in the form of a bimetallic disc 24 which is responsive to heater temperature and opens the contacts when a safe operating temperature has been exceeded. For example, if the outlet of the heater was obscured, temperature could build up in the heater, and this would be dangerous. Bimetallic disc 24, when the unsafe temperature is reached, snaps from a concave form (shown dotted 24a) to a convex form 24 in order to pull the contacts 18, 19 apart. The rod 25 connecting the bimetallic disc 24 with the switch blade 23 is made of metal. Because the bimetallic disc 24 is therefore live, it must be mounted a certain minimum distance (6mm) away from the metal panel 7.

This causes a problem compared to those thernlostatlc switches in which the rod 2! is insulating, or in which both contacts 18, 19 are carried on a movable arm, and are insulated from the conducting switch blade, such that the fixed member merely has to bridge the two movable switch contacts, because these can be mounted nearer to the metal panel 7 and hence receive more heat from it.

In accordance with the invention, a peripheral unperforated shield 26 surrounds the bimetallic disc 24. The shield is open at the bottom and closed at the top by virtue of the lower face ofthe moulding 13. The shield 26 is moulded integrally with the remainder of the lower housing.

The advantage of the shield 26 is that it shields the bimetallic disc 24 from stray air currents, and also to some extent confines the heat rising from the panel 7. The result of this is that the bimetallic disc 24 can be manufactured to snap over at a higher temperature, say, 1300C instead of 1 15"C as hitherto in a thermostatic switch with an unshielded clicker disc mounted more than 6mm away from the panel, and this permits a much greater travel ofthe operating rod 25, and therefore much more reliable opening of the contacts.

With this design, the shield 26 may extend to within lmm ofthe panel 7.

Of course variations may be made without departing from the scope of the invention.

Thus, the thermostatic switch could be mounted in another position on the heater, and the first temperature sensor need not be of the hydraulic bulb type, but could if desired be a bimetallic device or even a thermistor type sensor.

Claims (4)

1. A thermostatic switch for an electric storage heater, comprising a pair of contacts having means for connection to electrical heating elements of the storage heater so that charging of the heater can be controlled by opening and closing of the contacts, a first temperature sensor responsive to room temperature and arranged to control the opening and closing of the contacts, a second temperature sensor comprising a bimetallic element responsive to heater temperature and arranged to open the contacts in response to a predetermined heater temperature, and an unperforated shield surrounding the bimetallic element which shield is open at the bottom and closed at the top, in order to shield the bimetallic element from stray air currents.

2. A thermostatic switch as claimed in claim 1, in which the shield is made of insulating material.

3. A thermostatic switch as claimed in claim 1 or claim 2, in which the thermostatic switch is adapted to be mounted on a metal panel above a store of the storage heater.

4. A thermostatic switch substantially as hereinbefore described with reference to the accompanying drawings.