GB2460645A

GB2460645A - Bathtub Heater

Info

Publication number: GB2460645A
Application number: GB0810036A
Authority: GB
Inventors: Plamen Spassov Vassilev; Nashim Imam
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-06-02
Filing date: 2008-06-02
Publication date: 2009-12-09
Also published as: EP2307815A2; GB0810036D0; WO2009147374A2; WO2009147374A3

Abstract

A system for heating a bathtub 10 comprising a heating element 11 applied to at least one wall of the bathtub. The heating element such as an electric resistive wire (12, fig.8) can be applied on the exterior wall or within the wall of the bathtub. The bathtub may be manufactured from an acrylic or metal material. Preferably the system includes sensors for measuring the bathtub temperature and also ambient temperature which may be used for measuring a rate of change in temperature of the bathtub and correlating this rate of change to an amount or depth of water in the bathtub. The rate of change can be a rate in rise of temperature or a rate of cooling of the bathtub. A database or look-up table is established showing the different rates of change in temperature to the amount or depth of water in the bathtub. Techniques such as 'fuzzy logic' can be used to continually update the database and to correlate the rate in change of temperature of the bathtub to the amount or depth of water in the bathtub.

Description

Bathtub Heater

Field of Invention

The present invention relates to a water heating system for a bathtub. More particularly, the present invention relates to a system for controlling and regulating the temperature of water in a bathtub.

Introduction

It is commonly recognised that is difficult to precisely set or maintain the temperature of the water in a bathtub. Traditionally, the temperature of the water during filling is adjusted by periodically adjusting the hot and cold water taps. However, this necessitates continually checking the temperature of the water and manually adjusting the flow of hot or cold water so as to ensure that the correct proportions of hot and cold water are supplied. A mixer tap may be used whereby both the hot and cold water supply enters through a single spout. In GB2322717 a bath water temperature regulating device is provided connected via conduits to a hot and cold water supply. The bath water temperature is regulated by controlling the relative flow of hot and cold water through the conduits, e.g. by motorised valves.

In the case where the water is accidentally left running causing the bathtub to overfill, valuable hot water is lost through the overflow drain. In an extreme case, where the overflow is not adequate to cope with the influx of water, the bath may overfill resulting in flooding which in turn result in extensive damage and a very high insurance claim.

GB2322717 attempts to overcome this problem by providing means to detect the level of bath water. and regulate the water depth in response to the signal from the detecting means.

A further consideration is that, having run a bath, the temperature of the water in the bath will gradually fall over time through heat loss to the external surroundings. To bring the temperature of the water to the desired level, a person may therefore have to top up the bath with additional hot water. In order to do so and to prevent overfilling the bath, it is necessary to partially drain the water so as to provide room for the additional hot water.

Desirably, this step is performed automatically so as to continuously regulate the temperature of a filled bath without the risk of an overflow.

GB2 174219 discloses a bath having an automated hot and cold water inlet mixer valve and an automated outlet for waste water. A control system senses the level and temperature of the water in the bath during the filling and regulates the mixer valve to achieve a user-specified depth and temperature at a point when the mixer valve is closed.

The user can also adjust the water temperature during bathing by activating the control system to add hot or cold water. When the level of water in the bath becomes excessive, the control system opens the waste valve until the surplus water has been drained off The bath is also provided with a water recirculation pipe connected to an intake in the side of the bath and having an outlet end connected to a pump and conventional Jacuzzi jets. The recirculation pipe is provided with a through flow water heater which can be used to maintain the water at a pre-set temperature once the bath has been filled. Similarly, JP63153357 describes a water temperature regulator for a bathtub which regulates the temperature of water in a bathtub automatically in accordance with the choice of a user.

Thus when the water temperature is lower than the set value, a controller drives a circulating pump to circulate the cold water to a heat exchanger so as to increase the temperature of the water, which is then re-circulated to the bathtub.

Such systems suffer from the disadvantage that additional apertures need to be cut into the wall of the bath tub spoiling the appearance of the tub and providing a. further potential leak path. A further problem is the cost of additional equipment to operate such a temperature regulating system such as the circulation pump. Moreover, the need for additional mechanical devices such as the pump renders the systems prone to breakdown * and in need of regular servicing. The additional circulation components provide regions * where water can stagnate and are difficult to clean. This leads to hygiene problems, particularly in hotels, retirement homes and similar multi-user installations.

An improvement to the previous systems is the provision of heating means acting through the walls of the bathtub. DE10019937 describes a bath tub comprising a layer of electrical resistance heater material in contact with outer surface of the bath tub. In an alternative-arrangement, JP07222692, JP200723681 8 and JP03082416 all describe bathtubs in which the electrical heating elements instead of being on the exterior of the bath tub are buried or provided within the walls of the bath tub. The heat emitted from the heating elements thereby helps to keep the water and the bather warm.

Although such a heating means helps to keep the bathtub and water warm, an effective control system is required for (i) controlling and regulating the water temperature in the bathtub, (ii) controlling the flow of water into the bath tub and (iii) regulating the amount of electrical heat energy required to heat the water to a desired temperature for a given depth of water. This can be particularly important where conservation of energy is paramount, i.e. to limit any wasteful energy being expended in heating the water particularly in an environment where energy costs are continually rising.

Summary of Invention.

The present applicant has mitigated the above problem by providing a system for heating a bathtub comprising a heating element applied to at least one wall of the bathtub. Having a bathtub with heating means removes the need to re-circulate the water to any external heating means such as a heat exchanger and any pump system associated with the heat exchanger. This also eliminates any costs associated with a re-circulating system such as maintenance or service costs and costs associated with any peripheral components, e.g. the pump system. Preferably the heating means is a resistance heating element.

Optionally, the heating element is applied on the exterior wall of the bathtub or within the wall of the bathtub. For example, for baths composedof thermally conductive material * such as steel or cast iron, the heating element is preferably on the exterior wall of the bathtub. In the case where the bathtub is made from low thermal conductivity material such as acrylic, it is preferable that the heating element is contained within the wall of the bathtub so as to transfer as much heat as possible to the water. Optionally, the heating element can be applied to the wall of the bathtub by means of an adhesive or simply printed onto the wall of the bathtub.

More. preferably, the system comprises a controller for controlling and regulating the temperature of the bathtub. By means of a temperature sensor, the system measures the temperature of the bathtub, T and correlates this temperature to the temperature of the water. Since the temperature of the bathtub is taken to correlate with the temperature of the water, the rate of change in temperature of the bathtub and thus of the water is dependent upon the amount of water in the bathtub. The rate of change in temperature could be the rate in rise of temperature of the bathtub or the rate of cooling of the bathtub.

For example, when there is very little water in the bathtub, the amount of heat energy required to raise the temperature of the bathtub to a desired value would be much smaller than if the bathtub is full of water. In the latter case, the temperature rate will be represented as a relatively shallow slope whereas in the former case when the bathtub contains less water, the temperature/time slope will be much steeper. This may provide a mechanism for filling the bathtub to a pre-determined depth. Hypothetically, consider raising the temperature from an ambient temperature T1 to a set temperature of T2.

Neglecting heat losses, the relationship between the amount of energy required to raise the temperature of the water is approximated by the simple equation:-IxVxt=massx(T2-T3xSHC Eq.l Where I is the current in amps V is the voltage in volts.

t is the time in seconds mass is the mass of wateriri the bathtub T2 -Ti is the rise in temperature of the water which approximates to the rise in temperature of the bathtub.

SHCis the specific heat capacity of water in J kg' 0C1. The specific heat capacity of water measured at 25°C is taken to be 4.184 J g K' (wikipedia).

* Thus by correlating the rate of change in temperature of the bathtub to the amount of * water in the bathtub, the depth of water in the bathtub can be determined for a given size of bathtub. Preferably, the system uses fuzzy logic' to correlate the rate of change in temperature of the bathtub to the amount and thus depth of water in the bathtub. More preferably, the system is calibrated by measuring the rate of change in temperature of the bathtub for a given depth of water and this is repeated for different depths of water in the bathtub. A complete chart is thus built up relating the rate of change in temperature of the bathtub to the measured amounts and thus depths of water in the bathtub. Typically, the rate of temperature change of the bathtub is non linear due to heat losses. The low specific heat capacity and low mass of the bathtub in comparison to the volume of water would mean that energy lost through heating the bathtub alone can be neglected. For example, the measured specific heat capacity of iron at 25°C is taken to be 0.450 J g' K whereas the specific heat capacity of water at 25°C is taken to be 4.1813 J g K (wikipedia). In the case of acrylic bathtubs, the low mass of the bathtub material would mean that very little energy is required to heat the bathtub in comparison to that of water.

As a result, the present system is sensitive to detect small variation in depth of water in the bathtub.

The system preferably comprises an environment tempcraturc sensor for measuring the * temperature of the surrounding area which represents a baseline or zeroing' temperature whose value is used to determine the change in temperature of the bathtub. By using a baseline temperature, the system is able to compensate for heat loss of the heated water to the surrounding area as the water tries to euilibrate with the temperature of the surrounding area.

The system also preferably has a built in safety check for determining when the bathtub is * * empty. Forexample, the controller can be set to switch off power to the heater when it detects a rapid rate of rise in temperature of the bathtub corresponding to when there is substantially no water in the bathtub.

The simplification of the heating system provided by the present invention enables the bathtub incorporating a system for controlling and regulating the temperature and depth * of water to be manufactured cheaply without any external mechanical components.

Preferably, the present invention provides a method of making a bathtub comprising the step of applying a heating element to at least one wall of the bathtub. More preferably the method.of making a bathtub comprises the step of (a) applying the heating to a finishing layer, (b) laying a fibrous mat over the heating element and (c) impregnating the fibrous mat with a resinous material.

Optionally, water is supplied to the bathtub by means of a mixer tap or separate hot or cold water taps.

In use, the present invention provides a method of filling a bathtub to desired depth through the steps of: a. filling the bathtub with water for a fixed time period; b. determining the rate of change in temperature of the bathtub; c. determining the amount and depth of water in the bathtub by correlating the rate of change in temperature of the bathtub to a pre-determined known amount of water in the bathtub; d. calculating the flOW rate of water and determining the time period for filling the bathtub to the desired level; e. filling the bathtub with water for the calculated time period.

Preferably, the rate of change in temperature is measured by determining the rate in rise * of temperature of the bathtub or the rateof cooling of the bathtub. The initial filling of * * the bathtub for a predetermined fixed time enables the system to establish a temperature profile of the water in the bathtub and correlate this to a known depth of water in a stored database orlookup table. If the temperature profile does not correlate tothe depth desired * by the user, then the system calculates the flow rate for the water in the bathtub and then determines.the time period required to fill the bathtub to the desired fill depth. It then fills the bathtub for that calculated time period to the desired depth. More preferably, the present invention has a calibration mechanism for correlating the rate of rise of temperature to the amount of water in the bathtub by the method of (a) filling the bathtub with a known amount of water; (b) determining the rate of change in temperature of the bathtub and repeating steps (a) and (b) for different known amounts of water. These rates of change in temperature may also be determined at various differences in temperature between the bath and its surroundings: For the case where the water is supplied to the bathtub by means of separate hot and cold water tap, the flow rates of water over a predetermined period of time is calculated by measuring the temperature of the hot and cold water supply and the temperature of the water in the bathtub. Assuming the flow rate of the hot and cold water supply does not change, their respective flow rates can thus be determined. From the measured flow rates, the time taken to fill the bathtub to the desired depth can thus, be calculated.

Specific Description

Further preferred features and aspects of the present invention will be apparent from the claims and the following illustrative description made with reference to the accompanying drawings in which:-Fig. 1 is a schematic plot of temperature versus time for different depths of water in the bathtub.

Fig. 2 is a side view of a bathtub showing the heating element applied to the bottom external wall of the bathtub.

Fig. 3 is a plan view of the heating element applied to the bottom external wall or under body of the bathtub. * Fig. 4 is a plan view of the heating element, Fig. 5 is a cross-sectional view of the heating element along the line X-X.

Fig. 6 is a cross-sectional view of a bathtub in another embodiment of the present invention where the heating element is incorporated within the wall of the bathtub.

Fig. 7 is a plan view of a bathtub where the heating element is incorporated within the wall of the bathtub.

Fig. 8 is an expanded view of a cross-section of the bathtub wall showing the heating element within the wall of the bathtub.

Fig. 9 is a block diagram showing a control system for controlling and regulating the temperature and depth of water in the bathtub.

Fig. 10 is a flowchart showing the sequence of processing steps to calibrate the system.

Fig. 11 is a flowchart showing the sequence of processing steps to fill the bathtub. with water to a desired depth.

Theoretical Consideration The amount of energy to heat water from an ambient temperature T1 to a set temperature T2 is dependent upon the amount of water in the bathtub and is given by the simple equation 1.

The greater the mass of the water in the bathtub, the greater the amount of energy, given * by. IV; required to heat the water. For a constant voltage supply say, 230V to 240V and * operating. within a temperature range of 25°C to 40°C;: time, t, would be the factor determining how much energy is transferred to the water. Typically the bathtub would be operating within a narrow temperature range from as low as the. temperature of the cold water supply up to the temperature that the human body can confortably stand, i.e. 37.5°C. The time taken to heat a small quantity of water to a desired temperature T2 * * would be much less than a significant quantity of water. Heat losses increase the amount of electrical energy required to raise the temperature of the water by a given amount.

Heat losses increase as the temperature difference between the water and its surroundings * * increases. By monitoring the rate of change of temperature of the bathtub for different bath fills and at different temperatures above ambient, an unknown amount of water in the bathtub can thus be determined. A steep temperature rise with respect to time would indicate very.little water in the bathtub whereas a shallow temperature rise with respect to time would indicate a significant amount of water in the bathtub all at a given temperature difference above ambient. Fig. 1 shows a hypothetical plot of the temperature rise with respect to time for different bath fills, i.e. where the bathtub is empty I, partially full 2 and when it is full 3. If the bathtub is initially filled with hot water, the temperature rise determining measurements will just start at a higher temperature but the temperature curve will still be characteristic of the depth of water in the bathtub. Ln such a situation, the temperature of the water will reach a steady state condition as heat is lost to the bathtub and eventually the temperature of the bathtub will equilibrate to the temperature of the water. The temperature curve measured by the system is characteristic to the depth of water in the bathtub. Optionally, once the system has detected that hot water has been added to the bathtub, e.g. by means of the temperature sensor incorporated in the bath, it can measure the rate of cooling of the water over a fixed period of time and correlate this to the amount or depth of water in the bathtub. For example, the rate of cooling can be determined once the system detects that the temperature of the water is higher than ambient temperature. By setting up a database or look up table containing various temperature profiles for different amounts of water in the bathtub, an unknown quantity of water and thus its depth in the bathtub can be determined by simply analysing its temperature profile curve and correlating this profile to a stored profile of known water depth. Correlation can involve measuring and comparing the gradient of the temperature curve to more sophisticated methods such as polynomial curve fitting techniques. Any heat loss to the surrounding area can also be factored into the equation by measuring the temperature of the surrounding area and.

using this value as a zeroing' or baseline temperature for determining the change in temperature of the bathtub.

Apparatus In a first embodiment of the present invention, the heating elements are applied to an exterior wall of the bathtub. In this way heat generated by the heating elements is thermally conducted through the wall of the bathtub to the water. In this instance, it is desirable that the bathtub is fabricated from a thermally conductive material such as steel or iron as found in conventional metallic-type baths. In the example shown in Fig. 2 and 3, the heating elements II are appliedto the bottom wall or base of the bathtub 10. The heating elements 11 are secured onto the exterior surface of the bathtub wall by any means known in the art, e.g. a suitable adhesive or printing.

The heating element 11 shown in Fig. 4 is in form of a flexible heating blanket so that it can be easily moulded around any curved shaped wall thereby providing maximum intimate contact with its exterior wall. A cross-sectional area along the lines X-X of the heating element 11 shown in Fig. 4 shows that it comprises a two wire electrical heating cable 12 (not shown in detail) sandwiched between a copper foil 13 and a flexible insulating blanket 14 (see Fig. 5). The copper foil 13 provides a surface of high thermal conductivity from which heat from the heating cable 12 is efficiently thermally conducted to the exterior wall of the bathtub The insulating blanket 14 prevents any heat loss to the surrounding area. Typically, any thermally insulating blanket can be used such as glass/refractory fibre/mineral wool or suitable heat/flame resistant polymer foams. Tn the particular example, the heating element is secured onto an aluminium backing 15 so as to provide a reflective surface for radiating the heat towards the bathtub wall. One side of the aluminium backing or foil 15 is provided with an adhesive (not shown) so as to secure the heating cable 12 to the flexible insulating blanket 14. In the particular embodiment, the electrical heating cable 12 is located in depressions formed in one face of the flexible insulating blanket 14. 0 To measure the temperature of the bathtub and thus of the water, a temperature sensor 16 * such as a thermistor can be.built into the heating blanket. Alternatively, the temperature * measuring sensor can be separate of the heating blanket 11. It is assumed that the temperature of the bathtub is equivalent to the temperature of the water. For a perfectly conducting bathtub all heat generated in the walls of the bathtub is transferred to the * water. A steady state condition will be reached where the temperature of the water in the bathtub will be in equilibrium with the temperature of the bathtub.

To attach the heating blanket to the bathtub wall, the top face of the copper foil 13 covering the heating cable 12 is provided with an adhesive. However, the use of the copper foil 13 covering the heating cable 12 is optional and the heating cable12 can be secured directly onto the wall of the bathtub. The reflective foil and insulating blanket are likewise and independently optional.

In an alternative arrangement, the heating elements can be buried within the walls of the bathtub (see Fig. 6). In the particular embodiment, the heating element 11 is buried within the bottom wall of the bathtub 10 (see Fig. 7). This can be particularly beneficial for low thermal conductivity baths such as acrylic-type baths where the heating element needs to be as close to the water as possible to reduce the insulating effects of the bath * material and to transfer as much heat to the water as possible. Traditionally; in a typical * manufacturing process for an acrylic type bathtub, fibrous mat (chopped or woven fibre) * is applied to a finishing layer 17 which forms the smooth inside surface of the bathtub and the fibrous mat is impregnated with a resinous type material such as PMIMA as is conventionally known to the skilled person. In order to incorporate the heating elements 11 within the wall of a bathtub, the heating elements 11 can be laid onto the finishing layer 17 prior to laying up the fibrous mat. Alternatively, the heating element can be sandwiched between the layers of the fibrous mat.

To improve the efficiency of heat transfer from the heating cable 12, similarly to the * * heating blanket described earlier, the heating cable 12 is optionally sandwiched between * a copper foil 13 or other thermally conductive material and aluminium backing layer 15 * * or ether reflective coating for radiating the heat towards the bathtub wall (see Fig. 8). As * ::with the heating blanket, a sensor 16 such as a thermistor for measuring the temperature of.the bathtub can be incorporated within the heating element 11 or situated separately of the heating element. As a substitute to the fibrous blanket, the resinous material 18 of the bathtub provides the similar insulating properties for preventing escape of heat to the * * surrounding area. Optionally, exterior thermal insulation can also be used. * II

Fig. 9 shows a block diagram of a system used for controlling and regulating the temperature of the water in the bathtub. A central processing unit (CPU) monitors the temperature via an analogue-to-digital convertor from the temperature sensor of the heating element and that of the environment sensor. The CPU could be a personal computer or other similar processing unit. Through a built in internal clock, the central processing unit calculates the rate of change in temperature of the bathtub and thus of the water during a typical heating cycle. By means of the environment sensor, the system can also take into consideration any heat loss to the surrounding area and factor this into the rate determining step. For example, by assigning the environment temperature to a baseline' or zeroing' temperature, the system can compensate for changes in the bathtub temperature due to heat loss by basing the calculation of the change in temperature of the bathtub from this baseline' temperature. The desired temperature and depth of water in the bathtub is set by the user using a heater controller connected to the CPU. The heater controller can be operated locallythrough a local user interface or alternatively remotely.

through a remote user interface such as bluetooth, internet. or other telecommunication svsterri.

The system is first calibrated so as to formulate a database or look-up table showing various rates of change of temperature or temperature profiles for different known water * depths at various difference in water temperature and ambient temperature. Fig. 10 is a * S. * flowchart showing the sequence of ste.ps for calibrating the system. At the first stage, the * * . * system is initialised and then the.bathtub is filled with a known amount of water. A steady state. is soon reached as the temperature of the bathtub is in equilibrium to the * * . . * temperature of the water. In some instances, a time delay can be, incorporated into the * * ;*. . *. * calibration, system so allowing sufficient time for equilibrium to be established. The * power. to the heating elements is then switched on and the CPU monitors the temperature * rise of the bathtub and thus of the water. In certain circumstances as will be explained later, the system can measure the rate of cooling of the bathtub and correlate this to the depth of water in the bathtub. From the temperature measurements, the CPU calculates the rate in change of temperature for that particular known water depth at various temperatures above ambient. This is repeated for different water depths. The rate in change in temperature of the bathtub can be determined by determining the gradient of * the temperature profile or by fitting a curve to the temperature profile and determining its fitting equation. A database or look up table is thus built up consisting of the rate in change of temperature for varying depths of water in the bathtub and various differences in water temperature/ambient temperature. The system may use fuzzy logic' to regularly update its database for different known depths of water in the bathtub each time the user uses the bathtub.

* Typically, water can be supplied to the bathtub from a mixer tap. However, the system * would work equally well when the water is supplied to the bathtub from separate hot and cold water taps. Mixtures of a known quantity of hot and cold water are first supplied to the bathtub for a fixed period of time: Temperature sensors attached to the hot and cold water system preferably at the supply exit measure the temperature of the hot and cold * water supply as it exits the taps. This will help to negate any heat losses or heat gain of * the water travelling through the pipes and thus, provide a true representation of or nearest to the temperature of the water in the bathtub. Assuming a perfectly closed system where no heat is lost to the external environment, the measurements of the temperatures of the hot and cold water supply and the final depth and temperature of the mixture in the bathtub can be used to determine the flow rates. of both the hot and the cold water supply.

The quantity and thus depth of water.in the bathtub over a fixed time period can be * determined by measuring the rate of cooling of the water or the rate of rise in temperature * . of the water and correlating this rate toa known depth of water in the bathtub from a look-up table. or calibration chart as discussed above. From the amount of water in the * .. * bathtub, the system can calculate the overall flow rate of water from the hot and cold.

* * water supply. This is assuming the flow rate from both the hot and cold water supply does not change. For example, hot water from a hot water supply at a flow rate of Fh and fixed temperature Th and cold water at a flow rate of F and at fixed temperature T result in a mixture having a temperature of T and depth, D, then the relationship between the temperature of the bathtub and the flow rate of the hot and cold water can be given by a very simplified equation. * * + F;x7, (2) Th+7, Where Fh is the flow rate of hot water; F is the flow rate of cold water; Th is the temperature of hot water; T the temperature of cold water.

Since the flow rate, F, is determined by the sum of the flow rate of the hot and cold water supply:-S * FP+] (3) Substituting equation (3) into equation (2), the individual flow rates from the hOt and cold water supply can thus be calculated. It is assumed that the flow rates from both the hot and cold water supply remain constant. Any variation in the flow rates is considered marginal and therefore neglected. Numerical techniques can be used to calculate the supply flow rates even where Ihe supply temperatures are changing with time. Knowing the flow rates of the hot and cold water supply and the quantity or depth of water in the * bathtub, the system can then calculate the amount of hot and/or cold water necessary to achieve the desired depth and temperature of water. This is assuming a totally closed * system where no heat is lost to the outside environment. In reality, heat is èontiriually being lost to the external environment and the determination' of the flow rate may hot' be totally accurate. In one particular instance; the determined flow rate from the cold water supply, F may be higher than the actual flow rate resulting in the' supply Of less cold * water than is actually needed to mix with' the hot water resulting in a hOtter than desired * * bathtub water temperature. This may be' compensated by the heat losses frOm the hot water in the bathtub. In order to make sure that the temperature of the water reaches the * desired depth and temperature, the bathtub is filled at periodic intervals and temperature measurements are made periodically to check that the desired temperature is maintained.

If the temperature of the water is too high, the'n the system calculates the amount of cold water necessary to lower the temperature to the desired temperature. Likewise, if the temperature of the water is too cold, the system calculates the amount of hot water necessary to raise the temperature to the desired temperature.

Once the system has been calibrated, the system is ready for use. Fig. II is a flowchart showing the sequence of steps of operation of the system according to the present invention during a typical filling cycle. Firstly, the user via the heater controller sets the desired temperature and depth of water in the bathtub: This can be presented to the user as a touch screen display. For example, a graphics screen can be presented to the user.

showing various bath fill options and temperatures. The user simply selects the required bath fill and temperature setting. Once the user selects the desired bath fill and temperature, the system fills the bath for a predetermined time and powers up the heating elements. This pre-determined time is a fixed time limit and is sufficient to enable the CPU to obtain an initial reading of the rate of change of temperature of the water in the bathtub from the measured temperature profile. During filling, the relative hot and cold supply flow rates can also be determined as discussed above. Depending on the supply flow rate, the predetermined time period can range from 30 seconds to a couple of minutes so as to provide sufficient water in the bathtub for an initial temperature rate reading. A further time delay can be introduced before the system powers up the heating elements to allow the temperature of the bathtub to equilibrate with the temperature of the water; By means of the measured rate of change of temperature, the CPU then correlates this measured rate to a similar rate in the database or look-up table at the particular difference in water temperature/ambient temperature and thereby establishes a corresponding. depth of water in the bathtub. From the established depth.of.water, the * CPU then calculates the flow rate and thus the time taken to fill the bathtub to the desired water depth. If separate hot and cold supplies are provided, the. methods described earlier * are used to calculate the respective flow rates and hence the further individual times that these hot and cold flows must be "switched. on" to achieve the desired final water depth and temperature. Note that using this temperature and depth control system, it is only * . necessary to provide an onloff valve (e.g. solenoid valve) to control-the supply flows, and not a variable flow rate control valve. No depth measurement sensor as such (e.g. a pressure transducer) is required either. It then fills thebathtub to the desired level by keeping the supply water on for the calculated time period.

It is assumed that the rate of water flow does not vary significantly during a bath fill. Any changes to the rate of water flow as a result of an interruption in the main water supply such as flushing a toilet or opening a tap connected to the same supply is considered to be marginal and would not greatly affect the overall flow rate. Once the time period has elapsed, the system stops the water supply. In more sophisticated' arrangements, the system can monitor the flow rate by means of a flow rate sensor built into the water supply network. Once the system realises that there is an interruption or flow variation in the supply of water to the bathtub, it can factor this into the time period for discharging water in the bathtub, e.g. by allowing more time to compensate for the interruption in flow rate. Additionally the system can provide a safety mechanism for preventing heating the bathtub when there is no water present. This is accomplished by correlating the rate of change of temperature of the bathtub when the bathtub is empty and turning the power off to the heating elements when a similar rate of change is measured.

Once at the desired water depth, the system maintains the temperature of the water in the bathtub to the set value. This removes the need to continually top up the bathtub with additional hot' water as the water cools. Upper and lower temperature control limits are set within the system, say +1-0.5°C. When the temperature of the water falls below the lower control limitthe system activates the heating elements to heat the bathtub. Hence, the system periodically monitors the temperature of the bathtub so as to keep it within the control limits. , * * The present invention is not restricted to a bathtub and is applicable to other bathing systems such as a Jacuzzi� bathtub, hydrotherapy pool or the like. The present invention can also be used in conjunction to re-circulation heating system as is conventionally known in the art.

Claims

Claims I) A system for heating a bathtub comprising a heating element applied at least one wall of the bathtub.
2) A system as claimed in Claim 1, wherein the heating element is applied on the exterior wall of the bathtub.
3) A system as claimed in Claim 2, wherein the heating element is attached. to the exterior wall of the bathtub.
4) A system as claimed in Claim 1, wherein the heating element is provided within the wall of the bathtub.
5) A system as claimed in any of the preceding claims, wherein the heating element is a resistance heating element.
6) A system as claimed in any of the preceding claims, comprising a controller for * -controlling and regulating the temperature of the bathtub. 0
7. A system as claimed. in Claim 6, comprising a temperature sensor for measuring * the temperature of the bathtub. * * . 0 8) -A system as claimed in Claim 7, wherein the temperature sensor is built into the heating element.* 9) A system as claimed in anyone of Claims 6 to 8, wherein the system comprises an * environment temperature sensor for measuring the temperature external of the bathtub.10)A system as claimed in anyone of Claims 6 to 9, wherein the controller measures the rate of change in temperature of the bathtub.11) A system as claimed in Claim 10, wherein the controller measures the rate in rise of temperature of the bathtub.12)A system as claimed in Claim 11, wherein the controller measures the rate of cooling of the bathtub.13) A system as claimed, in any of Claims 11 or 12; wherein the controller correlates the rate of change in temperature of the bathtub to the amount of water in the bathtub and to the depth of water in the bathtub.14) A system as claimed in Claim 13, wherein the controller uses a fuzzy logic to * correlate the rate of change in temperature of the bathtub to the amount of water n th. htkhih 15) A system as claimed in any of the claims 6 to 14, in which the system is calibrated by:- * . a. measuring the rate of change in temperature of the bathtub for a given depth of water in the bathtub; * b. repeating step (a) for different depths of water in the bathtub..16)A system as claimed in Claim 15, in which the systeñtis further calibrated to * * . . . determine the rate, of change.' in temperature. of the bathtub when there is. * substantially no water in the bathtub.l7)A system as claimed in Claim 16, in which the system does not activate the * heating element when the rate of change in temperature corresponds to when there is substantially no water in the bathtub.18)A bathtub comprising a body for containing waterand a heating element applied at least one wall of the body.19) A bathtub as claimed in Claim 18, wherein the body is metallic.20) A bathtub as claimed in Claim 19, wherein the body is acrylic.21)A bathtub as claimed in any of the claims 18 to 20, wherein the heating element is applied to the body by means of printing.22)A bathtub as claimed in any oftheclaims 18 to21, wherein the heating element is applied to the body by means of an adhesive.23) A bathtub as claimed in any of claims 18 to 22, wherein the heating element is applied onto at least one exterior surface wall of the bathtub.24)A bathtub as claimed in any of claims 18 to 22, wherein the heating element is incorporated within at least one wall of the body.25) A bathtub as claimed in any of the claims 18 to 24, comprising a mixer tap.26) A bathtub as claimed in any of the claims 18 to 24, comprising a separate hot and cold water tap.27) A method of making a bathtub comprising the step of applying a heating element to at least one wall of the bathtub.28) A method of making a bathtub as claimed in Claim 27, comprising the step of: a. applying the heating element to a finishing layer; b. laying a fibrous mat over the heating element; c. impregnating the fibrous mat with a resinous material.29) A method of filling a bathtub to a desired depth comprising the step of:-a. filling the bathtub with water for a fixed time period; b. determining the change in temperature of the bathtub; c. determining the amount and depth of water in the bathtub by correlating the change in temperature of the bathtub to a pre-determined known amount of water in the bathtub.d. calculating the flow rate of water and determining the time period for filling the bathtub to the desired level; e. filling the bathtub with water for the calculated time period.30) A method of filling a bathtub as claimed in Claim 29, wherein the change in temperature is measured by determining the rate of rise in temperature.3 1) A method of filling a bathtub as claimed in Claim 29 wherein the change in temperature is measured by determining the rate of cooling of the bathtub.32)A method of filling a bathtub as claimed in Claim 30 or 31 wherein the water is from a hot and a cold water tap.33).A method of filling a bathtub as claimed in Claim 32, wherein the flow rate of the hot and cold water supply is calculated by measuring the temperature of the hot * and cold water supply and the temperature of the water in the bathtub.34) A method of calibrating a system for correlating the change of temperature of the bathtub to amount of water in the bathtub as defined in any of Claims 29 or 33 comprising the step of: a. filling the bathtub with a known amount of water; b. determining the change of temperature of the bathtub; V * c. repeating steps (a) and (b) for different known amounts of water.35)A System as claimed in any of claims I to 17, and substantially as described herein with reference to the accompanying drawings.36)A bathtub as claimed in any of the claims 18 to 26, and substantially as described herein with reference to the accompanying drawings.37)A method of making a bathtub as claimed in claims 27 or 28, and substantially as described herein with reference to the accompanying drawings.38)A method of filling a bathtub as claimed in any of claims 29 to 33, and substantially as described herein with reference to the accompanying drawings.