GB2306631A - Domestic water heating apparatus - Google Patents

Abstract

In domestic thermal storage water heating apparatus, a heat exchanger (33) is used for transferring heat between primary water pumped from the thermal store (10) and secondary water which is mains driven (38) through the heat exchanger. To control the temperature of the secondary water which flows from the heat exchanger, the flow of primary water through the heat exchanger is controlled dependant upon that temperature, the temperature being measured by a thermistor (42). Specifically, the primary flow is either throttled dependant upon that temperature or the pump speed is controlled dependant upon that temperature, but in either case, the pump (34) is supplied with sufficient voltage that it will be driven even if the demand for secondary hot water is very low, e.g. in the order of two or three litres per minute or less.

Description

Improvements Relating to Heating Apparatus This invention relates to heating apparatus for the heating of a fluid (herein the secondary fluid) by means of another, hot fluid (the primary fluid) by heat exchange.

In the particular application in which we are interested, both fluids are water, but the invention is not to be considered as being limited to this medium, as other liquids could be used, and the liquids could be different and also gases or gas and liquid combinations could be used. As the main use of the invention is the control of the heating of secondary water by means of primary hot water, the further description given herein will be limited to this medium, but the wider application of the invention should be borne in mind.

A main application of heating o! water by heat exchange is in the heating of domestic hot water which is used for washing purposes. This water is in fact referred to as secondary water in domestic water heating systems and the water which is used for space heating by way of central heating radiators, is referred to as primary water. The primary water is heated by means of a boiler or immersion heater.

Recently in the United Kingdom, the use of pressurised secondary hot water has been permitted, and this has led to the development of pressurised supply systems. One such system in which we are interested is known as " thermal storage", and in such a system, a body of water, supplied from the mains, is heated by the boiler and forms a thermal store. The heating takes place on the basis of supplying the heat to the store depending upon the heat stored, and not depending on instantaneous conditions of demand of the space heating and the hot water consumption. Thus, the boiler may be adding heat to the store in the evening when there is no demand from the space heating or for hot water, and equally, the boiler may not necessarily fire when the space heating is switched on (under thermostat control) or when someone in the house opens a hot water tap.Of course, if either space heating or the hot water demand continues, eventually the store heat supply capacity will reduce to such an extent that the boiler will fire adding heat to said store.

We are particularly interested in thermal storage installations and the present invention has particular application to such an installation in which the primary water is circulated through a heat exchanger which is external to the storage tank which holds the store water.

The secondary water also passes through this exchanger, but in the opposite direction from toe primary water so that the primary water gives up its heat to the secondary water.

It is important in such systems that the secondary water which has contact with atmosphere be kept at as low a temperature as possible because there is a tendency for the inside of the heat exchanger to scale up on the secondary side, such scaling being proportional to the temperature of the water and the speed at which it passes and volume which passes through the exchanger. There is not the same problem on the primary side in that there is only a limited amount of water in the primary side which is not consumed in the same way as primary water and scale build up is limited.

Another reason for keeping the temperature of the secondary water down is to avoid scalding of users of the hot water when they place their hands under a tap from which the hot water is issuing. Against the requirement to keep the temperature of the secondary water down is the requirement to ensure that the temperature of the secondary water is not too low, as otherwise the whole system would be ineffective.

To give an example by way of reciting some figures, the store temperature typically may be in the order of 80"C, whilst the required temperature of the secondary hot water may typically be 55"C.

To arrange for the secondary water to be controlled to be at the lower temperature, various steps have been taken in the past. In one arrangement, the pump which circulates the primary water through the heat exchanger is speed controlled dependent upon the sensed temperature of the secondary water issuing from the heat exchanger. If the sensed temperature is too high, the pump is slowed down, and if the sensed temperature is too low, the pump is speeded up. As a result the temperature of the issuing secondary water is kept constant, at least in theory.

In fact, an unexpected problem exists in that having regard to the voltage applied to drive the pump, when there is a demand for only a very small flow rate of hot secondary water, the pump is asked to run at a slow rate, and consequently only a small voltage is applied thereto, in the order of 40-50 volts. Such a small voltage often is insufficient to rotate the pump and there can be a complete loss of circulation of primary water, with the result that there is no heating of the secondary water at all.

Another step which has been taken which fails to solve the problem of scaling, is to allow the secondary water to heat up close to that of the primary, but to mix cold water from the mains by means of a thermostatic mixing valve, which controls the degree of addition of the cold water to ensure that the water which is supplied to the taps is modulated to the desired temperature.

It can be seen that neither arrangement is satisfactory. In the first case, failure, undesirably, of the pump to rotate would simply lead to endless complaints from users and in the second case, the scaling of the heat exchanger would mean expensive and frequent maintenance.

The present invention proposes an arrangement whereby the aforesaid disadvantages are obviated or mitigated.

In accordance with the present invention there is provided a heat exchange system wherein the primary fluid is circulated by means of a pump through a heat exchanger through which secondary fluid is also circulated, and including sensing means for sensing the temperature of the secondary fluid issuing from the heat exchanger to thereby control the rate of heat supplied to the heat exchanger via the primary fluid, and wherein the said control is overridden at low secondary fluid flow rates when the pump is driven at a rate higher than would normally be demanded at such flow rates.

As stated the primary fluid and the secondary fluid are preferably the primary and secondary water of a domestic thermal storage water heating system.

The arrangement may be such that at said low flow rates (which may typically be in the order of 2 to 3 litres per minute) the primary water discharged by the pump may be allowed to circulate through the heat exchanger unrestricted, in which case the secondary water may be heated to a temperature which is higher than required, but the consequences of this are minimal insofar as the low flow of secondary water issuing from the heat exchanger would not be sufficient to cause scalding to a user and the amount of scaling it would cause would be minimal.

By the use of an electronic control board, the override can be removed at a predetermined hot water demand level above said low flow rates, and when the override has been removed, the pump speed can be arranged to follow the demand set by the sensing of the temperature of the secondary water. This will provide that although at low flow rates, the temperature of the secondary water issuing from the heat exchanger may be higher than a preset maximum level, at the higher flow rates, the said preset maximum temperature will not be exceeded.

In a preferred feature, the primary side is controlled so that the flow of primary water is throttled depending upon the temperature of the secondary water, in which case the pump can be set to run at all times at its maximum speed, which has the clear advantage that there is little danger of the pump sticking as aforesaid; indeed some pumps are at their most efficient when running at full speed.

In this case, the restriction which performs the throttling, may be a simple mechanical temperature sensitive valve which closes progressively as the temperature of the secondary water rises and opens up as the said temperature falls. The temperature sensitive element of the valve may be linked to the sensor which detects the temperature of the secondary water. The linking can be electrical or by any other means.

The invention in one embodiment will now be described, by way of example, with reference to the accompanying drawings, wherein Fig. 1 is a circuit diagram of a heating apparatus according to the invention; and Fig. 2 is a circuit diagram of an alternative embodiment of the invention.

Referring to Fig. 1, reference 10 indicates a thermal storage tank which contains the body of primary water which is heated by means of a boiler 12. To effect this heating, the water is circulated from the boiler through pipe 14 by means of the boiler pump 16. The pump draws the water from the store 10 and delivers it to boiler 12. From the boiler 12, the heated water is returned to the store via the pipe 18.Heating of the water in the store is carried out under the control of a pair of store thermostats 20 and 22 of which 20 is a limiting thermostat and controls the maximum store temperature, whilst the thermostat 22 serves to control the temperature of the store accurately to 82so, plus or minus 3"C. It will be seen that there is a manual reset and overheat cut out device 24 in line 18, which operates to cut out the boiler 12 should it overheat to the extent that the water in line 18 reaches a temperature in the region of 105"C. The device 24 can be used manually to reset the apparatus after the cut out.

The device is linked to the thermostats 20 and 22 and also to the pump 16, so that the device 24 or the thermostat 20 or 22 by detecting an appropriate temperature can cause the pump 16 to stop.

Connected to the store 10 is a twenty litre expansion vessel 26 which serves to accommodate expansion of the water in the tank 10.

The apparatus illustrated is an integrated thermal storage system in that the water in the tank 10 serves to heat the dwelling in which the apparatus is located, and to supply the heat for the secondary water which is connected at the dwelling taps.

For the heating circuit, the water in the tank 10 is circulated through pipe 28 via a central heating pump 30.

The pipe is connected to the appropriate number of radiators 32.

For the heating of the secondary water, a plate heat exchanger 33 is used. A circulating pump 34 serves to pass water from the tank 10 through line 35 and to return it to the tank 10 via line 36. The secondary water is supplied from the mains via line 38 and it passes through the heat exchanger in contra flow to the primary water from the tank 10. The heated secondary water emerges on line 40, which contains a sensing thermister 42, and is delivered to the dwelling consumption points. The line 38 contains a flow switch 44 and electrical control lines 46 and 48 connect the sensor 42 and the flow switch 44 to a speed controller 49 which in turn controls the speed of the pump 34 via electrical control line 51.

Finally, the circuit may also include an integral or external clock and room thermostat; these items are indicated generally by the reference 50.

The basic operation of the circuit illustrated will now be described.

The boiler, under thermostatic control, supplies heat to the store by circulating water from the boiler through the tank 10, in known manner. The heat is supplied depending upon the condition of the store 10, and not upon the instantaneous demand for space heating or hot water. The room thermostat and/or clock 50 dictates when the space heating circuit is operational, and the opening of a hot water dispensing point i.e. the opening of a tap, dictates when the heat exchanger 33 is operational.

The embodiment of the invention is concerned with the control of the pump 34 when there is a demand for hot water at say a tap. This will be detected by the actuation of the flow switch 44, which causes starting of the pump 34 to circulate hot water through the heat exchanger 33. The pump speed control 49 detects the level of demand at the open tap and signals to the pump 34 accordingly. However, in accordance with this embodiment, if the level of demand at the tap is low e.g. in the order of 2 to 3 litres per minute, the pump is not set at the speed which theoretically would be correct to provide hot water at the tap at a predetermined temperature as this would involve the pump being driven by a voltage (in the order of 40 to 50 volts) which has been shown to be, practically, too low for reliably turning the pump 34, which can stick at such low drive voltages.Instead, the drive voltage is maintained at a higher voltage, in the order of 80 volts which although not maximum voltage, is sufficient to ensure that the pump 34 will not stick when demand is present, but low.

The driving of the pump 34 at a higher than needed voltage, results in the heat supplied to the heat exchanger being greater, which results in the hot water issuing from the tap at a temperature greater than the preset desired temperature (typically 550) but the danger of scalding is minimal because the flow is so low; also low flow at a higher temperature only produces a marginal scaling problem, if any.

When demand for flow of hot water is greater however, for example for the running of a bath, the speed control 49 becomes operational and ensures that the voltage applied to drive the pump will be increased to ensure that the temperature of the secondary water will be kept at the preset required value. As demand decreases, so the drive voltage applied to the pump 34 is decreased but only to the minimum level to ensure that the pump will keep turning at the low flow hot water demands which sometimes prevail.

In the embodiment described above, the apparatus includes the appropriate electronic controls to ensure the functioning described and indeed the apparatus includes a printed circuit board which controls all of the functions of the apparatus, including the control for the driving of the pump 34 as described above.

In an alternative embodiment however, as illustrated in Fig. 2 (which shows only the relevant components), the voltage of the pump 34 need not be varied during periods of different demands, but rather the flow of primary water is throttled or restricted. By this means, the pump 34 can in fact run at full voltage at all times that there is a demand for hot water. In such case, it is the amount of throttling which is controlled dependent upon the demand for hot water.

The control of the degree of throttling may be by electrical means or simple thermo-mechanical means such as are used in thermo-mechanical control valves for central heating radiators.

Referring to Fig. 2, parts already described with reference to Fig. 1 have the same reference numerals as in Fig. 1 and are not described further in detail. In Fig. 2, some of the parts have been omitted for clarity, but will be present in the apparatus as used.

The major difference between Fig. 1 and Fig. 2 is that the speed controller 49 and the lines 46 and 48 have been omitted and are replaced by a flow restrictor 52 in line 35.

The flow restrictor 52 serves to throttle the primary water flow through the heat exchanger 33 when there is a demand for hot water at a tap, and that demand is less than the maximum demand. The degree of throttling varies with demand so as to ensure that the temperature of the secondary water issuing from the tap remains at or as near to a preset level as possible. Thus, as the demand falls or rises, this will be detected by the sensor 42 which will in turn adjust the setting of the flow restrictor 52 to adjust to amount of primary water which flows through the heat exchanger 33. By this operation, the pump 34 can be set to run at its maximum output as long as there is a demand for hot water, however great or small, which is better for pump efficiency and indeed may enable the use of a less expensive form of pump.

It can be seen that by virtue of the invention the problem of pump sticking can be overcome and various methods can be adopted, whilst there is no sacrificing of the obtaining of effective control of the temperature of the secondary water to keep it at or within close limits of a preset required value. Scaling and scalding are also avoided.

Claims (8)

1. A heat exchange system wherein the primary fluid is circulated by means of a pump through a heat exchanger through which secondary fluid is also circulated, and including sensing means for sensing the temperature of the secondary fluid issuing from the heat exchanger to thereby control the rate of heat supplied to the heat exchanger via the primary fluid, and wherein the said control is effected at low secondary fluid flow rates when the pump is driven at a rate higher than would normally be demanded at such flow rates.

2. A heat exchange system according to claim 1, wherein the control is overridden at low secondary fluid flow rates.

3. A heat exchange system according to claim 2, including an electronic control board by which the override can be removed at a predetermined hot water demand level above said low flow rates, such that when the override has been removed, the pump speed follows the demand set by the sensing of the temperature of the secondary water.

4. A heat exchange system according to claim 1, wherein the primary side is controlled so that the flow of primary water is throttled depending upon the temperature of the secondary water, the pump being set to run at all times at a higher speed than required without the throttling, to provide secondary water at the required output temperature.

5. A heat exchange system according to claim 4, wherein a controllable restriction performs the throttling, and it may be a simple mechanical temperature sensitive valve which closes progressively as the temperature of the secondary water rises and opens up as the said temperature falls.

6. A heat exchanger according to claim 5, wherein the temperature sensitive element of the valve is linked to the sensor which detects the temperature of the secondary water.

7. A heat exchange system according to any preceding claim, wherein the primary fluid and the secondary fluid are the primary and secondary water of a domestic thermal storage water heating system.

8. A heat exchange system substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.