GB2269443A

GB2269443A - Control for water heating systems

Info

Publication number: GB2269443A
Application number: GB9214035A
Authority: GB
Inventors: Bernard John Patrick
Original assignee: CALEDONIAN CONTROL TECHNOLOGY
Current assignee: CALEDONIAN CONTROL TECHNOLOGY
Priority date: 1992-07-01
Filing date: 1992-07-01
Publication date: 1994-02-09
Anticipated expiration: 2012-07-01
Also published as: GB9214035D0; GB2269443B

Abstract

In control systems for boilers for central heating, electric showers and other water heating systems, the method of controlling the water heating system comprises measuring the volumetric flow of water, producing an analogue output signal proportional to said volumetric flow and using the said analogue output signal to control the heating of the water. A magnetic field may be induced in an area through which the water flows, and the electrical signal caused by the flow of water within the field measured. A water temperature signal may be electronically integrated into the analogue signal.

Description

IMPROVEMENTS IN CONTROL FOR WATER HEATING SYSTEMS The invention relates to improvements in control systems for boilers for central heating, electric showers and other hot water systems.

Modern hydronic central heating systems are progressing towards the total integration of the central heating boiler with an additional high efficiency heat exchanger.to provide "instantaneous" domestic hot water. These units are often referred to as combination boilers.

The advantages of the aforementioned systems lie in the cost saving associated by dispensing with the need to store large volumes of domestic hot water for intermittent use, and also in space saving due to the said hot water storage vessel being redundant.

A further advantage is that the installation costs are considerably lower than with conventional installations as the systems described are normally sold fully "packaged".

The main disadvantages of the systems are in an inherently slower response time to demand, poor temperature control, coupled with the hazard of scalding if conventional techniques are used to improve the above shortcomings.

There have been many attempts to produce combination boilers with a small quantity of stored hot water to overcome the response time problem.

However, these are not entirely successful with regard to the temperature control problem where there is a constantly varying demand for hot water which modern household appliances such as dishwashers, washing machines, as well as normal sanitary usage impose.

Conventional systems utilise a primary heat exchanger to convert the energy from, typically, a fossil fuel source such as natural gas into primary hot water which is then normally pumped directly around a circuit fitted with heat emitters to provide space heating. For many reasons it is not practical to use the primary water for secondary purposes such as washing etc. It is therefore an essential part of all combination boilers to include a water to water heat exchanger.

Conventional systems utilise a water flow switch to indicate when domestic hot water is being consumed. This is typically a micro-switch and paddle arrangement. The combination boiler must respond by redirecting the primary water from the central space heating circuit either in full or in substantial part to the water to water heat exchanger. This can be effected by any of several methods; for example by the operation of a water diverting valve or by the controlled switching on or off in sequence of circulating pumps.

In order to maintain an adequate temperature rise in the secondary water it is common to also increase or boost the firing rate of the primary heat exchanger. This is often initiated by the water flow switch.

The somewhat crude system described suffers from very poor temperature control as a constantly variable load is basically being controlled by an on/off device, namely the primary heat exchanger.

Improvements have been attempted to -the type of system described above, including the modulation control of the primary water into the water to water heat exchanger or the use of a modulating fuel valve to more closely match the fuel input to the demand.

These devices are typically controlled by a temperature sensor situated in the discharge pipe from the water to water heat exchanger.

It is possible to have fast acting temperature controls but there is a danger that scalding may occur if the response to a sudden drop in domestic hot water consumption is not responded to instantly.

For example, if a shower in a residential situation is being used and an automatic washing machine demands filling, the domestic hot water demand can quadruple in an instant. If the combination boiler responds to the sudden drop in water temperature that such events will cause, by increasing its input fourfold after the thermal lag period of the temperature sensing device, then it is reasonable to assume that when the automatic washer has filled there will be a sudden drop in water consumption to a quarter of the rate that existed. This sudden drop in flow will result in excessive water temperature for a short period equal to the thermal lag of the temperature control being discharged through the shower, thus giving rise to the scalding hazard referred to earlier.

Attempts to improve the above conditions normally result in less than perfect operation of combination boilers in the interests of safety.

Similar problems with temperature control and scalding occur with electric showers because of the lack of fast response of the heating unit and inability to deal with external factors such as incoming water temperature.

It is an object of the present invention to provide improved control of water in systems such as central heating systems, showers and the like without compromising the safety thereof.

According to the invention there is therefore provided a method of controlling a water heating system comprising measuring the volumetric flow of water, producing an analogue output signal proportional to said volumetric flow, using the said analogue output signal to control the heating of the water.

According to an alternative embodiment of the invention there is also provided a control unit for a water heating system comprising means to measure the volumetric flow of water, processing means for producing an analogue signal proportional to said volumetric flow, means connecting the output of the control unit to means for heating the water to control energy input into the water.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic of the basic layout of a typical combination boiler; Fig. 2 is a plan view of a flow sensor of a control unit according to the invention (without its housing) for controlling the combination boiler of Fig. 1; Fig. 3 is an pictorial representation of the flow sensor of Fig. 2 without its housing; Fig. 4 is a plan view of the electromagnet of the flow sensor of Fig. 2; Fig. 5 is an end elevation of the electromagnet version of Fig. 4; Figs. 6 and 7 are schematic representations of a permanent magnet version of Fig. 4; Fig. 8 is a schematic representation of a typical control circuit for a shower; and Fig. 9 is a schematic representation of a control circuit for a control unit for a shower according to the invention.

Referring to Fig. 1 there is shown a basic layout of a typical combination boiler 10. Fresh or cold water is fed into the system via a fresh or cold water feed 11. The water enters a water to water heat exchanger 12. The heat exchanger 12 has an outlet 13 which supplies domestic or secondary water on demand.

Water also circulates in a closed loop system.

This is the primary water flow, which flows through pipe 14 in the direction of the arrows shown in Fig.

1. The primary water flow is directed to flow by diverter valve 15 either via pipe 16 through the water to water heat exchanger 12 to warm the secondary water or to radiators and the like 17 to effect space heating. The primary water flow is returned via pump 18 to the primary heat exchanger 19, where the water is heated. The heating is effected by means of a burner 20 which is fed from fuel inlet 21 and the quantity of fuel supplied to the burner 20 is controlled by fuel control valve 22.

The flow sensor 25 shown in Figs. 2 and 3 is used according to the invention to control the operation of the boiler when located at position A or B on Fig. 1.

The flow sensor 25 can be a single or multipart device. The sensor 25 comprises a tube 26 for conveying water through the sensor 25. The tube 26 is held by a ferromagnetic core 27, which also supports a magnet 28, which can be either a permanent magnet 28a as shown in Figs. 6 and 7 or an electromagnet 28b as shown in Figs. 4 and 5. The water tube 26 is made from a non-metallic body which provides a water-tight seal and incorporates electrical pick-up electrodes 29. The tube 26 may be integrally formed with the flow sensor casing 32 (see Fig. 3).

The sensor 25 also includes an electronic circuit 30, preferably a printed circuit, which gives an analogue output signal relating to the flow of water through tube 26. The electric circuit 30 can either be integral with or external to the flow sensor 25, whichever is more convenient. The sensor 25 is electronically linked to the fuel control valve 22 and the diverter valve 15.

In use, a magnetic field is set up by the magnet 28, which field encompasses the tube 26. The water passes through the magnetic field in the tube 26 and as it is an electrically conductive fluid this causes an electrical signal to be induced within the water according to known principals of electronmagnetism. The flow tube 26 may be flattened at 26a to provide improved generation of the magnetic field within the tube 26. The signal is picked up by the electrodes 29 and fed to the electronic circuit 30, which converts the signal to an analogue output signal relating directly to the volumetric flow of water either entering or leaving the water to water heat exchanger 12 depending on the location of the sensor 25.This analogue signal is used to control the heating of the domestic hot water production cycle by directly controlling the modulating fuel valve 22 and/or the diverter valve 15 depending on the level of flow i.e. the demand of secondary water. The sensor 25 functions in a similar manner to a water flow meter and more precisely and immediately matches the load characteristics on the boiler 10 than other known methods. When commanded to operate in domestic hot water production mode, the primary hot water is diverted to flow into the water to water heat exchanger 12 by means of the diverter valve 15, which as mentioned above is controlled by the analogue output signal from the sensor 25.

In another embodiment of the invention, a temperature sensor is also provided within the flow sensor 25 to allow for offset compensation for seasonal temperature changes in the incoming cold water supply. This gives a more perfect control of the heated water temperature with instant response to load changes, not possible with systems which are controlled by temperature only. This also greatly reduces the hazard of scalding and greatly increases the safety in the system. As the flow sensor would normally be incorporated within the casing of the combination boiler 10, it would be influenced in the quiescent state by the warmth of its surroundings.

This would normally force the sensor 25 to start its sequence at a low rather than a high input rate, until colder water moved through the flow device.

The use of a temperature sensor allows a more controlled energy input, thus increasing safety and economy still further.

The temperature sensor output signal can be electronically integrated into the analogue output signal from the flow sensing part of the device 25.

The analogue output is thus modified to compensate for external conditions which may affect the required temperature raise of the water for the particular level of flow of.water reached.

The electrical pick-up electrodes 29 can have a second function as temperature sensors or there can be separate temperature sensors in addition to the electrodes 29.

In another embodiment of the invention, the sensor 25 may be provided with a fixed or adjustable analogue to digital circuit with a digital output, which helps to minimise distortion by outside disturbances. This digital output is a useful addition to the analogue output and can be used to switch the system from the heating cycle to the hot water production cycle only at a predetermined flow rate. Thus the water to water heat exchanger 12 only commences operation when a minimum flow rate has been achieved through the sensor 25, thus preventing parasitic or spasmodic cycles, which may happen with a purely analogue circuit.

Once the domestic hot water production has been initiated by the digital output, the analogue output is used alone to control directly the fuel valve 22 or diverter valve 18. The analogue to digital circuit can have an option for adjustment of the threshold for initiating the hot water production cycle.

In yet another embodiment of the invention using an electromagnet, the coil may be pulsed with electrical signals. This allows the reversal of the electrical supply to invert the output, which enables a faster response of the diverter and fuel valves.

The invention may also be used in a water heater or shower application having only one water circuit. Fig. 8 shows a conventional set up where when the manual control valve 40 is opened the presence of water is detected conventionally, by a pressure switch or water flow switch 41. This causes the heating element 42 in the water container or tank 43 to be powered via a temperature controlling device such as a thermo-stat or electronic proportional temperature control 44. Variations of incoming water temperature from the cold water feed 45 and or water flow rate cause fluctuations in the discharge water temperature at the shower head or tap 46 even in the electronically controlled versions equal to the response time or thermal lag of the temperature sensor. This is uncomfortable, inefficient, and possibly hazardous.

Fig. 9 shows diagrammatically a shower or water heater with a volumetric control system. In practice the flow sensor 50 controls the energy input to the heating element 42 directly according to the flow of water through the system. The flow of water through the shower is measured by the sensor 50 and when it reaches a given flow, the digital output initiates the water heating cycle and the analogue signal thereafter controls the amount of energy put into the heating elements according to the flow of water through the unit. It is also possible to set a desired temperature at any flow rate which is impractical on some existing systems. It is also possible to compensate for variations as mentioned previously in the incoming water temperature by combining a temperature sensing device within the flow sensor. A further improvement can be made by the addition of a computing device 51 to allow all parameters of flow, incoming temperature, desired out going temperature, supply voltage etc. to be taken into account. It is further possible with good effect to use the volumetric method of control to determine the energy requirement for cooling applications, beverage dispensing equipment such as beer pumps could use continuous metering of the liquid to give optimum temperature control from one cooler supplying several beer taps.

Claims

CLAIMS:

1. A method of controlling a water heating system comprising measuring the volumetric flow of water, producing an analogue output signal proportional to said volumetric flow, using the said analogue output signal to control the heating of the water.

2. A method as claimed in claim 1 in which the volumetric flow is measured by inducing a magnetic field in an area through which said water flows and measuring an electrical signal caused by the flow of water within said field.

3. A method as claimed in claim 1 or claim 2 further comprising measuring the temperature of the water and producing an electrical signal proportional to said temperature.

4. A method as claimed in any one of the preceding claims in which the temperature of water is measured before it is heated.

5. A method as claimed in claim 3 or claim 4 in which the temperature signal is electronically integrated into the analogue output flow signal.

6. A method as claimed in any one of the preceding claims in which a digital signal is produced by the analogue output signal to control the initiation of the heating cycle at a predetermined flow rate after which the analogue output signal controls the energy input into the water.

7. A method as claimed in any one of the preceding claims in which the analogue output signal is used to control the direction of flow of water in one or more water circuits.

8. A control unit for a water heating system comprising means to measure the volumetric flow of water, processing means for producing an analogue signal proportional to said volumetric flow, means connecting the output of the control unit to means for heating the water to control energy input into the water.

9. A control unit as claimed in claim 8 in which the means to measure the volumetric flow comprise a magnet adjacent to a pipe through which the water flows, and at least one electrode at least partly located within the pipe to pick up electrical signals within the water.

10. A control unit as claimed in claim 9 in which said pipe is partially flattened.

11. A control unit as claimed in claim 9 or claim 10 further comprising at least one temperature sensor for measuring the temperature of the water and means for producing a signal proportional thereto.

12. A control unit as claimed in claim 11 in which the temperature sensor is mounted within the electrode.

13. A control unit as claimed in claim 11 in which the electrode is also the temperature sensor.

14. A control unit as claimed in any one of claims 11 to 13 in which the temperature sensor measures the temperature of water incoming to the system.

15. A control unit as claimed in any one of claims 11 to 14 in which the processing means integrate the temperature signal with the analogue flow signal to compensate for conditions affecting the required temperature raise of the water.

16. A control unit as claimed in any one of claims 8 to 15 further comprising an analogue to digital circuit for producing a digital signal from the analogue output signal for controlling the initiation of the heating cycle at a predetermined flow rate, after which the analogue output signal controls the energy input into the water.

17. A control unit as claimed in any one of claims 8 to 15, which heating system comprises a primary water circuit for space heating and heating water in a secondary water circuit for domestic consumption, in which the analogue output signal controls the direction and proportion of flow of primary water to space heating means and/or to heat exchanger means for effecting the heating of the secondary water.

18. A control unit as claimed in any one of claims 8 to 17 in which the analogue output signal controls the energy input into the primary water.

19. A control unit as claimed in any one of claims 8 to 18 in which the analogue output signal controls the supply of fuel to the water heating means.

20. A method of controlling a water heating system as hereinbefore described, with reference to and as shown in the accompanying drawings.

21. A control unit for a water heating system as hereinbefore described with reference to and as shown in the accompanying drawings.