GB2043305A - Control arrangement in a circulating fluid heating system - Google Patents

Abstract

A heating system, such as a domestic central heating and hot water supply system, including a gas or oil fired boiler 10 and a pump 14, is provided with thermostatic valves M and 16 which control water flow in response to desired room or water temperature respectively, and a flow sensor 17. An electrical monitoring unit is arranged to switch OFF the pump 14 via a triac 22 Figure 7 for a set period determined by a timer circuit 21 Figure 7, when the boiler has been switched OFF in response to detection of low flow rate. The pump is then switched ON again for a shorter period to determine whether any of the thermostatic valves have opened permitting fluid flow which would be detected by the flow sensor -(since the sensor is inoperative if the pump is off). The sampling procedure is repeated until heat demand is detected whereupon the pump will be run continuously. This switching may be over-ridden if the boiler temperature exceeds a preset maximum, in which case the pump operates continuously. The flow rate sensor Figure 3 comprises a magnetically operated reed switch A, the magnet D moving with a spring biassed piston member C in response to flow and the reed switch being eccentrically mounted on a tube member B transverse to the flow so that rotation of the member B alters the relative positions of the magnet and the reed, and hence the sensitivity of the sensor. <IMAGE>

Description

SPECIFICATION Fluid heating and circulation system This invention relates to a fluid heating and circulation system and particularly, though not exclusively, to a domestic central heating hot water system.

As a result of the ever increasing cost of domestic energy the average householder now demands greater efficiency in a domestic central heating system than in the past, coupled with minimal installation and maintenance costs and ease of control.

Thermostatic control valves both for radiators and hot water systems provide an effective and efficient means of controlling central heating systems.

However such systems suffer from one drawback, which occurs when all valves are closed due to their set temperatures being reached. In such a condition of no-flow or low-flow in the system, the boiler of the system short-cycles as there is no electrical interconnection between the valves and the boiler, to switch the boiler off.

To overcome this problem, it has been proposed to include in the system a flow sensor to detect the overall flow condition of a thermostatic valve circuit, and to switch off the boiler when the flow drops below a certain low-flow value i.e. when the heat demand of the system is satisfied. The flow sensor operates to switch the boiler on again as the thermostatic valves open.

Although such a flow sensor contributes considerably to the efficiency of thermostatic radiator valve controlled systems by eliminating short-cycling of the boiler, it still requires the pump to run continuously during a no heat demand period, in order to maintain potential. Such continuous running of the pump is, of course, an undesirable unproductive use of energy.

In combined direct hot water service heating systems where one or both services are pumped and under the control of thermostatic valves, a problem may arise in that the thermostatic valves, in particu lathe thermostatic valve controlling the cylinder, may modulate open although the heat demand is satisfied. The constant running pump will either encourage the boiler to cycle under the control of the boiler thermostat as excess heat is pumped through the pipework of the system, and the modulating valve or valves, or the pump, may dissipate heat away from the cylinder or radiators through the cooling boiler circuit. In either case the boiler will operate more frequently than the thermostatic valves require, with consequential over heating from the system pipework and the cylinder and all radiators where the thermostatic valves are modulating.

Accordingly it is an object of the invention to provide a fluid heating and circulation system in which these problems are overcome, or at least reduced.

According to the invention there is provided a fluid heating and circulation system comprising a heat generating unit including a burner, a pump for circulating fluid heated by said heat generating unit through the system, a heat exchanger for dissipating heat from the system, a flow sensor operatively associated with the burner so that, in use, the burner will automatically be switched off or on in accordance with the rate of flow of fluid detected by said flow sensor, and a monitoring unit arranged to switch off said pump for part of the time during which said burner is switched off by said flow sensor and to run the pump continuously once the burner is switched on again.

Preferably the fluid heating and circulation system is a central heating hot water system comprising a boiler having a burner unit operatively associated therewith, a central heating circuit including at least one radiator fitted with a thermostatic valve, a hot water supply storage cylinder, which is also fitted with a thermostatic valve, a pump unit for circulating water through the system, a flow sensor operatively associated with the burner so that, in use, the burner will automatically be switched off or on in accordance with the rate of flow of water detected by said flow sensor, and the system also includes a monitoring unit arranged to switch off the pump when the flow sensor has switched off the burner, the pump thereafter remaining off for a predetermined period of time and then automatically being switched on for a relatively shorter period of time to determine whether or not there is a heat demand created by one or more of the valves opening, thus establishing a flow when the pump runs which is in turn detected by the flow sensor, this procedure being repeated until it is determined that there is a heat demand, whereupon the pump will continue to run until the flow sensor detects no heat demand due to the flow rate falling below a pre-set level of the flow sensor.

In a further construction, the monitoring unit has sole control of the pump, so that when a thermostat associated with the boiler isolates the boiler at maximum temperature, the pump is arranged to run continuously, irrespective of whether or not the flow sensor has switched off the boiler, until such time as the boiler temperature drops to that at which switching of the thermostat occurs. This feature is especially relevant to low water content boilers.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 illustrates in a schematic block form, a domestic central heating system to which the present invention is to be applied.

Figure 2 is a diagrammatic electrical circuit for the system of Figure 1.

Figure 3 is a preferred arrangement of a flow sensor.

Figure 4 shows the detail of the flow sensor adjustment featured at its minimum flow setting.

Figure 5 shows the detail of the flow sensor adjustment featured at its maximum flow setting.

Figure 6 is a diagrammatic electrical circuit for the system using a monitoring unit (shown in block form).

Figure 7shows the complete electronic circuit of the monitoring unit of the systems of the present invention.

Referring to the accompanying drawings, the central heating system illustrated is of the type using an oil or gas fired burner and a water boiler.

Naturally any other conventional type of burner could be used.

The system shown in Figure 1 comprises a heat generating unit in the form of a conventional boiler/oil or gas fired burner unit 10 with a flow pipe circuit 11 including heat dissipating devices in the form of radiators 12, and a water storage cylinder 13.

The circuit further includes a conventional pump 14 on the downstream side of the boiler to propel the water through the system. A suitable fluid other than water could be used.

With the circuit of Figure 1 the remote sensor unit 15 will operate to close the valve 16 when the water in the cylinder is sufficiently hot. Likewise when the temperatures in the respective rooms containing the radiators are sufficiently high, the radiator thermostatic valves will close. With all such valves closed the water flow drops below the predetermined level, and the flow sensor 17 opens to switch off the burner. When a thermostatic valve is opened, the rate of flow of water in the circuit attains a predetermined level again and the flow sensor 17 closes to switch on the burner. Thus the flow sensor ensures that the burner is only switched on when there is sufficient demand for heated water in the pipe circuit 11.

Figure 2 shows the electric circuit for the system of Figure 1.

Whilst the use of a flow sensor eliminates shortcycling of the boiler, the pump is still running continuously during no heat demand period, thereby wasting energy. Moreover, with the system of Figure 1, if the thermostatic valve of the cylinder is prone to modulation or leakage, then the constant running of the pump could dissipate the heat away via the relatively cold boiler circuit. The use of a monitoring unit N as hereinafter described is added to the system to overcome these problems.

A block diagram of the monitoring unit circuit system is shown in Figure 6. This circuit is similar to that shown in Figure 2 except that the pump operation is controlled by the monitoring unit N which monitors the system state with regard to heat demand (flow sensor operation) and burner overheating (burner thermostat operation).

To carry out this control, a priming system is proposed which will come into operation when there is no heat demand, i.e. when the flow sensor has switched off the burnerboiler. With no heat demand, the pump will be switched off for a period of typically a few minutes and then switched on again for a few seconds to sample if there is any heat demand. This pump on/off priming sequence will continue until such time as a heat demand is detected by the flow sensor whereupon the priming sequence will cease and the pump will then startto run continuously.

The period of time for which the pump is switched off (priming quiescent) is an arbitary choice determined by efficiency verses comfort response of the heating system, whilst the period oftimeforwhich the pump is switched on (priming sampling) to sample any heat demand, is determined by the time taken for a system to assume its natural flow condition.

Should the flow sensor close during the quiescent priming mode, the pump would not switch on until the period is completed. It must be noted, however, that the above condition cannot occur during normal operations, as the pump is required to be operating to establish a flow and therefore as the pump is switched off during the quiescent priming phase, the flow sensor cannot operate.

As the monitoring unit has sole control of the pump, a further feature may be provided so that when the boiler thermostat K has isolated the boiler at maximum temperature this condition is recognised and the pump will run continuously, irrespective of the flow sensor state, until the boiler temperature drops, i.e. the boiler thermostat Closes. This pumping out feature will only come into operation if the boiler circuitry controlled by the flow sensor, is completely isolated when the boiler thermostat K operates, and it is especially relevant to low water content boilers.

The circuit for the monitoring unit is shown in Figure 7 and is built around a linear integrated timer circuit 21. This is used in conjunction with triac 22 which is used to switch the pump 14 in and out of the circuit.

The circuit operates directly from the mains A.C.

voltage without using any costly transformers or heat producing dropper resistors.

A switch L is provided for over-riding the monitoring unit when servicing or commissioning of the system is required.

Low voltage required to power the timer circuit is derived via a series capacitor 23 which, whilst acting as a dropper, does not produce any appreciable heat as its impedance is reactive. Capacitor 23 is connected to the diode pair 24 & 25 which feeds the positive cycles to the power supply and sinks the negative cycles. The half wave D.C. thus produced is smoothed by capacitor 26. The negative rail of this supply is at mains A.C. line potential.

The circuit is dormant during the heat demand periods (flow sensor closed), reverting to a timing sequence when there is no heat demand (flow sensor open). There are therefore 3 circuit states as follows: (a) Dormant (b) Timing - charging (short sampling period) (c) Timing - discharging (longer time interval between samples) When dormant or charging the timer provides a positive output at pin C of the integrated circuit 21, which is used to fire the triac 22 via resistor N.

The charging circuit is formed by a diode 27, resistor 28, diode 29 and capacitor 30 and comes into operation when the flow switch is open circuit. The diode 27, the resistor 28 and the capacitor 30 are connected in series, with the diode 29 connected in parallel with a resistor 31 also connected in series with the diode 27 resistor 28 and capacitor 30. A charging current is then derived via the boiler circuit to the A.C. neutral rail which charges up capacitor 30 with positive going cycles until it reaches the threshold voltage level. At this point the circuit then reverts to its discharging state and discharges capacitor 30 via resistor 31. Whilst discharging, the potential at pin C drops down to the negative rail and inhibits triac firing.

During a 'heat demand' period the flow switch is closed and the charging circuit is connected to the negative rail. No charging current is therefore available and the timer is dormant with the triac fired continuously.

During a 'no heat demand' period (flow sensor open) when the boiler is above maximum temperature, the boiler circuit is itself open circuit and there is no path for the charging circuit. The timer will, therefore, remain dormant until such time as the boiler is 'pumped out' and the temperature reduces to a point where the boiler circuit is closed.

Triac 22 is operated in quadrants I and IV and therefore an appropriate sensitive gate device (logic triac) is required for satisfactory operation.

As there is no useful load during the discharge cycle (triac off), resistor 32 is provided to give a dummy load to regulate the supply voltage which could be arranged to be constant between charge and discharge cycles. However, resistor 32 has been designed to give a higher load and thus pull down the supply voltage. This has the effect of increasing the charge and discharge time thus keeps down the values of capacitor 30 and resistors 28 and 31 to reasonable levels.

A capacitor 33 is included to protect the integrated circuitfrom damaging high voltage spikes.

Resistor 39 with capacitor 40 is a snubber network for protection of the triac 22.

The monitoring unit included in the system of the invention thus ensures that no wasteful running of the pump occurs, for example when there is no heat demand the boiler is below maximum temperature and also that with no heat demand no leaching of heat from the hot water cylinder occurs.

It is convenient if the flow sensor is capable of adjustment in order to suit the characteristics of a particular thermostatic valve and installation, as well as facilitate the initial commissioning of the device.

Such adjustment also allows a desired comfort economy requirement to be met.

Figure 3 shows a preferred form of adjustable flow sensor in which the reed switch A is mounted eccentrically in a cross tube B disposed transverse to the pipe circuit 11. Disposed axially in a portion of the pipe circuit adjacent to the cross tube B, is a piston C incorporating a magnet D. When there is little or no flow through the pipe circuit, the piston is biased by a spring E away from the cross tube B so that the magnet D has no effect on the reed switch A.

The reed switch blades are thus out of contact with each other and the burner is switched off.

The Piston C has a disc shaped portion which operates in tapered profile Section F of the pipe circuit 11. The shape of the profile F together with the spring tension determining the flow characteristic of the flow sensor. The piston C is supported axially in pipe circuit 11 by the spring E and a circlip G.

If the rate of the flow increases, due for example to the opening of a thermostatic valve of the system, the piston is moved against the spring E so that at a predetermined higher flow rate, the end of the magnet D adjacent to the cross tube is sufficiently close to cause the blades of the reed switch A tocome into contact there by closing the switch and switching off the burner.

Adjustment of the switching point is achieved by rotating knob H which in turn rotates the cross tube shaft J and the eccentrically mounted reed switch A.

This eccentric mounting of reed switch A means that manual rotation of the knob H moves the reed switch A relative to the free end of the magnet D carried by the piston C thereby giving adjustment of the switching point. Figure 4 shows the eccentric positioning of reed switch A to give the minimum flow rate switching point whilst Figure 5 shows the eccentric positioning of the reed switch A two give the maximum flow rate switching point. Intermediate switching points can be set by suitable reed switch positions between the minimum and maximum settings.

Claims (7)

1. Afluid heating and circulation system comprising a heat generating unit including a burner, a pump for circulating fluid heated by said heat generating unit through the system, a heat exchanger for dissipating heat from the system, a flow sensor operatively associated with the burner so that, in use, the burner will automatically be switched off or on in accordance with the rate of flow of fluid detected by said flow sensor, and a monitoring unit arranged to switch off said pump for part of the time during which said burner is switched off by said flow sensor and to run the pump continuously once the burner is switched on again.

2. Asystem comprising a boiler having a burner unit operatively associated therewith a hot water storage cylinder fitted with a temperature responsive control valve and/or a central heating circuit including at least one room heat emitter controlled by a temperature responsive valve, a pump unit for circulating water through the system, a flow sensor operatively associated with the burner so that, in use, the burner will automatically be switched off or on in accordance with the rate of flow of water detected by said flow sensor, the system also including a monitoring unit arranged to switch off the pump when the flow sensor has switched off the burner, the pump thereafter remaining off for a predetermined period of time and then automatically being switched on for a relatively shorter period of time to determine whether or not there is a heat demand created by one or more of the valves opening, thus establishing a flow when the pump runs which is in turn detected by the flow sensor, this procedure being repeated until it is determined that there is a heat demand, whereupon the pump will continue to run until the flow sensor detects no heat demand due to the flow rate falling below a pre-set level of the flow sensor.

3. A heating system according to Claim 2 wherein the boiler has a thermostat associated therewith and so connected to the monitoring unit that the pump operates continuously whilst the boiler tem perature is above a preset maximum.

4. A system according to any preceding claim wherein the rate of flow at which the flow sensor switches on and off is adjustable..

5. A system according to Claim 4 wherein the flow sensor includes a magnetically operated reed switch eccentrically mounted on a rotatable member disposed transverse to the flow through the sensor.

6. A heating system substantially as hereinbefore described with reference to figures 3, 4, 5 and 6, or figure 6 or figure

7.