GB2423141A

GB2423141A - Diagnostic circuit in a condensing boiler

Info

Publication number: GB2423141A
Application number: GB0602953A
Authority: GB
Inventors: Gordon Smith; David Anthony Clark; Wayne Kenneth Aldridge; Graham Richard Roberts; Huw John Gaskill; Micheal Gerard Mcardle; James Robert Lowrie
Original assignee: Microgen Energy Ltd
Current assignee: Microgen Energy Ltd
Priority date: 2005-02-14
Filing date: 2006-02-14
Publication date: 2006-08-16
Anticipated expiration: 2026-02-14
Also published as: GB2423141B; GB0503039D0; GB0602953D0

Abstract

A diagnostic system 20 includes an ac power supply 22, a flame probe 24, an overflow detector 26 in a condensate trap 28, first 30 and second 32 testers and a controller 34. A flame electrode 36 is positioned in a flame 38 with a first earth connection separated such that the flame, when present, provides an electrical path to earth. An overflow electrode 42 projects into internal volume 40 of the condensate trap. Condensate provides an electrical path to earth when in contact with the overflow electrode. The flame and overflow electrodes are provided with an ac signal source. An output may receive a signal from the ac signal source modified by the flame detector and overflow detector. Diagnostic means use the output signal to determine whether there is an overflow in the condensate trap and, if there is no overflow in the condensate trap, whether there is flame or not. The diagnostic circuit may be used in a method of monitoring a condensing boiler with a burner.

Description

DIAGNOSTIC CIRCUIT IN A CONDENSING BOILER

The present invention relates to a diagnostic system for a condensing boiler having a flame probe for one or more installed burners and a condensate trap with an overflow detector. The diagnostic system provides information relating to the flame and any overflow in the condensate trap. The present invention has particular application in a domestic combined heat and power unit that has two burners.

Condensing boilers, i.e. boilers with heat exchangers that actively condense water vapour in exhaust gases, are becoming increasingly popular due to their improved efficiency. A condensate trap is provided to drain liquid away from the heat exchanger. This is especially important because the condensate is often mildly acidic and so will prove corrosive if left to stand within the heat exchanger.

A trap containing a siphon is often used as this allows condensate to drain from the heat exchanger without allowing the passage of exhaust gases. It also ensures that the condensate exits the trap in regular volumes, as opposed to constant drips which may be more susceptible to freezing at the exit.

However, condensate traps can become blocked by, for example, freezing of the condensate and/or aggregation of particles carried by the condensate. Such a blockage leads to the level of the condensate rising in the condensate trap and, if no action is taken, the corrosive condensate eventually overflows and comes into contact with nearby components of the heat exchanger. Where other boiler components are located close to, or below, the heat exchanger these could be at risk of coming into contact with the cool, corrosive condensate in the event of an overflow.

Where such a component were the heater head of a Stirling engine, for example in a combined heat and power design of boiler, this could result in damage.

To prevent overflow, condensate traps may be provided with an overflow detector that provides a signal when the condensate reaches a predetermined level, chosen to be indicative of a potential blockage and either imminent overflow or an actual overflow. The overflow detector provides this signal to a controller that then responds by causing the burner to shut down, i.e. the flames are extinguished by interrupting the fuel supply. This prevents further exhaust gases being produced that will in turn lead to more condensate entering the trap.

Burners are also often provided with a flame probe to sense whether or not a flame is present in the burner. This acts as a safeguard in that it allows the fuel supply to be cut off if the absence of a flame is detected. British patent application, published as GB-A-2,146,157, describes a system for monitoring both the flame and the condensate trap that is shown in Figure 1 herein.

An ac power supply provides an ac signal via a capacitor, thereby acting as a high-impedance source. The ac signal is connected to the flame probe, overflow detector and a filter. The filter acts to filter the ac components out of the signal and passes any remaining dc components to a controller.

The flame probe comprises an electrode to which the ac signal is supplied. The electrode is positioned to project into the flame, when a flame is present. An earth is provided by the metal casing of the burner head. It is a well known property of flames that they provide an electrical conduction path to earth and will partially rectify an ac signal.

In addition, the flames also produce a direct current, adding a dc offset to the partially-rectified ac signal.

Thus, if a flame is present, the ac signal provided by the ac source is partially rectified and has a dc offset added; if no flame is present, the flame probe forms an open circuit and there is no modification of the ac signal.

The condensate trap may comprise a metal housing that provides an earth or a dedicated earth connection such as an earth electrode where, for example, the housing is insulating. The overflow detector comprises the live electrode shown in Figure 1 that is supplied with the ac signal and that projects into the trap at a height above the level of any condensate during normal operation. In this situation, the overflow detector forms an open circuit and does not affect the ac signal. However, a blockage in the outlet from the trap will cause the condensate level to rise within the trap. When the condensate contacts the live electrode, a conducting path to earth is formed, providing a short circuit for the ac signal (as rectified or not by the flame probe, and including any dc offset).

As mentioned previously, the filter removes ac components from the signal it receives to leave only any dc offset added by the flame probe and passes this signal to the controller. The controller tests only for the presence of the dc offset: if it is detected, the controller knows the flame is present and allows operation of the burner to continue; if it is not detected, the controller knows that there is a problem in that either the flame is extinguished or there is a blockage in the condensate trap (or both), and so interrupts the fuel supply to the burner.

The system of GB-A-2,146,157 suffers a disadvantage in that it cannot distinguish between an overflow in the condensate trap or a lack of flame in the burner. This means that no guidance is provided to a service engineer when attending the defective boiler.

Against this background, and from a first aspect, the present invention resides in a diagnostic circuit for a condensing boiler comprising: at least one burner arranged to burn a flame; a condenser arranged to condense gas produced by the at least one burner; a condensate trap arranged to collect condensate produced by the condenser; a flame probe, suitable for use in the at least one burner, comprising a flame electrode and a first earth connection separated such that the flame, when present, provides an electrical path to earth; and a condensate overflow detector, suitable for use in the condensate trap, comprising a second earth connection and an overflow electrode positioned in the condensate trap to indicate an overflow such that condensate provides an electrical path to earth when in contact with the overflow electrode.

The diagnostic circuit comprises: an ac signal source adapted to provide an ac signal to the flame and overflow electrodes; an output operable to receive an output signal from the ac signal source as may be modified by the flame detector and overflow detector; and diagnostic means operable to use the output signal to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not.

This allows more precise identification of faults within the condensing boiler, thus allowing better remedial action to be taken.

The diagnostic means may comprise any two of a positive cycle detector, a negative cycle detector and a dc offset detector. The use of a only a single detector does not provide enough information to allow a distinction between an overflow and a flame-out condition to be made. However, using two detectors allows an unambiguous resolution of overflow and flame-out conditions, Of course, three types of detectors may be used if desired.

Preferred, but optional, features of the dc offset detector, the positive cycle detector and the negative cycle detector are defined in the appended claims. A particularly preferred feature is to provide a positive cycle detector that is operable to compare peaks in the positive cycle to two thresholds. This allows a distinction between three flame conditions to be made, namely (i) no flame or an unacceptably weak flame, (ii) a weak but acceptable flame and (iii) a normal flame.

From a second aspect, the present invention resides in a condensing boiler comprising: a burner arranged to burn with a flame; a flame probe provided in the burner, comprising a flame electrode positioned to be in the flame, when present, and a first earth connection such that the flame provides an electrical path to earth; a condenser arranged to condense gas produced by the burner; a condensate trap arranged to collect condensate from the condenser; a condensate overflow detector comprising an electrode positioned in the condensate trap at a height for indicating an overflow and a second earth connection such that condensate provides an electrical path to earth when present; and any of the diagnostic circuits described above.

Optionally, the condensing boiler further comprises one or more further burners each arranged to burn with a flame; and further flame probes, one provided in each of the further burners, each flame probe comprising a flame electrode positioned to be in the flame, when present, and an earth connection such that the flame provides an electrical path to earth; and wherein: all flame electrodes are provided an ac signal; an output is provided for each burner, wherein each output is adapted to receive a respective output signal comprising an ac signal as may be modified by the respective flame detector and the overflow detector; and the diagnostic means are operable to use the output signal to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not in each of the burners.

The above is intended to describe boilers having both a plurality of burners of which all burners have flame probes, and also boilers having a plurality of burners of which a subset of burners have flame probes.

The above arrangement is beneficial in boilers that comprise two or more burners, such as domestic combined heat and power units that comprise both a main burner and a supplementary burner.

Optionally, a separate diagnostic circuit is provided for each burner. Each diagnostic circuit may have its own ac signal source and its own overflow electrode, each ac signal source being operable to supply an ac signal to its flame and overflow electrodes. Each burner may have its own condensate trap, with each overflow electrode being positioned in that condensate trap, or alternatively a shared condensate trap may be used. In the latter arrangement, the overflow electrodes are all associated with the shared condensate trap. The electrodes may all be positioned at the same height where all burners should be controlled with respect to the same overflow condition.

Alternatively, some overflow electrodes may be located at different heights where remedial action should be taken earlier or later as an overflow condition is approached.

Where the boiler comprises more than a single burner, separate diagnostic means may be provided for each burner that are operable to use the output signal provided on its respective output to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not in its burner. Alternatively, a shared diagnostic means may be provided that is operable to receive the outputs as inputs and to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not in each of the burners.

Preferably, the condensing boiler further comprises a controller operable to stop a fuel supply to the burner in response to the diagnostic means determining the presence of an overflow or an absence of the one or more flames. Where an overflow is detected, this conveniently stops the generation of further condensate. Where a flame-out condition is detected, this prevents the supply of further combustible fuel that may otherwise present a risk of explosion.

From a third aspect, the present invention resides in a domestic combined heat and power unit comprising any of the condensing boilers described above.

From a fourth aspect, the present invention resides in a method of monitoring a condensing boiler comprising: a burner arranged to burn with a flame; a flame probe provided in the burner, comprising a flame electrode positioned to be in the flame, when present, and a first earth connection such that the flame provides an electrical path to earth; a condenser arranged to condense gas produced by the burner; a condensate trap arranged to collect condensate from the condenser; and a condensate overflow detector comprising an electrode positioned in the condensate trap at a height for indicating an overflow and a second earth connection such that condensate provides an electrical path to earth when present. The method comprises: supplying an ac signal to the flame and overflow electrodes; and using an output signal, derived from the ac signal source and as may be modified by the flame detector and overflow detector, to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not.

In order that the invention can be more readily understood, reference will now be made by way of example only, to the accompanying drawings, in which: Figure 1 is a schematic representation of diagnostic system for a condensing boiler according to the prior art; Figure 2 shows a diagnostic circuit according to an embodiment of the present invention for a boiler with a single burner; Figures 3a to 3c show signal outputs of a diagnostic system corresponding to three operational conditions of a condensing boiler; Figure 4 shows a modification of Figure 3a; Figure 5 shows a dc offset detector; Figure 6 shows an ac positive cycle detector; Figure 7 shows an ac negative cycle detector; Figure 8 shows a modified ac positive cycle detector; Figure 9 shows three waveforms that may be detected by the ac positive cycle detector of Figure 7; Figure 10 is a schematic representation of a domestic combined heat and power unit in which the present invention may be worked; Figure 11 shows a diagnostic system according to an embodiment of the present invention intended for use with a dual burner system such as the domestic combined heat and power unit of Figure 10; and Figure 12 shows a condensate trap also intended for use with the dual burner system such as the domestic combined heat and power unit of Figure 10.

A first embodiment of the present invention is shown in Figure 2 that provides a diagnostic system 20 for a condensing boiler having a simple arrangement of a single burner, and that is provided with an ac power supply 22, a flame probe 24, an overflow detector 26 in a condensate trap 28, first 30 and second 32 testers and a controller 34.

The flame probe 24 corresponds to that already described with respect to GB-A-2,146,l57. An electrode 36 is positioned to project into a flame 38 when present. The flame 38 provides partial rectification of the ac signal and adds a dc offset.

Although details of the condensate trap 28 may vary, all traps share a common feature of having an internal volume 40 that is used to contain condensate. In the event of a blockage, condensate will rise in this volume 40 until eventually it overflows. An overflow detector 26 is provided in this particular condensate trap that comprises an electrode 42 projecting into such a volume 40 at a height appropriate for indicating an imminent overflow. Rising condensate will eventually complete the circuit between - 10 - flame probe electrode 42 and an earth electrode 43, to provide an electrical path to earth. Earth may be alternatively provided by the trap 28 itself if conducting.

An ac power supply 22 acts as a high impedance source providing an ac signal through a capacitor 44. This ac power supply 22 is connected to the flame probe 24, the overflow detector 26 and the first 30 and second 32 testers.

The first 30 and second 32 testers receive an electrical signal that may be modified depending upon the operating conditions at the flame probe 24 and the overflow detector 26.

Figure 3a shows the ac signal 50 supplied by the ac power supply 22. An unmodified signal 50 (having the same waveform) will be received by the first 30 and second 32 testers when (a) the flame probe 24 does not detect a flame 38 and (b) no overflow is present. Thus, detecting such an unmodified signal 50 indicates a fault condition of a lack of flame 38 in the burner.

Figure 3b shows the signal 52 received by the first 30 and second 32 testers when (a) a flame 38 is present and (b) there is no overflow. Thus, Figure 3b corresponds to normal operation of the associated burner. This signal 52 is caused by the partial-rectification effect displayed by flames. As can be seen, the flame 38 offsets the signal to reduce the positive cycle from the ac signal 50 and to enlarge the negative cycle to be received by the first 30 and second 32 testers.

Figure 3c shows the signal 54 received by the first 30 and second 32 testers when an overflow is present at the overflow detector 26 (irrespective of whether or not a flame 38 is present) . The overflow detector 26 provides a lower - 11 - resistance short circuit such that a zero signal is received by the first 30 and second 32 testers.

The signal 50 shown in Figure 3a would be an ideal case for the signal 50 supplied by the ac power supply 22.

Figure 4 is a more realistic reproduction of the ac signal 56 provided by the ac power supply 22, showing an offset that biases the signal 56 to the negative cycle. Thus, the waveform of Figure 3b would change to reflect this.

The first 30 and second 32 testers test the signal they receive and provide the results to a controller 34 that performs logic operations to determine the operating condition of the condensing heat exchanger and associated burners. The first 30 and second 32 testers are chosen from three possibilities that will now be described with reference to Figures 5, 6 and 7.

Figure 5 shows a dc offset detector 60 akin to that described in GB-A-2, 146,157. Essentially, the dc offset detector 60 has a filter circuit comprising two resistors 62, 64 and a capacitor 66 to remove ac components. The remaining dc signal is amplified by an amplifier 68 to produce an output signal 70 that will indicate the presence or not of a dc offset.

Figure 6 shows an ac positive cycle detector 80. A buffer 82 ensures that the detector 80 has no effect on the signal that is being measured. A diode 84 then half-wave rectifies the signal and, in combination with a capacitor 86, acts as a peak detector. The resulting positive cycle signal is passed to a comparator 90 via an amplifier 88.

The amplifier 88 amplifies the received signal to levels suitable for the subsequent comparator 90, e.g. to a range of 0 to by. A resistive divider network 92 provides a reference voltage VREF to the comparator 90. Therefore, the - 12 - comparator 90 operates to indicate whether the received signal contains a positive cycle component in excess of the reference voltage VREF as an output signal 94.

Figure 7 shows a negative cycle detector 100. As will be apparent from the Figures, the negative cycle detector has the same arrangement as the positive cycle detector 80 of Figure 6, save for the reversal of the diode 84. Thus, the components are provided with like reference numerals, except incremented by 100. The values of the resistors in the divider network 192 may be different in the negative cycle detector 100 to provide different reference voltages for the positive 80 and negative 100 cycle detectors. Thus, the negative cycle detector 100 provides an output signal 194 that is indicative of a peak in the negative cycle of the received signal that exceeds the reference voltage VREF.

As mentioned previously, the outputs 70, 94, 194 of the dc offset detector 60, the positive cycle detector 80 and the negative cycle detector 100 when used as the first 30 and second 32 testers are passed to the controller 34 of Figure 2. The controller 34 uses these output signals 70, 94, 194 to determine the operating condition of the condensing heat exchanger and associated burner and to take corrective action if necessary. As discussed above, Figures 3a to 3c show the ac waveforms received by the first 30 and second 32 testers under the following conditions: (a) no flame, no overflow; (b) flame present, no overflow; and (c) overflow present. In addition, a dc offset will also be present only in the case of Figure 3b where a flame 38 is present. This can be summarised as follows: - 13 -

TABLE 1

conditions ac signal? dc signal? flame, no overflow negative yes (normal) cycle only no flame, no overflow both cycles no (fault) overflow no no (fault) The outputs of the dc offset detector 60, the positive cycle detector 80 and the negative cycle detector 100 may be added to Table 1, as follows:

TABLE 2

ac dc dc 0S neg conditions signal signal offset cycle cycle det det det flame, no overflow neg yes high low high (normal) no flame, no overflow both none low high high (fault) overflow (fault) none none low low low The three columns on the right of Table 2 form a truth table. As will be evident from inspecting Table 2, the three conditions cannot be distinguished using any one detector alone, but a combination of any two detectors allows the three conditions to be separately identified (i.e. any pair of these three columns contains a unique - 14 - output combination for each operational condition). The controller 34 is programmed to allow the operating condition to be diagnosed from the output signals provided by the first 30 and second 32 testers in accordance with the truth table of Table 2. At a first glance, it may appear that the controller cannot distinguish between whether or not there is a flame if there is an overflow. However, for condensate to be produced, a flame must have been present. Thus, if the overflow condition is triggered, then it may be assumed that a flame was present before the overflow trigger occurred.

If a normal operating condition is diagnosed, the controller 34 does nothing more than continue to monitor the output signals from the first 30 and second 32 testers.

However, if either fault condition is diagnosed, the controller 34 interrupts the fuel supply to the burner and records a fault code. The fault code may be used by a service engineer to identify the condition that caused shut down of the burner.

Although a choice of any two from the dc offset detector 60, the positive cycle detector 80 and the negative cycle detector 100 may be made, inclusion of a modified positive cycle detector 110 like the one shown in Figure 8 has been found advantageous.

The positive cycle detector 110 of Figure 8 corresponds broadly to the positive cycle detector 80 of Figure 6 but includes a second comparator 90'. Thus, like reference numerals are used for like parts. The two comparators 90 and 90' are supplied with inputs taken from different points in the resistive divider network 92 and so make comparisons with two thresholds, VREF and yak, to provide two outputs 94 and 94'.

- 15 - Figure 9 illustrates how the two voltage thresholds VREF and VOK may be applied. The waveform obtained by the peak detector 84 and 86 (i.e. after half-wave rectification) is shown in Figure 9 for three operating conditions in the S burner. If there is no flame 38, then the strong peaks shown at (a) result. If there is a strong flame 38, then half-wave rectification removes the positive ac cycles such that the zero signal shown in (c) results. However, a weak flame 38 will lead to incomplete half-wave rectification such that attenuated peaks in the positive cycle remain, as shown in (b).

The threshold voltages VREF and VOK can be used to set three operating bands. The normal band corresponds to (c) where there is no signal: in fact, a very small signal or noise may remain and so VOK may be set slightly above zero.

Such a situation will result in both comparators 90, 90' giving low output signals 94, 94', and the controller 34 will interpret these as indicating that the condensing heat exchanger and associated burner is functioning normally.

The controller 34 takes no remedial action in this instance.

If matters are very bad and there is no flame or only a very weak flame that requires the burner to be shut down, then the strong peaks of (a) will be present. Thus, VREF is set to create a shutdown band where the strong peaks exceed VREF. Consequently, the output signals 94, 94' from both comparators 90, 90' will be high and the controller 34 can use this to initiate interruption of the fuel supply to the burner 34.

Setting VOK and VREF as above also defines an acceptable band therebetween that corresponds to where a weak but acceptable flame 38 is present, as shown at (b) in Figure 9.

The height of this acceptable band can be varied by - 16 - adjusting VREF and/or VOK. For operation in the acceptable band, the output signal 94' of the comparator 90' referencing V0 will be high and the output signal 94 of the comparator 90 referencing VREF will be low. The controller 34 can detect these values and initiate remedial action in response. Causes of a weak flame 38 may be a temporary blockage in the fuel supply lines, temporary dampness in the burner (e.g. at start-up) or carbon build up. These situations can be remedied by increasing the flow of fuel and air to the burner, and, if required, subsequent scheduled corrective maintenance.

The present invention finds useful application in a domestic combined heat and power unit that may provide a domestic residence or the like with heat, hot water and, to a certain extent, electricity. Such a combined heat and power unit may comprise a Stirling engine that operates to generate electricity using a domestic heating arrangement.

Figure 10 shows such a dchp unit 120 based around a Stirling engine 122. The engine 122 is preferably a linear free-piston Stirling engine, the operation of which is well known in the art.

The Stirling engine 122 is driven by a heat input from an engine burner 124. The engine burner 124 is provided with a flame probe (not shown in Figure 10) This engine burner 124 is fuelled by a combustible gas supply 126 that is mixed with an air supply 128 under the control of a valve 130. The mixed stream is fed to the burner 124 by a fan 132. This drives the Stirling engine 122 to generate an electrical output 134 from a linear alternator. The alternator is not shown in Figure 10, but may be located within the pressure vessel enclosing the engine 122 or it may be located externally and coupled to the engine 122 via - 17 - a drive shaft. Heat is extracted from the Stirling engine 122 at a cooler 136 that is essentially a heat exchanger through which water is pumped by a pump 138 along line 140.

The water passing through the cooler 136 is then further heated in a heat exchanger 142 by exhaust gas from the engine burner 124 that has heated the head of the Stirling engine 122. Of course, this heat exchanger 142 condenses water from the exhaust gas and this condensate is drained away via condensate trap 144 that is provided with an overflow detector (not shown in Figure 10).

In order to provide further heating of the water, and also to provide a degree of independence to heat the water when the Stirling engine 122 is not being operated, a supplementary burner 146 is provided to heat the water in the heat exchanger 136. The supplementary burner 146 is provided with a flame probe (not shown in Figure 10) The supplementary burner 146 is fuelled by the combustible gas supply 126 which is mixed with an air supply 148 under the control of a valve 150. The mixed stream is fed to the supplementary burner 146 by the fan 132.

The fan 132 feeds air to mixer valves 130 and 150 through a diverter valve that ensures the correct air flow to each mixer.

In an alternative design, separate fans have been used to feed air to the two gas/air mixer valves 130 and 150.

This removes the need for a diverter valve but, as described in our copending Application GB0130380.9, it does carry significant weight, cost and efficiency penalties over the single fan design.

Exhaust gases from the supplementary burner 146 give up their heat in theheat exchanger 136 along with the exhaust gases from the engine burner 124. Thus, condensate is - 18 - collected in the condensate trap 144 that comes from the exhaust gases of both engine burner 124 and supplementary burner 146. The remaining exhaust gases exit along flue 152.

In this manner, the Stirling engine 122 produces an electrical output 134 and a heat output 154 which may be used, for example, to provide the domestic hot water requirement, to feed a central heating system, or both of these in a combination arrangement ("combi" boiler) The dchp unit 120 is designed to provide up to 1kw of electricity (net) feeding directly into the domestic network and, hence, combining with the supply from the grid.

Moreover, electricity from the dchp unit 120 may be sold back into the grid.

Figure 11 shows a further embodiment of a diagnostic system 160 according to the present invention suitable for use in a condensing heat exchanger fed by two burners, such as the dchp unit 120 of Figure 10 that has both an engine burner 124 and a supplementary burner 146.

Each burner 124, 146 is also provided with its own diagnostic circuit 162, 164 connected to the respective flame probes 24, 24'. Each diagnostic circuit 162, 164 is provided with an overflow electrode 42, 42' of an overflow detector 26, 26'. Thus, apart from a shared earth electrode 43 that is provided in the condensate trap 28, the two diagnostic circuits 162, 164 are separate and each circuit 162, 164 separately corresponds to the diagnostic system 20 of Figure 2. If the two overflow electrodes 42, 42' are set to the same height in the condensate trap 28, both will trip at the same time to ensure both burners 124, 126 are extinguished together. Alternatively, both circuits 162, 164 could be connected to a common electrode 42. If, for - 19 - any reason, it is desired to extinguish one burner before the other if an overflow appears imminent, the electrodes 42, 42' could be arranged at different heights.

A condensate trap 28 for use with the present invention is shown in Figure 12 in perspective, and with the front and side faces cut-away to allow a view of the interior.

Condensate enters via an inlet pipe 200 to fill a first chamber 202. A cleaning port 204 is provided in the bottom of the first chamber 202.

The housing 206 of the condensate trap 28 forms the first chamber 202 and a similar second chamber 208, the two chambers 202 and 208 being separated by a dividing wall 210.

The dividing wall 210 extends from the floor 212 of the housing 206 but stops just short of the top 214 of the housing 206 to leave a gap 216. Thus, condensate can spill over the dividing wall 210 to enter the second chamber 208.

The floor 212 of the second chamber 208 is penetrated by a siphon tube 218 to allow condensate to flow from the condensate trap 28 and is also penetrated by an air intake 220 that allows regulation of the flow of condensate from the trap 28. The siphon tube 218 is received within a wider tube 222 to leave an annular gap therebetween. The wider tube 222 depends from the top 214 of the housing 206 and circumscribes a circular aperture 224 provided in the top of the housing 206. This arrangement of passageways provides the correct differences in pressure acting upon the fluid surfaces to permit the siphon to eject liquid in regular volumes as opposed to a constant drip. The aperture 224 is provided to allow overflow of condensate to escape from the condensate trap 28 such that the condensate flows onto the top outer surface 226 of the trap 28.

- 20 - In this example, the condensate trap 28 is made from an insulating material such as a plastic. Three conducting electrodes are attached to the top outer surface of the trap, each insulated from the others by the housing 206.

The central electrode 43 corresponds to the earth electrode 43, and the other two electrodes 42, 42' correspond to the overflow electrodes 42, 42' of the diagnostic system 160 of Figure 11. Thus, when there is an overflow, condensate flows over the top outer surface 226 of the trap 28 and completes a short circuit between the two overflow electrodes 42, 42' and the earth electrode 43.

It will be evident to the skilled person that variations may be made to the above embodiments without departing from the scope of the present invention.

For example, the above embodiments use two testers 30, 32 chosen from three possibilities. However, three testers comprising one each of the dc offset 60, the ac positive cycle detector 80 and the ac negative cycle detector 100 may be used. This would provide a degree of redundancy.

While the two testers 30, 32, and also the controller 34 are shown as separate entities, this need not be the case. They may be implemented in a single device (all three or any combination of two thereof) such as an integrated circuit board or as software operating on a computer. If testing positive and negative cycles is to be used, this may be performed conveniently by a single device.

The two-burner embodiment of Figure 11 shows separate diagnostic circuits, but various elements could be shared.

For example, a shared controller 34 could be used that receives four inputs from the four testers 30, 30', 32, 32'.

Alternatively, a shared controller 34 could be used with two testers 30, 32, each tester 30, 32 being shared and - 21 - receiving multiplexed signals from the two flame probe/overflow detector circuits for example.

The circuits shown are but merely one way of implementing a dc offset detector 60, a positive cycle detector 80 and a negative cycle detector 100. Other arrangements are possible that will provide the same function.

Claims

- 22 - CLA I MS

1. A diagnostic circuit for a condensing boiler comprising: at least one burner arranged to burn a flame; a condenser arranged to condense gas produced by the at least one burner; a condensate trap arranged to collect condensate produced by the condenser; a flame probe, suitable for use in the at least one burner, comprising a flame electrode and a first earth connection separated such that the flame, when present, provides an electrical path to earth; and a condensate overflow detector, suitable for use in the condensate trap, comprising a second earth connection and an overflow electrode positioned in the condensate trap to indicate an overflow such that condensate provides an electrical path to earth when in contact with the overflow electrode; wherein the diagnostic circuit comprises: an ac signal source adapted to provide an ac signal to the flame and overflow electrodes; an output operable to receive an output signal from the ac signal source as may be modified by the flame detector and overflow detector; and diagnostic means operable to use the output signal to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not.

2. The diagnostic circuit of claim 1, wherein the diagnostic means comprise any two of a positive cycle detector, a negative cycle detector and a dc offset detector.

- 23 -

3. The diagnostic circuit of claim 2, wherein the dc offset detector is operable to filter any ac signal component from the output signal and then to detect any dc offset present in the filtered signal.

4. The diagnostic circuit of claim 2 or claim 3, wherein the positive cycle detector or negative cycle detector comprises an appropriatelybiased diode operable to allow only the positive or negative cycle respectively of the output signal to pass.

5. The diagnostic circuit of claim 4, wherein the positive cycle detector or negative cycle detector further comprises a capacitor operable in conjunction with the diode to act as a peak detector.

6. The diagnostic circuit of claim 5, wherein the positive cycle detector or negative cycle detector further comprises a comparator operable to compare the peak of the positive or negative cycle respectively to a reference voltage.

7. The diagnostic circuit of claim 6, wherein the positive cycle detector or negative cycle detector further comprises a resistive divider network operable to provide the reference voltage.

8. The diagnostic circuit of claim 2 or claim 3, wherein the positive cycle detector comprises: an appropriately- biased diode operable to allow only the positive cycle of the output signal to pass; a capacitor operable in conjunction with the diode to act as a peak detector; two comparators, each comparator being operable to compare the - 24 - peak of the positive cycle to a reference voltage; and a resistive divider network operable to provide a different reference voltage to the two comparators.

9. A condensing boiler comprising: a burner arranged to burn with a flame; a flame probe provided in the burner, comprising a flame electrode positioned to be in the flame, when present, and a first earth connection such that the flame provides an electrical path to earth; a condenser arranged to condense gas produced by the burner; a condensate trap arranged to collect condensate from the condenser; a condensate overflow detector comprising an electrode positioned in the condensate trap at a height for indicating an overflow and a second earth connection such that condensate provides an electrical path to earth when present; and the diagnostic circuit of any preceding claim.

10. The condensing boiler of claim 9, further comprising: one or more further burners each arranged to burn with a flame; and further flame probes, one provided in each of the further burners, each flame probe comprising a flame electrode positioned to be in the flame, when present, and an earth connection such that the flame provides an electrical path to earth; and wherein: all flame electrodes are provided an ac signal; - 25 an output is provided for each burner, wherein each output is adapted to receive a respective output signal comprising an ac signal as may be modified by the respective flame detector and the overflow detector; and the diagnostic means are operable to use the output signal to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not in each of the burners.

11. The condensing boiler of claim 10, wherein a separate diagnostic circuit is provided for each burner, each diagnostic circuit having its own ac signal source and its own overflow electrode, each ac signal source being operable to supply an ac signal to its flame and overflow electrodes.

12. The condensing boiler of claim 11, wherein each burner has its own associated condensate trap and each overflow electrode is positioned in that condensate trap.

13. The condensing boiler of claim 11, wherein the overflow electrodes are positioned in a shared condensate trap at at least two different heights.

14. The condensing boiler of any of claims 10 to 13, wherein separate diagnostic means are provided for each burner that are operable to use the output signal provided on its respective output to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not in its burner.

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15. The condensing boiler of any of claims 10 to 13, wherein a shared diagnostic means is operable to receive the outputs as inputs and to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not in each of the burners.

16. The condensing boiler of any of claims 10 to 15, comprising a total of two burners and two flame probes.

17. The condensing boiler of any of claims 9 to 16, further comprising a controller operable to stop a fuel supply to the burner in response to the diagnostic means determining the presence of an overflow or an absence of the one or more flames.

18. A domestic combined heat and power unit comprising the condensing boiler of any of claims 9 to 17.

19. A method of monitoring a condensing boiler comprising: a burner arranged to burn with a flame; a flame probe provided in the burner, comprising a flame electrode positioned to be in the flame, when present, and a first earth connection such that the flame provides an electrical path to earth; a condenser arranged to condense gas produced by the burner; a condensate trap arranged to collect condensate from the condenser; and a condensate overflow detector comprising an electrode positioned in the condensate trap at a height for indicating an overflow and a second earth connection such that - 27 - condensate provides an electrical path to earth when present; the method comprising: supplying an ac signal to the flame and overflow electrodes; and using an output signal, derived from the ac signal source and as may be modified by the flame detector and overflow detector, to determine whether (a) there is an overflow in the condensate trap and (b), if there is no overflow in the condensate trap, whether there is flame or not.

20. A diagnostic circuit substantially as hereinbefore described with reference to any of Figures 2 to 9 and 11 of the accompanying drawings.

21. A condensing boiler substantially as hereinbefore described with reference to any of Figures 2 to 12 of the accompanying drawings.

22. A domestic combined heat and power unit substantially as hereinbefore described with reference to any of Figures 2 to 12 of the accompanying drawings.

23. A method of monitoring a condensing boiler substantially as hereinbefore described with reference to any of Figures 2 to 12 of the accompanying drawings.