AU679289B2 - Heater unit - Google Patents

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION For a Standard Patent

ORIGINAL

Name of Applicant: Actual Inventor(s): HEATMASTER TECHNOLOGY PTY LTD ARTHUR MAURICE MEREDITH p p p.

Address for Service: WRAY ASSOCIATES, Primary Industry House, Terrace, Perth, Western Australia, 6000.

239 Adelaide Attorney code: WR a

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p. p Invention Title: "HEATER UNIT" The following statement is a full description of this invention, including the best method of performing it known to me:- 1

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-2- THIS INVENTION relates to a heater unit. In particular, the invention relates to a heater unit for use in a domestic hot water system.

Most domestic hot water systems include a hot water storage vessel arranged integrally with a heating unit. Such systems must thus be designed with conflicting aims; firstly to allow heat transfer to the water so as to heat the water, and secondly to prevent heat transfer from the water so as to retain the heat prior to use.

In at least one prior art system this has resulted in the production of an annular storage vessel where a burner flame heats air as it passes through the bore of the vessel. However, when not in operation, the pilot flame for the burner causes a draught through the orifice, thus cooling (albeit only slightly) the contents of the vessel. This cooling significantly reduces the efficiency of the system.

In another prior art system, the integration of the heater unit with the storage .000.: 15 vessel often results in condensation, produced while the burner is on (unless o S"operation is at very high temperatures), entering the various components of the heater unit and causing rust or other operational difficulties. These systems are then difficult to repair and thus generally require replacement of the complete system.

An aim of the present invention is to provide a heater unit for use in conjunction i with a hot water storage vessel that are together capable of providing a domestic hot water system with a high efficiency.

The present invention provides a heater unit which comprises a substantially vertical shell and tube heat exchanger having a plurality of tubes arranged longitudinally along a cylindrical shell and having a tube-side for passage of combustion gases and a shell-side for passage of water to be heated, an upright burner located below the heat exchanger, a combustion chamber located f: U jJ~-

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-3between the burner and the heat exchanger, a ventilation means and a control means, wherein the burner is configured such that in use the products of combustion pass from the combustion chamber through the tube-side to heat water passing through the shell-side, the products of combustion then being ventilated to atmosphere via the ventilation means, and wherein the heater unit is contained in a housing, the housing having inlet and outlet lines for the supply of the water to be heated and for the delivery of heated water to a hot water storage vessel.

The shell and tube heat exchanger is arranged such that the tubes are supported substantially vertically. The burner is then be configured to be upright below the bottom end of the tubes of the heat exchanger. In this form, the combustion chamber is defined by a downwardly extending skirt which projects from the bottom end of the heat exchanger to preferably encompass at least the upper portion of the head of the burner.

15 The downwardly extending skirt preferably diverges outwardly, and externally includes a contiguous pre-heat pipe through which the water entering the heat exchanger will pass. Such a pre-heat pipe is preferably configured so as to rise the height of the skirt while passing about at least one full circumference of the skirt. By passing the entering water through the pre-heat pipe prior to entry to 20 the heat exchanger, the temperature within the combustion chamber is able to be controlled (thus preventing damage to the skirt) and further use may be made of heat transfer that would otherwise be unused, thus improving the efficiency of the unit.

The shell and tube heat exchanger preferably includes a frusto conical discharge passage forming part of the ventilation means, the discharge passage being integral with the top end of the heat exchanger to provide a convergent flow restriction extending away from the heat -4exchanger, through which the products of combustion may pass after exiting the tubes of the heat exchanger.

The heater unit is preferably fully contained in a housing, the housing having inlet and outlet lines for the supply of the water to be heated and for the delivery of the heated water, and of course having a fuel inlet for the supply of fuel for the burner.

In use, the housing may be mounted to a hot water storage vessel, in the form of a cylinder, having the inlet line for the heater unit connected at or near the bottom of the cylinder and the outlet line from the heater unit being connected at or near the top of the cylinder. However, the heater unit may be located remotely from the hot water storage cylinder simply by extending the water lines. In this way the cylinder may be hidden from view or may be placed in a location where its size does not cause an obstruction.

The ease of separating the heater unit from the hot water storage cylinder is also advantageous in readily allowing the production of a large number of different capacity hot water systems, all using the same heater unit but having different cylinder sizes. Apart from simplicity in manufacturing, this also means that regulatory approval is only required for one unit, the heater unit itself, rather 25 than for a number of separate systems together which might typically represent a manufacturer's range of systems.

S. Furthermore, the heater unit may be installed either internally or externally of a dwelling with the hot water storage vessel similarly being installed either internally or externally, or vice-versa depending on requirements.

For either internal or external applications the same heater unit may be used, the only adaptation required being to the ventilation means.

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5 For external use, the ventilation means of the heater unit preferably comprises an integral balanced flue which combines the operation of the converging discharge passage referred to above with an arrangement of open and closed louvres included in the housing of the heating unit. This allows the products of combustion to be exhausted at a pressure equal to the pressure of the air supplied to the burner, irrespective of wind conditions. Thus, the burner combustion is not effected by wind conditions.

For internal use the ventilation means of the heater unit preferably comprises a conventional flue fitted to the converging discharge passage of the heater unit, which passes through the ceiling space of the dwelling to exhaust the products of combustion outside the dwelling. The flue preferably includes a draught diverter located adjacent the converging discharge passage in order to prevent a downward thrust of air from causing disruption to the burner combustion which may result in the production of carbon monoxide or unburnt fuel. In the internal heater unit, the .ooe 20 housing of the unit will preferably be provided with louvres at or near the bottom thereof to allow air for combustion to be drawn into the unit.

The burner of the heater unit is preferably a natural draught burner for use with gaseous fuel such as natural 25 gas or liquid petroleum gas. In one form, the burner is a stainless steel barrel burner with a plurality of fine slots arranged about the entire circumference of the barrel.

The control means of the heater unit preferably comprises a burner ignition means and a thermostat which communicates with an electronic gas module to control the flow of gas and the activation of the ignition means. The burner

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6 ignition means is preferably provided by an arc electrode or a hot surface igniter.

Thus, by locating the thermostat within the hot water storage system, when the temperature of the stored water goes below a predetermined level, the thermostat signals to the electronic gas module to open a gas valve and to cause the arc electrode to ignite the burner. The burner continues to heat water as it passes through the shell side of the heat exchanger until the temperature goes above a predetermined level. The thermostat then signals to the electronic gas module to close the gas valve.

The driving force for the passage of the water to be heated through the shell-side of the heat exchanger will be determined by the size of the hot water storage cylinder.

In particular, it is envisaged that three basic designs of heater unit would be manufactured, to suit different hot water system requirements.

The first type of unit is preferably a small unit having a rating in the range of from 3 kW/hr to 10 kW/hr. On o 20 commencement of operation of the burner, the heated water 20 in the shell side of the heat exchanger thermo-siphons through the heat exchanger to return to the top of the hot water storage cylinder, causing more unheated water to enter the heat exchanger from the bottom of the hot water 25 storage cylinder. Such a thermo-syphon unit would be 9* applicable for burners capable of up to about 24 MJ/hr.

.to* The second type of unit preferably has a rating in the range of from 15 kW/hr to 40 kW/hr, the burners thus being capable of about 50 MJ/hr to about 180 MJ/hr (or more).

However, for a unit of this size, the thermo-syphon effect would be too slow and thus the unit would require a circulator pump. The circulator pump may be controlled by the electronic gas module referred to above, such that the I I 7 same thermostat that ignites the arc to the burner, and opens the gas valve, also starts the circulator pump.

The third type of unit is preferably an instantaneous model, requiring a high-fire/low-fire burner, preferably having a 4 to 1 turn down ratio controlled by two thermostats set a few degrees apart. The gas valve preferably has the feature of high-low gas regulation, while the same electronic gas module would control the sequence of operation. However, the instantaneous unit operates from a drop in pressure, the differential being caused by a tap (for instance) being turned on. From that point onwards the operation of the heater unit is similar to that described above for the previous two units. Of course, the instantaneous unit does not require a storage cylinder and would operate with a larger burner capacity such that an acceptable flow rate of hot water is available. For example, units having ratings of 24 kW/hr and 33 kW/hr are envisaged.

The heater unit of the present invention will now be S 20 described in relation to several preferred embodiments as a. illustrated in the accompanying drawings. However, it must be understood that the following description is not to limit the generality of the above description.

In the drawings:e.

25 Figure 1 is a schematic side view of a heater unit in use in a domestic hot water system; Figure 2a is a schematic representation of a preferred control embodiment of a heater unit in accordance with the present invention; Figure 2b is an alternative version of Figure 2a showing a preferred configuration; Figure 3 is an internal side view of the heater unit of Figures 1 and 2 in use in a first embodiment; and I- I Il 8 Figure 4 is an internal side view of the heater unit of Figures 1 and 2 in use in a second embodiment.

Illustrated in Figure 1 is a heater unit adapted for internal use within a dwelling, the heater unit including a shell and tube heat exchanger 12 having an inlet line 14 for the supply of the water to be heated and an outlet line 16 for the delivery of the heated water.

The shell and tube heat exchanger 12 may be clad in insulation material such as fibre glass cladding if required. The heater unit 10 also includes a ventilation means (generally indicated by the numeral 18) a burner and a control means 22.

A combustion chamber 24 is arranged between the shell and tube heat exchanger 12 and the burner 20 such that in use the products of combustion from the burner 20 pass directly into the combustion chamber 24 and then through the tube side of the heat exchanger 12. Heat transfer from the products of combustion as they pass through the tube side of the heat exchanger 12 causes the water passing through S 20 the shell side of the heat exchanger to be heated.

At the upper end of the heat exchanger 12 there is provided a frusto conical discharge passage 26 through which the products of combustion pass prior to entry to the flue 28.

The flue 28 is shown provided with an anti-down draft cowl 25 32 together with an anti-down draft diverter The heater unit 10 as illustrated in Figure 1 may be .mounted to the side of a hot water storage vessel such as a hot water storage cylinder (not shown) or may be located remotely from such a cylinder. In this respect, provided that the outlet and inlet lines 14 and 16 are in fluid communication with the cylinder, the heater unit and the cylinder may be located virtually anywhere within or external to a dwelling. Of course, the only qualification 9 on that is that, where operation requires thermo-siphon, the locations must not be such as to overcome the thermosiphon effect.

The preferred operation of the heater unit 10, dictated mainly by the operation of the control means 22 and its associated components, will be better described in relation to the schematic illustration of Figures 2a and 2b.

Following that, further detail will be provided in relation to the various operating parameters of the heater unit Illustrated in Figure 2a is the shell and tube heat exchanger 12 of Figure 1 having its inlet line 14 and its outlet line 16 arranged such that the cool water enters the shell of the heat exchanger 12 at the lower end thereof.

The combustion chamber 24 is a downwardly and outwardly extending skirt which, at its lower end, extends below the upper leading edge of the burner 20. This ensures that all of the products of combustion pass from the combustion chamber 24 through the tube side of the heat exchanger 12.

The heat exchanger 12 includes at its upper end the frusto conical discharge passage 26 which restricts the area for flow of the products of combustion.

The burner 20 is able to be ignited by an arc electrode 34 configured together with a flame detection sensor 36. The e.

electronic gas control module 38 is connected to an operational thermostat 40 which is operationally attached to the hot water storage vessel (not shown). However, in an alternative form, the burner 20 may be ignited by a hot surface igniter, rather than an arc electrode. In this respect, if the gas module is a 24 volt module, it would be preferred to utilise an arc electrode, while higher voltage modules may better utilise a hot surface igniter.

I I I I 10 Furthermore, the unit may include a flame sensor (now shown) which is provided for safety in that the lack of a flame would be detected and the sensor would ensure that the gas supply was closed within a certain short period of time.

The fuel for the burner 20 is provided via a gas supply service valve 42 operating in conjunction with a gas regulator 44, a gas solenoid valve 46 and the gas jet 48 itself. As will be seen, the gas solenoid valve 46 is also connected to the electronic gas control module 38.

Thus, by setting the thermostat 40 to trigger the electronic gas control module 38 when the temperature of the water in the hot water storage vessel moves below a predetermined level, the flow of gas to the gas jet 48 may be opened and then ignited by the arc electrode 34. The products of combustion of the gas pass through the tubes of the shell and tube heat exchanger 12 to heat the water located thereabout in the shell side of the heat exchanger 12. As the temperature of that enclosed water increases, .0 20 the water (at least in one embodiment of the invention) thermo-siphons through the heat exchanger 12 to pass through outlet line 16 and on to the hot water storage vessel. The continued flow of thermo-siphoning water through the heat exchanger 12 allows the cold water S 25 extracted from towards the bottom of the hot water storage vessel to be heated to a temperature suitable to cause the bulk of water in the hot water storage vessel to increase beyond the predetermined level referred to above. The o thermostat 40 then turns off, causing the electronic gas control module to close the gas solenoid valve 46 and thus end the combustion of the fuel.

Illustrated in Figure 2b is the shell and tube heat exchanger of Figure 2a, but with the inlet line 14 altered so as to provide the contiguous pre-heat pipe referred to ii C 1 I Li113 11 earlier. The pre-heat pipe is illustrated as providing a coil which is braized to the exterior of the downwardly and outwardly extending skirt such that it continuously rises up and about at least one circumference of the skirt. This pre-heat pipe assists in absorbing the heat of the combustion wall that is generated by the flame of the burner 20, gaining extra efficiency. Also, where the skirt is made of tinned copper, the pre-heat pipe assists in preventing the temperature of the skirt rising to levels that may melt the tinning.

Indeed, it has been found that such a pre-heat pipe is preferred where the vertical height of a skirt such as that illustrated in both Figures 2a and 2b extends beyond about 150mm. Thus, there are preferably a number of passes of the pre-heat pipe about the skirt such that there is one pass for about every 150mm of vertical height of skirt.

A preferred configuration of the heater unit as briefly :i described in relation to Figures i, 2a and 2b is better illustrated in Figures 3A and 3B. In Figures 3A and 3B, 20 the same reference numerals are utilised to denote the same features as used in Figures i, 2a and 2b.

Figures 3A and 3B show the control means 22 located internally of a housing 50 and adjacent the heat exchanger 12. The control means 22 includes the electronic gas 25 control module 38, the gas regulator 44, the gas solenoid valve 46 and the gas jet 48. All of these components are connected via a gas line 52.

The burner 20 is a stainless steel barrel type burner having a plurality of fine slots to enhance gas combustion.

The burner is a natural draft burner mounted such that the upper end thereof extends at least partly within the combustion chamber 24, the combustion chamber 24 being open at its bottom end to allow access of air to the burner.

-I 12 The heat exchanger 12 is of the shell and tube type having a plurality of tubes arranged longitudinally along a cylindrical shell. The water to be heated passes through the shell, about the tubes, while the hot combustion gases pass through the tubes to heat those tubes causing heat transfer therefrom to the water thereabout.

The heat exchanger 12 has a copper tube jacket or body and preferably utilises copper tubes of either 12.7mm or diameter. It will be appreciated that the body of the heat exchanger will include copper headers at each end, the tubes being braised or welded therein. The skirt which defines the combustion chamber 24, together with the frusto conical diverging passage 26 are each also of copper and are braised to the body of the heat exchanger.

The heat exchanger 12 and the control means 22 are each mounted within the housing 50 such that a service entry is provided and also so as to be reasonably airtight, The control means 22 is itself provided within a housing located internally of the housing 50, the internal housing including therein the control module, a power transformer, the solenoid valve or gas valve, the gas regulator and a!.

electrical junction box. The combustion chamber 24 is sized so as to allow complete combustion of the gases from the burner ee Figures 4A and 4B illustrate a second embodiment of a heater unit 10 in accordance with the present invention.

This second embodiment is a heater unit adapted for external use, thus not requiring the elongate flue as illustrated in the earlier embodiments. Indeed, the enting arrangement for the products of combustion of this heater unit relies on a louvre arrangement as will be described below.

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13 In general, the features of the heater u of this second embodiment are the same as those of the heater unit illustrated in Figures 3A and 3B. Thus, the heater unit of the second embodiment includes a shell and tube heat exchanger having inlet and outlet lines, a combustion chamber in the form of a downwardly extending skirt and a frusto conical discharge passage. The unit also includes a burner and a control means including therewithin the electronic gas control module, a transformer, a gas regulator and a solenoid valve.

The embodiment of Figures 4A and 4B includes an arrangement of louvres at the top thereof that, together with apertures in the frusto conical discharge passage, define a balanced flue for ventilation of the products of combustion and for S supply of fresh air for the burner.

The arrangement of louvres includes upper and lower louvre segments, 54 and 56, the segments 54 and 56 being separated by a wall 58 through which the discharge passage 26 extends. Thus, the aperture 60 in the discharge passage 26 S 20 opens to the lower segment 56 while the opening 62 of the discharge passage 26 opens to the upper segment 54. This allows the pressures in both segments to balance.

The upper louvre segment 54 is open at either end thereof and is closed at the front thereof. In this respect, the 25 "front" and "rear" of the housing of the heater unit are defined by reference to the rear of the unit being that side of the unit mounted to a wall or the like. The lower oee• louvre segment 56 is also open at both ends thereof but includes an open front face. The open front face of the lower louvre segment 56 is approximately aligned with the apertures 60 located in the front and rear of the frusto conical discharge passage. This arrangement allows combustion gases to vent from the open ends of the upper segment, and due to the balancing of pressures, inlet air r C _I _I_ 14 is caused to enter the three open faces of the lower segment. Once equalised, the inlet air is unable to bounce back to give back pressure. Thus, this arrangement may be utilised for an external unit so as to avoid high winds from entering the housing of the unit to extinguish the flame of the burner.

For the externally utilised embodiment of the heater unit of the invention (illustrated in Figures 4A and 4B) the area of the open ends of the louvre arrangements may be related to the size of the burner being used. In particular, for every 1 kW/hr burner input, each end must have an open area in the order of 14 sqcm. Thus, the total open area of the end louvre arrangements is in the order of 28 sqcm per kW/hr net burner input. In this respect, due to the possibility of the wind direction being from only one side, the area of the opening of one side must be capable of exiting the exhaust for the full burner rating.

Also, the external model is preferably configured such that .air drawn through the lower louvre segments (for S 20 combustion) has enough space within the housing to pass to the burner without excessive restriction. It is envisaged that this will be met by ensuring that the minimum available open cross-sectional area within the housing is in the order of 0.00419 m per one kW/hr net burner input.

25 In fact, the same type of figure would most likely also be applicable to the internal (flued) model. However, due to the air inlet 70 being configured adjacent the burner (see Figure 3B) the requirement will always be met.

o Furthermore, the area of the apertures in the frusto conical discharge passage may also be related to the net burner input. In particular, the area of an aperture on one side of the discharge passage may be about 2 sqcm per kW/hr of net burner input. Again, the total area thus becomes about 4 sqcm per net burner input.

15 The ratio of the heating surface area of the shell and tube heat exchanger to the burner kW/hr net input is preferably in the order of 0.0331 sqm of heating surface area to 1 kW/ hr net burner input. The use of such a shell and tube heat exchanger allows the use of a relatively large surface area for heating but with a lower ratio of heat applied per unit surface area. Thus, the heater is able to operate on water of a quality considered to be generally poor, having a relatively high calcium and solids content, particularly if the temperature of the hot water is kept below 60 0

C.

In this respect, the combustion chamber and the shell and tube heat exchanger are preferably constructed of tinned copper, the combustion chamber being braized to the heat exchanger in a manner which allows the transfer of heat from the combustion chamber wall to the jacket of the heat exchanger. The tubes within the heat exchanger are preferably substantially vertical having a diameter in the range of 12.7 to 9e999* The volume of the combustion chamber may also be related to the size of net burner input. In this respect, the ratio of the volume of the combustion chamber to the kW/hr net oo burner input is preferably in the order of 0.00139 cubic 990 metres to 1 kW/hr burner input. Furthermore, the combustion chamber is preferably sized and configured so 25 that the flame of the burner does not contact any part of the heat exchanger and does not contact any part of the walls of the combustion chamber. Further still, in a preferred form of the invention, the downwardly extending skirt which defines the combustion chamber extends downwardly from a position about one third of the length of the exterior of the heat exchanger itself. In this form, the products of combustion not only pass through the tubes of the heat exchanger but also are allowed to circulate about the exterior of the lower third of the shell of the i IO 16heat exchanger. This provides extra surface area through which heat transfer may take place from the products of combustion to the water being circulated through the shell of the heat exchanger.

The storage volume of the shell of the heat exchanger may also be related to the net burner input. Thus, the volume of the shell is preferably in the order of 1 litre per 6 kW/hr net burner input.

The flow rate of water through the unit may also be related to the net burner input. In particular, the minimum flow rate is preferably in the order of 1 litre per second per 58 kW/hr net burner input. Thus, for a 30kW/hr burner, a flow rate in the order of 0.517 litres per second (or about 31 litres per minute) minimum is preferred. However, it should be appreciated that flow rates higher than this may be utilised depending upon the increase in temperature required, the minimum simply having been determined on the S...basis of requiring a maximum temperature increase of 140C.

The frusto conical discharge passage preferably is sized S 20 such that its upper opening, through which the products of combustion exit, is of a diameter that approximately equals the combined cross-sectional area of all of the tubes of the shell and tube heat exchanger. Furthermore, the diameter of the shell of the shell and tube heat exchanger 25 is not specifically related to the diameter of the upper opening of the frusto conical discharge passage, other than to have a requirement that it be larger than that diameter oooo in order to provide a restriction in the passage to cause the venturi effect on the discharge on the products of combustion as referred to above.

With regard to the number and size of tubes utilised in the heat exchanger unit of the present invention, these will generally be determined by the above ratios once the size 1 II L I L b I 17 of the burner is selected. For example, for a 3 kW burner the discharge passage upper opening will be required to be about 43mm diameter, thus indicating that the crosssectional of all of the tubes be of the same order. Thus, fourteen tubes of half inch diameter will suffice.

The total heat transfer area may also be determined in relation to the net burner input, the above example producing tubes in the order of 180mm long, held in a shell having a diameter of about In a further form of the invention, an additional thermostat may be mounted on the surface of the shell of the heat exchanger. Such a thermostat may operate as a "lock-out" thermostat to measure the temperature of the water within the shell to ensure that that temperature does not reach boiling point. Thus, the thermostat could be set at, for example, 70 0 C such that if the water within the shell were to reach that temperature the thermostat would trigger the electronic gas module to close the flow of gas to the burner. This feature ensures that any blockages, or eo eo S 20 any other reasons for the flow of water to be stopped or decreased, do not cause the generation of hot spots or over heating within the system.

9ooo 9* Finally, it will be understood that there may be other modifications and variations to the configuration described herein that are also within the scope of the present invention.

9 9 9o 9.

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Claims (20)

1. A heater unit which comprises a substantially vertical shell and tube heat exchanger having a plurality of tubes arranged longitudinally along a cylindrical shell, and having a tube-side for passage of combustion gases and a shell-side for passage of water to be heated, and upright burner located below the heat exchanger, a combustion chamber located between the burner and the heat exchanger, a ventilation means and a control means, wherein the burner is configured such that in use the products of combustion pass from the combustion chamber through the tube-side to heat water passing through the shell-side, the products of combustion then being ventilated to atmosphere via the ventilating means/ and wherein the heater unit is contained in a housing, the housing having inlet and outlet lines for the supply of the water to be heated and for the delivery of heated water to a hot water storage vessel.

2. A heater unit according to claim 1 wherein the combustion chamber is 15 defined by a skirt extending downwardly and outwardly from the heat exchanger S. to encompass at least an upper portion of the burner.

3. A heater unit according to claim 2 wherein the skirt includes an externally located, contiguous pre-heat pipe which passes up and about the skirt, through which water entering the shell-side of the heat exchanger will pass. 20 4. A heater unit according to claim 3 wherein the pre-heat pipe is configured such that there will be one pass of the pipe about the skirt for about each 150mm of vertical height of skirt. A heater unit according to any one of claims 1 to 4 wherein the heat exchanger includes a frusto conical discharge passage which forms a part of the ventilation means.

6. A heater unit according to claim 5 wherein the discharge passage is integral with the top end of the heat exchanger to provide a convergent flow U7 I -19- restriction extending away from the heat exchanger, through which products of combustion may pass after exiting the tubes of the heat exchanger.

7. A heater unit according to any one of claims 1 to 6 wherein the housing is located either remotely from the hot water storage vessel or adjacent to the hot water storage vessel.

8. A heater unit according to any one of claims 1 to 7 wherein the housing is located internally of a dwelling, the heater unit including a flue fitted to the converging discharge passage, the flue passing through the ceiling space of the dwelling to exhaust the products of combustion outside the dwelling, the flue include a draught diverter located adjacent the converging discharge passage in order to prevent a downward thrust of air from causing disruption to the burner combustion.

9. A heater unit according to any one of claims 1 to 7 wherein the housing is S: located externally of a dwelling, the ventilation means of the unit combining with 15 the housing to provide an integral balanced flue. A heater unit according to claim 9 wherein the housing includes upper and lower louvre segments separated by a wall through which the discharge passage extends, the discharge passage having an upper opening which opens to the upper segment and the discharge passage also including at least one aperture which opens to the lower segment allowing the pressures in both segments to balance, the upper louvre segment being open at opposing ends thereof and closed at the front and rear thereof, the lower louvre segment being open at opposing ends thereof and also having an open front.

11. A heater unit according to claim 10 wherein the area of the open ends of the upper and lower segments is about 14 sqcm per 1/kW/hr burner input. 'l'7 C1 P" I I

12. A heater unit according to claim 10 or claim 11 wherein the minimum available open cross-sectional area within the housing is about 0.00419 sqm per 1kW/hr burner input.

13. A heater unit according to any one of claims 10 to 12 wherein the area of apertures in the discharge passage is about 4 sqcm per 1 kW/hr burner input.

14. A heater unit according to any one of claims 1 to 13, wherein the ratio of the heating surface area of the heat exchanger to the burner kW/hr input is about 0.0331 sqm of heating surface area to 1 kW/hr burner input. A heater unit according to claim 14 wherein the tubes within the heat exchanger are substantially vertical and have a diameter in the range of from 12.7mm to

16. A heater unit according to any one of claims 1 to 15 wherein the ratio of the volume of the combustion chamber to the burner kW/hr input is about 0.00139 cubic metres to 1 kW/hr burner input. oo S 15 17. A heater unit according to any one of claims 1 to 16 wherein the ratio of the storage volume of the shell of the heat exchanger to the burner kW/hr input is about 1 litre per 6 kW/hr burner input.

18. A heater unit according to any one of claims 1 to 17 wherein the ratio of the flow rate of water through the unit to the burner kW/hr is about 1 litre per 20 second to 58 kW/hr burner input as a minimum flow rate giving a maximum temperature rise of 140C.

19. A heater unit according to any one of claims 1 to 18 wherein the upper opening of the discharge passage is of an area that approximately equals the combined cross-sectional area of all of the tubes of the heat exchanger. r c r\ -21 A heater unit according to any one of claims 1 to 19 wherein the control means comprises a burner ignition means and a thermostat which communicates with an electronic gas module to control the flow of gas and the activation of the ignition means.

21. A heater unit according to claim 20 wherein the ignition means is an arc electrode.

22. A heater unit according to any one of claims 1 to 21 wherein the burner has a net burner input of from 3kW/hr to 10 kW/hr and the water circulates through the unit during operation of the burner by virtue of a thermo-siphon effect.

23. A heater unit according to any one of claims 1 to 21 wherein the burner has a net burner input of from 15 kW/hr to 40 kW/hr and the unit additionally includes a circulator pump to circulate water therethrough. 040

24. A heater unit according to any one of claims 1 to 21 wherein the burner is 15 a high-fire/low fire, instantaneous burner having a 4 to 1 turn down ratio controlled by two thermostats. 000

25. A heater unit according to claim 1 substantially as herein described in relation to the accompanying drawings.

26. A hot water system incorporating a heater unit in accordance with any 20 one of claims 1 to DATED this TWENTY-FIFTH day of MARCH 1997. HEATMASTER TECHNOLOGY PTY LTD Applicant Wray Associates Perth, Western Australia Patent Attorneys for Applicant 4' W r _l ii ABSTRACT A heater unit which comprises a substantially vertical shell and tube heat exchanger having a tube-side for passage of combustion gases and a shell-side for passage of water to be heated. The heater unit includes an upright burner located below the heat exchanger, a combustion chamber located between the burner and the heat exchanger, a ventilation means and a control means. The burner is configured such that in use the products of combustion pass from the combustion chamber through the tube-side to heat water passing through the shell-side, the products of combustion then being ventilated to atmosphere via the ventilation means. o« *o i a *oo -M