GB2188715A - Central heating boiler arrangement - Google Patents

Abstract

A central heating boiler arrangement has a flue gas condensation economiser 12 fitted to the exhaust gas flue 11 of the boiler 10, to recover heat from the exhaust gases. To maintain the economiser surfaces cool enough to ensure condensation takes place, the outlet flow from the boiler is divided into two portions one of which (from B to A) is circulated through the radiators and the other of which is fed along path 21 back to the boiler inlet. The return flow 14 from the radiators is passed through the economiser 12 and is then mixed with the portion fed back to the boiler, in a mixing chamber 15. The division ratio of the boiler outlet flow is adjusted, for example by valve 24 or by temperature-sensitive valve 23, to ensure that the flow rate through, inlet temperature to and temperature rise across the boiler are all within the design ranges therefor, whilst allowing a radiator return temperature substantially below the dew point of the flue gases. <IMAGE>

Description

SPECIFICATION Central heating systems This invention concerns central heating systems. In particular, the invention relates both to methods of operating and to boiler arrangements for hot water central heating systems using radiators for space heating.

Modern space heating systems using wellcontrollable boilers (such as natural gas or oilfired boilers) to heat water which is circulated through radiators are usually designed to operate in one of two ways. In the first, a boiler thermostat is used for control of the boiler outlet temperature and a room thermostat to switch on and off a water circulating pump and/or the boiler burner, or to switch off a section of the radiator system (zoning) when the required temperature is reached. Such a system can be arranged with an over-run device to keep the pump running after the burner is switched off. In normal operation, after the initial heating-up period the room thermostat switches out before the set boiler outlet temperature is reached, the room thermostat then switching on and off, so cycling the boiler and modulating its output.The boiler thermostat mainly operates when only domestic hot water is required or on very cold days when the full boiler output is continuously required to counter heat losses; hence the boiler outlet temperature hardly ever reaches the set point of the boiler thermostat. Such a system will hereinafter be referred to as a "room thermostat system".

In the second way, a boiler thermostat is used for control of the boiler outlet temperature and individual radiator temperature regulating valves are also employed, there being with a recirculation by-pass back to the boiler inlet to allow a minimum circulating flow. As the temperature in a room rises, the regulating valves throttle the flow of circulating water to the radiators in that room, thus reducing their heat output and controlling the room temperature. Throttling the circulation flow forces more water through the recirculation path, thus increasing the boiler temperature until the outlet thermostat operates. Such a system will hereinafter be referred to as a "boiler thermostat system".

A room thermostat system operates with a varying boiler outlet temperature, and thus the boiler efficiency increases marginally with a reduced heat load. By contrast, a boiler thermostat system always operates with the boiler output temperature within the thermostat control band whatever the heat load requirement, and thus with essentially constant boiler efficiency.

Whichever system is used, a gas or oil-fired boiler is designed for operation within a relatively narrow band of operating parameters and conditions, if optimum efficiency is to be obtained. Typically, the water temperatures at the inlet to and outlet from the boiler are designed to be 71.1"C and 82.2"C respectively that is, a temperature drop across the boiler (and hence also across the radiators) of 11.1"C. The flow rates through the radiators should be adjusted in order to give this temperature drop across the boiler, and the areas of the radiators should appropriately be selected such that they dissipate sufficient thermal energy to heat a room with these temperatures and temperature drop across them.

Most central heating systems, in terms of the sizes of the radiators, are over-designed and as a result the return temperature from the radiators is below the design point. When operating in this way, better economy can be achieved because less fuel is being consumed, and in fact the boiler efficiency may increase marginally due to the lower return temperature of the water.

The flue gases from a boiler are always relatively hot, as compared to the water outlet temperature from the boiler, even when operating under the above-described enhanced efficiency regime. Typically, the flue gases are at about 250"C, at a 75% boiler efficiency (nettover-gross calorific value basis). Not surprisingly therefore, there have been proposals for increasing the efficiency of a boiler by recovering heat from the flue gases, using a socalled "economiser". Such a device comprises a heat exchanger arranged to reduce the flue gas temperature whilst pre-heating either the central heating water or feeding heat to some other heat sink.This will not only recover the sensible heat energy of those gases, but moreover will extract the latent heat of condensation of the water vapour produced by the combustion of the fuel gas, provided the surfaces of the economiser are operating below the water dew point. In this case, it is found that a significant proportion of the recovered heat in fact comprises such latent heat of condensation.

The dew point for the water vapour in the flue gases of a natural-gas fired boiler is typically within the range of 52" to 59"C, which is below the design inlet water temperature to a boiler. If an economiser is therefore simply fitted in the flue of an existing central heating boiler with the intention of condensing heat recovery and the return flow from the radiators is passed through the economiser before entering the boiler inlet, the water temperature entering the economiser must be 30 to 40"C, resulting in a feed temperature to the boiler of below 50"C. The consequence is that the notional increased efficiency which could be obtained by using an economiser is unachievable because the heat provided cannot be used either by the installed radiator system or by a domestic hot water system, as the temperatures are too low.

In order to overcome the above disadvan tages, recently there have been certain new designs of natural gas boiler arranged to operate in a "condensation mode", wherein the flue gases leaving the boiler are at a temperature below the dew point of those gases. The designed flow rates, temperature differentials and so on for such new boilers are different from those for conventional central heating systems, and in particular the return flow from the radiators has to be lower than 50"C, and is preferably as low as 30"C. Moreover, since the moisture resulting from condensing the flue gases is acidic and highly corrosive, such boilers have to be made from corrosion-resistant materials, which lead to very high manufacturing costs.This may be contrasted with the use of a separate economiser, where only the economiser-which is relatively small--has to be made of such corrosion-resistant materials.

It is a principal object of this invention to provide an operating method for a central heating system, and also a central heating boiler arrangement, where use is made of a flue gas economiser whilst still having the boiler operating within its design parameters.

Accordingly, one aspect of this invention provides a method of operating a central heating boiler fitted with a flue gas condensation economiser, comprising dividing the hot water flow from the boiler outlet into two portions, one of which portions is circulated through the radiators and the other of which portions is fed back to the inlet to the boiler, passing the cold water return from the radiators through the economiser before mixing that water with said other portion fed back to the boiler inlet from the boiler outlet, and adjusting the division ratio of the boiler outlet flow into said one and other portions such that the flow rate through, inlet temperature to, and temperature rise across the boiler are all within the design ranges therefor whilst allowing a radiator return temperature substantially below the dew point of the flue gases.

In the operating method of this invention, the boiler is operated wholly within its design parameters-that is to say, the water flow through the boiler is not less than that proscribed by its rated output and design temperature rise nor is the boiler fed with water at an excessively low inlet temperature. In other words, it, is not apparent to the boiler that a flue gas economiser is fitted to the boiler.

Despite this, the economiser is able to operate properly so as to obtain condensation of the water vapour in the flue gases. In order to achieve this, the flow rate through the radiators is much reduced as compared to a central heating system operating conventionally, for the flow through the boiler is divided into two portions one of which is passed through the radiators and the other of which is simply fed back to the boiler inlet. By reducing the flow through the radiators, a greater temperature drop thereacross may be obtained, whereby the return flow from the radiators may be at a temperature well below the dew point of the flue gases. The economiser may then increase the temperature of the return flow by an amount whereby when mixed with the fed-back portion from the boiler outlet there results an inlet flow to the boiler within the design temperature range.

An economiser as employed in this invention produces a condensing effect which, provided the economiser is designed and fitted correctly to a gas fired boiler, can increase the boiler efficiency on a nett-over-gross calorific value basis to 969/0 with a return water (economiser inlet) temperature of 30"C. This may be compared with an existing modern domestic unit which typically operates at 75% efficiency (on the same basis), the economiser thus representing a 28% improvement. If an economiser is used as an add-on to an existing system or to a proprietory boiler, it will be seen the effect of fitting the economiser may be to provide the same amount of heat output but with a 22% less fuel consumption.In other words, a boiler fitted with such an economiser can have its gas control turned down so that itconsumes 22% less, yet still give the same heat output, provided the radiator system can provide a return temperature of 30"C.

If the economiser is fitted as an add-on to an existing system or to a proprietory boiler, the economiser presents a flow resistance to the flue gases leaving the boiler, which reduces the combustion excess air rate. However, if at the same time the gas requirement is reduced, as described above, then provided the stack buoyancy is as before fitment of the economiser-i.e. the same boiler outlet flue gas temperature as before fitting the economiser and the same system heights-then the difference between the stack buoyancy and the reduced friction losses of the combustion air and flue gases will provide an adequate pressure differential for the economiser in all types of boiler flue system.

When operating the method of this invention, the boiler and economiser may always be fed with water at the optimum temperature, to achieve maximum efficiency, by providing an automatic temperature-control arrangement for the recirculated portion. At start-up, the radiator return water is cold, and so is fed at full flow rate through the economiser thus achieving maximum heat recovery whilst providing preheat for the boiler itself and thus avoiding the possibility of condensation within this unit (which can occur under normal operation without an economiser fitted and which then has a deleterious effect on the boiler material). As the system warms up and the return temperature from the radiators rises, the recirculation path for hot boiler outlet water is established, for mixing with the return water leaving the economiser before en try to the boiler.The effect of this is to reduce the water flow rate to the radiators which consequently over-cool the water passing therethrough and provide the ideal return water temperature for the economiser.

In a room thermostat system, the effect of the boiler system of this invention will be automatically to drive the boiler outlet temperature upwards, ultimately until the boiler outlet setting is reached, but as all the waste heat is recovered in the economiser, the efficiency will be unaffected by the high operating temperature. Also, by so doing, the system will inherently adjust itself to compensate for the higher radiator temperature drop required for the system of this invention. In a boiler thermostat system, as the cold return water is fed directly back to the economiser without mixing with the recirculating water, the lowest possible temperature is provided for the economiser to create the required cold surfaces thereof and so to maintain its performance, and the boiler thermostat may be set to as high a temperature as required without loss of efficiency.

When operating a central heating system as described above in accordance with this invention, it is necessary for the radiators to be able to dissipate the required amount of heat to obtain effective space heating, whilst operating with flow rates well below their design values. In order to achieve this despite the high inlet temperature, it may be necessary to increase the surface area of those radiators (for example, by changing at least some of the radiators for larger ones) or preferably by adding fins to the existing radiators, in order to increase their surface area. However, as noted above, many radiator systems are overdesigned, resulting in the need to operate such systems at well below their designed parameters.For such cases, it is anticipated that the over-designing will result in radiators already of a sufficient area to accommodate the lower flow rate therethrough whilst still dissipating enough heat for satisfactory space heating.

The method of mixing of the return flow from the radiators with said other portion fed back from the boiler outlet to the boiler inlet (which said other portion is hereinafter referred to as the 'recirculated portion') preferably is chosen in dependence upon the basic operation of the central heating system. In the case of a room thermostat system, the mixing may simply be performed using a T-connector, the three ports of which are connected respectively to the radiator return flow, the recirculated portion and the flow to the boiler.

For such a case, the volume flow rate of the recirculated portion is advantageously controlled dependent upon the water temperature in a sensing chamber through which the economiser inlet flow and the recirculation flow both pass, but in separate compartments.

Conveniently, a temperature controlled valve for the recirculated portion flow is adjusted dependent upon the sensed temperature.

In the case of a boiler thermostat system, either of the above mixing method may be used, or a mixing chamber may be employed for the mixing of the return flow from the radiators and the recirculated portion. Such a mixing chamber may have three ports, one each for the radiator return flow, the recirculated portion and the boiler inlet flow respectively, and again it is advantageous to control the volume flow rate of the recirculated portion, but in this case by sensing the temperature in the mixing chamber. The sensed temperature may serve directly to control the volume flow into the chamber of the recirculated portion, by providing a temperature-controlled valve at the inlet to the chamber for the recirculated portion.

Whichever form of recirculated portion flow control is used, operation of the boiler as near as possible to its design parameters can be obtained for a broad range of operating conditions. In both cases, the systems operate to increase radiator water circulation rates to normal levels as the system temperature declines, and to reduce the circulation rate to the radiator system when the system temperature increases, whilst at all times maintaining 'normal' boiler circulation rates and temperatures.

The system may be used with fuels other than natural gas provided the materials of the economiser are sufficient to withstand the corrosive effect of the lower temperature flue gas. The action of the temperature control device always ensures that potentially corrosive thermal circumstances only occur in the condensing economiser, and not in the boiler itself.

In addition to the temperature-controlled flow rate valve for the recirculated portion, it may be desirable for some conditions-and especially those where the radiator system offers a low flow resistance-to have a throttling valve installed in the radiator return flow, to ensure the temperature of that return flow is low enough for the economiser properly to function. Depending upon the system used, such a throttling valve may be arranged in the sensing chamber or the mixing chamber, and be coupled to the temperature-controlled valve for the recirculated portion, whereby the radiator return flow is throttled to a greater extent as the temperature controlled valve opens more. The radiator return flow must never wholly be cut-off, and so the throttling valve should have minimum aperture size.

In addition to the valve arrangements mentioned above, a manually-presettable valve may be provided to permit setting of the maximum flow of the recirculated portion. This serves to allow the flow resistance of the recirculated portion to be appropriately balanced with the flow resistance of the radiator sys tem.

Most central heating systems are arranged also to heat domestic hot water, by circulating water heated in the boiler through a heat-exchanger coil in the hottank. Such circulation may be natural-that is, convection flow induced by the prevailing temperature differentials-or forced, using the same pump as does the radiator system.

When the method of this invention is to be used on a system also supplying domestic hot water, operation may be enhanced by passing the radiator return flow, following pre-heating in the economiser, through an auxiliary heat exchanger before mixing with the recirculated portion, the auxiliary heat exchanger being used to pre-heat the cold make-up water for the hot-tank. By providing suitable pipe-work and valves, advantageous operation can still be obtained in the event that only hot water is required (i.e. no central heating), by providing forced flow around a circuit containing the economiser and the auxiliary heat exchanger.

When operating, the boiler would heat the hot-tank in the normal way, and the heat recovered by the economiser may heat the cold make-up water, and it would be expected that that water should mostly be sufficiently cold to permit economiser operation in the condensing mode. Top-up heating to make up for heat losses from the hot water tank will not bring the economiser into operation.

As an alternative to using an auxiliary heat exchanger for the case where domestic hot water is supplied by the boiler, operation may proceed as has been described previously when both central heating and hot water is required, but when hot water only is required or is a priority, the flow through the economiser may be re-routed so as to be in series with, but down-stream of, the flow through the coil of the hot tank. This may be done by providing suitable pipe-work and valves to allow the change-over between space-and-water heating on the one hand and water heating only on the other, which valves are preferably electrically-operated automatically to provide the correct flow paths depending upon the user requirements of space and water heating.

When the economiser is arranged in this way in series for domestic hot water alone, condensation will not occur on the economiser other than at start-up from cold or after a heavy demand on the hot water supply.

According to a second aspect of this invention, there is provided a central heating boiler arrangement comprising a central heating boiler, an economiser fitted to the exhaust gas flue to extract heat from the flue gases, adjustable valve means to divide the boiler outlet flow into two portions one of which is circulated through the radiators and the other of which is fed back through a feed-back path arranged to feed back to the boiler inlet said other portion of the boiler outlet flow, and mixing means arranged to mix the flow along the boiler feed-back path with the radiator return flow and to feed the mixed flows to the boiler inlet, the radiator return flow passing through the economiser before entering the mixing means, whereby the boiler may operate within its design range whilst permitting the economiser to reduce the temperature of the flue gases to below the dew point thereof.

In order to achieve optimum operation, the boiler outlet flow should appropriately be divided into one portion for circulation through the radiators and- the other portion for feeding back directly to the boiler inlet (which said other portion is referred to herein as the "recirculated portion"). By proper division, the economiser may operate in a condensing mode as the return flow from the radiators will be at a temperature below the dew point of the exhaust gases from the boiler, but the radiator return flow, after pre-heating by the economiser and being mixed with the recirculated portion, will be at a suitable temperature for the boiler inlet, within the design range therefor. The adjustable valve means permits proper division to be obtained, and may be pre-settable, but preferably is dynamically adjustable to suit the operating conditions.

In the case of a boiler thermostat system, it is preferred for the mixing means to comprise a chamber having three ports, one each for the radiator return flow, recirculated portion and boiler inlet feed respectively. A temperature-controlled valve may then be adjusted automatically depending upon temperature sensed either at the boiler inlet or more preferably within the chamber, that valve adjusting the division ratio of the boiler outlet flow.

The valve preferably has a throttling effect on the flow through said feedback path and advantageously is mounted at the respective port of the chamber, for direct operation by temperature sensing means within the chamber. At the same time, it may be desirable to control flow through the radiator circuit with another valve; a throttling valve for operation in anti-phase with the valve for the recirculated portion may be mounted at the port for the radiator return flow.

In the case of a room thermostat system, the mixing means may comprise a simple Tconnector, with the three ports thereof connected as just-described for a mixing chamber.

However, the division ratio in this case preferably is adjusted by means of a temperaturecontrolled valve in the feed-back path, which valve is controlled by temperature sensed within a chamber through which both the radiator return flow and the feed-back flow pass but in separate compartments.

In either case, a separate pre-settable valve may be provided in the boiler feed back path, to limit the maximum flow rate therealong and so to allow balancing of the overall system having regard to the flow resistance of the radiator circuit.

With either system, domestic hot water may be produced in a conventional manner, either with natural or forced circulation. Preferably, however, an auxiliary heat exchanger is provided in the pipe-line from the economiser to the mixing means, the cold make-up water for the domestic hot water tank being piped through the auxiliary heat exchanger for preheating thereby.

When hot water only is required (i.e. no space heating) it is preferred for the flow through the economiser to be routed solely through the auxiliary heat exchanger, which thus acts as a heat sink for the recovered heat from the flue gases. In the case of a boiler thermostat system there may be provided an additional flow path from the pump outlet to the economiser inlet, with a first valve in that additional flow path and a second in the boiler feed pipe to the radiators, these two valves being operated in anti-phase so that when no space heating is required, the flow through the economiser is pumped solely through the auxiliary heat exchanger by opening the additional flow path, but when space heating also is required that flow path is closed and operation proceeds as has been described above.

In the case of a room thermostat system using a flow sensing chamber, a somewhat similar additional flow path and valve should be provided except that the additional flow path should connect to the inlet of the sensing chamber for radiator return flow, were the radiator system in operation. Again a valve in that additional flow path should be arranged for operation in anti-phase with a radiator circuit valve, as just-described above.

As an alternative the arrangements using an auxiliary heat exchanger for make-up water for the hot tank, the pipework may be arranged to have the economiser in series with the hottank heating coil whenever only hot water is required. In the case of either a boiler thermstat or room thermostat sysem, an additional valved flow path should be arranged from the down-stream side of the hot tank heating coil leading to the economiser inlet, the valve in this path being operated in anti-phase to two further valves one arranged in the normal return path between the hot tank heating coil and boiler inlet and the other in the radiator circulation path.

By way of example only, certain specific embodiments of central heating systems arranged to operate in accordance with this invention will now be described in detail, reference being made to the accompanying drawings, in which: Figures 1 and 1a are diagrammatic illustrations of a first embodiment of heating system of this invention which normally would be used with a boiler thermostat system (as hereinbefore defined), with either a natural circulation (Figure 1) or a forced circulation (Figure 1a) hot water system; Figures 2 and 2a are diagrammatic illustrations of a second embodiment of this invention and which normally would be used with a room thermostat system (also as defined hereinbefore), with either a natural circulation (Figure 2) or a forced circulation (Figure 2a) hot water system;; Figures 3 and 4 are illustrations of systems similar to those of Figures 1 and 2 respectively, but with the addition of a pre-heat arrangement for cold make-up water for a domestic hot water system; and Figures 4 and 5 are again illustrations of systems similar to those of Figures 1 and la respectively, but differently configured for advantageous operation when domestic hot water alone is required.

Throughout the following description of the arrangements shown in Figures 1 to 6, like reference characters denote like parts and such parts common to more than one system will be described only once.

Referring initially to Figure 1, there is shown a balanced-flue gas central heating boiler 10 of a conventional design and having an exhaust gas outlet 11. An economiser 12 is fitted to the outlet 11 by means of a transition duct 13. The radiators (not shown) are connected in parallel across points A and B, with the radiator return flow being fed into the economiser inlet through pipe 14. The water is preheated in the economiser and then is fed to a mixing chamber 15, through port 16. The outlet port 17 from the mixing chamber 15 is connected to the boiler inlet 18 through a pump 19 which causes circulation of water through the system.Hot water flow from the boiler outlet 20 is divided into two portions, one of which is supplied to the radiators and the other of which is passed through a recirculation flow path 21 leading to a third port 22 of the mixing chamber 15, that port 22 being fitted with a temperature controlled valve 23 which opens the port as the temperature in the mixing chamber 15 rises. The temperature-controlled valve 23 modulates the flow rate of the recirculated portion from the boiler outlet 20, such that the temperature in the mixing chamber 15 is allowed to rise to a maximum value once the temperature rises above a preset minimum. A manually adjustable valve 24 is fitted in the path 21, to permit pre-setting of the maximum recirculation flow rate.

In certain installations, it may be advantageous for valve 23 to be coupled to a throttling valve fitted to port 16, so that when valve 23 opens as the mixed temperature rises, the throttling valve closes-but the throttling valve must be arranged so as never completely to close port 16. This will permit there always to be a minimum flow through the radiator system.

The flow leaving outlet port 17 of the chamber 15, can thus be maintained at the original design flow rate for the boiler, drawing pre-heated return water from the economiser via port 16 as the system heats up from cold, up to the point at which the temperature-controlled valve 23 permits flow through the recirculation path 21. Thereafter, once the system has achieved normal working temperature, pump 19 draws a mixture of recirculated boiler outlet water via port 22 and pre-heated radiator return water via port 16. The recirculation flow path 21 has a low flow resistance as compared to the radiator system, and hence the recirculation water has a preference for this path; valve 24 is consequently provided to adjust the flow resistance of path 21 to suit the particular radiator circuit to which the system is connected.If the radiator circuit has a particularly low flow resistance, then a throttling valve fitted to port 16 and associated with valve 23 may be required, as mentioned above, to provide throttling of the cold (but pre-heated) return flow by positive means, to ensure the radiator cold return temperature is sufficiently low.

Also shown in Figure 1 is a domestic hot water system hot-tank 25 in a natural circulation system and having a cold make-up water inlet 26 and a hot water outlet 27, the tank 25 having an indirect heating coil 28 mounted therewithin. That coil 28 is connected by suitable pipework across the boiler inlet 18 and outlet 20, with a non-return valve 37 provided so as to prevent parallel flow through the boiler and hot water heating coil. A further valve 30 can be fitted in the pipe from the boiler outlet 20 to point B, to enable the radiator system to be closed down.

Figure 1a shows a modification of the arrangement of Figure 1, using a forced circulation domestic hot water system. In this case, the coil 28 is connected by suitable pipework between the boiler outlet 20 and pump 19 suction line, with a valve 29 in the coil inlet pipework to operate in anti-phase with the valve 30. Valves 29 and 30 are preferably both electricallyoperated and are provided to permit selection of whether the boiler is to provide domestic hot water.

The above-described arrangements are suitable for use in conjunction with a boiler thermostat system that is to say, a system where operation of the boiler burner and pump are primarily controlled by a boiler outlet thermostat, and the heating for individual rooms is controlled by flow thermostats on each radiator. For a room thermostat system, where operation of the boiler burner and pump is controlled primarily by at least one thermostat fitted at a suitable point, the arrangement of either Figure 2 (natural circulation domestic hot water) or Figure 2a (forced circulation domestic hot water) may be employed. Here, the mixing chamber 15 (Figure 1) is replaced by a simple T-connector 31, whereby the return flow from the radiators (at point A) may be mixed with the flow along the recirculation path 21, for feeding through the pump 19 to the boiler inlet 18.

The return flow from the radiators is arranged to pass through a temperature sensing chamber 32 before being supplied to the inlet of the economiser 12; that chamber 32 has a separate duct 33 passing therethrough, that duct being arranged to form a part of the recirculation flow path 21, as shown. At the inlet port 34 to the duct 33, there is arranged a valve 35, opened and closed depending upon the temperature sensed within the chamber 32. For this purpose, a suitable temperature-sensing means 36 may be mounted within the chamber 32, that temperature sensing means being coupled directly to the valve 35.

Referring now to Figure 3, there is shown a modification of the system of Figure 1, arranged to provide pre-heat for the cold makeup water for the domestic hot water system.

In this system, there is provided an auxiliary heat exchanger 40, the heat coil 41 of which is arranged in the radiator return flow path, between the economiser 12 and the mixing chamber 15. Cold make-up water for the hottank is passed through the auxiliary heat exchanger 40, before being fed to the inlet 26 of the hot tank 25. Also, there is provided an additional flow path 42, extending from the pump outlet 19 to the economiser inlet pipe 14, that additional flow path 42 being provided with a valve 43.

When the just-described system is to be used for both space heating and domestic hot water, valve 30 is opened and valve 43 is closed; operation then proceeds as has been described with reference to Figure 1, except that the cold make-up water for the domestic hot tank is to some extent pre-heated by the auxiliary heat exchanger 40. When however no space heating is required, valve 30 is closed and valve 43 is opened; the boiler then heats the hot tank 25 in the usual way but the heat recovered by the economiser is used solely to pre-heat the cold make-up water.

The economiser will not operate when the boiler runs simply to make good heat losses from the hot tank, but if hot water is drawn off resulting in the need for the supply of cold make-up water together with boiler operation, then the cold water in the auxiliary heat exchanger 40 may provide a heat sink at a sufficiently low temperature to enable the economiser again to operate in the condensing mode.

The valves 30 and 43 preferably are both electrically operated and suitably controlled electrically or electronically, in order to prevent both valves being either open or closed. At all times, these valves must be operated in antiphase.

Figure 4 shows the system of Figure 2 also fitted with an auxiliary heat exchanger 40, for pre-heating of cold make-up water for the hot tank. Operation of this system is essentially the same as that just-described above, with reference to Figure 3, in that opening of valve 43 and closing valve 30, allows heat recovered in the economiser to be used for preheating the cold make-up water when no space heating is required. When operating with a forced circulation domestic hot water system as shown in Figures 1a and 2a, the pre-heating of the cold make-up water for the hot tank is essentially as described with reference to Figures 3 and 4.

Figure 5 shows an alternative arrangement, to permit economiser operation when no space heating is required. In this system, the outlet from the hot tank heating coil 28 is connected to two flow paths 50 and 51, respectively provided with valves 52 and 53.

Flow path 50 leads back to the boiler inlet 18, whereas flow path 51 connects with pipe 14 leading to the economiser inlet; a nonreturn valve 55 may be necessary for the radiator circuit and may be fitted for example adjacent point A, as shown. A further flow path 54 is arranged between pipe 50 (downstream of valve 52) and the pipe connecting the economiser outlet with port 16 of chamber 15, equipped with a non-return valve 56.

When both space heating and hot water are required, valves 30 and 52 are opened and valve 53 is closed; operation then proceeds in the same manner as has been described above in connection with Figure 1. During this condition of operation, there is no flow along paths 51 and 54. When however no space heating is required, valves 30 and 52 are closed and valve 53 is opened. Then, the return flow from the hot tank heating coil 28 passes through the economiser 12 before entering the boiler inlet once more, along flow path 54, via nonreturn valves 56 and 37. Operation in this way will result in the economiser not operating in a condensing mode unless heating the domestic hot water from cold, but despite this there will always be some heat recovery by the economiser whenever the boiler operates to top up the domestic hot water, so improving the efficiency.

Figure 6 shows an arrangement similar to that of Figure 5, but as applied to a forced circulation domestic hot water system, as shown in Figure la. In this case, valve 52 is located in the flow path between the housing coil 28 and pump inlet 19.

When both space heating and hot water are required, valves 29, 30 and 52 are all opened and valve 53 is closed; operation then proceeds in the same manner as described with reference to Figure la, with no flow along path 54. In the case where no space heating is required or the domestic hot water has priority, valves 30 and 52 are closed and valves 53 and 29 are opened. The operation then proceeds as described with reference to Figure 5, with the economiser heating flow passing to the boiler, as described with reference to Figure 1a.

When operating with a room thermostat system as shown in Figures 2 and 2a, the valve and pipe arrangements are essentially the same as shown in Figures 5 and 6.

Claims (20)

1. A method of operating a central heating boiler fitted with a flue gas condensation economiser (as defined herein), comprising dividing the hot water flow from the boiler outlet into two portions one of which portions is circulated through the radiators and the other of which portions is fed back to the inlet to the boiler, passing the cold water return from the radiators through the economiser before mixing that water with said other portion fed back to the boiler inlet from the boiler outlet, and adjusting the division ratio of the boiler outlet flow into said one and other portions such that the flow rate through, inlet temperature to, and temperature rise across the boiler are all within the design ranges therefor whilst allowing a radiator return temperature substantially below the dew point of the flue gases.

2. A method according to claim 1, in which the return flow from the radiators is mixed with the recirculated portion (as defined herein) by using a T-connector, the three ports of which are connected respectively to the radiator return flow the recirculated portion and the flow to the boiler.

3. A method according to claim 2, in which the volume flow rate of the recirculated portion is controlled dependent upon the water temperature in a sensing chamber through which the economiser inlet flow and the recirculation flow both pass, but in separate compartments.

4. A method according to claim 3, in which, a temperature controlled valve for the recirculated portion flow is adjusted dependent upon the sensed temperature.

5. A method according to claim 1, in which a mixing chamber is employed for mixing the return flow from the radiators and the recirculated portion, which mixing chamber has three ports, one each for the radiator return flow, the recirculated portion and the boiler inlet flow respectively.

6. A method according to claim 5, in which the volume flow rate of the recirculated portion is controlled, depending upon the sensed temperature in the mixing chamber.

7. A method according to any of the preceding claims, and in which a temperaturecontrolled flow rate valve is provided for the recirculated portion, whereon a throttling valve is installed in the radiator return flow, to ensure the temperature of that return flow is low enough for the economiser properly to function.

8. A method according to claim 7, in which the radiator return flow is never wholly cut-off by the throttling- valve.

9. A method according to any of the preceding claims, wherein a manually-presettable valve is provided to permit setting of the maximum flow of the recirculated portion.

10. A method according to any of the preceding claims and wherein domestic hot water is also supplied thereby, in which the radiator return flow, following pre-heating in the economiser is passed through an auxiliary heat exchanger before being mixed with the recirculated portion, the auxiliary heat exchanger being used to preheat the cold make-up water for the hot-tank.

11. A method according to any of claims 1 to 9, in which the flow through the economiser is re-routed so as to be in series with, but down-stream of, the flow through the coil of the hot-tank.

12. A method according to claim 11, in which the rerouting is performed by providing suitable pipework and valves to allow the change-over between space-and-water heating on the one hand and water heating only on the other, which valves are electrically-operated automatically to provide the correct flow paths depending upon the user requirements of space and water heating.

13. A central heating boiler arrangement comprising a central heating boiler, an economiser fitted to the exhaust gas flue to extract heat from the flue gases, adjustable valve means to divide the boiler outlet flow into two portions one of which is circulated through the radiators and the other of which is fed back through a feed-back path arranged to feed back to the boiler inlet said other portion of the boiler outlet flow, and mixing means arranged to mix the flow along the boiler feed-back path with the radiator return flow and to feed the mixed flows to the boiler inlet, the radiator return flow passing through the economiser before entering the mixing means, whereby the boiler may operate within its design range whilst permitting the economiser to reduce the temperatures of the flue gases to below the dew point thereof.

14. A boiler arrangement according to claim 13, wherein the mixing means comprises a chamber having three ports, one each for the radiator return flow, recirculated portion and boiler inlet feed respectively.

15. A boiler arrangement according to claim 14, wherein a temperature-controlled valve is provided to adjust the division ratio of the boiler outlet flow, the valve being adjusted automatically depending upon the temperature sensed either at the boiler inlet or more preferably within the chamber.

16. A boiler arrangement according to claim 15, wherein a second valve is provided to control flow through the radiator circuit, the second valve operating in anti-phase with the temperature-controlled valve and being mounted at the port for the radiator return flow.

17. A boiler arrangement according to any of claims 13 to 16, wherein a separate presettable valve is provided in the boiler feed back path to limit the maximum flow rate therealong and so to allow balancing of the overall system having regard to the flow resistance of the radiator circuit.

18. A boiler arrangement according to any of claims 13 to 17, wherein an auxiliary heat exchanger is provided in the pipe-line from the economiser to the mixing means, the cold make-up water for the domestic hot water tank being piped through the auxiliary heat exchanger for pre-heating thereby.

19. A method of operating a central heating boiler fitted with a flue gas condensation economiser, substantially as hereinbefore described with reference to the accompanying drawings.

20. A central heating boiler arrangement including a flue gas condensation economiser, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.