GB2304876A - Control of water flow in a condensing boiler heating system - Google Patents

Abstract

Water flow is directed in a heating system with a condensing boiler such that on start up substantially all the flow of primary water from the boiler is used to heat water in a secondary store 10 via a heat exchanger 20 until a first degree of heating has occurred, then dividing the flow of water between the secondary water store and a space heating system and finally, after a second degree of heating, directing substantially all the flow of primary water to the space heating system. The return water is passed through a heat exchanger with the flue gases and the unit operates most efficiently when the return water is cool enough to condense the steam in the flue gas; the first degree of heating may be when the return water is too warm to condense the steam. A heat sink may be provided either in the secondary store or the primary water circuit to help reduce the temperature of the return water.

Description

IMPROVEMENTS IN WATER HEATING This invention relates to improvements in water heating, and in particular to indirect wet heating systems which also provide a supply of hot water.

The rising cost of energy, environmental concerns and the higher levels of comfort demanded by consumers have led to continuing increases in the efficiency of domestic and commercial heating systems. Condensing boilers are among the most efficient designs currently widely available. A condensing boiler includes two heat exchangers, one of which is heated directly by the burning fuel, typically natural gas, while the second is heated by the exhaust or flue gases after they have passed over the first heat exchanger. The larger secondary heat exchanger operates most efficiently when it surface temperature is sufficiently low to cause the steam in the flue gas to condense, which occurs at a temperature of around 54 degrees centigrade.This transformation of latent heat to sensible heat provides a significant increase in boiler efficiency, and manufacturers claim that efficiencies higher than 92% may be achieved when using such boilers in full condensing mode. This requires that the temperature of the return water to the boiler, which flows through the secondary heat exchanger, is relatively low, however in most existing heating systems the return temperature is too high (around 60 degrees centigrade) to achieve this effect, and the corresponding higher efficiency levels.

This failure to achieve the potentially high efficiency levels is particularly acute in systems where the primary water from the boiler is used to heat secondary water to provide a hot water supply. In conventional water heating, the heated primary water is fed from the boiler through a heating coil located within an insulated water cylinder: The volume of heated secondary water contained in the cylinder is then available for use. In warmer weather, when the boiler is being used solely to provide hot water, the primary water is circulated only between the boiler and the hot water cylinder. During much of this time the return temperature of the primary water to the boiler is relatively high, such that the boiler operates well below its potential efficiency.

When a conventional space and water heating system is switched on, the system is arranged to first heat the secondary water to the desired temperature, and only then direct the heated primary water to the radiators for space heating. Thus the consumer may have to wait some time before obtaining the benefit of the space heating, and initially at least the boiler is likely to be operating inefficiently.

It is among the objects of embodiments of the present invention to obviate or mitigate these disadvantages.

According to the present invention there is provided a control system for a heating system having a condensing boiler for heating primary water for use in space heating and for heating secondary water in a store via a heat exchanger to provide a hot water supply, the control system comprising: valve means for controlling the flow of heated primary water from the boiler to the space heating system and to the secondary water store; temperature sensing means for detecting one or both of the return temperature of the primary water from the secondary water store and the secondary water temperature; and means for actuating the valve means, the arrangement being such that, (i) on start up, the valve means is configured to direct substantially all of the flow of primary water to the secondary water store, and (ii) on the temperature sensing means detecting a temperature indicating a predetermined first degree of heating of the secondary water the valve means is actuated to divide the flow of primary water between the secondary water store and the space heating system, and (iii) on the temperature sensing means detecting a temperature indicating a predetermined second degree of heating of the secondary water the valve means is actuated to direct substantially all of the flow of primary water to the space heating system.

The invention also relates to a secondary water store in combination with a control system, to a heating system and to a method of operating a heating system.

In use, the control system allows initial heating of the secondary water in a relatively efficient manner, the difference in temperature between the primary water and the secondary water providing for efficient heat transfer through the wall of the heat exchanger, typically a heating coil, and assuring that the return temperature to the boiler is relatively low, such that the boiler will operate in condensing mode. After a short period, typically only a few minutes, the rising temperature of the secondary water causes the efficiency of the heat transfer across the coil wall to fall, and the return temperature of the primary water to rise.At or before this point the valve means is actuated such that the primary water is also circulated through the space heating system, such that the return temperature to the boiler remains relatively low, and efficiency high, and space heating is available within a relatively short period. Once the temperature of the secondary water reaches the desired temperature the valve means directs all of the primary water into the space heating system. Of course once the room temperature reaches the desired level the boiler will shut down.

Preferably, the system includes means for controlling the primary fluid circulating pump and (i) at start-up the pump is operated at a relatively low rate which is increased when (ii) the valve means also directs primary water into the space heating system.

Preferably also, the control system is provided in combination with means for agitating the secondary water during heating, thus providing a more even temperature distribution through the secondary water and also improving heat transfer from the primary coil. This allows provision of a relatively small volume of heated secondary water; as the average temperature of the water is higher than in a conventional hot water cylinder less hot water is required to, for example, fill a bath at a desired temperature.

Further, in conventional hot water cylinders the water tends to stratify, with a volume of relatively warm water sitting above a volume of relatively cold water. The average temperature in the water is thus likely to be lower than the temperature measured at the upper portion of the cylinder, and the colder body of water in the lower portion of the cylinder will tend to cool the warmer water, causing more frequent firing of the boiler. It is desirable that the boiler be fired infrequently as, at each firing, the boiler structure must be heated before water heating commences. The provision of agitating means also prevents stagnation within the secondary water store and thus protects occupants from legionella bacteria.

Preferably also, the agitating means is in the form of a circulating pump for bringing water from an lower portion of the cylinder to an upper portion of the cylinder.

Preferably, also, the control system is provided in combination with heat rejection means or a heat sink, typically for use when only secondary water heating is required, to maintain a low primary water return temperature, and maintaining the high operating efficiency of the boiler. The heat sink may receive primary or, most preferably, secondary water. Conveniently, the heat sink is in the form of an arrangement that may be used for drying washing, such as a towel rail, or a heating arrangement for a loft space. The rejected heat may thus be employed usefully and, for example, may reduce dependence on tumble dryers and the like. The supply of secondary water to the heat rejection means may be initiated by pressurising the hot water cylinder, for example by increasing the speed of a circulating pump.

Preferably also, the control system is provided in combination with a secondary water store, typically in the form of a hot water cylinder, in a unitary form. Most preferably, the unit is provided within a casing, which may be wheel or castor mounted. The connections between the unit and the boiler and radiators are preferably provided through a single jig.

These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an elevational view of a hot water cylinder in accordance with a preferred embodiment of the present invention, shown within a casing; and Figure 2 is an elevational view of a cylinder of Figure 1, shown removed from the casing.

The drawings illustrate a hot water cylinder 10 which is mounted, by appropriate brackets 12, within a casing 14 with a removable front cover 16. The casing 14 is mounted on lockable wheels 18, which may be raised inside the lower end of the casing 14 once the casing 14 is in a desired position. In this preferred embodiment the casing 14 contains the cylinder 10 and the control system to allow the cylinder 10 to be used in combination with a heating boiler having relatively straightforward controls. The cylinder 10 and associated controls thus forms a self contained unit which is relatively easy to install and remove. To facilitate installation, the connections between the cylinders 10 and the boiler and wet heating systems are provided through a single jig (not shown).

Heated primary water from the boiler is circulated through a heating coil 20 linked to the boiler by primary ports 22, 23. Alternative or supplementary heating may also be provided by an electric immersion heater 24. The secondary water is supplied through a suitable port 26 in the lower portion of the cylinder, and is drawn off from a port 27 at the top of the cylinder, a leg of the draw-off pipe 28 leading to a suitable expansion vent. In this particular embodiment the cold water supply to the port 26 is direct from the mains supply, and this is passed through a double check valve 30, a pressure regulator 31, and a non return valve 32. In certain jurisdictions such a supply is not permitted, and the secondary water must be supplied from a feed tank, isolated from the mains supply.

The primary water is circulated by means of an electric pump 34, and downstream of the pump 34 is a primary flow temperature sensor 36. From the pump, the primary water passes through a flow control valve 38 which determines whether the primary water flows to the port 22, and into the coil 20, or to the wet heating system, through pipe leg 39. A by-pass balancing valve 40 is also provided to link the primary water inlet with the primary return piping 42 leading from the port 23. The return piping also includes a primary water return temperature sensor 44.

Mounted on the cylinder 10 is an agitating pump 46 which circulates secondary water between the upper and lower portions of the cylinder 10. The cylinder 10 also provides auxiliary ports 48, 49 which allow secondary water to pass to a heat sink (not shown) which may be in the form of a towel rail, heating for a drying cupboard, or a loft space heater.

On firing the boiler, heated primary water is drawn from the boiler by the pump 34. Initially, the valve 38 is configured such that all of the heated primary water passes through the port 22 to be circulated through the heating coil 20. The significant temperature difference between the secondary water in the cylinder 10 and the heated primary water is such that there is efficient heat transfer and the temperature of the water passing from the coil 20 through the port 23 is relatively low, as detected by the temperature sensor 44.

The boiler thus operates efficiently, in condensing mode, and the temperature of the secondary water rises rapidly. However, as the temperature of the secondary water (as detected by the immersion sensor 50) rises the temperature of the primary return water will also rise, soon reaching a level at which the boiler no longer operates in full condensing mode. Before this situation occurs, the valve 38 redirects a portion of the flow of the heated primary water to the wet heating system. On this occurring the rate of the pump 34 is also increased. This results in the primary water return temperature dropping once more, such that the boiler will continue to operate in condensing mode.The valve 38 remains in this configuration until the temperature of the secondary water reaches the desired final level, at which point the valve 38 is reconfigured such that all of the primary water passes into the wet heating system. On the room temperature within the dwelling reaching the desired level, the boiler shuts down. The boiler will also shut down prior to this point if the primary water temperature sensor 36 detects that the primary water flowing from the boiler is higher than a predetermined temperature, typically 820C.

The control of the valve 38 ensures that the boiler will operate in the condensing mode over an extended period, and will thus operate more efficiently. Also, space heating is available relatively soon after the boiler fires up, in contrast to conventional systems which initially will only supply heated primary water to the hot water cylinder until the secondary water has reached the desired final temperature.

While the secondary water is being heated1 the agitating pump 46 operates to circulate the secondary water around the cylinder 10. This results in a more even temperature distribution over the height of the cylinder 10, such that heat transfer from the coil 20 is more efficient. Also, the average temperature of the water within the cylinder 10 will tend to be higher than in a conventional hot water cylinder, in which temperature stratification may occur; in a conventional cylinder a relatively warm body of water sits above a cooler body of water and the hot water sensor, such as sensor 50, will detect the temperature of the warmer water, which may not be representative of the average temperature of the secondary water in the cylinder.

In warmer weather, when space heating is not required, the valve 38 will remain in a configuration such that all of the heated primary water is supplied to the coil 20.

Thus, after an initial heating period, the temperature of the primary return water will rise to such an extent such that the boiler will begin to operate less efficiently. In the event of such an occurrence, the circulation rate of the pump 46 may be increased to pressurise the cylinder 10, and cause secondary water to pass from the auxiliary port 48 to the heat sink. The cooler return water from the heat sink passes into the cylinder through the port 49, thus lowering the temperature of the secondary water and allowing the boiler to operate more efficiently. As noted above, this rejected heat may be used, for example, to dry washing.

From the above-described embodiment it will be clear to those of skill in the art that the system will provide significant increases in operating efficiency of a condensing boiler, and will also provide increased comfort and convenience for the system users.

For ease of understanding, the operating controls forming part of the system have not been described in detail, as these may be based on conventional controls, such as the Satchwell Controls Ltd BAS 2000 Series control system in combination with a Satchwell IAC 400 unit, set to operate the system as outlined above.

It will also be clear to those of skill in the art that the above-described embodiment is merely exemplary of the present invention, and that various modifications and improvements may be made thereto, without departing from the scope of the present invention. In one modification to the illustrated embodiment, the heat sink may be connected to the primary water, such that primary water passing from the coil 20 is used to heat a towel rail or the like before returning to the boiler. Further, the control of flow of primary or secondary water to the heat sink may be achieved by a suitable valve arrangement externally of the cylinder controlled by, for example, the primary water return temperature, and may utilise convection or an auxiliary pump to circulate water through the heat sink.

Claims (16)

1. A control system for a heating system having a condensing boiler for heating primary water for use in space heating and for heating secondary water in a store via a heat exchanger to provide a hot water supply, the control system comprising: valve means for controlling the flow of heated primary water from the boiler to the space heating system and to the secondary water store; temperature sensing means for detecting one or both of the return temperature of the primary water from the secondary water store and the secondary water temperature; and means for actuating the valve means, the arrangement being such that, (i) on start up, the valve means is configured to direct substantially all of the flow of primary water to the secondary water store, and (ii) on the temperature sensing means detecting a temperature indicating a predetermined first degree of heating of the secondary water the valve means is actuated to divide the flow of primary water between the secondary water store and the space heating system, and (iii) on the temperature sensing means detecting a temperature indicating a predetermined second degree of heating of the secondary water the valve means is actuated to direct substantially all of the flow of primary water to the space heating system.

2. The control system of claim 1, further including means for controlling a primary fluid circulating pump such that (i) at start up the pump is operated at a relatively low rate which is increased when (ii) the valve means also directs primary water into the space heating system.

3. The control system of any of the preceding claims in combination with means for agitating the secondary water during heating.

4. The control system of claim 3, wherein the agitating means is in the form of a circulating pump for bringing water from an lower portion of the secondary store to an upper portion of the store.

5. The control system of any of the preceding claims in combination with heat rejection means or a heat sink, to maintain a low primary water return temperature, and maintaining the high operating efficiency of the boiler.

6. The control system of claim 5, wherein the heat sink receives primary water.

7. The control system of claim 5, wherein the heat sink receives secondary water.

8. The control system of claim 5, 6 or 7, wherein the heat sink is in the form of an arrangement that may be used for drying washing or a heating arrangement for a loft space.

9. The control system of claim 7, wherein the supply of secondary water to the heat rejection means is initiated by pressurising the secondary water store.

10. A secondary water store in combination with a control system in accordance with any of the preceding claims.

11. The combination of claim 10, wherein the control system and secondary water store are in a unitary form.

12. The combination of claim 11, wherein the unit is provided within a wheel-mounted casing.

13. The combination of claim 11 or 12, wherein connections between the unit and the boiler and radiators are provided through a single jig.

14. A heating system in combination with a control system in accordance with any of the preceding claims.

15. A method of operating a heating system having a condensing boiler for heating primary water for use in space heating and for heating secondary water in a store via a heat exchanger to provide a hot water supply, the method comprising the steps: (i) on start up, directing substantially all of the flow of primary water to the secondary water store; (ii) following a predetermined first degree of heating of the secondary water dividing the flow of primary water between the secondary water store and the space heating system; and (iii) following a predetermined second degree of heating of the secondary water directing substantially all of the flow of primary water to the space heating system.

16. A secondary water store in combination with a control system substantially as described therein and as illustrated in the accompanying drawings.