GB2131526A

GB2131526A - Space heating system and method

Info

Publication number: GB2131526A
Application number: GB08235386A
Authority: GB
Inventors: Geoffrey White
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-12-11
Filing date: 1982-12-11
Publication date: 1984-06-20
Also published as: GB2131526B

Abstract

A heating system has a water storage tank 1 and heated water may be pumped from the top of the tank through piping 3 to hot water radiators 4, while cooled water from the radiators is returned to the bottom of the tank through piping 5. The combined capacities and positions in the tank 1 of immersion heaters 6, 7 are selected so that water can be heated to a pre-set maximum temperature during a cheap rate power supply period only by initially using both heaters, the immersion heaters 7 being usable to boost the water temperature during a peak rate power supply period if required. Thermostat T1 is at a calculated distance from the top of the tank to switch on immersion heaters 7 when appropriate. <IMAGE>

Description

SPECIFICATION Space heating system and method This invention relates to a space heating system and method and more particularly to such a system having a store for a fluid heating medium, heat being transferred from the medium to spaceheat exchanger means, for example hot water radiators.

In such systems in which the fluid heating medium is heated by an electric heater it is known to make use of cheap rate off-peak electricity which is usually supplied for a specified time period during the night to provide heating during other periods, and in particular during high rate peak periods of the day. The use of such off-peak power supplies is assuming a greater role of importance with current rising fuel prices and emphasis on saving the World's energy supplies.

It is known to utilize off-peak electricity during the night to heat a store of water which is subsequently drawn off or pumped to radiators during the day and circulated around the system back to the store. In order to provide a system which does not have a large, cumbersome storage tank a system has been proposed in which the amount of hot water stored will not meet a full day's demand on the coldest days of the year but having supplementary electric heaters which utilize the more expensive rate electricity during the day to provide any extra heat required to meet the consumers need during the day, and preventing the store from becoming exhausted of heat during the day by an amount which is greater than can be replenished during the following off-peak period.

It is an object of the present invention to provide a space heating system and method in which the need for a heating arrangement supplementary to the heating arrangement required to heat the fluid heating medium in the store to a pre-set maximum value is obviated.

According to the present invention there is provided a space heating system comprising a store for storing a fluid heating medium, space heat exchanger means, for example hot water radiators, first and second heating arrangements being provided in the store to heat the fluid heating medium, the first heating arrangement, preferably being positioned at or towards the bottom of the store, the second heating arrangement being positioned, preferably, in between the bottom and top of the store at a level, preferably, above the second heating arrangement, the combined capacities of the first and second heating arrangements and their positions in the store being selected so that the fluid heating medium in the store may be heated on combined operation of the first and second heating means to a pre-set maximum temperature during a first time period, for example an off-peak cheap rate power supply period, each of the first and second heating arrangements being individually insufficient to heat the fluid heating medium in the store to said pre-set maximum temperature during the first time period, the second heating arrangement being utilizable to boost the temperature of fluid medium thereabove in the store at a stage during a second time period, for example a peak high rate power supply period, so that the heat being drawn off from the store to the heat exchanger means is sufficient for the requirements of the system.

By the present invention there is available an advantageous saving in the total heater capacity needed for particular system operating requirements than hitherto available, and where the heating arrangements are mains electrical heaters there may be additional savings in costs of controls, heater cables and fittings and accessories necessary to equip the heaters in the store.

Preferably, the fluid medium itself is drawn off from the store to the heat exchanger means and, preferably, a thermostat is provided at a calculated distance from the top of the store (usually but not essentially located vertically above the second heating arrangement) which switches on the second heating arrangement at said stage during the second time period when a predetermined amount of fluid heating medium has been drawn off from the store. This distance is calculated so that the heated fluid medium in the store is not completely exhausted when the second heating arrangement is switched on, so that the supply of fluid medium at the correct temperature to the heat exchanger means is not interrupted and enough time is allowed for the second heating arrangement to heat more fluid in the store to the required temperature before being drawn off from the store.

In one embodiment of the system the store comprises a storage tank in which both the first and second heating arrangements are located.

Preferably, the first heating arrangement is offset to one side of the tank and the second heating arrangement is offset to a side of the tank opposite said first mentioned side, the first and second heating arrangements, preferably being in non-overlapping relationship. A baffle plate may be provided which is arranged generally vertically in the tank to divide off a compartment within the tank above the second heating arrangement yet allow communication of fluid heating medium from the top of the compartment to the remainder of the store. Conveniently, the volume of the compartment may often be about one quarter of the volume of the store but of course this will depend upon the technical characterisation of the system. The volume of the compartment may conveniently lie in the range of 2 to 1/6 the volume of the store or even less in some embodiments.

In another embodiment of the system the store comprises two storage tanks connected in parallel, the first and second heating arrangements being provided in respective tanks.

Preferably, the two tanks are of identical size and a compartment is defined by one of the tanks above the second heating arrangement, which compartment is, conveniently in one embodiment about one quarter the volume of the two storage tanks. Once again the volume of the compartment may be in the range 2 to 1/6 the volume of the store or even less.

Further according to the present invention there is provided a method of heating space comprising: (a) heating fluid heating medium in a store by first and second heating arrangements provided in the store to a pre-set maximum temperature value during a first time period, for example an off-peak cheap rate power supply period, (b) during a second time period, for example a high rate peak power-supply period, drawing off heat from fluid heating medium in the store to space heat exchanger means to heat a space, for example a room of a house, whilst both the first and second heating arrangements are inoperative to supply heat to the fluid medium in the store, (c) at a stage in the second time period before the heat being drawn from the store to said heat exchanger means is insufficient to meet required needs, boosting the temperature of fluid medium above the second heating arrangement in the store, using the second heating arrangement, sufficiently to meet the required needs and so that flow of heat at the required temperature to the heat exchanger means is not interrupted.

Preferably, the method also comprises: (d) drawing heated fluid medium from the store to the space heat exchanger means and switching on the second heating arrangement at said stage in the second time period when the store is nearly exhausted of fluid heating medium at the pre-set maximum temperature, and thereby heating only part of the cooled fluid medium in the store returned from the space heat exchanger means in a compartment of the store which is, preferably, within the range 2 to 1/6 the volume of the store.

Conveniently, the fluid heating medium in the compartment may only be heated to a system flow temperature rather than to said pre-set maximum flow temperature.

Still further according to the present invention there is provided a space heating system comprising a store, which in use, holds a fluid heating medium for circulation around the system and which medium is heated in the store, a first heating arrangement positioned towards the bottom of the store and offset to one side thereof, a second heating arrangement positioned between the top and bottom of the store and above the first heating arrangement and offset to a side of the store opposite to said first mentioned side, the arrangement being such that the first and second heating arrangements are adapted to heat the fluid heating medium in the store to a predetermined maximum value during a first time period, and at some stage during a second time period when the fluid heating medium is being drawn off from the store and circulated through the system while the first heating arrangement is inoperative the second heating arrangement is adapted to boost the temperature of some of the fluid heating medium in the store which is located in a compartment thereof defined above the second heating arrangement, the return of fluid heating medium from a space heat exchanger means of the system being input towards the bottom of the store to stratify the fluid heating medium in the tank until the temperature of fluid heating medium in the compartment in the store above the second heating means is boosted thereby during said second time period whence the stratification in the compartment above the second heating arrangement is degraded.

The present invention also provides a method of heating space using the space heating system as described in the immediately preceding paragraph in which the fluid heating medium in the compartment is raised to a system flow temperature below the maximum pre-set temperature during said boost period.

An embodiment of a space heating system and method in accordance with the present invention will now be described, by way of example only, with reference to the accompanying much simplified schematic drawings in which: Figure 1 shows an overall view of the space heating system, and Figure 2 shows a modified heated water store of the system shown in Figure 1.

Figure 1 shows a store in the form of a single tank 1 for storing a fluid heating medium 2, in this instance water which is circulated around the space heating system.

As appropriate, heated water may be drawn off or pumped from the top of the tank 1 through piping 3 to space heat exchanger means in the form of hot-water radiators 4 where heat is given up from the water to the space surrounding the radiators, for example a living room of a house.

The water which has given up heat by way of the radiators 4 is returned to the bottom of the tank 1 by way of piping 5.

While heated water is supplied via a pump (not shown) to radiators 4 from the tank 1, cooled water in return pipe 5 may be mixed with heated water in the supply pipe 3 to reduce the initial temperature of water supplied to the radiators if this is too high and by way of appropriate piping (not shown) and mixing valve (not shown) in a manner known per se.

The tank 1 is provided with a first heating arrangement 6 comprising, in this instance, immersion heaters which are located at the bottom of the tank. A thermostate T, is positioned just above as shown, or alternatively, to one side of, heating arrangement 6. The heating arrangement 6 is offset to one side of the tank (to the left in Figure 1) and a second heating arrangement 7 (also immersion heaters) is located at a level about halfway up the tank; the arrangement 7 is offset to the side of the tank opposite the first-mentioned side (to the right of Figure 1) and the heater arrangements 6 and 7 do not overlap. A thermostate T2 is positioned just above or adjacent to heating arrangement 7.

A further thermostate Td is located vertically above thermostate T2 at a pre-set distance from the top of the tank 1.

A vertical baffle plate 8 is located in the tank 1 which divides the upper region of the store into two compartments 2a, 2b. The plate 8 is spaced from the top at the top of the plate 8 to allow a limited control flow of water in the upper region of the tank from compartment 2a to compartment 2b. The water outlet from the tank 1 which communicates with piping 3 communicates with compartment 2a. Heating arrangement 7 and thermostats T2 and Td are within compartment 2a; the baffle plate 8 extends at least as far as or, in this instance, just below the heating arrangement 7. The baffle plate 8 may be located in the tank 1 by welding two opposed side edges of the plate to the walls of the tank.

In operation of the space heating system, during a cheap rate off-peak electricity power supply period (which is usually during the night) the water in the tank is heated to a pre-set maximum temperature which is, in this instance, about 950C. Of course in some systems this preset temperature may be adjustable. The heat exchanger means or radiators 4 are arranged to operate on a lower system flow temperature which is typically 700C. In order for the radiators 4 to be supplied with water at about 700C when the temperature of water in the tank is 950C the water in piping 3 is mixed as aforesaid with cooled water returning from the radiators to the tank 1. The cooled water is typically at temperatures of about 600C or 550C. During the off-peak period both heating arrangement 6 and 7 need to be switched on to heat the water to the pre-set maximum temperature of 950C.

During the off-peak period, and when the space heating system is not supplying heat to the radiators as is usual during the night, the system functions as follows. Both heating arrangements 6 and 7 are switched on and initially the water in compartment 2a above heater arrangement 7 heats up more quickly than the remainder of water in the tank heated by arrangement 6. This is because the volume of water above heater arrangement 7 is much smaller than the volume of water being heated primarily by heater arrangement 6. A temperature inbalance thus develops across the baffle plate 8 which results in a current flow through the space or opening between the plate 8 and the top of the tank 1.The current flow is from compartment 2a to compartment 2b so that the hotter water above heating arrangement 7 flows away from compartment 2a at the top thereof and is replaced by cooler water rising to the heating arrangement 7 from the lower region of the tank 1. In this way the heater arrangement 6 and 7 cooperate to heat the water in the tank until the maximum pre-set temperature is reached. The thermostats T1 and T2 finally switch off their associated heater arrangements 6 and 7 respectively as the maximum pre-set temperature is attained.

Once the off-peak period is over, for example during the day, and when there is a consumer requirement for the space heating system to be operational the system functions as follows.

Water is drawn from the tank at the maximum pre-set temperature (950C in this instance) from the top of the tank via piping 3 where it is mixed with cooler water returning from the radiators 4 via mixing valve (not shown) as aforesaid until it is at the system flow temperature (700C). Water at the system flow temperature is supplied to the radiators 4 where it is cooled and returned to the bottom of the tank via piping 5. During this period neither heater arrangement 6 or 7 is switched on.

As the store of heated water is used up from the top it is progressively replaced by cooler water returning from the radiators 4 at the bottom of the tank. If the tank were filled with returned water at the temperature of 550C or 600C, all the heated water would have been used up and a flow of water at the system flow temperature of 700C would no longer be maintained to the radiators.

However, in order to prevent such a situation occuring, when the heated water from the tank is nearly exhausted the change of temperature of the water is detected by thermostat Td (which is at a pre-arranged distance from the top of the store) which is used to switch on the second heater arrangement 7. The thermostat Td is arranged at a calculated distance from the top of the tank so that the supply of heated water at the correct temperature to the radiators 4 need not be interrupted i.e. the store of heated water is not allowed to be completely exhausted. The amount of heated water left in the tank when the thermostat Td is switched on is enough to mix with cooler water in the compartment while water in compartment 2a is heated so that water at the required temperature may be drawn off from the tank without interruption of water at the required temperature to the radiators.

As the tank 1 of heated water is replaced progressively by cooled water returned from the radiators 4 to the bottom of the tank the water does not tend to circulate or mix within the tank and hence the temperature of the water is different in different regions of the store. In fact, the water in the tank 1 becomes stratified so that effectively a division surface between water at the maximum pre-set temperature (say 950C) and between water at the return temperature (55 OC to 600C) moves progressively up the tank as more water is return from the radiators 4. Once said division surface has moved upwardly beyond the thermostat Td, the thermostat Td detects the dramatic temperature drop and switches on heating arrangement 7.Therefore, the thermostat Td is arranged at a calculated distance from the top of the tank so that the heating arrangement 7 is switched on before it is too late to continue to supply water to the radiators at the required temperature in an uninterrupted fashion.

When the heating arrangement 7 is switched on the stratification in compartment 2a above the arrangement 7 is degraded and the water tends to circulate and mix in the compartment. The temperature setting and positioning of the thermostate Td is so arranged that the heated water above thermostat Td at the pre-set maximum temperature and the cooled water between thermostate Td and the heating arrangement 7 can be mixed to give water of a resuitant temperature which is approximately equal to the system flow temperature of 700C.

This ensures that water of the required temperature may be fed to the radiators and, of course, from this stage onwards the water drawn off the tank is no longer mixed with cooled water returned from the radiators since the water now drawn from the tank is at system flow temperature rather than at the maximum pre-set temperature. The heating capacity of the heating arrangement is designed to be equal to or more than the design capacity of the radiator system so that the required system flow temperature can be maintained for as long as necessary.

Compartment 2a comprises a volume which is less than one quarter the volume of the tank and of course, the temperature of the water in the compartment can therefore be raised that much more quickly than, for example, trying to heat the whole volume of water in the tank using the heating arrangement 7.

When the heating arrangement 7 is switched on by the thermostat Td as aforesaid when the tank is nearly exhausted of heated water, as the stratification in compartment 2a is degraded the stratification in the remainder of the tank (to the left of baffle plate 8 in Figure 1) is retained.

The level of the temperature of the water to the left of the baffle plate 8 remains substantially constant as the water at the system flow temperature is provided from compartment 2a.

Inclusion of the baffle 8 in the tank 1 tends to improve performance.

Figure 2 shows modified store comprising two water storage tanks S" S2 connected in parallel by way of piping P which communicates the lower zones of the tanks S" S2 with one another and by way of piping P' which communicates the upper zones of the tanks S" S2 with one another.

Such an arrangement may be preferred for ease of transportation of the various parts of the space heating system or for other reasons of convenience.

The arrangement functions similarly as that shown in Figure 1 with no baffle plate being provided. Instead compartment 2a is defined by the upper walls of storage tank S2. Water is drawn off from the store by way of piping P" (see Figure 2) and returned to the store by way of piping P"'.

Since the arrangement in Figure 2 functions similarly as that already described no further description is deemed necessary.

Claims (Filed on 1/12/83) 1. A space heating system comprising a store for storing a fluid heating medium, space heat exchanger means, for example hot water radiators, first and second heating arrangements being provided in the store to heat the fluid heating medium, the combined capacities of the first and second heating arrangements and their positions in the store being selected so that the fluid heating medium in the store may be heated on combined operation of the first and second heating arrangements to a pre-set maximum temperature during a first time period, for example an off-peak cheap rate power supply period, each of the first and second heating arrangements being individually insufficient to heat the fluid heating medium in the store to said pre-set maximum temperature during the first time period, the second heating arrangement being utilizable to boost the temperature of fluid medium thereabove in the store at a stage during a second time period, for example a peak high rate power supply period, so that the heat being drawn off from the store to the heat exchanger means is sufficient for the requirements of the system.

2. A system as claimed in Claim 1 in which the first heating arrangement is positioned at or towards the bottom of the store.

3. A system as claimed in Claim 1 or Claim 2 in which the second heating arrangement is positioned in between the bottom and top of the store.

4. A system as in Claim 3 in which the second heating arrangement is at a level above the first heating arrangement.

5. A system as claimed in any one of the preceding claims in which the fluid medium itself is drawn off from the store to the heat exchanger means.

6. A system as claimed in Claim 5 in which a thermostat is provided at a calculated distance from the top of the store which switches on the second heating arrangement at said stage during the second time period when a predetermined amount of heated fluid heating medium remains in the store.

7. A system as claimed in Claim 6 in which the thermostat is located vertically above the second heating arrangement.

8. A system as claimed in any one of the preceding claims in which the store comprises a storage tank in which both the first and second heating arrangements are located.

9. A system as claimed in Claim 8 in which the first heating arrangement is offset to one side of the tank and the second heating arrangement is offset to a side of the tank opposite said first mentioned side.

10. A system as claimed in Claim 9 in which the first and second heating arrangements are in non-overlapping relationship.

1 A system as claimed in any one of Claims 8 to 10 in which a baffle plate is provided which is arranged generally vertically in the tank to divide off a compartment within the tank above the second heating arrangement yet allow communication of fluid heating medium from the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

arrangement 7 is degraded and the water tends to circulate and mix in the compartment. The temperature setting and positioning of the thermostate Td is so arranged that the heated water above thermostat Td at the pre-set maximum temperature and the cooled water between thermostate Td and the heating arrangement 7 can be mixed to give water of a resuitant temperature which is approximately equal to the system flow temperature of 700C.

This ensures that water of the required temperature may be fed to the radiators and, of course, from this stage onwards the water drawn off the tank is no longer mixed with cooled water returned from the radiators since the water now drawn from the tank is at system flow temperature rather than at the maximum pre-set temperature. The heating capacity of the heating arrangement is designed to be equal to or more than the design capacity of the radiator system so that the required system flow temperature can be maintained for as long as necessary.

Compartment 2a comprises a volume which is less than one quarter the volume of the tank and of course, the temperature of the water in the compartment can therefore be raised that much more quickly than, for example, trying to heat the whole volume of water in the tank using the heating arrangement 7.

When the heating arrangement 7 is switched on by the thermostat Td as aforesaid when the tank is nearly exhausted of heated water, as the stratification in compartment 2a is degraded the stratification in the remainder of the tank (to the left of baffle plate 8 in Figure 1) is retained.

The level of the temperature of the water to the left of the baffle plate 8 remains substantially constant as the water at the system flow temperature is provided from compartment 2a.

Inclusion of the baffle 8 in the tank 1 tends to improve performance.

Figure 2 shows modified store comprising two water storage tanks S" S2 connected in parallel by way of piping P which communicates the lower zones of the tanks S" S2 with one another and by way of piping P' which communicates the upper zones of the tanks S" S2 with one another.

Such an arrangement may be preferred for ease of transportation of the various parts of the space heating system or for other reasons of convenience.

The arrangement functions similarly as that shown in Figure 1 with no baffle plate being provided. Instead compartment 2a is defined by the upper walls of storage tank S2. Water is drawn off from the store by way of piping P" (see Figure 2) and returned to the store by way of piping P"'.

Since the arrangement in Figure 2 functions similarly as that already described no further description is deemed necessary.

Claims (Filed on 1/12/83)

1. A space heating system comprising a store for storing a fluid heating medium, space heat exchanger means, for example hot water radiators, first and second heating arrangements being provided in the store to heat the fluid heating medium, the combined capacities of the first and second heating arrangements and their positions in the store being selected so that the fluid heating medium in the store may be heated on combined operation of the first and second heating arrangements to a pre-set maximum temperature during a first time period, for example an off-peak cheap rate power supply period, each of the first and second heating arrangements being individually insufficient to heat the fluid heating medium in the store to said pre-set maximum temperature during the first time period, the second heating arrangement being utilizable to boost the temperature of fluid medium thereabove in the store at a stage during a second time period, for example a peak high rate power supply period, so that the heat being drawn off from the store to the heat exchanger means is sufficient for the requirements of the system.

2. A system as claimed in Claim 1 in which the first heating arrangement is positioned at or towards the bottom of the store.

3. A system as claimed in Claim 1 or Claim 2 in which the second heating arrangement is positioned in between the bottom and top of the store.

4. A system as in Claim 3 in which the second heating arrangement is at a level above the first heating arrangement.

5. A system as claimed in any one of the preceding claims in which the fluid medium itself is drawn off from the store to the heat exchanger means.

6. A system as claimed in Claim 5 in which a thermostat is provided at a calculated distance from the top of the store which switches on the second heating arrangement at said stage during the second time period when a predetermined amount of heated fluid heating medium remains in the store.

7. A system as claimed in Claim 6 in which the thermostat is located vertically above the second heating arrangement.

8. A system as claimed in any one of the preceding claims in which the store comprises a storage tank in which both the first and second heating arrangements are located.

9. A system as claimed in Claim 8 in which the first heating arrangement is offset to one side of the tank and the second heating arrangement is offset to a side of the tank opposite said first mentioned side.

10. A system as claimed in Claim 9 in which the first and second heating arrangements are in non-overlapping relationship.

1 A system as claimed in any one of Claims 8 to 10 in which a baffle plate is provided which is arranged generally vertically in the tank to divide off a compartment within the tank above the second heating arrangement yet allow communication of fluid heating medium from the

top of the compartment to the remainder of the store.

12. A system as claimed in Claim 11 in which the volume of the compartment lies in the range of-21 to 1/6 the volume of the store or even less.

13. A system as claimed in Claim 12 in which the volume of the compartment is about one quarter of the volume of the store.

14. A system as claimed in any one of Claims 1 to 7 in which the store comprises two storage tanks connected in parallel, the first and second heating arrangements being provided in respective tanks.

1 5. A system as claimed in Claim 14 in which the two tanks are of identical size and a compartment is defined by one of the tanks above the second heating arrangement.

1 6. A system as claimed in Claim 1 5 in which the volume of the compartment may be in the range of 2 to 1/6 the volume of the store or even less.

1 7. A system as claimed in Claim 16 in which the volume of the compartment is about one quarter the volume of the two storage tanks.

1 8. A method of heating space comprising: (a) heating fluid heating medium in a store by first and second heating arrangements provided in the store to a pre-set maximum temperature value during a first time period, for example an off-peak cheap rate power supply period, (b) during a second time period, for example a high rate peak power-supply period, drawing off heat from fluid heating medium in the store to space heat exchanger means to heat a space, for example a room of a house, whilst both the first and second heating arrangements are inoperative to supply heat to the fluid medium in the store, (c) at a stage in the second time period before the heat being drawn from the store to said heat exchanger means is insufficient to meet required needs, boosting the temperature of fluid medium above the second heating arrangement in the store, using the second heating arrangement, sufficiently to meet the required needs and so that flow of heat at the required temperature to the heat exchanger means is not interrupted.

1 9. A method as claimed in Claim 18 also comprising: (d) drawing heated fluid medium from the store to the space heat exchanger means and switching on the second heating arrangement at said stage in the second time period when the store is nearly exhausted of fluid heating medium at the pre-set maximum temperature, and thereby heating only part of the cooled fluid medium in the store returned from the space heat exchanger means in a compartment of the store.

20. A method as claimed in Claim 19 in which the compartment volume is within the range 2 to 1/6 the volume of the store.

21. A method as claimed in any one of Claims 18 to 20, in which the temperature of fluid heating medium above the second heating arrangement is boosted to a system tiow temperature rather than to said pre-set maximum flow temperature.

22. A space heating system comprising a store, which in use, holds a fluid heating medium for circulation around the system and which medium is heated in the store, a first heating arrangement positioned towards the bottom of the store, a second heating arrangement positioned between the top and bottom of the store and above the first heating arrangement, the arrangement being such that the first and second heating arrangements are adapted to heat the fluid heating medium in the store to a predetermined maximum value during a first time period, and at some stage during a second time period when the fluid heating medium is being drawn off from the store and circulated through the system while the first heating arrangement is inoperative the second heating arrangement is adapted to boost the temperature of some of the fluid heating medium in the store which is located in a compartment thereof defined above the second heating arrangement, the return of fluid heating medium from a space heat exchanger means of the system being input towards the bottom of the store to stratify the fluid heating medium in the tank until the temperature of fluid heating medium in the compartment in the store above the second heating means is boosted thereby during said second time period whence the stratification in the compartment above the second heating arrangement is degraded.

23. A system as claimed in claim 22 in which the second heating arrangement is offset to one side of the store.

24. A system as claimed in claim 23 in which the first heating arrangement is offset to a side of the store opposite to said first mentioned side.

25. A method of heating space using the space heating system as claimed in any one of claims 22 to 24 in which the fluid heating medium in the compartment is raised to a system flow temperature below the maximum pre-set temperature during said boost period.

26. A system or method as the case may be as claimed in any one of the preceding claims in which the heating arrangements are mains electrical heaters.

27. A space heating system substantially as herein described with reference to Figure 1 of the accompanying drawings.

28. A space heating system substantially as herein described with reference to Figure 2 of the accompanying drawings.

29. A method of heating space as claimed in Claim 1 8 or Claim 25 and substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawings.