GB2489017A - A control system having three thermostats that is suitable for controlling a heating system - Google Patents

Abstract

The control system is suitable for a heating system and comprises a primary 140, secondary 150 and tertiary 160 thermostat, and further comprises a first switch responsive to the primary thermostat, a second switch 180 responsive to the secondary thermostat, and a third switch 190 responsive to the tertiary thermostat, wherein the first, second and third switches are arranged in series with one another. The thermostats may each have respective ranges of operation wherein the ranges of operation of the secondary and tertiary thermostats both fall within the range of operation of the primary thermostat and wherein the range of operation of the secondary thermostat is lower than that of the tertiary thermostat. The system may be used to control operation of a boiler 130 of a central heating or hot water system such that the boiler is actuated continuously when the primary and secondary thermostat call for heat and intermittently when the primary and tertiary thermostat call for heat, the intermittence being provided by a timer 110. The control system improves the efficiency of a heating system that it controls.

Description

MODULATING PROGRAMMER

This invention relates to heating systems and in particular to devices and methods for improving the efficiency of a heating system by careful control of various component elements of the system.

Background to the Invention

Condensing boilers are now considered to be the most efficient boilers in the marketplace. In fact, Building Regulations have recently been changed so that all new boiler installations must have condensing boilers unless there are extenuating circumstances which are very rare.

Three operational aspects make a condensing boiler much more efficient than standard on/off boilers, namely; the condensing process, burner modulation and dual heat exchange.

The Applicant's copending patent application publication number GB 2462183A describes an electronic circuit for a hot water heating system, which conserves fuel consumption and reduces carbon dioxide emissions. In a heating system including a burner that heats water and a room thermostat, is provided an electronic circuit which comprises an override thermostat having settings for a lower temperature limit and an upper temperature limit which are below those of the room thermostat. The override thermostat is operable to activate continuous operation of the burner when the indoor temperature falls below the lower temperature limit of the room thermostat and until the upper temperature limit of the override thermostat is reached. The electronic circuit also includes a timer which is operable to deactivate and reactivate the burner while the room thermostat is calling for heat.

This previously disclosed invention has been found in practice to improve the efficiency of a domestic central heating system incorporating a standard on/off boiler by in excess of 50%.

The present invention provides for further improvements in that efficiency.

In accordance with the present invention there is provided a control system for a healing system comprising; a primary adjustable thermostat, a secondary adjustable thermostat and a tertiary adjustable thermostat, the primary thermostat having an operating range which is a multiple of the operating range of both the secondary and tertiary thermostats, a first switching means responsive to the primary thermostat to switch a water heater of the heating system on and off, a second switching means arranged in series with the first switching means and responsive to the secondary thermostat to open or close a circuit containing the first switching means and the water heater, a third switching means arranged in series with the first and second switching means and responsive to the tertiary thermostat to open or close a circuit containing the first switching means, the second switching means and the water heater.

Optionally, the control system additionally includes a programmable timer for restricting the duration of continuous operation of the circuit containing the first, second and third switching means and the water heater when the switching means are all closed.

Desirably the control system may be activated or deactivated by means of an associated programmable timer.

The control system may be provided as an integral part of a new boiler or alternatively might be provided in the form of a retrofittable unit. In both cases operational energy efficiency is substantially improved over prior art systems.

The control system is suited to use in any heating system based on the principle of heating and circulating fluid and is not restricted to use with a domestic gas operated central heating system. For example, the controller can be incorporated into oil fired systems and electrical water heating systems. The invention is suited to use both on a domestic and industrial scae.

By way of example, an embodiment of the invention is now described with reference to the accompanying circuit diagram in Figure 1. -3,-

The components of the Programmer are as shown in the circuit diagram in Figure 1.

Different combinations of room thermostats, temperature switches and timers can be configured to achieve burner modulation through intermittent burner operation.

Figure 1 is just one such arrangement and the components of this are as foflows: The circuit of Figure 1 relates to one embodiment of the invention and its use in controlling a gas operated central heating system in a domestic household with specified temperature and heat duration requirements is described! Setting of water and room heating temperatures is achieved through a programmer panel represented by the rectangular box at the top of the page. The box is safely connected to a mains supply (N, L) and presents controls (1, 2, 6) to adjust the primary, secondary and tertiary thermostats as well controls (3, 4) to switch room heating and water healing on or off independently.

Primary room thermostat (140) takes the place of a conventional room thermostat and has an operating range adjustable in fraction of degree increments over a multiple of one degree centigrade between lower and upper temperature limits.

Secondary room thermostat (150) operates in much the same way as the primary and is also adjustable in fraction of degree increments. It is adjustable to give the desired upper and lower temperature limits.

Tertiary room thermostat (160) in much the same way as the primary and is also adjustable in fraction of degree increments. It is adjustable to give the desired upper and lower temperature limits. These limits are set higher than those of the Secondary room thermostat but lower than those of the primary room thermostat.

Water temperature switch TI (170) controls the circulating water temperature between upper and lower limits, both of which are adjustable.

Room temperature switch T2 (180) switches between positions A and B in response to the secondary room thermostat (150).

Room temperature switch T3 (190) switches between off and on positions in response to the tertiary room thermostat (160).

Timers 1 and 2 (110 and 120) are both adjustable to provide a wide range of on times and off times.

The values and settings stated in the following procedures are purely to demonstrate how the system works. All the key components are adjustabUe and the values and settings are able to be set to suit specific user requirements.

The lower and upper temperature flmits of the primary room thermostat are respectively 20.5°C and 21.5°C.

The lower and upper temperature limits of the secondary room thermostat are respectively 20.7°C and 20.8°C.

The lower and upper temperature limits of the tertiary room thermostat are respectively 21.0°C and 21.1°C.

Timer 1 (110) when energised alternates between the on and off positions. For the purposes of this example it is set to be off for 3 minutes and on for 2 minutes.

Timer 2 (120) when energised alternates between the on and off positions. For the purposes of this example it is set to be on for 1 hour. The off period should be set for a period longer than the Fongest period between programme times so that the pump doesn't come on again until the start of the next programme.

Operation 1 The programmer is in the central heating mode and one of the pre-programmed heating periods commences and heat is called for. Assuming the starting room temperature is 17°C, all three room thermostats take up the demand (D) position and call for heat.

The circulating water pump (100) switches on and remains on all the time the programmer is calling for heat. Room temperature switch T2 (180) responds to the secondary room thermostat (150) and goes to position A. This energises water temperature switch Ti (170). This is on when the circulating water temperature is below 72°C and goes Off when the circulating water temperature rises to 75°C.

Since the water temperature switch Ti (170) is already in the on position, the boiler burner (130) is energised.

The boiler burner (130) receives its electrical supply via the secondary room thermostat (150), room temperature switch T2 (180) and water temperature switch Ti (170). The circulating water pump receives its electrical supply via the primary room thermostat (140).

Operation 2 When the room temperature rises to 20.8°C, the secondary room thermostat (150) takes up the satisfied (5) position. This changes the room temperature switch T2 (180) from position A to position B. A connection is then made with room temperature switch T3 (190) and because the tertiary room thermostat (160) is in the demand (D) position, the room temperature switch T3 (10) is in the on position.

A connection is then made with Timer 1 (110). Once energised, this timer alternates between 3 minutes in the off position and 2 minutes in the on position repeatedly.

Under most load conditions, the burner-on time will not be enough to maintain the room temperature at 20.8°C. The room temperature will therefore gradually fall until it drops to 20.7°C. At this point the secondary room thermostat (150) reverts to the demand position, room temperature switch T2 (180) moves to position A and Timer 1 is taken out of the circuit. Operations I and 2 will then be repeated until and unless the room temperature continues to rise and reaches 21.1°C. Between room temperatures 20.8°C and 21.1°C, Timer 1 (110) wiU continue to operate.

Operation 3 At lighter loads, the room temperature may continue to rise and reach 21.1°C.

When this happens, the tertiary room thermostat (160) will be satisfied and the room temperature switch T3 (190) will go to the off position. This cuts off the electrical supply to Timer 1 (110) and the burner (130). When the room temperature falls to 21.0°C, the tertiary room thermostat (160) will take up the demand (D) position and Operation 2 will be repeated.

Operation 4 At the end of the programmed period when the heating is shut down, Timer 2 (120) switches to the on position which ensures that the circulating water pump continues running for the period the Timer 2 is set for; in this case 1 hour.

It is important to emphasise that during the burner-off periods, the heat exchanger's residual heat is being reclaimed and passed on to the room air by virtue of the continuous operation of the circulating water pump. The average temperature of the water is maintained at a high level during intermittent burner operation thus sustaining high heat transfer efficiency.

The following analysis compares the efficiency of a heating system incorporating the novel control system with conventional heating systems.

Why a central heating system incorporating the invention and a standard boiler is so much more effident than one incorporating a condensing boiler A central heating system with a standard on/off boiler is known to be much less efficient than a system with a more modern, condensing boiler. The huge improvement in efficiency of a central heating system incorporating a standard on/off boHer as demonstrated in this report is largely achieved by a combination of improved heat transfer due to elevated circulating water temperature and intermittent burner operation.

The burner on and off periods of the standard on/off boiler are relatively short and if it is performing properly, the circulating water temperature does not rise to a very high level by the time the burner cuts out. Since the water temperature starts off low, the average temperature is also ow. As a consequence, heat transfer efficiency is compromised. Also, during the off period, the reservoir of heat that has built up in the heat exchanger is lost to the flue. The lost heat is otherwise known as standing losses. The control system of the present invention addresses both of these issues.

The circulating water is allowed to rise to a high level and is maintained at this level and because the burner does not cut out on room temperature during the whole time the control system is calling for heat, there are no standing losses. The high differential between the room air temperature and the circulating water temperature means that the rate of heat transfer to the room air is much higher. Once the water temperature has reached a high level after the initial continuous burner on period, the burner begins its intermittent operation. Using the example in the accompanying table marked Appendix B, the burner is on for 3 minutes and off for 3 minutes. It can be seen that the water temperature is maintained within a few degrees thus sustaining high heat transfer. During the burner off periods, the circulating water pump continues to run. As a result the circulating water draws from the reservoir of heat in the heat exchanger and passes the heat on to the room air.

This reservoir of heat, which is wasted in any current central heating system, ensures a high rate of heat transfer by maintaining a high water temperature, but with the burner on only half the time. When the burner comes on again, the reservoir is topped up and the three or four degrees drop in the water temperature is restored. This process is repeated and the status quo is maintained.

Comparisons between the modulating performance of a condensing boiler and that of a standard on/off boiler incorporating the control system of the invention The beneficial effects of intermittent burner operation can be seen in Appendix B (copy attached) of the Fuel Econorniser submission. In the period between 23 hours minutes (twenty-fourhour clock time) and 00 hours 48 minutes 11 seconds which is a period of 53 minutes 11 seconds, the circulating water temperature rises from 51.2°C to 56.1°C. This is despite the burner only being on for 25 minutes of the 53 minutes, i.e. 47.S% of the time.

Regulations require a new boiler to be sized to meet fuD load conditions when the indoor temperature is 21°C and the outside temperature is 3°C below zero. This is a differential temperature of 24°C. The boiler will be sized so that the maximum fuel flow rate will exactly meet these requirements. The differential between inside and outside temperatures is direcdy proportional to the heat loss from the building which in turn is directly proportional to the heat energy supplied to the indoor air that is necessary to maintain the indoor temperature of 21°C. In other words, the fuel flow rate at less than full load conditions as a percentage of the maximum fuel flow rate equals the corresponding differential temperature as a percentage of the maximum differential temperature.

As stated in the last paragraph, a boiler is required to be sized to meet conditions where the outside temperature is 3°C below zero and the inside temperature is 21°C, i.e. 24°C differential. However, recent winters in the UK have seen outside temperatures as low as 22°C below zero with a differential of 43°C. Clearly sizing in accordance with the Regulations would mean the boiler being unable to sustain an indoor temperature anywhere near 21°C in such extreme conditions. In order to cope with such conditions the boiler size would need to be much greater. Unfortunat&y the boiler would effectively be oversized most of the time and consequently much less efficient. The modulating programmer of the invention overcomes this problem.

The boiler would be sized to suit a temperature differential of 43°C but by virtue of the intermittent burner operation and improved efficiency during modulation, the boiler would effectively supply only the amount of heat energy necessary to meet -.9-Regulation requIrements. It would therefore be operating as a more efficient smaller boiler reducing the fuel usage and CO2 emissions. In the event of the indoor temperature falling below the minimum, the Override Thermostat described in the Applicant's earlier patent publication number GB 2462183A would enable the boiler to operate at full firing rate, The indoor temperature would then quickly rise to the point when the modulation once again takes over.

The heat exchange efficiency of a boRer will determine the size of the boiler in that the maximum fuel flow rate will be less than the fuel flow rate from the burner. For example, a condensing boiler with a heat exchange efficiency of 90% and a maximum heat energy rating of the burner of 16.2 kilojoules per second will only deliver a maximum of 14.58 kilojoules per second, Conversely, the same boiler required to deliver a maximum of 14.58 kiloJoules per second of heat energy will require a burner heat energy rating of 16.2 kiloJoules per second, i.e. 1.11% more.

Using all the above information, comparisons can be made between the modulating performance of a condensing boiler and that of a standard on/off boiler converted to a modulating boiler, Modulating performance of a condensing boiler under the conditions prevaiilng during the test detailed in,lppendiv S Heat exchange efficiency: 90% Maximum temperature differential is: 24,0°C Actual temperature differential is: 20.4°C Percentage of full load is: Q5!I x 100% = 85% 24.0°C Rate of heat energy required to meet full load conditions at 90% heat exchange efficiency is: 14.58 kJ/sec Rate of heat energy required from the boiler burner at full load is: 14.58 x 1.11% = 16.2kJ/sec Rate of heat energy required from the burner at 85% load is therefore: 16.2 x 0.85 = 13.77 kJ/sec Duration of the modulating period is: 53 minutes Heat energy required during modulation is: 53 x 60 x 13.77 = 43,789 kJs Modulating performance of a converted standard on/off boiler under the conditions prevailing during the test detailed in Append/v S Maximum temperature differential is: 24.0°C Actu& temperature differential is: 20.4°C Rate of heat energy supphed by boiler burner at fufi load is: 16.2 Id/sec Duration of the modulating period is: 53 minutes Burner-on time during the modulating period is: 25 minutes Heat energy required during modulation is: 25 x 60 x 16.2 = 24,300 kJs The saving in fuel and reduction in C02 emissions are calculated as follows Savings in fuel and emissions during modulation are: = 44.5% The initial burner on period in Appendix B when the burner is on continuousy lasts for 14 minutes 30 seconds. With a condensing boiler, the initial full firing rate can be much shorter, say 3 minutes, before modulation kicks in. Taking this into account, the revised calculations are as follows.

Modulating performance of a condensing boiler under the conditions prevaiilng during the test detailed in Append/v S taking into account the initial burner-on period Duration of the initial burner-on period is: 3 minutes Duration of the modulating period is: 53 + (14.5 -3) = 64.5 minutes Heat energy required during the initial heating period is: 3x60x16.2 =2,916kJs Rate of heat energy required from the burner at 85% load is: 13.77 Id/sec Heat energy required during modulation is: 64.5 x 60 x 13.77 = 53,290 kJs Total heat energy supplied is: 53,290 + 2,916 = 56,2O6kJs Modulating performance of a standard on/off boiler incorporating the con trol system of the present invention under the conditions prevaiilng during the test detailed in Append/v S. taking into account the initial burner-on period Duration of the initial burner-on period is: 14.5 minutes Duration of the modulating period is: 53 minutes Rate of heat energy supplied by boiler burner at fufl load is: 16.2 kJ/sec.

Heat energy supplied at maximum fuel flow rate is: 14.5 x 60 x 16.2 = 14,094 kJs Burner-on time during the modulating period is: 25 minutes Heat energy required during modulation is: 25 x 60 x 16.2 = 24,300 kJs Total heat energy supplied is: 24,300 + 14,094 = 38,394 kis The saving in fuel and the reduction in C02 emissions using the converted standard on/off boiler are calculated as follows: Saving in fuel and emissions is: 506 -38,394 x 100% = 31.69% The typical heat exchange efficiency of 9O% for a condensing boiler takes account of the benefits of the condensing process and two-stage heat exchange. The percentage reductions in fuel usage and CO2 emissions calculated in this section therefore represent net savings and are what would actually be achieved in practice.

In Appendix B, the test period of 70 minutes used in the above calculations is short when compared with the durations of those heating programmes used in most households. Usually, these are several hours long. In these cases, savings would be even more than calculated in the examples because the initial heating period would take up a smaller proportion of the total programme time.

This reasoning also applies when the system starts up from cold. The initial heating period for the condensing boiler would take longer for the room temperature to rise to the start of modulation.

The main intended use of the invention is to convert standard on/off boilers currently in the marketplace to modulating boilers by retrofitting the control system of the invention. The control system not only enables more efficient performance of central heating systems due to improved heat exchange and improved heat transfer but -12-more efficient modulation than current condensing boilers. Some of the advantages the invention has over current modulating boilers are as follows.

a) No turn-down ratio. The control system modulates efficiently over a very large range of outside temperatures. Modulating boilers with turn-down ratios of 5 to I do not modulate efficiendy above outside temperatures of 14°C to 16°C.

b) The control system modulates at much higher circulating water temperatures.

The resulting increase in heat transfer efficiency more than makes up for the absence of the condensing process.

c) Fluctuating fuel flow rates make it difficult for the condensing, modulating boiler to sustain optimum combustion efficiency without the influx of excess air. With the control system of the invention, the combustion efficiency can be optimised using natural air infiltration. Because the fuel flow rate is constant, deviation from optimum efficiency is less likely.

d) With natural air infiltration, no fan is necessary. Consequently the exit velocity of the hot gases is less and the dwell time of the hot gases in the vicinity of the heat exchanger is longer.

e) No fan means reduced electricity consumption.

f) The control system is much more compatible with thermostatically controlled radiators. As these radiators come in and drop out of circuit there can be large fluctuations in circulating water temperatures. Current modulating boilers have problems in responding to these fluctuations which can result in their burners cutting out on water temperature or room temperature resulting in increased standing losses. The water temperature switch Ti (170) irons out these fluctuations and stabilises the circulating water temperature. -13

g) When the control system goes off, the circulating water pump continues running thus transferring the residual heat in the heat exchanger to the room air. It has been proved in tests that the room temperature can be sustained within 1°C for as long as 2 hours even with the outside temperature as low as 2°C. With current modulating boilers, most of this residual heat is lost to the flue when the burner goes off at the end of the programme. The programme time when using the Programmer can therefore be shortened to aflow for the benefit of the extra heat.

When taking into account the above advantages the control system of the invention has over the condensing boiler, the savings in fuel and the reduction in CO2 emissions are even greater than those already summarised above.