GB2222006A

GB2222006A - Space heating control

Info

Publication number: GB2222006A
Application number: GB8825979A
Authority: GB
Inventors: Ian David Burgess; Royston George Dann
Original assignee: British Gas PLC
Current assignee: British Gas PLC
Priority date: 1988-08-15
Filing date: 1988-11-07
Publication date: 1990-02-21
Anticipated expiration: 2008-11-07
Also published as: GB8825979D0; GB2222006B; GB8819344D0

Abstract

Space heating apparatus comprises a boiler (1) operatively connected to a programmed controller (2) arranged to switch the boiler on and off according to a predetermined programme in response to inputs from a room temperature set point device (3), a room temperature sensor (4), a boiler flow temperature sensor (5) and a boiler return temperature sensor (6). The controller (2) is arranged to switch the boiler off when measured room temperature approaches an upper desired value, to prevent overshoot and to switch the boiler on when room temperature falls towards a lower desired value. The controller (2) does this by calculating the rate of change of room temperature and comparing an estimated future room temperature with the set point. The estimation may be improved by a learning process. <IMAGE>

Description

SPACE HEATING CONTROL This invention relates to space heating control and in particular, but not exclusively, to a boiler operated central heating system.

Generally central heating systems have been based on a simple on/off concept with control of the boiler in response to a thermostat device to switch on when the working fluid temperature is below a preset lower threshold and to switch off when the working fluid reaches a preset upper threshold.

In recent years microprocessors have been used in boiler control for example to obtain optimum start ie. to bring room temperature up to a desired level at a preset time, or in externally compensated systems which monitor external conditions and modify system behaviour.

Externally compensated systems normally use a self adjusting or preprogrammed relationship between outside temperature and the required system temperature, and additional room controls may be used to trim operation locally when required. In this way local effects such as solar gain which would distort the predetermined general relationship may be alleviated.

It is an object to provide an improved method and apparatus for controlled space heating.

A controller according to the invention is arranged to operate in response to room temperature, rate of change of room temperature and system temperature, and to operate the system to maintain the room temperature with a desired range.

In a boiler system heating fluid to be pumped through a radiator system and also heating a domestic hot water supply, the controller may be arranged to control room temperatures in response to a single point of temperature reference and to control the hot water supply and operation of the pump.

The controller is adapted to monitor actual and desired room temperatures, rate of change of actual room temperature and boiler system water temperature. System water temperature is suitably monitored by outlet and return temperature and computing the mean.

The invention will now be described, by way of example, with reference to the accompanying partly diagrammatic drawings, in which: Figure 1 is a schematic diagram of a boiler and controller set up according to the invention, Figures 2A to 2F constitute a flow diagram of the control programme of the controller in the set up of Figure 1, and Figure 3 is a graph of room set point temperature RSP and boiler cut off against rate of room temperature rise.

Referring to Figure 1 a boiler 1 is operatively connected to a programmed controller 2 arranged to switch the boiler on and off according to a predetermined programme, set out in the flow diagram of Figures 2A to 2F in response to inputs from a room temperature set point RSP device 3, a room temperature TR sensor 4, a boiler flow temperature sensor 5 and a boiler return temperature sensor 6. The controller is suitably programmed to compute a boiler system temperature TS = Boiler Flow TemPerature+Boiler Return Temperature 2 Reaction point and Switch off During warm up or start conditions the controller is arranged to allow the boiler water flow temperature to rise until a temperature is reached when the room temperature approaches the desired value and to switch off the boiler to avoid room temperature overshoot.In this manner high temperature cycling on the boiler thermostat can be limited or avoided. To this end the controller responds to the rate of change of room temperature and the difference between required and actual room temperature. The relationship between the difference between the required and actual room temperatures and the rate of change of room temperature is an empirical one established from observed data. The rate of room temperature rise is faster if any of the following apply:a) the system is oversized with respect to the house or room being heated b) the last period of heating has only recently finished c) other factors exist which reduce the required heat output from the boiler.

Under any of these circumstances the boiler would need to be shut off sooner to avoid room temperature overshooting its set point or desired value. A boiler cut off line can be established in a graph of set temperature and rate of room temperature rise as shown in Figure 3, and which will depend on a number of factors including the thermal mass of the heating system in relation to the design heat loss and the time delay between heat output from the radiator registering at the room temperature sensor in any particular installation. However if the cut off line is set on the low side to ensure avoidance of overshoot remedial steps can be taken in the control cycle. If however the boiler were to be turned off too late the room temperature would inevitably overshoot the set value RSP.

Cool-Down and Evaluation of Equilibrium Temperature The reason for switching the boiler off before the set point temperature is reach is to enable heat stored within the system to be usefully distributed via the radiator to the air without causing overshoot and allowing natural cooling of the radiators at such a rate that when room temperature reaches the set point the system temperature has fallen to such a value that the heat output matches the required output.

In order to predict the switch point a simple self compensating control is adopted. The profile of temperature rise in a room can be approximated by the following mathematical relationship: Temperature = time2 + b time + C once the particular variant of this general form is established for the particular installation it becomes possible to predict the temperature after any length of time has elapsed. If the particular variant is established over a long enough time interval an approximation of the equilibrium temperature is established and a profile of the general form determined by the mathematical relationship may be fitted to observed data points.

If TE = RSP the boiler is switched off to prevent overshoot. When the boiler is switched off the anticipated equilibrium temperature starts to drop as the system cools and output from the radiator falls. The control system is arranged to monitor anticipated equilibrium temperature from room temperature data received until the anticipated equilibrium temperature drops below the set point temperature (TS). When this occurs the mean system temperature is measured and the boiler switched on to maintain it at or close to this value.

An alternative to the establishment of TE through profile fitting as discussed above other means such as: a) straight line fit b) fixed curve c) self learning may be employed.

A particular house uses essentially a single heat-up curve, operating on different parts depending on the final equilibrium value and an intelligent controller can build up control data from sensed data under different weather conditions where rate of heat loss would vary.

It is of advantage to evaluate the value of TE after a particular time period (+) rather than its final value in order to avoid long warm up periods.

Switch on and attainment of set point RSP The object of this phase of operation is to attain the set point temperature RSP in a reasonble time (t). The estimated equilibrium temperature will have reached the set point temperature value before the set point temperature RSP is attained, and the observed mean system temperature Ts is stored. Under these conditions the system output is sufficient to achieve the set point temperature RSP under equilibrium conditions or at time t, whichever is sooner.

The measurement loop of the control programme is set out at the head of the flow diagram in Figure 2. At start the inputs of the devices 3, 4, 5 and 6 are measured namely RSP = Room temperature set point TR = Room temperature Boiler flow temperature -T1 and boiler return temperature -T2 and TS - boiler system temperature computed from T1 and T2 2 The rate of change of room temperature dTR/dt is measured over a period and a predicted room temperature TE after a time t is estimated.

If TE < RSP then the boiler continues to run until TE = RSP when the control is reset to start on the run loop which follows below the measurement loop in the flow diagram of Figure 2. Positive action is now taken to maintain the required mean system temperature by establishing a cycling regime to maintain the required system condition.

According to one arrangement the boiler is cycled through a set bandwidth either side of the required mean system temperature TS and is switched off when the upper mean system temperature of the bandwidth is reached and restarted when the lower mean system temperature of the bandwidth is reached. In this arrangement the cycle length is variable.

In an alternative arrangement a fixed cycle length i.e.

on-off period is used and a preprogrammed relationship between the percentage on time and mean system temperature controls the cycling, a feed back loop observing the actual mean system temperature at the end of a complete cycle and adjusting the cycling appropriately. A 15 minute cycle may be used.

A check is maintained on the projected final value of equilibrium temperature TE and if this begins to deviate from the set temperature RSP the target mean system temperature is adjusted. An approximate r-31ationship between deviation from set point temperature and the magnitude of adjustment of target mean system temperature is established in a look-up table and the feed back loop ensures that the correct value will eventually be arrived at.

When the desired set point temperature TS is achieved this phase on control ends.

Maxntenance of set point temperature TS Once the required set point temperature is reached it is maintained by adjusting system temperature to match heat demand. Any changes in heat load produced by solar gains, external temperature changes, ventilation changes, or internal gains are accommodated automatically when a deviation from the required set point temperature is sensed and the required mean system temperature adjusted accordingly. If the room temperature rises, mean system temperature and hence system output is reduced, and vice versa.

A modified form of the control type used in the switch on phase is employed. Instead of using the deviation of estimated equilibrium temperature TE from set point temperature RSP, the deviation of room temperature TR from set point temperature is used. The values of TR are either mean values over a complete cycle period or from a comparable point in each cycle. Use of mean or average values avoids some of the seasonal problems associated with some existing thermostat types. The control cycle used to maintain the required mean system temperature follows the same approach as in the previous phase.

In the run loop, as shown in the flow chart the predicted equilibrium temperature after time t is compared with the sum of room set point temperature RSP and room comfort band width temperature and if TE > RSP + RCB the boiler is switched off. If TE is not greater then room temperature TR is compared with the difference between the room set point temperature and the comfort band and if TR > RSP - RCB the boiler is switched off, but if TR < RSP - RCB the boiler is maintained in a switched on condition.

There then follows a cooling regime during which TR is monitored against room set point temperature, and when TR < RSP the predicted equilibrium temperature after time t is compared with the sum of the room set point temperature and the comfort band and when: TE < RSP + RCB the cycling or attainment regime is started.

If it is the first cycle of the attainment regime the required system temperature TS is set and stored and a load obtained from an assured load against system temperature relationship in a look up table and a calculation made of boiler on-time and the boiler set to run accordingly.

If on the other hand the attainment regime cycle is not the first the required system temperature Ts is adjusted by reference to a look up table according to whether TE > or < TR and a calculation of the observed load against system temperature relationship is made to update the overall load against system temperature relationship from which further cycles of boiler operation calculate boiler on-time while the boiler is in a run condition.

TR is compared with the sum of the room set point temperature and the room comfort band and if TR > RSP + RCB the boiler is switched off, otherwise TR is compared with the difference and if TR > RSP-RCB there is a wait until the start of a fresh period during an equilibrium phase to be described below.

If TR is not greater than RSP-RCB a check is made to establish whether it is time to start a further cycle if it is the attainment phase recommences. If it is not time then TR is again compared with RSP plus RCB until a recycle time is established.

During the equilibrium phase there is a wait until the start of the next period during which the required system temperature TS is adjusted, and the observed load v system temperature relationship calculated to update the average load v system temperature relationship from which the required boiler load is calculated and the boiler on-time calculated. The boiler is run for the required time and the room temperature is compared with the sum of the room set point temperature the room comform band and if TR > RSP + RCB the boiler is switched off, whereas if not, the room temperature is compared with the difference. If TR is not < RSP - RCB it is calculated whether it is time to start a fresh cycle and if not TR is again compared with the sum of and difference between room set point on room comfort band and when TR < RSP - RCB a fresh cycle of the attainment phase is initiated.

If TR is not < RSP-RCB and it is time to start a new cycle, adjustment is made to the required system temperature for future cycles.

The whole of the above described control procedure is followed whenever a new heating period is triggered by the programmer except when room temperatures are already above the boiler cut off line as describect in the reaction point and switch off phase as described above. In the latter case the control will start at the switch-on and attainment of set point phase described above with an estimated value of mean system temperature based on the deviation of room temperature TR from set point temperature TS.

The above described control strategy has the following advantage compared with a conventional room thermostat control system:i) No temperature overshoot during warm-up ii) Reduced operation at high boiler temperature iii) Precise control at room temperature allowing for a reduction on set point temperature iv) Overall lower system temperatures for a given output.

A controller incorporating the above described method of control could also be set up to control hot water, pump over-run and time keeping functions.

Claims

1. A method of controlling the temperature of a space heated by a space heater which comprises measuring rate of change of space temperature and space heater system temperature and operating the space heater to maintain the space temperature within a desired range by switching off the space heater when the measured room temperature approaches an upper desired level to prevent overshoot and operating the space heater when the space room temperature approaches a lower desired level.

2. A method as claimed in claim 1 applied to a boiler system heating a room through a radiator which includes monitoring actual and desired room temperature, rate of change of room temperature and boiler flow temperature.

3. A method as claimed in claim 2 in which boiler system temperature is computed from the mean of boiler outlet and return temperatures.

4. A method as claimed in any preceding claim in which during start up conditions the difference between required and actual room temperature and the rate of change of room temperature are compared with pre-determined data to determine switch off of the boiler sufficiently in advance of actual room temperature reaching desired room temperature.

5. A method as claimed in claim 4 which includes estimating an equlibrium room temperature from the actual room temperature and the rate of change thereof.

6. A method as claimed in claim 5 in which the profile of temperature rise in the room is established from the following relationship Temperature = a time2 + b time + c fitted to observed data points.

7. A method as claimed in any of claims 4 to 6 in which when the boiler is in an off condition and the anticipated equilibrium temperature drops below a set point value, the mean system temperature is measured and the boiler switched on to maintain the room temperature at or close to the set point value.

8. A method as claimed in any of claims 4 to 7 in which after boiler start up and in a normal running phase a cycling regime is established to maintain the required system condition for maintaining desired room temperature.

9. A method as claimed in claim 8 in which the boiler is cycled through a bandwidth either side of a required system temperature.

10. A method as claimed in claim 8 in which the boiler is cycled over a fixed cycle length and the ratio between on and off periods within a cycle is preprogrammed, a programme feed back loop observing the actual mean system temperature at the end of a complete cycle and adjusting the cycling appropriately to maintain the desired system temperature.

11. A method as claimed in claim 10 in which in response to deviation of the projected final value of equilibrium temperature from the set temperature the target mean system temperature is adjusted to compensate for the deviation.

12. A method as claimed in any of claims 4 to 11 in which once the set point temperature is reached it is maintained by adjusting system temperature to meet heat demand in reponse to deviation of room temperature from set point temperature.

13. Apparatus 'for carrying out the method of any of claims 2 to 12 which comprise a boiler operatively connected to a programmed controller arranged to switch the boiler on and off according to a predetermined programme in response to inputs from a room temperature set point device, a room temperature sensor, a boiler flow temperature sensor and a boiler return temperature sensor.

14. Apparatus as claimed in claim 13 in which the controller is programmed to carry out the method according to any of claims 1 to 12.

15. A method according to claim 1 and substantially as described with reference to the accompanying figures.

16. Apparatus according to claim 13 and substantially as described with reference to the accompanying figures.