GB2027949A

GB2027949A - Central heating systems

Info

Publication number: GB2027949A
Application number: GB7921184A
Authority: GB
Original assignee: Servotomic Ltd
Current assignee: Servotomic Ltd
Priority date: 1978-08-09
Filing date: 1979-06-18
Publication date: 1980-02-27

Abstract

In a central heating system in which a heat transfer liquid is circulated (by a pump 13) in a piping circuit 11, 12 between a boiler 23 and heat dissipating appliances (not shown), the boiler has a cut-out 28, 29 sensitive to the temperature of liquid leaving the boiler and a delay device (31) is associated with the cut-out to prevent the boiler coming into operation again until a pre-set time has elapsed after the cut- out has operated. Rapid on-off cycling of the boiler, which is a relatively inefficient operating condition, is thus avoided. As shown the cut-out switch 28 is a change-over switch and the delay device is a thermal relay having its heating resistor 32 and normally closed contact 30 connected to the two output contacts of the switch 28. The pump 13 and a solenoid 26 of boiler fuel valve 25 are thus normally energised from supply 27 via 28 and 30. When sensor 29 detects an excessive liquid temperature, switch 25 opens the pump and solenoid circuit and instead energises resistor 32, which causes contact 30 to open. When the cut-out returns to its normal position, the thermal constant of the device 31 imposes a delay before the contact 30 recloses. <IMAGE>

Description

SPECIFICATION Central heating systems This invention relates to Central Heating Systems.

In central heating systems of the type in which a heattranfer liquid is heated by a boiler and circulated to heat dissipating appliances such as room radiators and a heat exchanger for hot water supply, an operating condition which arises relatively frequently is that in which the heat output of the boiler is greater than can be dissipated in the system. Such a condition can arise, for example, when the room thermostat or thermostats are satisfied and the radiators are isolated from the boiler by a valve operated in response to such thermostat, in which case the entire heat output of the boiler is directed to the hot water supply system.Under these conditions, with the full heat output of the boiler not being capable of being absorbed, the temperature of liquid leaving the boiler may exceed the value predetermined for the safe operation of the boiler and system, and to avoid this it is usual to provide a temperature responsive cut out for the boiler which senses the temperature of liquid leaving it and is operative to cut off the supply of fuel to the boiler when such temperature exceeds a predetermined value. As soon as the fuel supply is cut off, the circulating liquid cools and when it has cooled below the relevant temperature the fuel supply to the boiler is re-established to commence heating operation once again.The effect of this mode of operation, particularly in boilers which have a low thermal capacity (low liquid content) is that the boiler can cycle between its "on" and "off" conditions very rapidly, which can lead to excessive wear of the boiler and can also be irritating to a person who can hear the boiler operating.

With the object of overcoming or reducing this disadvantage, the present invention provides a central heating system comprising a boiler, a piping circuit, a plurality of heat dissipating appliances and means for circulating a heat transfer liquid in the system, the boiler including cut-out means for sensing the temperature of heated liquid leaving the boiler and operative to cut off the supply of fuel to the boiler upon said temperature exceeding a predetermined value and delay means associated with the cut-out means and operative to prevent reestablishment of the fuel supply until a predetermined time has elapsed.

By arranging such predetermined time to be appreciably longer than the time which it would take for the system to cool sufficiently to re-establish supply of fuel to the boiler without such a time delay, the excessively rapid off-on cycling of the boiler can be avoided.

The cut-out means may operate in a simple on-off mode according to the sensed liquid temperature, or may be of a type which progressively decreases the fuel supply to the boiler as the sensed temperature approaches the predetermined value, thereby reducing the heat output of the boiler as the heat dissipated by the system decreases. With the latter type of control usually, in the case of a gas fired boiler, the fuel supply may be reduced to about 30% or so of its maximum value, representing the minimum value at which combustion can reliably be sustained in the boiler, and then on further output liquid temperature rise snap to the "off" condition.

The delay means may then operate to provide that, after cutting off of the fuel supply, the supply is resumed at a level above the minimum sustained level above referred to. This reduces operation of the boiler under its minimum load condition, which for many types of boiler is a relatively inefficient condition.

The invention is particularly applicable to a central heating system in which the radiators are provided with individual thermostatically controlled valves, which operate proportionally in accordance with sensed temperature, so that the heat dissipating capacity of the system is, in effect, gradually reduced.

The invention will now be described by way of example with reference to the accompanying drawings of which: Figure 1 is a diagrammatic representation of a central heating system to which the invention may be applied.

Figure 2 shows diagrammatically elements associated with the boiler of the heating system.

Referring firstly to Figure 1 ,there is illustrated a heating system which comprises a boiler 10, and a piping circuit comprising main flow and return pipes 11, 12, respectively. A pump 13 is provided in the pipe 12 to circulate heat transfer liquid, usually water (possibly with the addition of an anti-corrosive agent), through the piping circuit between the boiler 10 and heat dissipating appliances in form of radiators 14 and an immersion heating coil 15 in a hot water cylinder 16, all these appliances being connected in parallel with one another. Liquid flow through the radiators 14 is controlled by respective thermostatic valves 17 responsive to ambient temperature, and flow through the coil 15 by a valve 18 controlled by a phial 19 fixed to the exterior of a cylinder 16.The valve 18 is a three way valve, and opens a by-pass pipe 20 when it cuts off liquid flow from the coil 15. The system also includes a pipe 21 leading to an expansion tank 22, and a venting pipe 34.

Referring now to Figure 2, the boiler 10 includes a combustion chamber and heat exchanger unit 23 supplied with fuel, e.g. gas, by a pipe 24 incorporating a valve 25. The valve 25 is controlled by a solenoid 26, which is connected electrically in parallel with the motor of pump 13. The electrical components are connected to an electrical supply at 27, under the control of a change over switch 28 which is itself operated in response to a sensor 29 which senses the temperature of hot liquid leaving the boiler. When the temperature of liquid is below a predetermined value, the switch 28 connects solenoid 26 and pump 13 to the electrical supply by way of the normally closed contact 30 of a thermal relay 31, the heating resistance 32 of which is connected when the change over switch 28 changes its state in response to an excessive temperature being sensed by sensor 29.

In operation of the system, when the valves 17 associated with radiators 14 are open the capacity of the system to dissipate heat will at first equate with the output of the boiler, so that the temperature of liquid leaving the boiler will not exceed the predetermined value. As the thermostatic valves 17 close after the heating system has been in operation long enough for the heated rooms to approach the required temperatures or if hot water only is required during summer conditions, the output of the boiler will exceed the capacity of the system to dissipate heat. In this case, the temperature sensed by sensor 29 will cause change over switch 28 to operate, thereby causing pump 13 to cease operating and cutting off the supply of fuel to the boiler by solenoid 26 controlling valves 25.The heating resistance 32 of thermal relay 31 is also brought into operation under this condition, to open, e.g. by expansion of a by-metal strip, contact 30. When the temperature sensed by sensor 29 has decreased, switch 28 is operated to its previous condition which would bring the pump of the system and burner of the boiler into operation again except for contact 30 being held open. Closing of contact 30 is dependant on the cooling time of thermal relay, thus delaying restarting of the boiler. The effect of this is that the boiler does not cycle rapidly between on and off conditions, which could occur were the delay in recommencing operation not present.

The valve 25 may be of a type which, in addition to solenoid control, is arranged to modulate fuel supply to the boiler by way of a mechanical or electrical connection 33 from sensor 29 or another sensor responsive to the temperature of liquid leaving the boiler. In this case, the effect of the delay is that after the boiler has shut down it recommences operation at a higher level of heat output, thereby avoiding excessive operation under the relatively inefficient low output condition.

Claims

1. A central heating system comprising a boiler, a piping circuit, a plurality of heat dissipating appliances, and means for circulating a heat transfer liquid in the system, the boiler including cut-out means for sensing the temperature of heated liquid leaving the boiler and operative to cut off the supply of fuel to the boiler upon said temperature exceeding a predetermined value, and delay means associated with the cut-out means and operative to prevent re-establishment of the fuel supply until a predetermined time has elapsed.

2. A system according to Claim 1 wherein the cut-out means progressively decreases the fuel supply to the boiler as the sensed temperature approaches the predetermined value, and the delay means provides a time interval sufficient for the supply of fuel to the boiler to be resumed art a level above the minimum sustained supply level to the boiler.

3. A system according to Claim 1 or Claim 2 wherein the heat dissipating appliances include room radiators provided with individual temperature responsive valves which operate proportionally in accordance with sensed temperature parameters.

4. A central heating system substantially as hereinbefore described with reference to and as shown in the accompanying drawings.