fig1 is a schematic diagram illustrating a heating boiler 1 controlled by a control unit 10 according to the prior art , and more particularly the “ microtherm ” delayed cycle control unit mentioned above . the boiler 1 comprises a water tank 3 , an inlet conduit 4 , an outlet conduit 5 , and a burner 2 controlled in an on - off manner by a delayed cycle controller 11 in the control unit 10 . a thermal relay 6 provides a temperature responsive signal 7 which is switched on when the temperature in the boiler underpasses a lower temperature level . the signal 7 is further switched off when the temperature in the boiler exceeds a higher temperature level . the delayed cycle controller 11 is further arranged to produce a control signal 8 for input to the burner 2 , in order to switch the burner 2 on and off as a function of the thermal relay signal and the time . more specifically , the control signal is switched off immediately when the thermal relay signal 7 is switched off , and the control signal is switched on at a delayed point of time after the thermal relay signal 7 is switched on . the delay is calculated by the controller 11 as a certain percentage of the boiler &# 39 ; s cooling time , i . e . the time elapsed from the on - off transition to the off - on transition of the signal 7 provided by the thermal relay 6 . the control unit 10 is further adapted to communicate with an external computer 13 . a temperature sensor 15 is arranged to measure the temperature of the tap hot water in the outlet 5 from the tank 3 . the prior art controller 11 is arranged to compare the temperature measured with the temperature sensor 15 with a preset lower temperature level , and to start the burner if the measured temperature becomes lower than the lower temperature level . the cooling time delay percentage or “ e - factor ”, stored in the percentage register 12 , is a fixed value , which may be preset by the external computer 13 . fig2 is a graph illustrating the boiler temperature as a function of time , in order to illustrate the operation of the delayed cycle controller . at time τ 1 the signal 7 provided by the thermal relay 6 is switched off , and the controller 11 switches off the burner 2 . the cooling time of the thermal relay is measured by the controller 11 as the period ( τ 2 − τ 1 ). at τ 2 , the signal 7 provided by the thermal relay 6 is switched on . the delayed cycle controller 11 will not start the burner immediately , at time τ 2 , but delay the start until the time τ 3 . the delay ( τ 3 − τ 2 ) is calculated by the controller 11 as the stored delay percentage of the measured cooling time ( τ 2 − τ 1 ). fig3 is a schematic diagram illustrating a heating boiler controlled by a control unit 20 according to the invention . a boiler 1 comprises an oil burner 2 and a water tank 3 containing water heated by the burner 2 . the heating plant further comprises a hot - water conduit 5 which may be further connected to a number of taps ( not shown ) and / or a room heating system ( not shown ) provided with a shunt valve , circulation pump and radiators . the heating plant further comprises an incoming conduit 4 for supply of cold water as well as a chimney ( not shown ) for discharging fluid gases resulting from the combustion of oil . the control unit 20 comprises an improved delayed cycle controller 21 and a time delay percentage register 12 . similarly to the delayed cycle controller 11 of the prior art illustrated in fig1 , the improved delayed cycle controller 21 is arranged to input a temperature responsive signal 7 provided by a thermal relay 6 which is arranged to sense the temperature of the water present in the water tank 3 . the thermal relay 6 causes the temperature responsive signal 7 to be switched on when the temperature in the boiler underpasses a lower temperature level and to be switched off when the temperature in the boiler exceeds a higher temperature level . further in correspondence with the prior art illustrated in de 10057942 , the delayed cycle controller 21 is arranged to perform a delayed cycle control function which is described in the following . when the thermal relay signal 7 is switched off , the controller 21 deactivates the burner control signal 8 , which leads to that the burner 2 is turned off . this occurs substantially immediately , such as within a couple of seconds , i . e . with no essential or intentional delay . the delayed cycle controller 21 is further arranged to calculate the cooling time , i . e . the time ( τ 2 − τ 1 ) of the temperature decay from the high temperature level t 2 to a low temperature level t 2 . this is performed by measuring the time elapsing between the on - to - off transition and the off - to - on transition of the signal 7 provided by the thermal relay . the delayed cycle controller 21 is further arranged to read a percentage value from the time delay percentage register 12 , and to calculate a time delay , which equals the cooling time multiplied by this percentage value . when the water temperature indicated by the temperature signal 7 falls below the low temperature level , the signal provided by the thermal relay is switched on . this off - to - on transition of the thermal relay signal does not lead to that the burner is switched on immediately . instead , the controller is arranged to switch on the burner when the calculated time delay has elapsed . in this way , an extension of the off - time of the heating boiler is obtained , which leads to a reduction in the total consumption of energy , i . e ., oil , by the burner 2 . if the percentage value is set to zero , the delayed cycle controller will act as a conventional on - off controller , i . e . with the same function as if the thermal relay controlled the burner directly . the temperature control unit 20 is further arranged to communicate with an external computer 13 via a communication connection 14 . the communication connection 14 may be based on a wired connection , such as a serial communication , or a local , wireless connection such as an infrared or a radio based ( e . g ., bluetooth ) connection . alternatively , the communication connection 14 may include a telecommunication network such as the public switched network or a cellular network . the percentage value may be set and / or read out by the external computer 13 via the interface 26 . a novel and distinct feature of the invention is the percentage value calculating module 23 , which is a part of the improved delay cycle controller 21 . the module 23 is arranged to change dynamically the delay time percentage value stored in the register 12 , in accordance with changes in the temperature measured by the temperature sensor 15 . the function of the percentage value calculating module 23 is further described with reference to fig4 below . in a preferred implementation , the delayed cycle controller 21 , including the percentage value calculating module 23 , is implemented as a software module , for execution by a microcontroller employed in the control unit 20 . the time delay register 12 is a part of a memory connected to the microcontroller . the memory comprises a random access memory for variable data and a non - volatile memory containing fixed data and program instructions . the implementation of the program instructions is an ordinary task for a person skilled in the art , based on the description given in this specification . fig4 is a flowchart illustrating the process steps performed by the improved delayed cycle controller according to the invention . the process starts at the initial step 31 . in step 32 , the temperature t 1 at time τ 1 , i . e . when the thermal relay 6 is switched off , is measured and stored . then , in step 33 , initiated by the thermal relay 6 being switched on , i . e . at time τ 2 , the temperature t 2 is measured and stored . then the time τ 3 is determined by calculating the cooling delay as ( τ 2 − τ 1 ) multiplied with the delay percentage stored in the register 12 . at time τ 3 , in step 34 , the temperature t 3 is measured and stored . next , the calculating step 35 is performed . the temperature difference t 2 − t 3 is calculated , and so is the temperature difference ( t 1 − t 2 ) multiplied with a predetermined , stored constant ed , the value of ed is typically between 0 . 25 and 0 . 75 . most preferred , ed is about 0 . 50 . then , in the decision step 36 , the results of the preceding step 35 are compared . if t 2 − t 3 & lt ; ed ·( t 1 − t 2 ), step 38 is performed , wherein the percent value stored in the register 12 is increased by an increment value es . else , the percent value stored in the register 12 is decreased by a decrement value , typically identical to the increment value es . the increment value is typically between 2 % and 10 %, and most preferred about 5 %. in either case , the process continues at step 39 , which brings the process control back to the initial step 31 for the processing of a next temperature cycle . although the heating device mention in the detailed description is an oil burner , the skilled person will also realize that the invention is applicable with other heating devices as well , e . g . an electrical heater or a gas heater . further variations and alternative embodiments will be evident to the person skilled in the art .