Manually controlling the boiler thermostat of your central heating system according to the season will require the boiler to be turned on and off too many times, especially during the winter days, as indicated in the following graph.
The solution for this is the circuit explained below that prevents the switching on of the boiler for a certain period td after the threshold temperature for switch-off, T2 has been attained.
The boiler temperature T would have fallen far below the critical temperature for switch-on, T1 by the time td has passed.
How the Circuit Works
Referring to the circuit diagram below the result is obtained by extending that of the central heating monitor, connecting the make contacts of a relay into the boiler circuit of 24 V.
Depending on the state of the N2-N3 bistable, the transistor T1 will be on or off, or in other words, the 24 V circuit will be open or closed. T1 starts conducting, td starts and the IC2 reset is cancelled from the moment the bistable gets set.
When IC2 reaches maximum value after a period, the logic level selected by S1 at the output changes and this resets the bistable through N4. This marks the end of td.
N1, R3-C2 and R2-C1 connect the set input of the bistable to the collector of T3 in the central heating monitor circuit. This transistor controls the LED that acts as the indicator for the interrupted heat request for the thermostat.
The time range for td is wide and can be controlled using P1 and S1. A value of 10 minutes for td restricts the turning on and off of the boiler to 6 times in an hour.
How to Set up the Central Heating Controller
You can, of course, change td by changing P1 and S1 by noting how many times the LEDs in the central heating system for this lights up. You must set the bistable by connecting the R3-C2 junction to earth for a moment to extinguish D3 and start td.
Then change P1 to set the time after which D3 lights to 5, 10 or 20 minutes. Other possibilities are 4, 8 or 16 and 6, 12 or 24 minutes.
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