See wiring diagrams and manuals at following link:
http://waterheatertimer.org/Paragon-timers-and-manuals.html#defrost8145-20 is 240V time initiated, pressure or temperature terminated with back-up time termination.
8045-20 is 240V time initiated, time terminated.
So 8145-20 has pressure or temperature limit switch that connects to terminal X that turns timer off, and ends defrost cycle. This timer is for hot gas defroster that has pressure/temperature limits. Check and see what the back-up time termination is set at on 8145-20. Look at small inner dial.
The 8045-20 timer does not use pressure/temperature limit switch, and instead you have to estimate length of time you want defrost cycle to run, and set inner dial to that time. Use same time as back-up time termination on 8145-20. Of course the pressure/temperature limits would not be exact and defrost cycle might not work correctly.
Here's the change-over wiring from 8145-20 to 8045-20.
There are other wiring variants depending on equipment.
240V hot wires from breaker box are L1 and L2
a) 8145-20 L1 is at terminal 1 > move L1 to terminal N on 8045-20
8145-20 L2 is at terminal N > move L2 to terminal X on 8045-20
b) 8145-20 hot gas defroster is at NO terminal 3 > move defroster to NO terminal 3 on 8045-20
Complete the 240 circuit to defroster by connecting to L2 at terminal X
c) 8145-20 refrigeration is at NC terminal 4 > move refrigeration to NC terminal 4 on 8045-20
Complete the 240 circuit to refrigeration by connecting to L2 at terminal X
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