1) Fireman's switch automatically turns off heater few minutes before turning off pump.
Not all pool timers have fireman's switch.
If old timer did not have it, then don't worry about it.
P1353 has wires marked for fireman's switch, so simply don't use them if you don't want to.http://waterheatertimer.org/pdf/Intermatic-P1353-manual.pdf
page 21 of manual shows wiring for fireman's switch.
2) T104 and P1353 comparison
Difference 1: T104 turns OFF both hot wires, while P1353 turns Off 1 hot wire.http://waterheatertimer.org/Compare-120-and-240.html
Difference 2: T104 controls 1 240V load with 1 time schedule, while P1353 controls 3 different 240V loads, each with different time schedule.
Check that wire(s) coming from breaker can handle all 3 loads.http://waterheatertimer.org/Color-codewire.html
3) 240Volt T104 is wired as follows:
Hot1 from breaker connects to T104 terminal 1
Hot2 from breaker connects to T104 terminal 3
There is constant power on terminals 1 and 3, each terminal has 120V when combined makes the 240V circuit.
Load1 wire connects to T104 terminal 2
Load2 wire connects to T104 terminal 4
Terminals 2 and 4 have power whenever timer turns ON
In electric wiring, Line means power from breaker, and Load means wire going to light,fan, motor, pump.
5) Converting T104 to P1353http://waterheatertimer.org/images/P1353-timer-350.jpghttp://waterheatertimer.org/images/P1353-terminals-566.jpg
Hot1 from breaker connects to P1353 terminal 1
Hot2 from breaker connects to P1353 terminal 2
This will power timer electronics and timer clock and switches.
Jumper wire is placed from Hot1 at terminal 1 to each Line terminals 3 5 and 7.
This will put power at the switch, and when switch turns ON, then power will flow from Line wires to Load wires and then on to the Load.
Load 1 connects to terminal 4, Load 2 connects to terminal 6, Load 3 connects to terminal 8
To complete each 240V circuit, Hot2 is connected directly to each Load.
6) You say 'salt' system connects to terminals 1 and 3 on T104, then those wires are carrying constant 240V to that device. So Salt system connects to terminals 1 and 2 on P1353
7) Get a second opinion:
I frequently contact Intermatic for various problems.
P1353 is fairly new product, and manual is long, and I might not fully understand your problem or wiring.
Intermatic answers their email.http://www.intermatic.com/customer%20service.aspx
Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, expert speaks with you over phone while you work on timer or any do-it-yourself project.
Fixya is always less expensive than a service call.