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How to connect fireman switch to pool heater
Pentair pool
Hayward pool

How to wire 2-speed poool pump
Some Intermatic 24-hour mechanical timers have a fireman switch that comes pre-installed, or can be purchased separately.

The main function of the Intermatic timer is turning the pool pump on-off. The pump circulates water through the pool filter, and if the pool has a heater, the water also passes through the heater.
Timers with the fireman switch have added function and can turn off pool heater 20 minutes before pool pump.

You do not want the heater running when the pump is off.
But the heater does not require a timer with fireman switch. The heater has it's own control system and thermostat that operate completely independent of the Intermatic timer. The heater has a pressure switch that turns off automatically when water pressure in the water line drops. So when pump turns off, the water slows down, and heater turns off.

In addition to the pressure switch, the heater also has high limit switch that automatically turns off heater if water inside pipe is too hot. The limit switch gives heater a back up system if the pressure switch or thermostat fail.

However in some cases, you want the timer to turn off the heater before the pump turns off, so hot water is not left inside the pipe.
Purpose of fireman switch is to turn off heater 15-20 minutes before the pump.
That ensures maximum utilization of hot water, and ensures heater is not ON unless timer is ON.

The fireman switch provides additional layer of fail safe for the heater. And then both pump and heater work together to maximum efficiency and protection.

Note: The timer and heater are electrically separate.
They do not share same circuit breaker or control mechanism.
The timer is a simple motor-driven device like a clock. The heater has a circuit board that is controlled by thermostat and pressure switch.
The timer can be 120 volt and heater 240 volt, or reverse. It is not necessary for timer and heater to have same voltage.
There is only one common connection between the timer and the heater: using the fireman switch if heater has fireman's connection.

The fireman switch connection must be available on the heater, and looks like two small terminals, or two wires, inside a connection compartment on heater. The product manual will show if the heater has fireman switch connection.

Small 18 gauge thermostat-size wires are installed from the connection on the heater, over to the fireman switch on the timer.
The small wires carry 24 volt provided by the heater power supply.
These wires are NOT connected to 120-240 volt terminals on timer. The only thing the two small wires connect to are the fireman switch wires.
It is advisable to keep the small wires separated, and not touching the 120-240 volt wires that are present inside the timer wiring compartment. It is preferable if the two compartments were separated by a barrier, but this is not the case.

Basically the fireman switch is a simple on-off  SWITCH. When it is ON, it opens an override on the heater that lets the heater run. When it is OFF, then the heater is OFF.
What if heater does not have connections for fireman switch?
Not every heater has a fireman switch connection.

What if heater manufacturer says a fireman switch is not necessary? That means the manufacturer believes the pressure switch is reliable. It says they believe the cost of a bit of inefficient water heating is better than answering 1000s of questions from people wanting to know what a fireman switch is. Plus the omission of a fireman connection reduces complexity of manufacturing and cost of product.
After all we need less efficiency, cheaper products, and modular simplicity to meet public expectations.

How can a fireman switch be connected to the heater if there are no fireman switch connections?

You can look at manual and wiring schematic to determine if heater has connections for fireman switch.
For example the wiring schematic will show temperature limiter switches, and water pressure switch.

A good method for connecting fireman switch if no connections exist:
If you have access to the limit and pressure switch wires, then cut one of those wires .... that creates two ends ... connect each of those ends to the fireman's switch.

There is another way.
For example, most heaters have electronic ignition system that runs on 24 volt power from a transformer or power supply... usually there is junction box, or control access door. located on the heater where you connect 120 or 240V to the 24 volt power supply.

Cut one of the 24 volt wires and you have two ends. Connect those ends to fireman switch.

Interrupting power to the circuit board runs risk that control board will not complete the firing sequence, and could damage circuit board over time.... but I don't know this for fact.
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