There is no need to rewire the internal of the heater!
Just bring in the electrical supply to the heater, it can be either 120 or 240 volts.
If the heater you have is like the picture associated with your post, then all then have to do is change the supplied plug from 240 to 120. It comes pre set for 240. That plug is inside the cabinet.
If any other brand, then you need to change the wires at the transformer of the heater. All heaters need to be supplied with primary voltage: 120 or 240. Then the control system is supplied with secondary voltage out of the transformer. The secondary voltage is no more than 28 volts. and these wires are normally colored as blue or yellow.
Secondary power is already wired within the heater. NO need to change where they are. You will only need to deal with the Primary wiring.
There are many varieties of transformers that transform high primary voltage to a low secondary voltage of 28 volts. and these transformers have many wires: primary & secondary.
I will try to make this easy to understand.
If you have a Gas Heater Transformer with these colors:
- White with Red Stripe
- White with Black Stripe
These are Dual Voltage Transformers, you can bring in either 120 Volts or 240 Volts
If you bring in only 120 Volts...
The Red wire will not be used! Hot wire to the Black, and the Neutral/Common, to the white wire. Green to ground.
Connect the Red wire with the White with Red Stripe together and connect these to the incoming Neutral/Common wire, normally White.
Connect the Black Wire with the White with Black Stripe to your incoming Hot Wire, which is normally Black.
Green to ground
If you bring in 240 Volts...
1 Hot wire to the Red, the Other Hot wire to the Black, and White wire is not used. Green to ground
Wire nut the two Striped wires together. then one incoming Hot wire to the Black, the other incoming Hot wire to Red.
Green to ground
Or This is only a Low voltage - 120 Volts Only
Or either any of these combinations...
- 2 Blacks w/Green
- 2 Reds w/Green
- 1 Black, 1 Red w/green
This type will only be a high voltage, 240 volts only
Picture below is what a transformer looks like and it is where your incoming supply wires go. The smaller leads with the spade connections will go to a specific location & is the 'Secondary" .