Timer for house ventilation switch - high/low voltage wiring
This looks like a standard double pole switch.



Essentially, it is two independent switches both controlled by a single toggle. The two sides of switches are not connected to each other.

In your box, it looks like you have two separate switch loops, one going to each side of the switch. The cable on the left is supplying a hot lead an a return switched hot (probably to the 120V bathroom fan). The white wire is not a neutral, it is being used as a hot. Code requires that the white be marked with black tape or a marker to show it is hot.

The right cable (probably 24V furnace) uses red and black for the hot and the switched hot. This is standard color code. It seems that the right wire (lighter gauge) is the low voltage circuit. 

The specific questions:


There is no transformer in the switch. The low voltage is provided by a transformer elsewhere, probably near the furnace.
There does not appear to be any neutral wire in this box. While that does not conform to current code, up until recently it was common to run a switch loop with only hot wires, using the white wire (properly marked) as a hot. Current code requires running a true neutral to each switch box (with a few exceptions), even if it is not currently needed.
It is likely that power is provided directly to the box that the fan is connected to, but the hot line is not connected directly to the fan, it is connected to one of the wires running to this switch and then back to the fan on the other wire from this switch box.


While this setup works, I am not sure if current code allows a high and low voltage switch combination, or even high and low in the same box without a barrier between the sides. (NEC 725.136(B))

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