This is going to completely depend on the make and model furnace you have. If you simply wire the humidistat to the 24V transformer, the humidifier will be active only based on the humidistat. This is not ideal, since the furnace has to be moving air for the humidifier to function properly. You want the humidifier to be active when the humidistat is closed (calling for humidity), and the furnace is on.
Locate the control terminals
If you check the schematic on the furnace (often found on the inside of the access panel cover), you may find there are separate terminals on the control board for a humidifier. In some models these are 24V terminals, in others they're line voltage.
24 volt terminals
If they're 24V terminals, you're in luck. All you have to do is connect the humidistat wiring to these terminals, and you're done.
Line voltage terminals
If they're line voltage terminals, you're going to need a second step down transformer. You'll connect the primary side of the transformer to the terminals on the control board, then connect the humidistat to the secondary side of the transformer.
No humidifier control terminals
If there are no humidifier control terminals present, you'll have to get creative. Check the furnaces user manual, as it may have instructions on how to connect a humidifier. Though a common solution to this problem, is to connect one wire to the
C terminal, and the other to the
W terminal. This causes the humidifier to be active only when the furnace is calling for heat.
- Always follow all manufacturer installation instructions.
- Do not forget to turn off the power to the furnace while working.