I have a 25 year old gas furnace without A/C. I had a 2 wire basic thermostat that I have replaced the wiring with 5 wires and also a new Nest 3rd generation thermostat. On the wall beside the thermostat is a humidifier control to run the fan that connects to terminal "G" in furnace. The new thermostat also requires terminal "G" for the furnace… Is it possible to have both the nest and humidifier control connected to the "G" terminal in the furnace or will something go up in smoke? The nest does not automatically run the fan when humidity increases, it only has a setting for a timed control.
Wiring – Possible to have more than one wire connected to “G” terminal in furnace
furnacewiring
Related Solutions
According to the documentation for your unit.
The HUM terminal is a 24 VAC output, energized when the blower is operating during a call for heat.
Connect an accessory 24 VAC, 0.5 amp. maximum humidifier (if used) to the ¼ in. male quick-connect HUM terminal and COM-24V screw terminal on the control board thermostat strip.
NOTE: If the humidifier has its own 24 VAC power supply, an isolation relay may be required. Connect the 24 VAC coil of the isolation relay to the HUM and COM/24V screw terminal on the control board thermostat strip. (See Fig. 32.)
So if the humidifier is less than 0.5 amperes, then you can simply use the HUM and COM terminals. Otherwise you'll need an alternate power supply, and an isolation relay.
Technically, there should only be one wire connected to the screw terminal. However, it's common practice in the HVAC industry to connect more.
If you want to do it the proper way, you should use a twist-on wire connector (or other connector), and a length of scrap wire to make a pigtail.
- Disconnect the wires that are currently attached to the terminal.
- Group together those two wires with the wire you want to add, and a short bit of scrap wire of about the same gauge.
- Use the twist-on wire connector (or other connector) to connect all the wires together.
- Attach the other end of the scrap piece of wire to the screw terminal.
You may also want to consider why there are already two wires. It's common in a furnace to have the A/C contactor coil terminated there, but you'll want to figure out where the other wire goes. The concern here, is that you could overload the transformer. If the transformer is operating at or near maximum capacity, the additional load of the thermostat could push it beyond the limit. Best case here would be a blown fuse, worst case being a blown transformer.
If the transformer is powering a humidifier or air cleaner, you'll want to make sure it can handle the additional load of the thermostat (though thermostats are typically a small load).
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Best Answer
As long as both thermostats are also using the same R wire, that should be fine. Internally, the only thing the thermostat does is connect R to G with a switch. Have you looked at all the documentation for the Nest? There may be a different, "best practice" way to connect the humidifier control.