Wiring a Gas Boiler and a standalone whole-house humidifier (and mini-splits?!?!)

[ibcdave]

I recently moved into a new house with an older (1994) gas-fired boiler and hydronic baseboard heat, currently controlled by a simple two-wire Honeywell manual globe thermostat. The boiler has a C terminal, so I intend to run new wiring and install my Nest 3rd-gen thermostat that came with us from our old home. 

Also in this house is a standalone AprilAire 350 whole-house ducted humidifier that connects to a two-wire manual humidistat, directly adjacent to the manual thermostat. It runs on its own 120V circuit and has no wired connection to any other system.

My primary question is: can I run a new 18/5 thermostat wire and use 3 wires for the boiler (Rh, W1, and C) and the other two for the humidifier? If so, I know one would use the Star terminal, but what other terminal would be used? 

Of less urgency since it is winter now: this house also has a Fujitsu mini-split system with 4 heads for air conditioning. However, each head seems to only be controllable by its individual infrared remote, so I don't have much hope of connecting that to my Nest thermostat unless anyone has ideas!

I am still learning how these systems work together in this new home. My previous house had a simple central forced-air furnace and AC setup. 

[inb8264 · Accepted solution]

Hi there,

This is a very interesting question. The learning thermostats can control a humidifier/HVAC if the system is 24 Volts: https://support.google.com/googlenest/answer/9246656

I think you would need some sort of controller to stepdown the voltage so it can control the humidifier and power the humidifier. I recommend consulting a local professional to help with the install to the older system. I will note that a gas boiler is not currently listed as compatible with Nest Thermostats.
Please consults the compatibility checker if you have questions: https://store.google.com/intl/en_us/widget/compatibility/thermostat

[ibcdave]

Thanks for your reply. While I could not find a manual for the humidifier, I did find a label that states control power is 24V, so I don't think I need an additional transformer.

Based on the current wiring, my system does appear compatible with Nest gen3 thermostat as a "heat only" option. I haven't found anything that explicitly states it would be incompatible.