Just grabbed 2 Honeywell whole house humidifiers: HE360A1075. It's the best model Lowes has and recommends.

I have 3 floors and a basement in an old (1890s) victorian house. We have cast iron radiators with a gas boiler for heat on the bottom 2 floors (original) and separate ducting for the AC; then a regular AC/gas furnace ducting for the top 2 floors.

For the top two floors the install should be easy, I assume, as we have water and drain locations up there.

For the lower floors, can I install the humidifier in the AC ducting and have the thermostat power just the blower, and not actually turn on the AC?

I have 2 Nest gen 2 thermostats, which I believe can control the humidifiers too.

So - key question:

Can I install the humidifer on the AC ducting and use the AC blower, without turning on the AC? This should be controlled by the Nest to turn on whenever humidy is low.

Note - there is no heating furnace, just an AC unit.

  • Please improve your question. Your title asks about AC ducting, but you never actually describe the ducting in the body of your post. If you're asking about how to wire them up to work with your air handler blowers, then please re-title your question and put a question in the body. If you're asking about how to mount the humidifiers in the ducting, then please ask a question about that. If you want to ask both questions, then I think you should post two separate questions.
    – longneck
    Commented Nov 18, 2013 at 13:49
  • Is there a question here? Commented Nov 18, 2013 at 14:13
  • Yes, there is..
    – Alex
    Commented Nov 18, 2013 at 16:51
  • While it's possible to do, I don't think it's going to be very effective. Humidifiers work by evaporating water into the air to increase the moisture level. Your setup would be using room temperature air, and the evaporation effect is going to be greatly diminished compared to using heated air. You may have better luck with a whole-house steam humidifier, but they're also much more expensive. I think that coupled with the additional expense of running the blower motor all the time (when you otherwise wouldn't) might make using 1+ stand-alone model(s) a better (and cheaper) solution.
    – gregmac
    Commented Nov 18, 2013 at 17:26

2 Answers 2


Almost all thermostats have a FAN switch, that can be set to ON or AUTO. In the AUTO position, the blower comes on when the furnace needs it (during heat/cool calls). In the ON position, the blower will always be on. This is usually controlled by the G terminal on the furnace and thermostat.

I'm not sure exactly what you're trying to do, so I'll cover a couple different setups

Blower on when you want

For this situation, simply connect the G terminal on the thermostat to the G terminal in the furnace. When you want the blower on, set the thermostats FAN setting to ON.

Blower on when house is too warm

If you want the blower to come on when the temperature in the house raises above a set temperature, you'll simply connect the Y terminal of the thermostat to the G terminal in the furnace. This way whenever the thermostat calls for cool, the blower fan will come on.

Depending on the furnace, you may be able to connect the Y terminal on the thermostat to the Y terminal in the furnace, and simply not connect any control wires from the furnace to the A/C unit. Most furnaces activate the blower after a predetermined (possibly programmable) delay, when the thermostat calls for cool.

Blower speed (Multi-speed blower)

Blower speed may vary based on the different functions. For example, heat might be medium low, fan might be low, while cool may be high speed. So you'll want to take that into consideration as well.

Without more information about your specific equipment, and what exactly you're trying to accomplish. It's difficult to provide more accurate, specific information.


I don't know about NEST specifically however other thermostats with humidity control have the ability to switch on FAN while there is a call for humidity. Would check your product documentation for a similar setting. With this approach the NEST would both control the water solenoid turning on the humidifier and also at the same time switch on blower FAN if not already running.

With cool air hooking up the humidifier to your hot water supply will yield much better results.

My understanding use of a humidifier at any time where you would also be using AC throughout the day must be handled very carefully to prevent possibility of condensation in ductwork. I wouldn't try it unless the NEST has a supply temperature sensor and is able to run the dew point calculation or there is enough margin in any manual calculation. I should also add in heating season old homes and humidifiers can still be particularly dangerous, make sure you have an outdoor temperature sensor and your stat is able to dial back humidity to prevent any condensation within walls, windows..etc.

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