I just moved into a new home and it has a standalone bypass style humidifier (Aprilaire 550). This already has a humidistat it connects to. DIY forums elsewhere make me think most Aprilaire humidifiers work with Nest, so I'm not particularly worried about having a humidifier without separate dehumidifier wiring. I know the Nest Learning Thermostat is needed, not the traditional. What does worry me is some text from Google's support answers regarding the topic below. This phrase, and others on the support page, make me think that the Nest Learning Thermostat is going to perform humidifier / dehumidifier functions if and only if my humidifier is integrated with my air conditioning / furnace / HVAC system. Has anybody successfully installed and used Nest Learning with a bypass humidifier, ideally an Aprilaire?
Important: If your thermostat has a DEHUM or dehumidifier wire, it doesn't mean that your air conditioner has a dehumidification mode. The wire could connect to a standalone dehumidifier that is separate from your air conditioner. From https://support.google.com/googlenest/answer/9255237
There are multiple ways to perform a de humidification process with an air conditioning system. Number one there is the ability to change the fan speed to increase the ability of the system to collect more moisture out of the air. Basically, a D humidification, standalone system is a very small air conditioner only it doesn’t blow air into the room what it does is create a very cold coil to condense the moisture blown slowly over that coil and allow it to drip into a collection tank. With a whole house, air conditioner, we can perform the same function by slowing down the blower speed and allowing the air to stay in contact with the coil longer thus removing more moisture. This is very common approach in the tropical areas of safe Florida now with a nest thermostat is wired with a D humidification function. It can be one of two things. It can signal the control board in the air handler to switch to low speed fan, or it can turn on a standalone dehumidifier.
The humidification system is only going to be activated when the gas furnace is turned on. Since your humidifier has a standalone humidistat, all the thermostat will do a signal dehumidifier to turn on all the control of humidity and the run will be done by the actual Aprilaire 550.
The AC Cooling Wizard
-oh that’s why the interesting, I have several of these installed and neither one of them has a problem reporting the relative humidity and I’ve even checked with a standalone humidistat and a standalone humidity monitor.
AC Cooling Wizard