As far as I know, the humidifier is controlled by the Infinity control. It was programmed to run only when the furnace runs (as opposed to running with the fan). I was wondering if there is a reason to choose one mode or the other.
You can do things the usual way- to run the humidifier only during a call for heat. This is normal for evaporative humidifiers (which are by far the dominant type).
You can also set the system such that it is allowed to run the humidifier anytime there is demand for humidification. The system will make sure to run the fan anytime it is attempting to humidify, so that the humidity can circulate and not just stick in the ductwork. This is normally only used for steam humidifiers, which are unusual because they are expensive to buy and operate.
Evaporative humidifiers blow hot air from the heating system across a wetted wicking surface; this of course results in evaporation, which humidifies the air. If you desperately need more humidifier capacity, you can do the same thing when the system isn't heating (ie using the second setting) without hurting anything, but you won't get much evaporation without the air being hot. So for a very small increase in the amount of humidifier output, you use a lot of water. Most evaporative humidifiers use 6 gallons of water per hour of operation. That's acceptable if you're at least managing to get good evaporation, but by running the humidifier with room temperature air, almost all of that water goes down the drain. If we assume that your humidifier isn't keeping up with your humidity setpoint, and your heating system runs for 4 hours a day, with the standard setting you're using 24 gallons a day for humidification. Switching settings, you're likely to have the humidifier run the other 20 hours of the day, using an extra 120 gallons per day, and probably only gaining 1-2% extra humidity in the process (if any).
Steam humidifiers actually boil water to make steam; room temperature air has no trouble absorbing steam. So with that type of system, it doesn't really matter if the heating system is running or not; the humidifier can run productively whenever it's needed, so long as the fan is running. They work very well, and are usually more efficient with water, but they are more expensive to buy, require a dedicated electrical circuit to run, and they use a great deal of electricity. Many people who have gotten them have commented that the steamer doubled their winter electricity consumption (and bill).
For most people, an evaporative humidifier (that runs only when the heating system is on) works fine. In most cases where that setup isn't adequate to humidify a house, it's because the heating system is oversized for the house, and as such is only running for a few minutes here and there. To get full output from an evaporative unit, the heating system needs to be sized properly so that it can do the job slowly and steadily (ie during cold weather, heating for more like 12 hours a day than 2).
An Aprilaire 600 humidifier comes standard with an "Auto-Trac" humidistat that will automatically shut down the humidifier when the outdoor temperature reaches 70 degrees to insure that it doesn't operate in cooling mode - if you installed the outdoor temperature sensor that came with it. You did, right?
My favorite way to wire up Auto-Tracs are to use the 110v EAC terminal on the furnace control board to power up the transformer that runs the 600 bypass humidifier itself, so it can operate in fan mode with no call for heat. On an Aprialire 700 power humidifier I power up the 110v outlet receptacle with the EAC terminal. In both cases this eliminates the need for a current sensing relay.
I've found that indoor humidity levels can be considerably higher when the humidifier is allowed to run during fan only operation rather than just on a call for heat (depending on the application). I have never had any problems with it and have wired hundreds of units this way. I have a lot of customers who were not happy with the indoor humidity levels when the humidifier operated only in heating mode and saw significant improvements when the humidifier was allowed to operate in fan only mode (many of them had basic hygrometers to tell the difference). The most important factor is that the humidifier is connected to hot water and not cold (or humidity output will be reduced even more). This helps somewhat to make up for the difference in air temperature. Also, make sure they keep their variable speed Infinity furnace in the Fan On mode for lots of reasons: better air circulation, better air filtration, and better humidity output.
Wyounger, thank you. That is very helpful to understand.
With temperatures having cooled off a bit, the humidity levels have taken a fall as well. They are within the expected range considering the outside temperature. I was not sure if you could effectively "overhumidify." It sounds like you can't do too much.