so I can't be sure I didn't screw something up.

For that reason, this is a lesson in futility. I can only imagine what you've done. I know I'd make more $$ being a psychic, but I can't be something I'm not.

The system will have to be diagnosed in person by someone who knows and an assessment provided to determine if you did something wrong or a part of the existing system has failed.

Short cuts have provided you with what you have now. This is why DIY is never recommended in the HVAC servicing realm. Rarely does one get ahead doing such things.

Here are the board installation instructions in case you don't have them

The fan delay is set for 90 seconds at the factory. Your board might set to 60 seconds which is the minimum. When you turn the thermostat to the "On" position, have you waited at least 60 seconds for the fan to start? Other than that I don't see why the fan should not be turning on.

Thank you for your helpful and supportive comments kevinande! Agreed, hiring a "pro" is hit and miss... last guy I had working on my HVAC damaged a sensor, which I fixed myself.

And thank you mike_home, I have previously referenced the directions to no avail. If the blower is set to "On" then the delay shouldn't apply. (I did set it to 180 seconds yesterday to help extend the on time, but it does eventually turn off.) One thing I thought was strange with my setup is that the "Fan" header on the control board has nothing plugged into it, though it shows in the directions that it should be connected to the blower. This was not changed as part of the humidifier installation so I assume it was installed this way originally, and may be correct.

Considering that the humidifier installation was fairly complex and works, and plenty of complex projects I've done in the past were successful, I'm reasonably confident that I didn't cause the issue. This is why I added the troubleshooting information, to show that the issue appears to be localized to the control board. I'm hoping this is a typical issue with this control board and someone else has seen it in the past.

I agree the fan should come on immediately with the "On" switch is engaged. I was suggesting waiting to see if that may have been the issue. It appears it is not.

I am not familiar with this board, but I will give you ideas for things to try. You stated the Fan header is not connected. Perhaps that is the problem. The blower may only be turning on when in heat or cool mode, but it cannot in the fan only mode since there is no connection. Are you able to put your voltmeter on the fan header connection? If you can, you should see it energize when setting the thermostat to "On". I also see connections labeled "Park". Are these extra wire connections? Does the blower have a wire that can be moved from "Park" to the "Fan" connection?

Don't assume the blower was wired correctly when it was installed. Even the 'pros' can make an improper installation. Improper installations are the majority of discussion of this forum.

however offering some help or advise, beyond call a professional would have proven more useful than cynical comments .

Kevinande, The problem with your comment is that HVAC is a skilled trade in which codes and procedures must be followed to prevent injury to unskilled person working on such equipment, damage to equipment in question, damage to the building in which the equipment is located and health related problems of occupants at a later date due to the lack and skill of those attempting these kinds of things.

The help I offer on this board is advice for those who appear to me, not attempting to do the work themselves but hiring professional help. This is a tough love situation.

Your home is one of the biggest liabilities you will have in your lifetime. In my experience you only make things worse attempting to do things you do not have the skill or knowledge to do.

I have been running HVAC service for 24 years. I will never support a DIY attitude because: in every situation in which the home owner has attempted such things before calling me has only made matters worse, increased the cost to fix the problem up to an including the need to pull equipment and replace it.

If you fail to heed my warning, you get the brunt end of the stick.

It's because I care I say these things. [Tough Love]

With Mike's suggestion, I dug into the "Fan" header and found that it was indeed providing power when I expected the blower to run. The blower had two unused wires (parked) and some research revealed that they are for different blower speeds, based on impedance between them and the neutral wire to the blower. I picked the higher impedance one (for "low" speed) and connected it to the "Fan" header. This worked perfectly. Now the blower runs on "low" at all times (I have my fan option set to "On"), and only runs "high" speed when the heat comes on, which is perfect for circulation and reduces wear on the blower motor since there will be fewer cold starts. So, turns out, the system was installed incorrectly in the first place. This is resolved! Thanks!!

Good work Darin! I am happy to hear you were able to trouble shoot this issue and easily resolve it. You may have a second career as a HVAC tech. :)

After paying on average of $500 to repair several A/C units in my rentals, I did a little research and discovered that all had capacitor issues costing between $9-$15 in parts and 15 min of my time. Hvac contractors are absolutely an asset to have for major issues but charging an hour and a half to replace a part that takes me 15 min is straight theft.

Why would someone get on a DIY forrum and go on and on about not DIY'ing?