mike_home, you may be on to something with your last paragraph. Perhaps the second stage is short cycling and that may have something to do with it.

Let me describe the situation in different words.

We previously had an old-fashioned (and I mean OLD) furnace that worked fine and the Aprilaire worked fine. The Aprilaire operated when the blower turned on and remained on if added humidity was required during blower operation. Keep in mind this is a bypass humidifier - we're only adding humidity when the blower is on because the furnace is on. The Aprilaire is controlled by the furnace (blower). The Aprilaire does not control the furnace.

So a few years ago we needed a new air conditioner - evaporator and condenser. The service people we received quotes from would not install a new air conditioner unless they also installed a new furnace. I hated to see that furnace go. It was half the size of a Volkswagen Beetle but it was rock-solid, easy to work on, low-tech, old-school. So I relented and allowed the installer to put in an new furnace as well as air conditioner. That's when the issue with the Aprilaire came up.

I immediately asked the installer to "fix" the operation of the Aprilaire but he insisted it was operating correctly.

Here's an idea of the timing (not precise):

T=0 minutes: the burner ignites and the Aprilaire solenoid opens allowing water to flow through the corrugated mesh. The blower is not on. All that water is wasted, it just goes down the drain.

T=2 minutes: the burner/baffles are hot and the blower turns on. The Aprilaire solenoid is still on and how humidity is being added to the system. Any excess water goes down the drain.

T=5 minutes: the burner turns off, the Aprilaire solenoid turns off. The blower is still running to extract heat from the system.

T=7 minutes: the blower slows and ultimately turns off.

In this scenario, the water has been running from T=0 to T=5 (5 minutes). However, humidity has only been added to the system from T=2 to T=5 (3 minutes). I want the water solenoid to be active, and therefore adding humidity to the system, while the blower is on from T=2 to T=7 (5 minutes).

After doing some additional research I realize Aprilaire sells a clip-on relay that attaches to the lead of the motor. This approach will allow me to power the water solenoid only when the blower is on. I'm going to investigate this approach.

Screw it. I tried twice to explain that the following is simply not correct! I'm just not up to the task.

"T=0 minutes: the burner ignites and the Aprilaire solenoid opens
allowing water to flow through the corrugated mesh. The blower is not
on. All that water is wasted, it just goes down the drain."

What is the thermostat model? You should have a 2-stage thermostat so it can control the low and high stages of the furnace. These thermostats have a W1 and W2 wire connection. If you don't then the staging is being done by the furnace control board. That is a poor set up for a 2-stage furnace in my opinion.

The thermostat is an Emerson 1F85-0422 and the instructions indicate it controls 2-stage units. However, the installer may not have used the correct wiring or otherwise took a shortcut...

The furnace board does not have any wire connected to W2. Here's a picture:

...and a picture of the thermostat and connections

If you have a spare wire in the bundle then it could be used for the W2 connection. If you don't then you will have to run new wiring. If you are going through the trouble of a wire, then you might as well run a new wire bundle so you can also connect the C wire in the future. You will need it if you decide to get a wireless thermostat with humidity control.

If you're going to run new wire, the best option would be to add wiring for a humidistat control, add wiring control for second stage heat of the furnace and control the humidifier indirectly and separate it from the thermostat. The thermostat connectivity control for a humidifier is like a shot in the dark.

Trying to control the humidifier this way is, for the lack of a better term... 'crazy talk'.

Your HVAC provider really dropped the ball on this, as this should have been part of the package they sold you.

If I've said it once, I've said it a thousand times... the devil is in the details.

If it were my house I would want the humidity sensor in the living space and not in the return duct. I also would want an integrated thermostat and humidity control.

Instead of spending money running more wires, you could get a wireless system. An example is the Honeywell Vision Pro with Redlink. With this you only need two wires for power or no wires if battery operated. You can use it to control the humidifier. You could also use it to control the AC in the summer with the cool to dehumidify feature. But if your AC is single stage only it may not be as effective as a 2-stage condenser.

There are several choices to control the HVAC equipment. Some are better than others and prices will vary. You have to decide what is important to you.

Thanks for the comments. I'll look at some of the options presented and give them some thought. I've also been in touch with the installer and will speak to them again next week. Hopefully I can get them to do the right thing by at least running the new wire bundle. The installer also installed the original Aprilaire and they knew there was a humidifier in the system when they sold us the new system AC and furnace.

You can install the one I mention above outside of the return, but you have to run the sensing tube into the air return. So it would have to be mounted on the wall of the air return with a hole that penetrates the return for the sensing tube to be mounted.

This may not be feasible, so this is why Honeywell also instructs you to mount it in return duct. Typically once you set it up, it's not a setting that will get changed because if the humidity is too high (if set up right) the humidifier will not come on.

But to each their own. I typically only set up wireless controls when there is no feasible way to run wires... as wire runs are longer term solutions with many more options to chose from in the form of different controls.

Remember, you're talking to someone who is called to trouble shoot problems with these kinds of systems. Nothing is better than a hard wired solution.

OP here... I forgot to take time to say that I purchased the AprilAire model 50 (24V) sensing relay and installed it as described in the directions. The furnace and humidifier now work exactly as I expect them to - that is, water is only provided to the humidifier when the fan is blowing. Lots of useful discussion above but in the end I simply followed the directions for the relay and it works as planned. I tried several times to get the original furnace installer to make this change and they wouldn't do it - claimed I didn't know what I was talking about. That may be true, time will tell. For now (the past two winters) it's worked like a champ!

Sometimes you have to do your own research to find a good solution. I am glad to hear it is working well for you.

I have the same issue, although I wired my humidifier so that the water is running ALL THE TIME. Still it doesn't humidify with the new two-stage furnace like it did with the old single stage furnace. Could it be airflow? I can't figure this out for the life of me and it's driving me nutso.