I got the Aprilaire 600a installed tonight. I went ahead and put the controller on the return trunk. I didnt have time to install the outdoor probe, so I set the unit for manual mode in the MAX setting.
I wired it to come on whenever the HP is running.
I did some testing and found out that if I rely solely on my Venstar T1900 and it calls for humidity, it opens the valve on the Aprilaire, even if the HVAC isnt running. I can set the Venstar thermostat to turn the fan on if there is a call for humidity, but I dont want to do that as I fear the blower will run most of the time to try to get the humidity up.
So instead, I am going to wire a relay in so that even if the Venstar calls for humidity, a relay wont open the valve unless the HVAC is running. This gives me the best of both worlds as my thermostat will control the call for humidity and the Aprilaire Controller will fine tune the humidity levels based on the outdoor temperature using its probe.
Fortunately for me, it will be easier to dial it all in with all the monitoring I have setup.
(Charts are at the link at the bottom of the page.)
Thanks all for your input.