Good to know that they don't all make the whining noise, I may dive deeper into mine and see if I can pinpoint the issue, but its honestly not a big deal, just something to note. The controllability and built in logging is really great for the price, probably the best option I have seen for a low cost ventilated setup. I have been toying with the idea of using a variable speed fan to cool a normally sealed flower room in the winter, with the idea that since the outside temps are in the 30's and 40's here for several months I could save on ac and some dehumidification costs by using a "smart" fan. I figure it would not take very much air exchange with air that cold to cool it, so I could still run co2 if I used a burner piped into my natural gas. would probably use more gas than normal but I think the electricity savings from not running air conditioning and using less dehumidification would offset this. I tried it once roughly 10 years back with a couple centrifugal fans that were switched on and off by a controller if the temps reached the setpoint, but there was too much variability in the outdoor temperatures for it to keep tight control of the environment, and the area I lived then was much warmer and more humid even in the winter so the fans had to run most of the lights on time to cool it, so running co2 was out. Also the outdoor humidity was high and all over the place so the dehumidifier actually had to work harder than when it was sealed. That was with hps lights though, the led's I have now produce less heat and I run the temps hotter so in theory I think it should work.
What size tent are you runnning your t6's and s6's in? are you using one for intake and one for exhaust? I am still dialing in the settings on the t8 in my new 4x8, I would like to run as much airflow as possible through the bottom vents as it seems like a great way to get some under canopy ventilation. However with the t8 cranked up and everything sealed up but the mesh vents there is huge negative pressure, like to the point I worry it could be detrimental to the lifespan of the tent. I havent moved plants in there yet as I am still testing different combinations of lights and fan speeds/ temperature. anyways I am interested in your setup, I have never really grown in a tent before and while it has some advantages there are some unique challenges as well.
My tent is fairly small... the rough equivalent of a 4x4, though it's 5x3 (to fit the space I have.
But yes, I have the T6 for exhaust up above (in tent), with a Terrabloom filter in front of it, then it ducts out the tent and to a basement level window (about 4' of ducting).
I have the S6 down at the bottom as an intake (outside the tent, with a bit of duct on each side of the fan). For that setup I need the S6 to run damped back a bit so it doesn't create positive pressure in the tent. I use MERV13 filter fabric over the open intake duct to not only filter the dust/dirt out of the air, but to reduce the airflow to keep the negative pressure. Depending on if I use 1 layer or a couple layers I can damp down the intake airflow though the fans are using the same settings with the Controller 67.
This with the 67 gives a reasonable amount of control over the grow environment, given I don't have AC. During the winter I have a small in-tent humidifier, then during the hotter weather when it's humid I run dehumidifiers in the 'lung room' (actually the whole basement level) where I want to control humidity up or down seasonally and not just for plants...we have a music room/studio space that holds my guitars, basses, amplification gear and electronics; a fairly decent ceramics studio & a large room for storage. I try to keep that floor in the 40-45 range which is comfortable for instruments, ceramics drying as well as giving the tent drier air when the fans kick in stronger.
In terms of running an in-duct with really cold air...that may cause you humidity issues. Cold air meeting hot creates condensation, so something to consider. Also, I use a backdraft damper on my exhaust duct at the window. That guarantees that cold air doesn't enter my exit, should weather conditions and air pressure push cold air in my exit.
Either way, the only time I really have difficulty in controlling conditions is the middle of summer when you're just dealing with lots of ambient heat and humidity.