Hello from Essex UK,
I’m a newbie and after many years dreaming of my own bees I installed a nuc in my flowhive brood box this morning: very exciting and all looks good with bees buzzing in and out.
I can’t seem to find a definitive answer on whether to place the white correx bottom board in the upper or lower position. There appears to be conflicting advice on this subject on the forum.
Any suggestions please?
Hello and welcome. I am also a newbie and was wondering this same thing when I got my bees. I keep mine on the very top slot up close to the screen.
I am sure there are many reasons for the 2 positions (ie, pest control, ventilation, etc) but my decision was based on a youtube video by HoneyFlow I watched. The link is below. The information I am talking about is at the minute 1 mark.
Again, this is what I decided to do and I am sure I will learn a lot more from other replies to your topic.
I put mine in the top slot now because the chalkbrood mummies were getting lodged in the wire screen and the bees could not remove them. There is no Small Hive Beetle or Varroa mite where I am, 
so take that into account.
Like the other 2 posters, I put mine in the top slot also, but then I generally prefer solid bottom boards, so the slider converts it to what I like.
In case you are interested, my detailed reasons for using the top slot are:
I love my Flow hive, but the slider is staying in the top slot. If it gets stuck, it can be loosened by inserting and wiggling a long metal rule above the white plastic. 
Thanks so much everyone who has replied. This all makes sense.
I’m heading down the garden to put it in the top slot. Have a great weekend
It depends on how hot it is as it’s for ventilation as well as a method of tracking veroa concentrations. Cooler weather upper warmer lower really hot remove as I understand it.
I think that is a controversial statement. I agree that it is the received wisdom, but I am not convinced that it is accurate.
Bees ventilate the hive by fanning. Over millions of years, they have developed a method for lining up and creating a “laminar flow” of air to ventilate the hive very efficiently. Laminar flow only works if the hive has all solid surfaces. Hollowed out tree trunk hives do not have screened bottoms. 
If you open up the bottom with a screen, the laminar flow becomes much less effective. @JeffH has a beautiful video of bees lining up to circulate air around his observation hive. I think a screened bottom would have made their efforts ineffective.
My summers are often in the 80s or low 90s Fahrenheit (28 to 35C), and with solid bottom boards or the screen in the upper slot, I have seen zero bearding, so I don’t think the bees are overheating. 
Of course my slatted rack may help (salted rat for @skeggley ) as it gives “hanging out” and fanning space at the bottom of the hive.
Jeff I saw this in my hive when I put an entrance reducer in the cold weather keeping the hive warm so it’s a great example how the bees do interior heating and air. 
HUH? the video didn’t mention if the bee emits the wax from the abdomen or what!!??? That was interesting! That was so cool!
Thank you Martha, the bees have 8 slots that the wax emits from. They convert honey to wax. However that takes many hours to happen. The bees hang off each other in long chains. The bees at the top of the chain eat lots of honey, it’s the weight of the bees pulling on the body after many hours that converts the honey into wax. Then comb builders grab those opaque wax flakes, mix it with saliva, that turns it white while she constructs the comb.
That’s the way I understand it. They show that in the video “City of Bees” on Youtube.
@JeffH, love the video of the wax flakes being secreted. At the very end when you are saying ‘thanks for watching’ the little bee waved her left hand to the viewers. So cute! What a cooperative bee!
Thank you Cathie, I didn’t notice that.
I put mine in the dumpster and nailed plywood to the bottom to seal it off.
That is fascinating! After I read this post yesterday I got on youtube and looked for other videos explaining this. However, you just summed it up all neat like! I’m going to check out the “city of bees.” The more I learn in my venture to #helpSaveTheHoneyBee the more addicting it gets. The bee civilization/colony is very interesting indeed and you make it so as well!
Wow, thank you Martha 