Ive done 2 grows now and its became obvious that i have temp problems. Ive tried a few things with little or no improvement now.
im now thinking of going from 6 inch to 8 inch extraction with a reducer attached to my 6 inch hps lights. will this improve things as I could do without upgrading my lights.
My rooms 3x2x2.5 metres and im running 2x1000 hps
if my conversion is accurate. your grow space is 9 feet x 6 feet by about 7 feet?
i have a similar size space running. its 10x10 feet. with 1000wHPS and a400wHPS.
what kind of fan(s) do you have running?
is duct running long distance? you will lose power the farther it has to push
what is temp of fresh air being drawn in?
right now, it is very cold where i live. so managing temps isnt hard. but i will have to buy an air conditioner before the warmer months arrive.
the fresh air being drawn into my space now is like 40degreesF. and my temps are almost too low. 77 with lights on. 66-68 with lights off. i actually had to put a heater in there for lights off time. or it would drop into the 50’s.
there are many ways to setup ventilation.
i have 1 fan drawing in cool air,…and blowing through lights and then out of grow area.
i have a 2nd fan that is strictly for pulling warm/stale air out of grow area.
upgrading your duct from 6" to 8" may improve slightly,…i dont think it would be major difference.
sounds like you need an A/C
There is also positive and negative air flow. Like for instance i have 2 fans running . one blowing in and second blowing out. The one blowing out was smaller and set up poorly. The first was larger and was working like it should. It’s a seal box so it started pressurizing the inside. So if there is anything obstructing the intake or exhaust and like he ^^^said the longer the duct it loses pressure. I use a light piece of paper or tape as a steamer to see which way and how strong the flow of air is. Hope that helps. Good luck 
a couple of things if you can ,try to match your exhaust fan with a intake fan…and in taking your word “FAN” literally ,try using blowers instead… AS to the fact that blowers develop pressure… Fans develop very little pressure when restricted. i also use shutter vents so it very importantant to me to be able to create enough press, to open them and exaust air freely… ps one more thought , ask Bergman’s lab folks for advice on a strain that handles higher temps … And or use CO2 . Hope this helps .an happy growing .