Need Help with Lowering Grow Room Temperature

Yeah I would def triple the output at least
If you go with a bigger fan than you will be able to upgrade lights in the future due to the higher powered fan

Do you think that is all that i will have to do to keep the temps down?

Can you attach a piece of ducting to your passive intake port and move the other end of the duct to a cooler area?
That way the fan will bring in cooler fresh air due to the negative pressure in the room

I could maybe do something like that... failing that ill just keep the window open.

Ive also been told that a small bathroom extractor fan would work well as an intake fan and they have a 100mm diameter which would slot in perfectly to the hole... watch this space.

It might actually restrict air flow if it is not powerful enough, the negative pressure created by the larger exhaust fan will make more air come in through the intake than an actual fan would

Ok well ive added it in just now tom and ill let you know how i get on with the temps etc... in the meantime i have turned my fan setting up a little bit as well (now does 187m3/hr) so i will see how it goes and if a stronger exhaust is needed then i will upgrade that as well...

In the meantime - heres some pics

Ok checked temps today and it seems to have stabilised around the 80F mark with humidity at about 55%

Im having the same problem, only because Im just starting out. Right now I have a little fan in my small closet and they have just been planted 3 days ago.