What temp is too high in a reef tank before deciding to need a chiller ??? Now that it's getting cold out and everyone is starting up their home heating.
A chiller is only necessary to deal with extremely high temperatures all the time or a tank that has fluctuating temp and cannot be corrected any other way.
If you have metal halides there is a good chance your tank will surpass 82 degrees almost every day of the year so you need a chiller.
If your tank for whatever reason fluctuates up and down and you can't correct it by say adding a reliable heater and or fan then you need a chiller and heater but that's kind of dumb, putting those two against each other to regulate temp, some ppl have tanks near drafty windows or doors and dont move them.
Your house heating will not overheat your tank or all of us would have chillers.
Buy a fan or two for summer time, set your tank heater at 78 and forget about it.