I'm subletting a loft for the summer and when I moved in a few weeks ago the central air was working fine. Then it kept getting stuck at 80. I noticed the water heat was on - there was firing bruning under it - so I thought that might be why it was not cooling down inside so I lowere the water heater to "warm" still my a/c is stuck at 80. My a/c has a heat and cool option so I have it on cool and it has a fan on and fan auto option - which one do I want to be on, fan on or fan auto?

Fan On = fan runs all the time
Fan Auto = fan runs when A/C is running

So, if it cool in one part of your loft (which I doubt) you can use Fan On to move the air around without running the A/C. More likely, you'll need the A/C on to cool things down.

An A/C unit will basically drop the outside temperature by "x" degrees. If the outdoor temp when you moved in was cooler than it is now, that might be why it seemed to work back then. As the outdoor temp rises, the A/C can only drop it by "x" degrees, and it cannot go lower than 80 at the current outdoor temp.

There could be a problem with the A/C unit that makes "x" too small. Since you are renting, I'd contact the super about it.

Best of luck,
WallyH