I had the distinct joy of coming home to a house problem
yesterday.  I got home and thought that the house was pretty
warm… so I checked the thermostat and it was reading 85, even though
the a/c (central air) should have been keeping it around 75.  So,
I went outside and examined the a/c unit.  Everything looked fine
outside.  Next, I went to the basement where the a/c line connects
to the furnace.  “damn…” I thought… there was a huge ice chunk
around the pipe and the connection was leaking (from the melting
ice). 

I called one of my really good friends, who just happens to know a
thing or two about houses, and he said that this is a pretty common
situation.  Apparently the a/c unit is low on freon. 
Everything iced up because there wasn’t a large enough difference
between the outside temp and the temp coming from the a/c unit.

Anyway, to remedy the situation, I turned off the a/c, but explicitly
turned on the fan (part of the furnace).   This allows the
ice to melt.  The next step will be to have a HVAC person to come
out and recharge the a/c units.