Heather is correct.  At two weeks old your chicks don't need it to be warmer than 85 degrees or so.  I live in the very sw part of Arkansas, so I know the type of temps you are dealing with in Texas.  I would say no heat during the day, as long as they are in a draft-free spot, and a lamp at one side of their brooder to use at night.  More important than watching the thermometer is to watch your chicks.  Cold chicks huddle under the heat source, hot chicks pant and spread their little wings to cool off.  They also get pretty noisey when they're hot or cold.  You want to see your chicks moviing around freely, without concern about where their heat source is.  Just have a heat source that they can get under at night should one of them feel a chill and they'll be fine.