Ruud UBHC-21J18SFA blowing only "luke...

Question

I have the temp set at 73, but the air at best is luke warm if not just a little cool. I have had the unit for 4 years (new). Temp outside is around 6 degrees and inside 69 although thermostat is at 73. This is a 208/220 unit. Any idea where I should start looking at?