Upon arrival customer stated that last weekend downstairs thermostat reach 68 degrees even though she had it set to 75. Stated it only happened that time but has since been heating. After inspection of 5 year old system, could not find any issues with equipment. Downstairs area had 102 degrees coming out of supply vent without heat strips on and 136 degrees with heat strips on. Noticed that room temperature at thermostat would fluctuate between 72-75 degrees and system does continue to run. Gave customer option to replace thermostat customer declined recommendations at this time. Changed (16x20x1) system filter. Checked outdoor unit and found no issues. Both zones opening properly, zone board is working properly, unit is heating at this time.