My warm air temperature measured at the plenum is about 79 degrees. The cold air return plenum is at 66.5. I keep the house temperature at 69. The system is having a hard time keeping up even in milder near freezing weather. I am thinking that the warm air temperature is too low. If that is the case what could be the cause(s). Any comments will be greatly appreciated and I can provide more information on the unit if needed. Thank you.