We had a new RUUD communicating system (UPRL-JEC 3 ton heat pump w/ RHPL-HM3621JC air handler) installed a couple of years ago. It has worked fine until this cold season. On a cold night, ice builds up around the upper half & top of the outside unit. It sounds like it is going into a defrost cycle, but the ice remains until the outside temperature rises above freezing the next day.The serviceman has been a couple of times to check it, but can find nothing wrong. He said it was not low in refrigerant. Any suggestions on what the problem could be??