We recently bought a house with a Coleman Evcon heat pump installed in 1997. Everything seemed to be working fine until the night time temperature dropped below freezing. The next morning we noticed the temperature drop about ten degrees in the house and frost collecting on heat pump. I am aware that frost sometimes develops but should it go away after some time if there is a defrost? Everything is running but it is blowing in cool air. Any input would be very helpful. Thanks.

Your auxiliary heat should kick in if your T-stat loses 2 degrees. The heat pump checks its self for frost every 30,60,90 min. of run time. (depends on defrost board set up) Not clock time actual accumulated time of the system running. If its not keeping up you have a problem. My heat pump ran around the clock a few weeks ago with the temp around 12 degrees but it kept the temp in the house.