I have a Heil Heating & Cooling system in one unit outside the house. It is an older system. It works great except when the temperture falls below 32 degrees the coils ice over. Never happens till below freezing. I have had several so called heating & air repairmen here to look at it. Eveything from the compressor to the veins being stopped up. So far 4 different answers and repairs, nothing works. Anysuggestions? It's suppose to be 18 day after tomorrow, hurry please!!!