Frost on a heat pump is not a problem.....this is normal. When the temp outside is 26 degrees Fahrenheit then your outside unit is not producing the main part of your heat. The back-up heat is providing heat for your home. If the back-up heat is undersized then the system ill not be able to keep up hen the temperatures are extremely low. Is 26 degrees F. uncommon here you live? If so then hopefully you ill not experience too many nights here the temperature drops below 35 degrees. You can always call an HVAC professional and have them check to make sure everything is fine in case I'm wrong about this plus you can have them check to see if the back-up heat is undersized.