There are a few things that can cause your HVAC lines to freeze. One possibility is that there is a blockage in the line, preventing the refrigerant from flowing properly. Another possibility is that there is too much moisture in the air, which can cause the refrigerant to condense and freeze. And finally, its also possible that there is a problem with your thermostat or furnace, which could be causing the system to not respond properly when it tries to turn on. If you are experiencing frozen HVAC lines, its best to call in a professional to help you troubleshoot the issue.