I arrived for a blue member no heat call on a heat pump. I first inspected the system and found the condenser fan blade to be completely frozen over due to the snow and ice dripping onto the condenser. I made this aware to the homeowner and and explained that the defrost board dose not come with a defrost sensor and that they are two separate components. And that the system will not build up 3â€ of ice on top of the system when the system is running normally. The system is in the off position when the top of the system freezes over. I melted the ice off of the system with a water hose and then cycled the systems defrost mode on. The system came on on defrost and successfully melted the rest of the ice on the unit. I recommended to install a thermostat that will shut off the heatpump when the outdoor temp drops below 36-34 deg. If you decide to install this there kit please let us know and we will do our best to help you in anyway we can. Your friend at Interstate.