Create an air-tight seal between the wet/dry vac and the drain line. This can be achieved by purchasing a specialized attachment for the wet/dry vac.
Use the wet/dry vac to remove all water and obstacles from the drain line.
Once clear, pouring a small amount of bleach down the drain line will help to keep it clear in the future.
For a visual example:
The system used in furnaces is slightly different. Without a compression cycle, condensation occurs differently, and is based on temperature differences. There’s a single drain tube running out of your furnace that guides condensation away from critical components. If this line is not installed correctly, or has become clogged (which can occur for the same reasons as the heat pump or due to extremely long on-cycles during cold weather), it will trigger a pressure switch that will shut off the system.
This pressure switch is what causes your furnace to turn off and remain off. Once the line clears, the switch resets and your furnace can resume operation, leaving little for a technician to investigate since the furnace typically “fixes itself” before one can arrive.
The best way to fix this is to clear the drain line. It’s typically a small plastic tube inside the furnace housing. Inspect it for blockages, and blow out any you may find from both the line and the drain trap.
Once you’ve successfully cleared any blockages, making sure that the drain line is installed again correctly is pivotal. The video below clearly shows what a properly installed drain line looks like.