It heats the engine aswell.
Its using its internal waterloop so no thermostat.
The heater has his own electric water pump.

The dbw heater warms the coolant, then it flows to the heatercore.
Then it flowes though the heatervalve(that will be open) back to the engine
From the engine it flows back to the dbw heater, and repeat process

When the coolant reaches 55 degree Celsius, the blower motor is activated.
Maximum time is one hour, if your fuel/battery last.
If your voltage drops below 10.5 volts is shut off.