There are several types of block heaters, and they are designed to keep the engines quite warm, much more than just avoiding freezing.
Some are the same idea as the water heater element, no thermostat or pump, will get really hot if you forget and leave it on.
Some are external and are usually plumbed in the heater hose loop, these are called a coolant heater not a Block heater. Most all of these have a thermostat but no pump, just check valves and work on the thermo-siphon principal. Some have pumps too.
Grigg
