What are the two types of low water fuel cutoffs used on hot water boilers?

The correct choice, which identifies the two types of low water fuel cutoffs used on hot water boilers, is based on the functionality and operation of the devices. Float and probe cutoffs are designed to monitor water levels effectively.

Float cutoffs operate using a buoyant device that rises and falls with the water level in the boiler. When the water level drops to a certain point, the float descends, triggering a switch that closes the fuel supply, thereby ensuring that the boiler does not operate without sufficient water, which could lead to overheating or damage.

Probe cutoffs, on the other hand, use electrical conductivity to detect the water level. They consist of one or more probes that extend into the boiler. When the water level falls too low, the probes sense a change in conductivity (as water is a conductor) and activate a switch to shut off the fuel supply.

This combination of float and probe mechanisms offers reliable safety features to prevent boiler failures and ensure safe operation. Other combinations listed do not reflect the standard practices for low water cutoffs in hot water boilers, as they either involve components not typically used together or refer to control mechanisms that do not directly relate to water level sensing.