A typical installation would use two units for level control and burner cut-off. The 157 would be used to control a pump to maintain water level and provide low water cut-off. The cut-off would auto reset when water level is restored. The 150 would be used as a secondary low water cut-off and be a manual reset model. This would require the unit be reset when the water level is restored.

While typical, boiler manufacturers can and do vary the type of control and location. Some will use a 157 with an electronic LWCO as the secondary. Others may install a 157 with a 63 or 64 as the secondary. On some smaller boilers, there may be a single 157 with a manual reset burner switch and no secondary.

Each set-up may be different but the purpose of the control is the same. Maintain a water level and protect the boiler.