An Aquastat is a temperature setting device that uses a sensor to turn on and off an electric element, pump, or other device in a heating system. 

In context of the SumPump model SP2.2 to SP2.3, it is a recommended solution to add a Johnston Controls a419 as the controller for activating the electric element function during the Winter season, but not the rest of the year. The SP3.0 and later have integrated controls to handle this function.

A high-level look at the a419 box and the sensor placed in the upper water sensor well.

This is the 24 volt AC model a419 GBF-1  This installer like pulling the power from a 24 VAC transformer - if close by. !120/240 V. models are common also. Do not be fooled by the simple 3 buttons, this unit has many functions and operational features including a neat power protection timer.

The 24 VAC wiring is simple for anyone with basic electrical skills. Can be done by a DIY or Trades.

Download the install manual PDF for quick reference.