Aquastat
An aquastat senses water temperature and can provide operating or limit functions depending on the boiler and control strategy.
Circulator
A circulator creates the pressure differential that produces flow through a hydronic circuit. It does not simply 'push hot water' independently of the piping system.
Zone valve
A motorized zone valve opens or closes a hydronic branch in response to the control sequence, allowing one circulator to serve multiple zones in many designs.
Expansion tank and relief valve
The expansion tank accommodates thermal expansion. The safety relief valve protects against excessive pressure. They perform different functions and neither should be defeated.
Low-water cutoff
A low-water cutoff is a safety control intended to prevent unsafe boiler operation when the required water level is not present. It is particularly fundamental on steam systems and may also be used on hot-water boilers.