Boiler glossary

Boiler & hydronic glossary

The boiler and hydronic-heating terms, defined in plain English. For cross-system terms like BTU and load calculation, see the master glossary.

Aquastat

The control that senses the boiler water temperature and switches the burner and circulator to hold it in range — the boiler's equivalent of a limit control.

Circulator pump

The pump that moves hot water from the boiler out to the radiators or loops and back. A failed circulator is a common cause of a boiler that fires but doesn't deliver heat.

Expansion tank

A small tank that absorbs the pressure change as water heats and expands, protecting the closed system. A waterlogged expansion tank shows up as pressure-relief valve dripping.

Hydronic

Heating that distributes heat with hot water rather than forced air — the family that includes boilers, radiators, and radiant floors. The water carries the heat; no ducts are involved.

Radiant heat

Warmth delivered from heated surfaces — floors, panels, or radiators — rather than blown air, giving even, quiet heat. It's a common hydronic application.

Zone valve

A valve that opens to send hot water to one heating zone when its thermostat calls, letting a single boiler serve rooms independently. A stuck valve leaves one zone cold.