- Closed expansion tank
- A sealed tank with a rubber diaphragm and a pre-charged air cushion that absorbs the extra volume of heated water in a closed loop. Its size depends on system volume, fill pressure, maximum pressure and average water temperature.
- Equivalent direct radiation
- An older rating unit for radiators and convectors, where each square foot stands for a fixed heat output. It still appears when older gravity or radiator systems are evaluated, and some expansion tank tables are organized around it.
- Boiler group
- A category in the mechanical code that sorts automatic boilers by fuel and by input range. The larger the group, the more safeguards it needs, such as assured fuel supply, low-fire start and automatic reset shutoff valves.
- Low-water cutoff
- A control that shuts off the burner when the water level falls too low, preventing a dry-fired and overheated boiler. On steam boilers it is exercised regularly by blowing it down to check that it is not stuck with sludge.
- High-limit control
- A safety that cuts the fuel when water temperature or steam pressure goes beyond a setting. A manual-reset type stays shut until a person resets it, forcing someone to find out why the limit tripped.
- Prepurge
- A timed burner fan cycle that sweeps unburned gas out of the combustion chamber and flue passages before ignition. It prevents a delayed ignition explosion and is among the added controls required of larger gas and oil units.
- Blowdown
- Draining part of the boiler water, from the bottom or the surface, to remove sludge and dissolved solids. It controls scale and foaming, and its discharge piping must end where hot, treated water cannot hurt anyone.
- Foaming
- A layer of froth on the boiler water surface, most often caused by a high concentration of dissolved solids or contamination. It carries water into the steam lines, makes the gauge glass hard to read, and is treated by blowdown and water treatment.
- Steam trap
- An automatic valve that lets condensate and air out of steam piping while holding live steam back. A trap that fails open wastes fuel, and one that fails closed floods the line and invites banging water hammer.
- Primary-secondary piping
- A layout in which the boiler loop and each distribution circuit are hydraulically separated by two closely spaced tees. Each pump then moves only its own flow, so zones do not disturb one another or the boiler's required flow.
- Air separator
- A fitting that strips dissolved and free air from circulating water so it can be vented. Removing air reduces noise, pump cavitation and oxygen corrosion, and keeps radiators and baseboards from becoming air-bound.
- Zone valve
- A small motorized valve that opens when a thermostat calls for heat, letting hot water into one zone while others stay closed. It allows a single boiler and one circulator to serve several independently controlled areas.
- Oxygen barrier tubing
- Plastic radiant floor tubing with a layer that stops oxygen from diffusing through its walls. Without that layer, oxygen enters the loop and corrodes iron parts such as the boiler, pumps and fittings.
- Radiant mixing valve
- A valve or injection arrangement that blends hot boiler water with cooler return water so a floor loop receives a much lower temperature than the boiler produces. It protects flooring and tubing from excess heat.
- Return-water temperature
- The temperature of water coming back to the boiler. Many boilers need a minimum value to keep flue gases from condensing on their heat exchanger, so piping and controls are designed to meet the manufacturer's figure.
- Spectacle blind
- A solid steel plate shaped like a pair of eyeglasses that sits between pipe flanges, with one solid disc and one open ring. Turning it to the solid side completely closes the line, which closing a valve alone cannot ensure.
- Double block and bleed
- An isolation arrangement of two closed valves with an open vented or drained gap between them. If the first valve leaks, the leakage escapes through the bleed instead of reaching a boiler that someone is working in.
- Hydrostatic test
- Filling new or repaired piping with water and raising pressure above working pressure to find leaks and weak points. The required level is a multiple of operating pressure, with a minimum floor, and plastic pipe has its own limits.
- Condensate return
- The piping and tank that bring condensed steam back to the boiler feed system. Returning it saves heat and treated water, so a leak or failed trap raises makeup water demand and chemical use.
