Hot water boilers and steam boilers use a variety of fittings and equipment; called boiler trim, to ensure a safe and smooth operation. This article will illustrate and describe the many parts that work together in a hot water or low pressure steam boiler.
Heat source – The boiler may be heated by oil, gas or electricity. Oil fired boilers use a “gun type” burner while residential gas burners are typically internal to the boilers housing. Electric boilers are very compact as result of the lack of fire and combustion gases.
Heat exchanger – The internal component of the boiler that holds the water. Heat exchangers can be made of steel, stainless steel, aluminum, cast iron and copper.
Tankless coil - Used to create domestic hot water by passing it through the heated water inside the boiler’s heat exchanger. This is an option for hot water boilers, steam boilers use an external converter to accomplish this.
Jacket – The external shell of the boiler that houses the heat exchanger, combustion chamber and combustion gases.
Exhaust components – The dangerous gases created by burner are vented to outside air. Some of the components may include: vent connector, draft hood, atmospheric damper and blowers depending on the boiler type. The venting system may also be made out of steel or in some high efficiency systems, plastic pipe.
Aquastat – Are the brains of the boiler, they sense the temperature of the water within the boiler and tell the burner when to start and stop. There are two types of aquastat: an immersion type that has a probe directly in the water and a strap-on type that mounts to boiler surface or distribution pipe close to the boiler. Aquastats also function as one of the safety limit controls, shutting the burner down in case of over heating.
Safety limits – Every boiler has safety features that prevent the water from over heating, some are simple electrical switches that open, disconnecting the power to the burner in the event of overheating.
Power supply – The main power supplied to the boiler is 120V and must pass through an emergency switch that can disconnect all power to the boiler (often the reason for service calls because it looks like a light switch).
Isolation valves - All boiler installations whether hot water or steam will utilize numerous valves for isolation and maintenance reasons. The most commonly used isolation valves are: ball valves, gate valves and globe valves.
Pressure reducing valves – Boilers typical operate under considerably lower pressure (usually 12psi cold and 20psi hot) than the make-up water supply, which is the same as the home’s domestic water pressure. A pressure reducing valve lowers the pressure of the make-up water.
Safety relief valves - Since water expands when heated a safety relief valve will prevent the boiler from creating excess pressure that could damage the unit. A hot water boiler relief valve will incorporate a temperature protection device, this called a T&P valve(temperature and pressure).
Backflow valve - A backflow valve will only allow water to pass through it in one direction. Some municipalities are now enforcing backflow valves be installed on the boiler make-up water lines in case chemicals are added to the boiler’s water. See various backflow preventers here.
Expansion tank – Are used to prevent pressure fluctuations in the heating system. Two main types of expansion tanks are commonly found: a precharged cushion tank that uses a diaphragm and pressurized air, and older style steel expansion tanks that simply use the trapped air in the tank as a cushion, they are usually found between the joist space or in the attic.
Air elimination devices – Air and other gases in a hot water heating system need to be removed for the smooth operation of the boiler and air is also responsible for a lot of the noise in a hot water system. There are a few different types of air elimination devices; such as: boiler fitting, air scoop, automatic air vent, manual air vent and micro bubble resorber are a few.
Site glass – Used on steam boilers to readily view the water level.
Low water fuel cut-off – A float or probe type device that shuts off the fuel to the burner if the water level falls below it. Most often seen on steam boilers and large commercial hot water boilers.
Zone valves - Electrically operated valves that are opened when a thermostat calls for heat.
Boiler drain – A sediment faucet threaded into the boiler or low piping near the boiler. Used to drain the boiler for maintenance.