Photograph of  a modern oil-fired heating boilerDictionary of Heating Boiler Components - Boiler Parts Guide

  • BOILER COMPONENTS & PARTS - CONTENTS: How to Identify the Components of Hot Water Heating Boilers - A Guide to Hydronic Heating System Parts. What are the basic components of heating boilers and hot water heat? List of heating system parts with definitions and links to detailed articles. How to inspect & repair central hot water heating boilers - hydronic heating
  • POST a QUESTION or READ FAQs about components & controls on hydronic (water) heating boiler systems

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Dictionary of heating boiler terms:

How to identify & name all of the parts & components of Hot Water Heating Boilers and hot water heating systems including oil burners and their parts, boiler parts, hot water baseboard and its parts, hot water radiators, and hot water convector units and their parts and controls.

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A List & Dictionary of Oil Fired Heating Boiler Parts & Components

Photograph of  a modern oil-fired heating boilerEach simple definition includes links to detailed articles discussing how to identify, inspect, diagnose, and repair each component of the heating system.

This article series answers most questions about central hot water heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.

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Below we list and describe the parts and controls on heating boilers.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs.

  • HEATING BOILER - Definition: an enclosed steel or cast iron container that uses an energy source to heat water (or make steam) that is sent through heat radiating devices in the occupied space to heat a building.

    use hot water to distribute heat through piping and baseboards or radiators.

    STEAM HEATING BOILERS produce steam to distribute heat through one pipe or two pipe steam heat systems and steam heat radiators.

    Some heating systems use a hydronic boiler to heat water that is circulated thorough a heating coil inserted in the plenum of a WARM AIR FURNACE heating system, using a water-to-air heating method.
  • AIR BLEEDER VALVES and AIR PURGE DEVICES vent out un-wanted air from hydronic (hot water) heating systems. Air in heating distribution piping, boilers, radiators, can prevent hot water circulation and thus stop heat from being delivered to the occupied space.
  • AIR SCOOPS SEPARATORS PURGERS - automatic air purging devices referred to as air scoops, air purgers, or air separators vary in internal design, but all are intended to capture air in hot water piping, sending it either to an expansion tank or to an air bleeder/air vent.
  • AUTOMATIC WATER FEEDER to provide makeup water in the system; note that different water feeder valves are
    used on
    a HYDRONIC HEATING BOILER (hot water circulates in pipes, radiators, baseboards, convectors) or on a STEAM BOILER (steam circulates in pipes and radiators to deliver heat to the occupied space)
  • BASEBOARDS - electric or hot water heat radiating devices transmit heat from hot water or steam into the occupied space; Leak stains, repairs indicating history of freeze-ups?

    Also see
    or "flue"
    and see the draft regulator
    or BAROMETRIC DAMPER - used to regulate draft seen by the oil burner
  • CHIMNEY & CHIMNEY FLUE - the flue vent connector sends gases into the chimney whose job is to vent these combustion products safely outdoors, above the building.
  • CIRCULATOR PUMPS force hot water through heating baseboards, radiators, or convector units
  • COMBINATION CONTROL - combining High Limit and "Low Limit" (which may or may not be in use depending on presence of a tankless coil)
    Also referred to as an AQUASTAT CONTROL
  • DRAFT HOOD assures proper exhaust gas venting on gas-fired heating boilers
  • ELECTRIC POWER SWITCH turning the heating system electrical power on and off
  • EXPANSION TANK to absorb initial system pressure increase
  • Flame sensing device: AQUASTAT - CAD CELL or STACK RELAY SWITCH(watch for sensitive relays and learn about the re-stepping lever)
  • HIGH LIIMIT & LOW LIMIT SWITCHES (if tankless coil) separate controls on older equipment
  • LOW WATER CUTOFF CONTROL LWCO, a safety device used on steam boilers and some hydronic heating systems to shut down the heating boiler if its water is lost.
  • MIXING VALVEto temper outgoing domestic hot water from a TANKLESS COIL or to control radiant floor heat temperature
  • OIL BURNER & oil burner parts: oil pump (fuel unit), blower, nozzle, electrodes, transformer, electric motor, air shutter, oil delay valve
  • Oil Burner reset switch & oil burner motor reset switch (these are two separate controls); a reset switch may be on the CAD CELL RELAY or on the STACK RELAY of the oil burner as well as a reset switch on the oil burner electric motor
  • OIL STORAGE TANK and OIL PIPING, and OIL CONTROL VALVES, and oil tank gauges
  • Piping on heating systems: Hot water (or steam) distribution piping and on steam systems, condensate return piping
  • Piping valves on heating systems: Hot water distribution piping drain and balancing valves
  • PRESSURE RELIEF VALVE safety device prevents boiler explosions by releasing temperature and pressure if other safety controls fail
  • PRESSURE & TEMPERATURE GAUGE -12 psi cold, less than 30 psi hot. Over 30 psi dumps relief valve.(Typical operating temperature settings LO-120-160 HI-180-200 °F. Typical operating temperature observed at the gauge will be below the high, and can be as low as nighttime room temperature in non-heating season if no tankless coil is in use. The temperature/pressure gauge may help in checking for normal conditions before and during boiler operation. However the gauge can be wrong!)
  • RADIATORS - hot water or steam. Steam: observe steam valves on each radiator and identify one pipe vs two-pipe distribution system. radiating devices transmit heat from hot water or steam into the occupied space; Leak stains, repairs indicating history of freeze-ups? Hot water radiators and baseboards may have a manually-operated air bleeder valve. Each steam radiator has a STEAM VENT to permit steam to rise into the radiator.
  • RESET BUTTON, ELECTRIC MOTOR on electric motors used on heating systems may require a manual reset
  • RESET SWITCH, CAD CELL RELAY on the cad cell or other primary control - press only once if the system is off on reset and does not keep runnining when you try this step
  • SIGHT GLASS: a glass tube used on steam boilers to give a visible indication of the boiler's water level
  • SPILL SWITCH, FLUE GAS DETECTOR are used to sense flue gas spillage at gas-fired equipment at the combustion gas exhaust venting systems and will turn off the heating appliance if gas spillage is detected.
  • TANKLESS COIL or a side-arm coil or other ALTERNATIVE HOT WATER SOURCES using the heating boiler; tips about leaks, missing mixing/tempering valve, relief valve (some jurisdictions). Risk of scalding water at the house sinks/tubs if there is no mixing valve.
  • ZONE VALVES which control hot water circulation through multiple heating zones



Continue reading at BOILER CONTROLS & SWITCHES or select a topic from the More Reading links shown below.


Or see FURNACE CONTROLS & SWITCHES if your building is heated by warm air

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BOILER COMPONENTS & PARTS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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