Hot water heated by boiler and piped to "fin-tube" baseboard units mounted along walls. The fins increase the surface area of heat dissipation making the unit more efficient.

Aquastat - The thermostat that regulates the boiler's temperature
Gas Valve and Burners - Combustion assembly that heats the water chamber
Combination Pressure / Temperature Gage (Tridicator) - Monitors water temperature and pressure
Water Feed Valve - Controls water flow to boiler
Pressure Reducing Valve - Automatically maintains correct water pressure at about 12-15 psi
Air Vent or air bleeder valve- Automatically purges unwanted air from hydronic system
   Guide to Air Bleeder Valves on Heating Systemse
Expansion Tank - Critical component that allows expansion of water as it heats.
Flow Control Valve - Regulates flow of hot water to system
Pressure Relief Valve - Relieves high water pressure
 Source: Troubleshooting a Hot Water Boiler - Components at
Troubleshooting a Hot Water Boiler - Poor Heating Performance

Scale build up is usually not a problem in hot water boilers. Although make up water contains dissolved minerals, system leaks discharge water with an equal amount of dissolved minerals. Since as many minerals are leaving the boiler system as enter it; minerals do not have the opportunity to accumulate in the boiler water.
Bleeding / Purging air from a system.
Turn off return valve
Open drain valve with  hose attached
Put hose in a bucket and let it drain until there are no more bubbles

Put the "balancing" screw slot in like with the pipe run for open and turn 90° to close.
Close to purge.
Adjust somewhere between open and closed to balance heat in each zone in a multi-zone system.
Bleeding a Hot Water Boiler - YouTube
Air Conditioning
last updated 10 May 2013