Switch to mobile site

Central Heating

Exam revision materials for Plumbing Level 2&3 and ACS Gas Safety test

Create an Article


Gas Boilers

Click to show/hide index


Where Should You Install a New Radiator?



Heating system water analysis



Circulating Pump

Heating System Controls

Minimum Requirements

Room Thermostats



Heat Emitters

Heat Loss Calculation

Radiator Sizing



Fully Pumped Hot Water and Central Heating

Gravity Hot Water and Pumped Central Heating

Gravity System

Open-vented System

Pipework Sizing

Pipework Systems

Sealed Systems

System Layout

Underfloor Heating


Boiler Sizing

Flue Types

Gas Boilers

Heat Pumps

Oil Fired Boilers


Solid Fuel Boilers

Range Rated Boilers

  • Adjustment to the heat output of the boiler is achieved by range rating of the boiler
  • They have different type of the burner
< Go Back2013-04-18

Condensing Boilers

  • have two heat exchangers
  • all new boilers fitted are condensing (Document L)
  • the flue create a plume
  • Discharge of condensate is required:
    • soil pipe:
      Condensate pipe termination into the soil pipe
    • waste pipe:
      Condense pipe termination into internal waste pipe
    • gully
    • soak way:
      discharge of condensate - soak way
  • min 25° - fall of a condensing pipe
< Go Back2013-04-18

Combination Boilers

combination boiler scheme, condensing boiler scheme

  • Pumped central heating
  • Pumped instantaneous hot water
  • All controls and components are internal
< Go Back2013-04-18

Boiler Components

  • Air pressure switch - safety device to prevent the boiler from functioning if the combustion fan is not operating
  • Pump
  • Diverting valve - motorised valve that alters the heated water flow between central heating circuit and hot water (priority)
  • Gas control (gas valve)
  • DHW thermistor
    • electrical resistor that reacts to temperature
    • found on HW side of combination boilers
    • Overheat Stat - safety device to prevent from overheating
  • Pilot assembly
  • Boiler control thermostat - senses the temperature and bring boiler on and turns it off
  • Pressure/temperature gauge
  • Timer
  • Printed Circuit Board (PCB)
    • governs the ignition sequence of the boiler
    • brings the fan and pump on
    • opens the gas valve if Flame Rectification senses the return signal
  • Plate heat exchanger (clorifier)
    • passes heat from the primary heat exchanger to the hot water
  • Primary Heat exchanger
    • Materials:
      • cast iron
      • high alloy steel
      • copper
      • aluminium
< Go Back2013-04-18

Regular Boilers

  • supply hot water to the radiators and the cylinder
  • obtain heat from a single heat exchanger
  • the key system controls are fitted externally
System Boilers
  • supply hot water to the radiators and the cylinder
  • the key system controls for a sealed heating system are built into the boiler
    • the pump
    • expansion vessel
    • by-pass
< Go Back2013-04-18