A Glossary of terms relating to boilers and central heating systems boiler glossary

Absolute Pressure – The pressure measured by a gauge plus a correction for the effect of air pressure on the gauge (l4.7 psi at sea level).

Active System – A solar heating or cooling system that requires external mechanical power to move the collected heat.

Ambient Temperature – Room temperature or the temperature of the surroundings; temperature of fluid (usually air) which surrounds object on all sides.

Atmospheric Pressure – The pressure that atmospheric gases in air exert upon the earth; measured in pounds per square inch or grams per square centimeter; one standard atmosphere equals 14.69 psi of pressure and measures 760 mm (29.92 inches) in a barometer of mercury.

Back Siphonage – The flow of water or other liquids, mixtures, or substances into the distributing pipes of a potable supply of water from any sources other than its intended course, due to a negative pressure in such pipe.

Bleed Valve – Valve with a small opening inside which permits a minimum fluid flow when valve is closed.

Boiler – A device for generating hot water for heating purposes or hot water supply. Heat from an external combustion source is transmitted to a fluid contained within the tubes in the boiler shell. This fluid is delivered to an end-use at a desired pressure, temperature, and quality.

Boiler Bow Off – An outlet on a boiler to permit emptying or discharging of water or sediment in the boiler.

Boiler Horsepower – Term now seldom used, meaning equivalent to a heating capacity of 33,475 Btu/hr. (983 watts).

BTU (British Thermal Unit) – A standard unit for measuring the quantity of heat needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit at or near 39.2 degrees Fahrenheit.

Building Codes – Community ordinances governing the manner in which a property may be constructed or modified.

Building Inspector – A public official that examines the work in the field to determine compliance with the applicable building laws.

Carbon Monoxide – Carbon monoxide is colorless, odorless gas resulting from the incomplete combustion of hydrocarbon fuels. Carbon monoxide interferes with blood’s ability to carry oxygen to the body’s tissues and results in numerous adverse health effects.

Celsius – International thermometric scale where 0.01 degrees represents the triple point of water and 100 degrees the boiling point; similar to Centigrade.

Celsius Temperature Scale – The temperature scale used in metric system in which the freezing point of water is 0° and the boiling point is 100°

Central Heating – Heating, usually in the form of radiators, which is supplied centrally form a boiler to all parts of a house.

Coefficient Of Heat Transmission – The rate of heat loss in BTU per hour through a square foot wall or other building surface when the difference between indoor and outdoor air temperatures is one degree Fahrenheit.

Cold water cistern – The large tank in your loft which feeds your cold taps and the hot water cylinder

Combi or Direct or Instantaneous Heating System – A floor or wall mounted boiler that heats water instantaneously on opening a hot tap. There are no tanks in the loft and no storage cylinder. The water is fed from the mains so the water pressure is generally stronger.

Combi boiler – A combi boiler is a compact high efficiency boiler, water heater and core of a central heating system.

Combustion Chamber– The part of a boiler, furnace or wood-stove where the burn occurs; normally lined with firebrick or molded or sprayed insulation.

Condensation Water – Temperature at which vapor condenses from the atmosphere and forms water.

Condensing Boiler – Condensing furnaces and boilers are the most energy efficient units on the market today. The combustion process produces gas by-products that include water vapor and carbon dioxide. In a conventional heating system, these by-products are vented out of the house.Condensing systems cool the combustion gases to the point that water condenses and the process releases additional heat that is captured and distributed to the home. Condensing gas furnaces are generally 10-15% more efficient than conventional units.

Controls – All boilers have controls that allow you to adjust the water temperature (thermostat). You may also find a water pressure gauge and a digital or mechanical timer.

Crawl Space – A shallow area below the floor, within a wall or in an attic or loft area in order to allow access, usually only accessible by crawling as the area of access would be very narrow and confining.

Design Heat Load – The total heat loss from a house under the most severe winter conditions likely to occur.

Design Temperature – The temperature close to the lowest expected for a location, used to determine the design heat load.

DHW – Domestic Hot Water.

Energy Efficiency Ratio(EER) – Energy Efficiency Ratio; is figured by dividing BTU hours by watts.

Energy Efficient – A property that has features that use energy (heat and electricity) in efficient levels.

Energy Guide Label – By law, most new appliances have to carry Energy Guide labels which provide an estimate of how much energy the appliance will use in one year – based on average household use patterns, or the energy efficiency rating of the appliance. Each label also has a bar scale, showing the range of efficiencies for all similar appliances and an arrow, indicating where in the range this model falls.

Exchange Rate Air -The number of times that the outdoor air replaces the volume of air in a building per unit time, typically expressed as air changes per hour. The number of times that the ventilation system replaces the air within a room or area within the building.

Expansion Vessel – As the water in the heating system gets hot it expands; in sealed systems the ‘expansion vessel’ (a small pressurised container) accommodates the extra volume.

F&E (Feed and Expansion Cistern) – The small tank (often in the loft) which feeds your radiator circuit.

Fire Brick -Brick that has been tested for fire-resistance and then graded for specific construction uses. Brick whose composition and characteristics make it suitable to use for masonry fireplace fire boxes and fire chambers.

Fireplace – An open recess in a wall at the base of a chimney where a fire can be built or a gas or electric fire can be installed.

Flow Rates– Generally the term ‘flow rates’ is associated with combi’s. It refers to how much water (usually measured in litres) can be heated as it flows through the pipes in a given time (usually per minute). A powerful boiler will heat more water therefore providing a better flow rate.

Flue – The hollow passage that carries smoke and heat to the outside from the fireplace or furnace.

Flue Chimney – A passage housed in a chimney through which smoke and gases are carried from a fuel burning appliance, fireplace or incinerator to the exterior.

Flue Collar – Round metal ring which fits around the heat flue pipe after the pipe passes out of the roof.

Flue Damper – An automatic door located in the flue that closes it off when the burner turns off; purpose is to reduce heat loss up the flue from the still-warm furnace or boiler.

Flue Liner – A liner used inside a chimney to extract smoke or gases from a property.

Flue Lining– Fire clay or terracotta pipe, round or square, usually made in all ordinary flue sizes and in 2-foot lengths, used for the inner lining of chimneys with the brick or masonry work around the outside. Flue lining in chimneys runs from about a foot below the flue connection to the top of the chimney.

Gas Meter – An instrument for measuring gas consumption.

Gravity Convection – The natural movement of heat that occurs when a warm fluid rises and a cool fluid sinks under the influence of gravity.

Heat Capacity – A property of a material denoting its ability to absorb heat.

Heat Exchanger – A device that is used to transfer heat from one fluid to another through a separating wall.

Heat Load Design – The total heat loss from a house under the most severe winter conditions likely to occur.

Heat Loss – A decrease in the amount of heat contained in a space, resulting from heat flow through walls, windows, roof and other building envelope components.

Heat Loss Standby – Heat lost though storage tank and piping walls.

Heat Pump – A heat and cooling source. Heat pumps extract heat from either the air or ground and transfer that heat by circulating a refrigerant through a cycle of alternating evaporation and condensation. The cycle can be reversed for cooling. The efficiency of an air source heat pump varies tremendously with climate while ground source heat pumps take advantage of stable ground temperatures to deliver consistent performance.

Heat Specific – The number of BTU’s required to raise the temperature of one pound of a substance 1° in temperature.

Heat Transmission Coefficient – The rate of heat loss in BTU per hour through a square foot wall or other building surface when the difference between indoor and outdoor air temperatures is one degree Fahrenheit.

Heater Wall Mounted – A heating unit mounted on or attached to a wall.


  1. Air-Conditioning The process of bringing air to a required state of temperature and humidity, and removing dust, pollen and other foreign matter.
  2. Central Heating A heating system in which a number of rooms or spaces are heated from a central source.
  3. Convector A heating device in which the air enters through an opening near the floor, is heated as it passes through the heating element and enters the room through an upper opening.
  4. Hot Water Heating. The circulation of hot water through a system of pipes and radiators either by gravity or a circulating pump.
  5. Panel Heating Coils or ducts installed in wall, floor or ceiling panels to provide a large surface supply of low intensity heat.
  6. Radiant Heating. A heating system in which only the heat radiated from panels is effective in providing the heating requirements.
  7. Warm Air Heating .A warm air heating plant consisting of a heating unit (fuel-burning furnace) enclosed in a casing, from which the heated air is distributed to various rooms of the building through ducts.

Heating Degree Days(HDD) – The number of degrees per day that the daily average temperature (the mean of the maximum and minimum recorded temperatures) is below a base temperature, usually 65 degrees Fahrenheit, unless otherwise specified; used to determine indoor space heating requirements and heating system sizing. Total HDD is the cumulative total for the year/heating season. The higher the HDD for a location, the colder the daily average temperature.

Heating Load – The amount of heating required to keep a building at a specified temperature during the winter, usually 65 deg F, regardless of outside temperature.

Heating Season – The period from early fall to late spring (in the northern hemisphere) during which additional heat is needed to keep a house comfortable for its occupants.

Heating System Passive – A solar heating or cooling system that uses no external mechanical power to move the collected solar heat.

Heating System Warm Air – Any heating system which depends upon the circulation of warm air.

Heating Warm Air – Any heating system which depends upon the circulation of warm air.

High Efficiency (condensing) -All High Efficiency boilers are environmentally friendly and operate at up to 96% efficiency compared to standard boilers at around 80% efficiency. They recover heat that is normally lost into the atmosphere via the flue gases.

High Pressure Boiler – boiler furnishing steam at pressures of 15 pounds per square inch gauge or higher (1.05 kg/cm2).

Hot water cylinder – The insulated hot water tank, normally in your airing cupboard.

HVAC – Abbreviation for Heating Ventilation and Air Conditioning.

Insulation – Generally, any material which slows down or retards the flow or transfer of heat. Building insulation types are classified according to form as loose fill, flexible, rigid, reflective, and foamed-in-place. All types are rated according to their ability to resist heat flow (R-Value).

Kilowatt (kw) – One thousand watts. A kilowatt hour is the base unit used in measuring electrical consumption, also see Watt.

Kilowatt Hour (KWH) – A measure of electrical energy equivalent to a power consumption of 1000 watts for one hour.

Landlord – Either the person who owns a property which you are renting and to whom you pay rent, or the freeholder of a property for which there is a leasehold tenure.

Local Planning Applications – These must be lodged by anyone wishing to build or extend a property to the local planning department of the council.

Lockshield Valve – The valve at one end of the radiator which you can’t turn

Main Water – Water supply pipe generally located at the street which may supply a number of buildings.

Megaflow or Unvented – A system which takes water straight from the mains and heats it up in a pressurised cylinder, with no tank in the loft.

NHBC – A governing body for new homes who will issue a 10 year structural guarantee. Their inspectors will ensure that your new home is built properly and is safe.

Open Vented – If your heating system has a feed and expansion tank (see above) then it’s called ‘open vented’ hot water. A hot water storage cylinder may also be open vented.

Output -The output ratings of boilers are measured in BTU’s (British Thermal Units) or kW (Kilowatts). A low output would be 30/60,000 BTU’s (10-17kW). A high output would be 60/100,000 BTU’s (17-30kW). Your installer should choose the output best suited to the size of your heating system, the type of property and, in the case of a combination boiler, the amount of hot water you require.

Pilot Light – A small, continuous flame (in a hot water heater, boiler, or furnace) that ignites gas or oil burners when needed.

Planning Permission – Permission granted by the Council or local authorities to progress with work on a building that required permission to make the changes to the structure required.

Plumbing – The practice, materials, and fixtures used in the installation, maintenance, and alteration of all piping, fixtures, appliances, in connection with sanitary or storm drainage facilities, the venting system, and the public or private water supply systems.

Plumbing does not include the trade of drilling water wells, installing water softening equipment, or the business of manufacturing or selling plumbing fixtures, appliances, equipment, or hardware.A plumbing system consists of three separate parts

  1. An adequate potable water supply system
  2. A safe, adequate drainage system
  3. And ample fixtures and equipment.

PSI (Pounds Per Square Inch) – A unit of measurement referring to the actual pressure exerted on one square inch of an objects’ surface.

Quotation – A document detailing all the costs involved in taking out a mortgage.

Quote – This is an amount a service provider estimates to be the cost of providing a service based on the available information. These can often vary so it can be worth getting a few quotes to ensure you get the best deal.

R Factor Or Value– A measure of a materials resistance to the passage of heat. New property walls are usually insulated with 4″ of insulation with an R value of R-13, and a ceiling insulation of R-30.

R Value – A measure of insulation that measures material resistance to the passage of heat, the higher the R value, the more insulating power it has.

Radiant Heat Transfer – The transfer of heat energy from a location of higher temperature to a location of lower temperature by means of electromagnetic radiation.

Radiant Heating – A method of heating, usually consisting of a forced hot water system with pipes placed in the floor, wall, or ceiling. Also electrically heated panels.

Radiant Panels – Panels with integral passages for the flow of warm fluids, either air or liquids. Heat from the fluid is conducted through the metal and transferred to the rooms by thermal radiation.

Radiation – Energy transmitted from a heat source to the air around it. Radiators depend more on convection than radiation.

Radiator – A heating unit which is supplied heat through a hot water system.

Sealed System – If you have a sealed (heating) system there’s no feed and expansion tank (see F&E tank) in the loft space. Instead it’s filled with water via a temporary connection and expansion is accommodated by a special vessel, which may be inside the boiler.

SEDBUK -Government database which lists the Seasonal Efficiency of all Domestic Boilers in the UK

Sludge – A relatively loose accumulation of material that has settled to the bottom of boiler.

Stop Valve – A device installed in a water supply line, usually near a fixture, that permits an individual to shut off the water supply to one fixture without interrupting service to the rest of the system.

Storage Cylinder – The storage cylinder supplies hot water for domestic use and is heated by the boiler. When the hot water in the storage cylinder has been used up it is refilled by the feed and expansion tank.

System – A system is the term used to describe the boiler and all of the components required to provide your home with heating and hot water.

System Water Supply – The water supply system of a building is composed of the water service pipe, the water distributing pipes and the various connecting pipes, control valves and fittings in or on private property.

Temperature Design – The temperature close to the lowest expected for a location, used to determine the design heat load.

Therm – The standard unit for measuring the amount of gas used, defined as the volume of gas needed to generate 100,000 BTU’s.

Thermal Insulation – Any material high in resistance to heat transmission that, when placed in the walls, ceiling, or floors of a structure, will reduce the rate of heat flow.

TRV (Thermostatic Radiator Valve) – Temperature controlled valve on a radiator which can be turned to the required setting

Unvented-The term unvented most commonly refers to cylinders storing and supplying hot water at mains pressure rather than via an F&E tank and therefore at flow rates which are usually far superior.

Vapor Condensation – Temperature at which vapor condenses from the atmosphere and forms water.

Vent Pipe – A vertical pipe of relatively small dimensions which protrudes through a roof to provide for the ventilation of gasses.

Vented or Gravity – A traditional system where cold water is stored in a tank in the loft and hot in a cylinder in your airing cupboard

Venting – Bleeding the air out of a radiator

Volume Air – Quantity of air in cubic feet, usually per minute, at atmospheric pressure.

Wall Heater – A heating unit installed in or on a wall.

Wall Mounted Heater – A heating unit mounted on or attached to a wall.

Water Gauge – An instrument to measure the depth of water, or to indicate the height of its surface, as in a steam boiler.

Water Heater – An apparatus for heating and storing water.

Water Vapour – Moisture existing as a gas in air.

Wheelhead valve – A valve at one end of the radiator which turns on and off