Central Heating.
This post lists alphabetically the elements of a typical domestic Open Vented Central heating (CH) systems for the benefit of the novice. Sealed systems may eventually be covered too, but initially this post is confined to Open Vented Systems. Please note that not every central heating system will conform in every detail to the descriptions here, but the majority of systems will be covered.
Boiler.
The boiler is the device where fuel (gas, oil, etc.) is burnt and the heat transferred via a heat exchanger to water which then circulates around the heating system. The key control on any boiler is the boiler thermostat which operates the gas valve or oil burner. Modern boilers are much more sophisticated and include a wider range of controls.
Central Heating (CH) circulation
The circulation to and from the radiators (rads) is always powered by a pump or circulator. The pipes from the boiler to the rads are flow pipes and those from the rads back to the boiler the return pipes.
Combi boilers
Combi boilers include a Domestic Hot Water (DHW) circuit and DHW heat exchanger so that DHW is produced within the boiler on demand instead of in a separate cylinder. The overwhelming majority of Combi boilers are on sealed systems.
Condensing boilers
Condensing or High Efficiency (HE) boilers are more efficient than Standard Efficiency (SE) boilers because some of the water vapour in the flue gases condenses back to the liquid state (water). This condensate has to be drained to waste via a plastic condensate pipe.
Controls
Heating systems require a range of controls to ensure that they operate safely, efficiently and according to consumer demand. The controls usually comprise the timer or programmer, the cylinder stat, the room stat, one or more motorised valves and the wring to link these together. Often there will be a junction box where the wiring connects.
Cylinder thermostats
In order to limit the temperature of DHW, and to avoid the boiler firing unnecessarily, DHW cylinders should have a thermostat fitted to the side, about 1/4 to 1/3 up from the bottom. This is usually set to 60C and consists of an electrical switch which feeds back into the control system.
DHW Cylinder
The DHW stored in a cylinder is usually heated by an internal coil heat exchanger through which heating system water circulates. There is usually an immersion heater in the cylinder to allow for heating of the DHW by electricity as an alternative to the heating system water.
Domestic Hot Water
The Domestic Hot Water (DHW) is the hot water that you draw from the taps. This is not the same as the water that circulates around the central heating system. DHW is usually supplied from a cylinder but can be supplied directly by a combi boiler or an independent water heater.
Feed and Expansion tank
Open vented systems are filled and vented to a Feed & Expansion (F&E) tank. This is usually a small plastic tank in the roof space which is fitted with a float valve to fill automatically from the mains supply. The outlet pipe is the F&E pipe and the vent pipe inverts (upside down U shape) over this tank.
Filters - Strainers
Some heating systems may have a filter or strainer on the return pipe to the boiler. Some combi boilers have filters built into them. The filter is intended to remove debris from the water which might otherwise cause problems elsewhere in the system. A recent development is a magnetic strainer (Magnaclean) which collects all iron oxide debris. Any filter or strainer will need to be cleaned at intervals.
Fully pumped Systems
Fully pumped systems use the power of the pump to drive the circulation to both the DHW cylinder and the radiator circuits. This system is taking over form the older gravity circulation formerly used for the DHW circuit.
Gravity Circulation
Gravity circulation was often employed on the DHW (primary) circuit, but has now been superseded by pumped circulation. Gravity circulation relied on the weight difference between hot and cold water to drive the circulation and required large (28mm/1") pipes to allow this.
Lock shield Valves
Radiators have two valves, one of which is used to open or shut the rad. The other valve is known as the lockshield valve and should be adjusted by the installer to balance the flow around the system. However in many cases they are simply left fully open.
Motorised Valves
Motorised valves (MV) or Zone Valves are used to divert the flow form the boiler to either the DHW cylinder, the CH radiator circuits or both. In some cases the CH circuit may be divided into 2 or more zones, each requiring an MV.
Open Vented Systems
Traditionally all heating systems were Open Vented, meaning the water was fed from a feed and expansion tank and the boiler was vented via the flow pipe and vent pipe. Pressures in such systems are typically low. Today sealed systems are becoming more common.
Primary (DHW) circulation
The circulation from the boiler to the cylinder coil heat exchanger was usually known as the primary circulation. Traditionally this would be gravity circulation which would take place in large pipes (28mm/1") powered only by the relative lightness of hot water compared to cold. Modern systems use pumped primaries to achieve faster heat transfer using smaller pipes.
Pump
Central heating pumps are very reliable and long lasting. they consist of an electric motor which turns an impeller which accelerates the water flow. CH pumps typically generate a maximum pressure or head of 5 or 6 metres. Pumps are often built into the boiler casing.
Programmable Room Thermostats
Room thermostats are available which combine the function of programmers. This has the advantage of putting the main user controls in one location, although as with simple room stats the location must be chosen with care to ensure that the air temp detected is representative of the whole house.
Programmers
Programmers are essentially timers which control switches linked into the control system. Typically they will have 2 channels (HW and CH) although Combi boilers will normally have single channel programmers. The simpler programmers are also known as timers.
Radiator Valves
Radiators have an isolating valve at each end. One is the main control valve (and may be a TRV) and the other a lockshield valve (although the 2 valves may look much the same, only one will actually open and shut without using a tool). If the radiator needs to be removed for decorating, the two valves can be closed and the union nuts released.
Room thermostats
The general temperature in a house should be controlled by a room thermostat fixed on a wall at eye level in a location where the temperature it senses will be representative of the general house temperature. the thermostat is an electrical switch that feeds back into the control system.
Sealed Systems
Sealed systems do not require a feed and expansion tank or vent pipe. The feed water is supplied manually using a filling loop. Expansion of the system water is accommodated by an expansion vessel. A Pressure Release Valve (PRV) is required as a safety device to release excess pressure. the boiler will require an overheat cutout to protect against thermostat failure.
System boilers
System boilers have the components of a sealed system (expansion vessel, PRV, pump) built into them to simplify the installation. It follows that they are always used on sealed systems and do not require feed and expansion tanks.
System controls
Heating systems require a range of controls to ensure that they operate safely, efficiently and according to consumer demand. The system controls usually comprise the timer or programmer, the cylinder stat, the room stat, one or more motorised valves and the wring to link these together. Often there will be a junction box where the wiring connects.
Thermostatic Radiator Valves
thermostatic Radiator Valves (TRVs) are often fitted to radiators to give a more responsive control of the radiator heat output according to the air temperature in the room. The head of the TRV has a sensor which responds to the air temperature around it. When the sensor expands on warming it pushes against a pin which closes the valve.
Three Port Motorised Valves
Some MVs have 3 ports (pipe connections) and can divert the water flow to either or both of the outlet ports. This allows one MV to control heating and hot water, instead of two separate (2 port) MVs. Most 3 port MVs have a mid position option which allows flow to both circuits simultaneously, but some (diverter valves) allow flow to only one or the other at any one time.
Timers
Timers are used to determine when the heating systems switches on and off. They are also known as programmers, particularly when controlling 2 channels (e.g. HW and CH).
Two Port motorised Valves
Two port MVs are either open or shut. I most cases they open when powered up and close automatically when power is cut off. When open they operate a microswitch which will in turn operate the boiler and pump.
Unvented cylinder
While most cylinders are open vented and are fed by a cold water storage tank in the roof space, some are now unvented and are fed directly from the mains. these supply hot water at much higher pressures than is normally possible from a tank fed vented cylinder.
Vent pipe
Open vented systems must have a vent pipe running from the top of the boiler, rising continually, to terminate over the F&E tank in an inverted U shape. The vent pipe allows air to escape from the system but more critically it is an escape route for steam in the event of the boiler overheating (e.g. failed boiler thermostat). Part of the vent pipe is usually combined with the flow pipe, but this section of pipe must not have any valves on it that might block the vent (e.g. the pump).
This post lists alphabetically the elements of a typical domestic Open Vented Central heating (CH) systems for the benefit of the novice. Sealed systems may eventually be covered too, but initially this post is confined to Open Vented Systems. Please note that not every central heating system will conform in every detail to the descriptions here, but the majority of systems will be covered.
Boiler.
The boiler is the device where fuel (gas, oil, etc.) is burnt and the heat transferred via a heat exchanger to water which then circulates around the heating system. The key control on any boiler is the boiler thermostat which operates the gas valve or oil burner. Modern boilers are much more sophisticated and include a wider range of controls.
Central Heating (CH) circulation
The circulation to and from the radiators (rads) is always powered by a pump or circulator. The pipes from the boiler to the rads are flow pipes and those from the rads back to the boiler the return pipes.
Combi boilers
Combi boilers include a Domestic Hot Water (DHW) circuit and DHW heat exchanger so that DHW is produced within the boiler on demand instead of in a separate cylinder. The overwhelming majority of Combi boilers are on sealed systems.
Condensing boilers
Condensing or High Efficiency (HE) boilers are more efficient than Standard Efficiency (SE) boilers because some of the water vapour in the flue gases condenses back to the liquid state (water). This condensate has to be drained to waste via a plastic condensate pipe.
Controls
Heating systems require a range of controls to ensure that they operate safely, efficiently and according to consumer demand. The controls usually comprise the timer or programmer, the cylinder stat, the room stat, one or more motorised valves and the wring to link these together. Often there will be a junction box where the wiring connects.
Cylinder thermostats
In order to limit the temperature of DHW, and to avoid the boiler firing unnecessarily, DHW cylinders should have a thermostat fitted to the side, about 1/4 to 1/3 up from the bottom. This is usually set to 60C and consists of an electrical switch which feeds back into the control system.
DHW Cylinder
The DHW stored in a cylinder is usually heated by an internal coil heat exchanger through which heating system water circulates. There is usually an immersion heater in the cylinder to allow for heating of the DHW by electricity as an alternative to the heating system water.
Domestic Hot Water
The Domestic Hot Water (DHW) is the hot water that you draw from the taps. This is not the same as the water that circulates around the central heating system. DHW is usually supplied from a cylinder but can be supplied directly by a combi boiler or an independent water heater.
Feed and Expansion tank
Open vented systems are filled and vented to a Feed & Expansion (F&E) tank. This is usually a small plastic tank in the roof space which is fitted with a float valve to fill automatically from the mains supply. The outlet pipe is the F&E pipe and the vent pipe inverts (upside down U shape) over this tank.
Filters - Strainers
Some heating systems may have a filter or strainer on the return pipe to the boiler. Some combi boilers have filters built into them. The filter is intended to remove debris from the water which might otherwise cause problems elsewhere in the system. A recent development is a magnetic strainer (Magnaclean) which collects all iron oxide debris. Any filter or strainer will need to be cleaned at intervals.
Fully pumped Systems
Fully pumped systems use the power of the pump to drive the circulation to both the DHW cylinder and the radiator circuits. This system is taking over form the older gravity circulation formerly used for the DHW circuit.
Gravity Circulation
Gravity circulation was often employed on the DHW (primary) circuit, but has now been superseded by pumped circulation. Gravity circulation relied on the weight difference between hot and cold water to drive the circulation and required large (28mm/1") pipes to allow this.
Lock shield Valves
Radiators have two valves, one of which is used to open or shut the rad. The other valve is known as the lockshield valve and should be adjusted by the installer to balance the flow around the system. However in many cases they are simply left fully open.
Motorised Valves
Motorised valves (MV) or Zone Valves are used to divert the flow form the boiler to either the DHW cylinder, the CH radiator circuits or both. In some cases the CH circuit may be divided into 2 or more zones, each requiring an MV.
Open Vented Systems
Traditionally all heating systems were Open Vented, meaning the water was fed from a feed and expansion tank and the boiler was vented via the flow pipe and vent pipe. Pressures in such systems are typically low. Today sealed systems are becoming more common.
Primary (DHW) circulation
The circulation from the boiler to the cylinder coil heat exchanger was usually known as the primary circulation. Traditionally this would be gravity circulation which would take place in large pipes (28mm/1") powered only by the relative lightness of hot water compared to cold. Modern systems use pumped primaries to achieve faster heat transfer using smaller pipes.
Pump
Central heating pumps are very reliable and long lasting. they consist of an electric motor which turns an impeller which accelerates the water flow. CH pumps typically generate a maximum pressure or head of 5 or 6 metres. Pumps are often built into the boiler casing.
Programmable Room Thermostats
Room thermostats are available which combine the function of programmers. This has the advantage of putting the main user controls in one location, although as with simple room stats the location must be chosen with care to ensure that the air temp detected is representative of the whole house.
Programmers
Programmers are essentially timers which control switches linked into the control system. Typically they will have 2 channels (HW and CH) although Combi boilers will normally have single channel programmers. The simpler programmers are also known as timers.
Radiator Valves
Radiators have an isolating valve at each end. One is the main control valve (and may be a TRV) and the other a lockshield valve (although the 2 valves may look much the same, only one will actually open and shut without using a tool). If the radiator needs to be removed for decorating, the two valves can be closed and the union nuts released.
Room thermostats
The general temperature in a house should be controlled by a room thermostat fixed on a wall at eye level in a location where the temperature it senses will be representative of the general house temperature. the thermostat is an electrical switch that feeds back into the control system.
Sealed Systems
Sealed systems do not require a feed and expansion tank or vent pipe. The feed water is supplied manually using a filling loop. Expansion of the system water is accommodated by an expansion vessel. A Pressure Release Valve (PRV) is required as a safety device to release excess pressure. the boiler will require an overheat cutout to protect against thermostat failure.
System boilers
System boilers have the components of a sealed system (expansion vessel, PRV, pump) built into them to simplify the installation. It follows that they are always used on sealed systems and do not require feed and expansion tanks.
System controls
Heating systems require a range of controls to ensure that they operate safely, efficiently and according to consumer demand. The system controls usually comprise the timer or programmer, the cylinder stat, the room stat, one or more motorised valves and the wring to link these together. Often there will be a junction box where the wiring connects.
Thermostatic Radiator Valves
thermostatic Radiator Valves (TRVs) are often fitted to radiators to give a more responsive control of the radiator heat output according to the air temperature in the room. The head of the TRV has a sensor which responds to the air temperature around it. When the sensor expands on warming it pushes against a pin which closes the valve.
Three Port Motorised Valves
Some MVs have 3 ports (pipe connections) and can divert the water flow to either or both of the outlet ports. This allows one MV to control heating and hot water, instead of two separate (2 port) MVs. Most 3 port MVs have a mid position option which allows flow to both circuits simultaneously, but some (diverter valves) allow flow to only one or the other at any one time.
Timers
Timers are used to determine when the heating systems switches on and off. They are also known as programmers, particularly when controlling 2 channels (e.g. HW and CH).
Two Port motorised Valves
Two port MVs are either open or shut. I most cases they open when powered up and close automatically when power is cut off. When open they operate a microswitch which will in turn operate the boiler and pump.
Unvented cylinder
While most cylinders are open vented and are fed by a cold water storage tank in the roof space, some are now unvented and are fed directly from the mains. these supply hot water at much higher pressures than is normally possible from a tank fed vented cylinder.
Vent pipe
Open vented systems must have a vent pipe running from the top of the boiler, rising continually, to terminate over the F&E tank in an inverted U shape. The vent pipe allows air to escape from the system but more critically it is an escape route for steam in the event of the boiler overheating (e.g. failed boiler thermostat). Part of the vent pipe is usually combined with the flow pipe, but this section of pipe must not have any valves on it that might block the vent (e.g. the pump).
