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Lovekin — Central Heating and Gas Boiler Terminology

Gas appliance and central heating controls terminology – please read our Legal Disclaimer


You’ll come across lots of terms on this site (and in the domestic gas industry generally) which can be confusing so we’ll try to define what some of them mean:

A multipoint water heater or multipoint is a less common type of boiler which heats only tap water, on demand. It is never used to heat central heating radiators.

Combi boilers (combination boilers) are boilers which heat tap water on demand and also heat the radiators (or underfloor heating). Combi boilers do away with the need for a hot water cylinder. If the central heating radiators are on when a hot tap is run, the combi boiler temporarily switches over to heating the tap water. Some combination boilers store a small amount of hot water within the boiler to shorten the time taken to get hot water to the taps.

Regular boiler is now a term used to mean a boiler which does not produce hot tap water on demand. Regular boilers are effectively non-combi boilers. They are used to heat radiators and to heat a hot water cylinder which stores hot tap water. Regular boilers are sometimes called conventional boilers, again to distinguish them from combi boilers. Confusingly, the term Regular Boiler was, in the past, used to describe a boiler which was not a condensing boiler but the term is rarely used in this way now.

Condensing boilers are the same as High Efficiency boilers (also known as HE boilers). These are boilers which are designed to recover some of the heat which would otherwise be lost in the flue. To do this they go into condensing mode which gives them a higher overall efficiency. If you want to know more about high efficiency boilers and about what gross and net efficiency mean, you can find it explained in an article we wrote on our lovekin.net site. Both combi boilers and regular boilers (non-combi boilers) can be high efficiency boilers. Now, nearly all new gas boilers fitted in the UK are high efficiency boilers.

Room-sealed gas boilers are boilers which draw their air for combustion from outside air, and pass their flue product back outside. Effectively, they are sealed boxes with a flue/air duct attached. Unless the boiler or flue is damaged, or incorrectly installed, it is not possible for the flue product to mix with room air. This makes room-sealed boilers inherently safer. There are also room-sealed gas fires.

Open-flued (conventionally flued) boilers are chimney-type boilers. These boilers draw their air for combustion and flue dilution from the room in which they are sited and then pass the flue product up a flue pipe to outside air (effectively like a chimney). Because there is no seal between the combustion chamber and the room air, open-flued boilers are inherently less safe than room-sealed boilers. Domestic open-flued boilers are no longer fitted in the UK and these boilers will gradually disappear. We are not suggesting that they cannot function safely, just that there is a higher risk of danger from carbon monoxide. It is vital that open-flued boilers are serviced regularly and thoroughly.

Flueless appliances do exist. Cookers are flueless and should not be used for heating a room. They are designed to be used for short periods in a well ventilated room and, if they are used heavily (Christmas!) the kitchen window should be opened too. There are still a few flueless gas-fired water heaters around. These are single-point water heaters (hot water from one spout only) and strict rules apply. They must not be run for more than 5 minutes at a time. Strangely, there are now flueless gas fires on the market. These use a catalytic converter to “scrub” the flue gases clean. Very strict rules apply to their installation. We don’t fit them as we think they make no sense. All the water vapour from combustion is passed directly into the room air and condensation is quite likely to be a problem. They also require a large air supply vent into the room to function safely and there is a risk that householders will cover this because of the cold draughts.

Back boilers are gradually disappearing. These are combination units which consist of an open-flued boiler fitted behind an open-flued gas fire and passing flue product up the chimney, through a liner. These draw their air for combustion and dilution from the room, low down. They are usually fitted in a living room which is carpeted and they draw fluff and fibre and pet hair across the floor and into the back boiler; this makes the air intakes more likely to block up and it is critical that back boiler units (including the fire fronts) are serviced frequently and thoroughly. There are still “back boilers” on the market but the ones we’ve seen are room-sealed, fan-flued boilers (with the flue/air duct contained in the chimney breast) and are fitted with electric fire fronts. They are targeting a niche sector and are generally too expensive to be taken seriously.

Programmers are usually electronic controls with a display and usually with lights. They usually allow you to set different times of day for the heating and for hot water to be heated. Older systems may not allow you to select only heating but having the hot water heated at the same time is not normally a problem. All systems should, however,allow you to select hot water on its own.

Older programmers may be electromechanical (rather than electronic). These don’t have an electronic display but have a round dial or similar indicator to show whether heating and hot water are on or off.

Time clocks are similar to programmers but usually only a single channel, so they either do just the heating or just the hot water or just a specific heating zone. Time clocks are often used to control the central heating when a combi boiler is used, as the hot water is not timed but always on demand.

Room thermostats (or room stats) are used to measure the air temperature in the room and to switch the heating on when the air temperature drops below a set temperature and to turn it off again when the set temperature is reached. Room stats were traditionally connected by wires going back to a central junction box but they can also be wireless with a wireless transmitter (usually fixed to a wall like a traditional room stat) and a receiver fixed to the wall near the central junction box. The wireless transmitter is not always fixed to a wall but may be moveable so it can be carried from room to room.

Programmable room stats are clever. They combine the central heating time control and temperature control into one unit and this allows you to automatically set different temperatures for different times of day. They are very often wireless and can be moved from room to room.

Cylinder thermostats are usually connected to the side of the hot water cylinder, against the metal side wall, usually about one-third of the way up from the bottom. They send power to a motorised valve which controls the flow of hot boiler water through the cylinder heating coil. When the cylinder stat is up to temperature the motorised valve closes, preventing the tap water in the cylinder from getting any hotter. Unvented hot water cylinders usually have a cylinder stat which has a probe that slips into a pocket which dips into the hot water cylinder through the side wall.

Motorised valves (also called zone valves or diverter valves) are valves fitted into central heating pipes and have a motor attached. Two-port motorised valves (zone valves) control the flow through a single pipe. Three-port valves (mid-position valves or diverter valves) are in a “T” configuration and have the flow coming in through one pipe and passing out through one or other of two pipes, or through both at the same time. Diverter valves are usually only either/or but mid-position valves can open to both outlet pipes at once. Motorised valves are most commonly used to control the boiler flow, passing it to either the heating coil of the hot water cylinder or to the radiator circuit or to both at the same time.

Legal Disclaimer:

Some information provided on this web site relates to safety and is given in good faith. It is only an indication of our understanding of some of the gas safety issues involved.  You must make your own judgements and should take the advice of a Gas Safe Registered engineer on site in your home. We can accept no liability or responsibility for any action or decision you take as a result of reading the information on this site, nor can we accept any liability arising out of misleading, incomplete or incorrect information on this site.