Technical Terms

Gas central heating terms are not always logical!

A combination boiler or combi is a boiler which heats mains cold water on demand, to produce hot water for the taps. It also heats the central heating water for the radiators or underfloor heating. With a combi fitted there is no need for a hot water cylinder to store hot tap water. If the combi is firing for central heating, a new demand for hot tap water takes priority and the boiler switches over to put all its power into heating the tap water.

Heating tap water on demand takes a lot of energy in a short amount of time and combis have power outputs which are much higher than would be needed just for the central heating system. For that reason, combis can heat up radiators from cold more quickly than regular boilers. Combi boilers are good for heating water for one tap or shower at a time. Only high powered combis can heat enough water for two strong showers to run simultaneously.

The stated power output of the combi boiler itself (typically between 25kW and 39kW) is not the only consideration, as the higher power output also requires a larger gas supply pipe. The resistance of the gas pipework slows down the flow of gas. As the length of the gas pipe increases, the resistance increases. Every bend, elbow or tee in the pipe also increases the resistance, reducing gas flow. For this reason there is a limit to how far from the gas meter the combi boiler can be sited. Increasing the diameter of the gas pipe helps. This reduces the resistance and increases the gas flow rate but there is a limit to how large a pipe can be used.

Cutting copper pipework with a wheel cutter can leave a significant burr obstructing the internal bore. A good installer will fully deburr the ends of each section of copper pipe, ensuring that the full bore is maintained.  There are occasions, however, when a combi boiler cannot be fitted in a certain location because it is too far from the meter and the long pipe cannot supply sufficient gas. A regular, heat-only boiler might be fine in that location because it has a much lower gas rate requirement.

Mains cold water flow rate is a factor for combi boilers too. Hot tap water cannot be produced faster than the mains cold water comes in, so if the cold mains supply pressure and flow rate are low, a combi may not be an option. Mains cold water flow rate can vary depending on the local demand at different times of day. If your kitchen mains cold tap can deliver at least 10 litres per minute at peak demand times, you are likely to be OK, at least with the smaller sizes of combi.

Regular boiler, conventional boiler and heat-only boiler are terms used for non-combi boilers. These boilers do not heat tap water directly on demand. Tap water is heated indirectly via a coiled pipe in the hot water cylinder. Central heating water from the boiler passes through this coil and indirectly heats the stored tap water in the cylinder. Regular boilers also heat the radiators or underfloor heating pipes.

Motorised valves are used to divert the flow from the boiler to either the radiators or to the hot water cylinder or to both simultaneously. Motorised valves are also known as diverter valves or zone valves.

A multipoint water heater or multipoint is an instantaneous hot water heater which heats tap water on demand. Multipoint water heaters do not heat the central heating system. Like combis, for heating tap water they are not truly instantaneous but take about 20 or 30 seconds to get the hot water to the taps. Multipoints are not as powerful as combis; they can produce an adequate shower but not torrents of hot water.

They only work well serving one tap, or shower, at a time. They are called multipoints because they can heat water for taps at multiple points in the house. Single point tap water heaters do exist, serving only one sink, but they are not common.

Back boilers are boilers fitted behind a fire. They originated as a water jacket fitted behind a coal fire and produced hot tap water and, maybe, heated a small radiator in the bathroom. Because they were fitted behind the fire, they were located in the chimney breast. As coal fires were replaced with gas fires, combined units were designed with a gas back boiler fitted behind a gas fire front, both sharing the same flue passing up through the chimney stack.

Initially, these gas back boilers only heated the water in the cylinder and a single radiator or towel rail in the bathroom. From the 1950s, central heating started to become an option for normal houses and, by adding a pump, radiators could be fitted to other rooms. Back boilers, still located in the builders’ opening in the chimney breast, became the standard. Back boilers were open-flued appliances and have not been made in that form since the late 1990s or early 2000s.

Open-flued appliances were the first to be developed and they evolved from open fires. When fires were first fitted in houses, a chimney was required and the flue gases from combustion passed up through the brick flue or chimney. As oil and gas heating appliances were developed they used the same open flue system. There was no physical barrier separating the combustion gases from the room air and the air for combustion was drawn directly from the room.

This open flue system was originally known as a conventional flue system and it means the same thing. Open-flued appliances are not limited to the fireplace. Many floor-standing and wall-hung open-flued boilers had a circular flue going up through the building to the roof, or out through a wall and then up to above roof level.

The characteristic that defines open-flued appliances is that there is no physical separation, of the combustion zone and flue gases, from the room air. There are risks with open flue systems. If the air supply for combustion is choked off, or it the flue becomes blocked, flue products may spill out into the room. These flue gases may be completely odourless and they are poisonous!

It’s vital to ensure that the air supply for the open flued boiler or fire is adequate and that the flue is not obstructed in any way. It’s also shrewd to fit an electronic carbon monoxide alarm, now freely available for less than £20.

Open flued appliances also draw extra air from the room, which dilutes the flue gases passing up the chimney and helps to reduce unwanted condensation in the flue (chimney). Unfortunately, the combustion air and dilution air used by open flued appliances is nice warm air we’ve already paid to heat, so it makes these appliances less efficient.

Room-sealed appliances were designed to address both the poisoning risk and the efficiency problem. They are called room-sealed because there is no air connection between the combustion chamber in the boiler and the air in the room. Room-sealed appliances draw their air for combustion directly from outside air and send the flue product back to outside air. They are inherently safer than open-flued appliances and they are more efficient too, because they don’t send a lot of warm room air out through the flue.

Earlier room-sealed boilers did not contain fans; they used the fact that the hot flue product going out was lighter than the heavier cold air coming in, to set up a gravity circulation of air and flue gases through the boiler. Room-sealed appliances without a fan were known as natural-draught.

Long flue runs were not possible with natural-draught boilers as the increase resistance of the longer flue stopped the natural, gravity-driven movement of flue gases through the boiler. Natural-draught room-sealed boilers (and fires) were fitted in the room but onto an outside wall. The flue/air duct passed through the wall to a rectangular (or for most fires, circular) stainless steel flue terminal on the wall outside.

The flue/air terminal had both the air inlet ducts and the flue outlet ducts close together on the wall outside. These were both in the same wind pressure zone and the flue system was know as a balanced flue because the pressure on the inlet and outlet was balanced. Wind pressure made very little difference to well designed balanced flues.

Adding a fan to a boiler flue system allowed the flue ducts to be longer and it became possible (but not necessary) to site the boiler away from an outside wall. These boilers are called fan-flued or sometimes, fanned-draught. All new boilers now use a fan. Originally, fans were used to blow fresh combustion air into the sealed casing (which effectively pushed the flue product out) or to blow the flue product out of the sealed casing (which effectively sucked fresh combustion air in).

New boilers now use a fan to blow a correctly proportioned mix of fuel gas and air directly into the burner. This type of fan is known as a pre-mix fan and it allows the pre-mix burner in the boiler to burn with the flames pointing downwards! This, in turn, allows the heat exchanger to be placed below the burner. Any condensate formed in the heat exchanger can then fall further down, into the condensate sump, without flooding the burner.

Originally the gas industry talked about flueways which passed through chimneys. We thought of the chimney as the brick stack through which the flue passed, sometimes in a flexible stainless steel flue liner. Now there have been official changes to the terminology for the gas industry at least, and we are told that metal flues will now be called chimneys. We don’t think it was deliberately designed to confuse!

Besides open-flued appliances and room-sealed appliances there are also flueless appliances. There were some single-point water heaters which were flueless and could only be used for a maximum of 5 minutes and in a larger, well-ventilated room. These have now almost entirely disappeared. The commonest flue-less appliances are gas cookers. Gas cookers rarely have perfectly clean combustion and should only be operated for a limited amount of time and always in a large enough, well-ventilated room.

Gas cookers should never be used for heating a room! Generally, gas cookers are only operated with two or three burners firing at once, and for relatively short periods, so the risks from flue gases are very small. However, at certain times like Christmas they may be heavily used and a window should be opened in the room, not least to stop the condensation from running down the windows or walls.

There are also fixed flue-less heaters. These use a catalytic converter to “scrub” the flue gases to remove toxic components. All the carbon dioxide and water vapour passes into the room air. These flue-less heaters have very tight rules applying to installation and use. They all require a permanent air vent into the room, from outside air, with a free air space of 100cm² (or 10,000mm²). This is equivalent to a 10cm wide hole in the wall (a 4″ wide hole) and it must be permanently open, even when the heater is not used.

The positioning of the air vent relative to the heater is critical, as is the minimum gas pipe sizing. All of the water from combustion also ends up in the room, possibly causing condensation problems. In short, we feel that flue-less gas heaters are a potentially dangerous nonsense! They may make sense in a commercial greenhouse where the carbon dioxide and water vapour are an added bonus, but not in a domestic room.

If you’re interested, we also wrote an article about radiator balancing, or balancing a central heating system:

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