Combi Boilers - Learn all About Combination Boilers and Condensing Boilers and how They Produce Hot Water and run the Central Heating in Your Home

Summary: Learn all about Combination Boilers, or combi boilers as they are also known. Find out how they are used to heat the hot water that supplies the taps in your home and also how they run your central heating. You will also find out about how condensing boilers recycle the heat produced by the boiler that would otherwise be lost and re-use it and also about the components and parts that make up a combi boiler, what filling loops are and how they are installed.

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What is a Combination or Combi Boiler

Combination boilers combine the functions of a central heating boiler and an instantaneous multi point water heater. They give priority, in the main, to the supply of domestic hot water (dhw).

The combination boiler has all its operating components contained within the casing and is typically designed for use within a sealed central heating system. A combination, or combi boiler, removes the need for separate water tanks as it's cold water supply is taken directly from the mains water and it heats the hot water instantaneously removing the need to store hot water unless you really want to.

Our projects section also contains lots of information about central heating of all kinds including oil fired.

How Does a Combination Boiler Work

In really simple terms the fuel supply (usually gas, sometimes oil) comes into the boiler and is directed to the burners in the boiler.

These burners are activated by messages from the valves in both the central heating and hot water systems in your home. If a valve (tap) is turned on which asks for hot water to a bath for example, the open tap allows the water to flow.

The flowing water in the boiler runs over something called a paddle switch which turns the burners on in the specific part of the boiler which heats the water for the taps (The Domestic Hot Water System).

Any surplus heat is stored in a heat exchanger, a small compartment within the boiler, so when (for example) the central heating is in full flow, the surplus heat in the heat exchanger is used to heat the hot water for the taps or domestic hot water system.

Both the domestic hot water pipes and the central heating pipes run through the heat exchanger so one can always heat the other. This allows for both systems to be hot at the same time.

How a combination boiler works

Diagram of a combination boiler at work

If the central heating is required indoors but no hot water is being called for the burners will heat the pipe directly to give the hot water to the central heating system in a more efficient way. If the hot water is needed but there is no central heating on and therefore no real heat in the heat exchanger, the burners will operate on the pipe in the same way.

In many cases turning on either the central heating or the domestic hot water heats up a small compartment of water in the heat exchanger.

Both the central heating pipes and the domestic hot water pipes run through the heat exchanger and the water inside of the heat exchanger heats the water in the pipes (indirect heating) so it does not matter which of the two water supplies is called for they are both heated in the same way through the heat exchanger.

Parts of a combi boiler - left hand view

Parts of a combi boiler - left hand view

Combi boiler construction - right hand side

Combi boiler construction - right hand side

Are Central Heating and Tap water Ever Mixed?

No, never. The central heating system and the domestic hot water system are two completely separate pipe-runs and the water contained within them is never mixed. When you think about it logically, with all the inhibitors and cleaning agents used in central heating systems, it would not take long to poison the entire UK population if they were mixed!

Will a Combination Boiler Save me Money?

Money saving expert has some views on many of the myths surrounding saving money with different forms of heating so it is as well to read the article to get an overview on this topic.

The answer to the specific question of whether combi boilers are cheaper to run than conventional boilers depends entirely on your lifestyle. If you have a large family, showering and bathing all the time then a combi is probably going to turn out more expensive than a conventional boiler. If you use little water however, a combi is the way to go. A condensing boiler changes this equation somewhat.

What is a Condensing Boiler

As fuel burns to heat the water in the boiler, it gives off exhaust gasses. These gasses are very hot but usually this heat goes straight out of the flue and is wasted. A condensing boiler however, recovers this heat by taking the water vapour from the heated exhaust gasses, condensing it into hot water and feeding it into a heat exchanger (see above) where it can be used as additional heat for the hot water or central heating water.

It is an incredibly efficient way of heating water and is now a building regulation requirement when domestic boilers are being exchanged.

What is a Heat Exchanger

A Heat exchanger is an area where heat from one medium such as gas, liquid or fluid is directed at another medium in order to use the heat from one, to warm the other, e.g. a place where heat can be exchanged.

In a combination boiler, the heat exchanger uses the heat from the central heating water to heat the water to the domestic hot water system and vice-versa.

Combi boiler heat exchanger

How a heat exchanger is used to take heat from one source and use it to heat another

How is a Combination Boiler Installed

Installation is made easier by incorporating an expansion vessel, thus eliminating the need for a feed and expansion cistern in the roof space. Like an ordinary boiler it will have a gas fired burner and a heat exchanger. The waterways of both form part of the central heating system.

In most of the early designs of combination boiler, hot water from the heat exchanger passes to a diverter valve, which directs the flow to a domestic hot water calorifier or central heating circuit.

The calorifier is tightly packed with small copper tubes, through which secondary water flows the moment the hot water draw off is opened. As the water flows it picks up heat from the surrounding primary water in the calorifier.

The central heating system is pressurised and needs a relief valve to protect against any build up of pressure in the system.

A pressure gauge on the appliance indicates the operating pressure. A by-pass, connected across the primary flow and return, ensures that an adequate flow of water is maintained. This by-pass may, or may not be a built in feature of the appliance.

The flue arrangements for combination boilers may be open, room sealed, fan assisted and room sealed, fan assisted open, miniature flue/condensing or se-duct.

The primary heating circuit of combination boilers should be pre-commissioned, cleansed and treated with a corrosion protector in the same way as any other heating system.

In hard water areas, the secondary domestic hot water should be treated with an in-line scale reducer or water softener (See projects section for info on both of these).

This will prevent the build up of limescale on the instantaneous water heater and the accompanying reduction in flow rate and water outlet temperature.

With a direct combination boiler the secondary water is heated in one of two ways. The water may pass through pipes which run down the inside, or next to the primary pipes in the heat exchanger, or by passing through a separate heat exchanger which is bolted on top of the main heat exchanger. A direct combination boiler does not need a diverter valve or a secondary calorifier.

What is a Dead Leg in Plumbing Terms?

In a domestic hot water system where the (condensing) boiler is a replacement for an existing storage cylinder system or a conventional boiler, it is essential that all redundant pipework is removed to eliminate “dead legs”.

Dead legs are sections of pipe which are now redundant and have been capped off. They still remain part of the system however and even though they go nowhere, water still flows into them.

Because it cannot flow out again, dead legs tend to trap any air that is in the system and this air makes the transfer of heat  much less efficient and an air lock  can sometimes stop a system working altogether. When a boiler senses an air lock (IE anything that is not water) it will shut off. The diagrams below identify most parts of a combination boiler.

Are Mixer Showers OK on Combi Boilers?

Well, one is!. Combi boilers feed mains water to the domestic hot water system. This means that a mixer shower (preferably a thermostatic mixer shower) works really well with a combi boiler providing the water pressure is OK in the first place.

See our projects on choosing the right shower and installing a pressure reducing valve).

The problem is that the shower needs all of the pressure from the mains water to work efficiently. If you have two mixer showers drawing water from one combi boiler you are, 99% of the time, asking the boiler to work much harder than it is capable of.

People who want a second shower when they have a combination boiler usually opt for a strong electric shower for the second one.

What is a Filling Loop in a Combi Boiler?

For centrally heated radiator systems to work efficiently the water needs to be pumped around the system. If any air gets into the system the effectiveness of the pump, the heating and the boiler is reduced. If enough pressure is lost (through a leak or a loose or rusty connection) the boiler will sense a fault and stop working.

Once the pressure in the boiler drops below a certain point it is essential that the pressure is reinstated and this is done via the filling loop. It is important to note that the filling loop is not part of the boiler but most often it is situated quite close to it for convenience.

There is a pressure gauge on the boiler which shows the pressure under which your boiler is operating at any one time. The pressure should not usually be allowed to drop under 1 bar, but there is usually a green segment within the gauges scale which shows you the range of pressures under which your boiler will operate most efficiently.

Pressure gauge on combination boiler

Make sure boiler pressure stays in green range

What Does a Filling Loop do?

The filling loop is connected between the mains water supply and the central heating return pipe from the central heating system. Both of these pipes go to the boiler, as well as other places so, most often it is the most convenient place to put a filling loop. The loop however can be put anywhere where these tow pipes are fairly close to each other.

Combi boiler filling loop

How a filling loop is used to top up a boiler

How is a Filling Loop Made?

A filling loop is simply a piece of pipe (usually flexible) with a pipe connector and isolation valve on each end. Filling loops can be bought all in one piece but it is perfectly OK to make your own.

It is important that one of the isolation valves is a non return valve as none of the pressured water from the radiator return pipe must be allowed back into the mains water.

Combi boiler filling loop with non return valve

A non-return valve should be fitted to a filling loop

Am I Allowed to Service my own Combination Boiler?

No! To do any work what so ever on a gas or oil boiler you must belong to the relevant competent workers scheme. For Gas, this is the Gas Safe Register. For oil fired appliances this is OFTEC.

Please stay safe and leave boiler work to the experts.

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