Condensing Boilers: How They Work

A condensing boiler is a boiler that recovers heat from its own waste gases before they leave the flue. Where an older boiler lets a large share of the fuel's energy escape up the flue as hot exhaust, a condensing boiler captures that heat and uses it to warm the water returning to the appliance, so noticeably less fuel is spent for the same warmth. New boilers in the UK are required to be condensing and are at least 90% energy efficient¹, a rule that has applied since 2005². Condensing gas boilers are now by far the most common boiler type in UK homes³.

The name comes from the physical process at the heart of the design. Burning gas, oil or LPG produces water vapour, and that vapour carries latent heat, energy locked up in the vapour itself. A condensing boiler is defined as one designed to make use of the latent heat released by the condensation of water vapour in the combustion flue products⁴. In other words, it deliberately turns that vapour back into liquid inside the boiler, harvesting the heat the change of state gives up. The liquid that results, the condensate, leaves through a small pipe to a drain, and that pipe is the feature most householders notice.

A boiler that takes heat back from its own flue

Every boiler burns fuel to heat water, and every boiler produces hot exhaust gases that have to go somewhere. The difference with a condensing model is what happens on the way out. A condensing boiler captures the heat from waste gases, using it to warm up the water returning to the boiler⁸, and its heat exchanger is powered by the heat generated by the flue gases⁸. The result is that a condensing boiler recovers more heat and loses less heat through its flue pipe than an older model².

This is why the design became the standard rather than an option. Since 2005, boilers have been legally required to be condensing boilers, which are much more efficient than older models². When a boiler in the UK is replaced today, the replacement will almost certainly be a condensing boiler⁹. Condensing boilers are the most common type of new boiler¹⁰, and condensing gas boilers are by far the most common boiler type in UK homes³.

For a householder, the practical test is straightforward. It is a gas or LPG boiler installed after 2005, or an oil boiler installed after 2007¹¹. The visible giveaway is the condensate pipe: a small plastic pipe, usually on the wall outside the home or underneath the boiler, carrying away the water the appliance produces¹². An older boiler has no such pipe, because it never condenses its flue gases on purpose.

The condensing process, step by step

The sequence inside the boiler runs as follows.

1. Fuel burns in the combustion chamber, heating the primary heat exchanger in the ordinary way, exactly as in any boiler.
2. The hot flue gases pass through a second heat exchanger before reaching the flue. This heat exchanger is powered by the heat generated by the flue gases⁸, and it transfers that heat to the water returning from the radiators, pre-heating it so the burner has less work to do⁹.
3. The water vapour in the flue gases condenses. As the gases cool against the heat exchanger, the water vapour from the flue gases is captured and turned into a liquid⁸. This change of state releases latent heat, the energy the boiler is designed to harvest⁴.
4. The condensate is discharged. The waste water exits the system via the boiler's condensate pipe⁸, which runs to a suitable drain.
5. The cooled gases leave through the flue, carrying far less heat away than they would from a non-condensing boiler.

The whole cycle depends on one condition: the water returning to the boiler must be cool enough. If the returning water is warm, the flue gases never fall to the temperature at which the vapour condenses, the latent heat stays locked in the vapour, and the boiler runs as an ordinary boiler with an ordinary flue loss. That is why the system's design and settings matter as much as the appliance itself, a point developed under flow temperature below.

Efficiency: a minimum of 90% of fuel turned into usable heat

The headline figure is consistent across official and independent sources. New boilers in the UK are all required to be condensing boilers, which are at least 90% energy efficient¹. A new condensing gas boiler would achieve an efficiency of 90%¹³, and condensing boilers are capable of using around 90% of their heat³. Modern condensing appliances can exceed 90% efficiency¹⁴.

The comparison with what came before is where the saving lies. Condensing boilers recover waste heat that normally passes out into the air, making them up to 97% efficient, as opposed to the 60 to 70% efficiency of an older boiler¹⁵. Condensing boilers are 10 to 20% more efficient than regular ones¹⁰. The main advantage, as the maker Ideal puts it, is that a condensing boiler wastes much less energy, lowering both carbon footprint and bills⁸.

The same efficiency is available off the gas grid. The latest liquid fuel-fired condensing boilers have efficiency ratings of over 90%, equal to gas boilers¹⁴, so an oil-heated home replacing an old boiler gains a similar step change rather than a lesser one.

Wales states the standard in building regulations terms: a condensing boiler with a SEDBUK rating of A or B should be installed unless an assessment carried out by a Gas Safe Registered engineer shows an exception applies¹⁶. The A or B rating is the efficiency band that the 90%-plus figures sit within. More on what the ratings mean is on boiler efficiency and ErP ratings.

Flow temperature: how reduced settings improve efficiency

A condensing boiler only condenses when the water coming back to it is cool. The water returning to the boiler has to be below a certain temperature, around 54°C, for the process to happen⁵. Above that, the vapour stays vapour and the latent heat goes up the flue.

This is where most UK installations fall short of their potential. Most condensing boilers in use across the UK are set with flow temperatures of 70 to 80°C⁵, and many are installed to have a flow temperature of between 70°C and 80°C⁶. Condensing combi boilers tend to have heating flow temperatures set to 75 to 80°C, but 50 to 70°C is enough for many homes¹⁷. Ideally, these boilers should be set to a flow temperature of 60°C or lower to ensure they are running as close as possible to their design efficiency⁶.

The measured gains are worth having. Research from Nesta shows that turning down the flow temperature to 60 degrees improves a boiler's efficiency by nearly 4%¹⁸, and households could reduce their gas use, costs and emissions by 8% by lowering the flow temperature on condensing combi boilers⁵. Boilers are more efficient when the return temperature is low, and this can save both energy and money¹⁸.

The official calculation methods recognise the same physics. Default efficiency data for condensing boilers is based on the design flow temperature of the water in the heat distribution system being 55°C or higher, and where a system has been designed to operate at a lower temperature, the space heating efficiency of a condensing boiler is increased by an applicable efficiency adjustment, with 45°C or 35°C design flow temperatures stated on the commissioning certificate¹⁹. For design flow temperatures below 35°C, the efficiency correction for 35°C is used, and for design flow temperatures above 80°C, the efficiency correction for 80°C is used⁴. In plain terms, the cooler the system is designed to run, the more efficient the boiler is credited as being, because it genuinely is.

The heat emitters govern what is possible. Water underfloor heating uses water at a lower heat than a radiator, making the boiler more efficient²⁰, which is one reason wet underfloor systems pair naturally with condensing boilers. Radiators can often run cooler than their historic settings too, provided the home still reaches temperature. The practical steps are on boiler flow temperature and heating controls.

The condensate pipe: its purpose and drainage point

A condensate pipe is a small pipe that carries away the water produced by your boiler¹². It is usually connected to the boiler and made of plastic¹², and it is usually found on the wall outside your home or underneath your boiler¹². The water it carries arrives in bursts rather than a continuous flow, which is why the pipe often appears to spit or gurgle: the boiler collects the condensate and releases it in stages.

The liquid is not plain water. Condensate must be disposed of in the right way, into a suitable drain, because it contains contaminants that make it slightly acidic²¹. That acidity is mild, and normal drainage handles it without any special treatment, but it is the reason the pipe must terminate in a proper drain rather than onto a path or into an open container.

The pipe's one well-known weakness is cold weather. In cold weather, the water inside the condensate pipe can freeze, and this will stop the boiler from working¹². The water in the pipe can freeze if the temperature outside drops below freezing, causing the boiler to stop working or display an error¹⁰. If the pipe gets blocked with a build-up of ice, the boiler will stop working²². The signs are that water is not coming out, you can see ice or the pipe feels very cold, or there is a gurgling noise from the boiler or pipes¹².

The fix is simple enough for most households. Hold a hot water bottle, warm cloth or microwaveable heat pack around the frozen part, or pour hot, but not boiling, water over the frozen end of the pipe, then reset the boiler if needed¹⁰. Northern Gas Networks and Wales & West Utilities give the same method: hold a hot water bottle or heat wrap around the affected pipe, or pour hot, but not boiling, water over it²¹. For external pipes, pour hot, but not boiling, water on the pipe; for internal pipes, hold a hot water bottle against the pipe¹².

Prevention is better than thawing. Insulate the plastic pipe with foam pipe insulation to keep it warmer and prevent the condensate water from freezing²², and the most energy-efficient way to protect it is to wrap the condensate pipe in foam pipe insulation¹⁰. Insulation can be bought from any DIY store¹². Where freezing recurs, the longer-term remedies are to move the pipe inside, replace it with one of a wider diameter, fit foam pipe insulation, or retrofit a manufacturer antifreeze¹⁰.

Combi, system and heat only: every one is a condensing design

Condensing is not a shape of boiler but a way the boiler works, and it comes in all three domestic forms. Homeowners have a choice of three different condensing boiler types: combi boilers, conventional boilers and system boilers⁸. Modern condensing appliances are available as regular, system and combination boilers¹⁴, and condensing boilers come in both combination and system models⁹.

The choice between them is about hot water, not efficiency.

• Combi boilers are the only type of condensing boiler that provides both central heating and instant hot water in one package⁸. There is no cylinder, and hot water arrives on demand.
• System boilers heat a hot water cylinder, with the major components built into the boiler itself.
• Regular, heat-only or conventional boilers also heat a cylinder, with a separate feed and expansion arrangement in the loft.

Because all three are condensing, the efficiency figures on this page apply whichever form the home takes. The differences that matter to a household are the ones covered on combi boilers, system boilers and regular, heat-only boilers. One point worth weighing at the choice stage: a combi has no cylinder, and that shapes what the home can do later with solar water heating, as covered below.

Picking the right output: sizing by bedrooms, radiators and bathrooms

A condensing boiler only delivers its efficiency if it is the right size for the home. The guidance on output is straightforward: a 24 to 30kW boiler is suitable for any home with up to three bedrooms, whereas larger houses require a boiler that is 35kW or more²⁴.

For a combi, the hot water load matters as much as the heating load, because the same appliance has to supply both, and a household running two showers at once needs more hot water output than one that never does. For system and regular boilers, the cylinder does the hot water work, so the output is sized to the radiators and the heat loss of the building. Oversizing wastes fuel by making the boiler cycle on and off; undersizing leaves the home cold on the coldest days. The full method is on boiler sizing.

Installation, running costs and potential savings

Installing an A-rated condensing gas boiler plus a full set of heating controls will cost about £3,900⁷. That figure is an independent estimate for the whole job, appliance and controls together, and actual quotes vary with the appliance, the flue run and the work needed on the system.

The savings come from two directions. Using a high efficiency condensing boiler with heating controls could save you a substantial amount of money over a year²⁵. Lowering the flow temperature alone could reduce gas use, costs and emissions by 8%⁵. In Northern Ireland, upgrading an oil boiler can save £440 a year⁷, which shows the scale of the gain when an old, inefficient oil appliance is replaced with a modern condensing one.

One honest caveat comes from the official measurement data. Analysis of installations in England and Wales between 2011 and 2015 found that the savings from new condensing boilers decreased by around a tenth between Year 1 and Year 5 after installation²⁶. The first year's saving is the biggest; the benefit persists but narrows somewhat over time.

On the rules side, the condensing requirement is close to absolute. Non-condensing boilers can only be fitted if the building qualifies for an exemption, which is based upon an assessment of the technical difficulty and cost associated with installing a condensing model¹⁴. In practice that exemption is rare, and a replacement will almost certainly be condensing⁹. What installation itself involves is covered on boiler installation, and the current prices across the market on new boiler cost.

Lifespan, servicing and maintaining efficiency

A condensing boiler holds its efficiency only if it is looked at. It is recommended that a boiler is serviced at least once a year, in other words every 12 months¹⁰, and the boiler should be serviced annually to ensure that it stays in good working order and maintains its efficiency⁹. Servicing is also the point at which an engineer checks the condensate route and the flue, the two things most specific to a condensing design.

Age is the other consideration. If a boiler is over six years old, replacing it with a modern condensing boiler can significantly improve energy use¹⁵, which places the economic replacement window well before a boiler physically fails. A boiler that is six years old today was likely installed after the 2005 condensing requirement, so it is condensing already, but the newest appliances sit at the top of the efficiency range while an aging one drifts down it. More on the working life of an appliance is on boiler lifespan, and on the routine itself, boiler servicing and maintenance.

Pairing with solar water heating

A condensing boiler and a solar thermal panel can share the hot water job, but the fit depends on the boiler type. If you have a conventional boiler with a hot water cylinder, it is often compatible with solar water heating²⁷, though the existing cylinder generally has to be replaced with one designed for solar water, or a dedicated solar cylinder²⁸. Solar water heating is easier to work with system boilers, although combi boilers can be adapted too¹¹.

The combi case is the difficult one. Few combi boiler models are compatible with solar thermal systems, as they are not designed to heat water in a cylinder²⁸. A combi household wanting solar water heating will likely need to add a solar hot water cylinder and replace the boiler with a conventional boiler, or a heat pump²⁷. That is a significant change, and it is the main reason a household weighing a combi against a system boiler may want to read solar thermal water heating before choosing.

Solar never covers everything. Because the amount of sunlight varies throughout the year, solar water heating will not give 100% of hot water needs, and it is best used alongside a boiler or an immersion heater to make up the difference¹¹. Solar thermal systems are usually used alongside a conventional boiler or immersion heater²⁷. The boiler remains the backbone of the hot water system through winter; the panels trim the fuel bill in the brighter months.

What stays the same with a condensing boiler

For all its efficiency, a condensing boiler does not change what the home depends on. It burns the same gas, oil or LPG, bought from the same suppliers, at prices the household does not control. It does not reduce exposure to fuel price rises; it reduces the volume of fuel needed to ride them out. A gas condensing boiler remains tied to the gas network, and an oil condensing boiler to delivered fuel and a tank, with everything that involves for off-grid heating.

Nor does the efficiency gain hold automatically. The official data showing savings falling by around a tenth by the fifth year²⁶ is a reminder that the benefit erodes without attention, and a boiler run at 75 to 80°C flow temperature¹⁷ forfeits most of the condensing advantage it was bought for. The efficiency is in the settings and the servicing as much as in the appliance.

Finally, the condensate pipe adds a small but real winter dependency: a plastic pipe on an outside wall that can freeze and stop the boiler working¹². It is easily protected with foam insulation²², but it is a maintenance item an older boiler never had. A condensing boiler is a more efficient version of the same heating, not a step towards independence from fuel, and households weighing that larger question will find it on heating and energy independence.