• A combi boiler.

• A system boiler.

• A regular or heat-only boiler.

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The word condensing explains how the appliance recovers heat. It does not describe how the boiler provides domestic hot water or connects to the wider heating system.

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APN Boilers helps homeowners find experienced professionals who can select, install and commission a condensing boiler around the needs of the property rather than simply replacing the appliance on a like-for-like basis.

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How Does a Condensing Boiler Work?

 

When gas, oil or LPG is burned, hot combustion gases pass through the boiler’s heat exchanger.

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Older non-condensing boilers allow these gases to leave at a relatively high temperature. A condensing boiler contains a larger or secondary heat-exchange area that removes more heat before the gases are discharged.

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As the gases cool, water vapour within them condenses into liquid. The boiler collects this condensate and discharges it through a pipe to a suitable drain or purpose-designed disposal point.

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A condensing boiler will normally have:

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• A combustion flue.

• A condensate drainage pipe.

• One or more heat exchangers.

• A burner.

• Heating flow and return connections.

• Electronic controls.

• Safety and monitoring components.

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Energy Saving Trust advises that a boiler is likely to be condensing where it has a plastic flue and a plastic condensate pipe running from the bottom of the appliance to a drain. Gas and LPG boilers installed after 2005 and oil boilers installed after 2007 are also likely to be condensing models.

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Condensing and Non-Condensing Boilers

 

A non-condensing boiler loses more heat through its flue because it does not recover as much energy from the combustion gases.

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A modern condensing boiler is designed to:

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• Recover heat that would otherwise be lost.

• Reduce the amount of fuel required for a given heating demand.

• Operate efficiently at lower heating-water temperatures.

• Work with modern controls and compensation systems.

• Reduce emissions compared with many older boilers using the same fuel.

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However, fitting a condensing boiler does not guarantee that the entire heating system will operate efficiently.

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Performance will also depend on:

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• Correct boiler sizing.

• Heating flow and return temperatures.

• Radiator output.

• Heating-system balance.

• Pipework design.

• Thermostat settings.

• System cleanliness.

• Property insulation.

• Occupant behaviour.

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The boiler should therefore be assessed as one component of the complete heating installation.

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Condensing Gas Boilers

 

A condensing gas boiler uses mains natural gas and can be supplied as a combi, system or regular boiler.

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Gas boilers are widely used in properties connected to the mains gas network. They can supply conventional radiators, wet underfloor heating and domestic hot-water systems.

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A gas-boiler installation should consider:

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• The required heating output.

• Domestic hot-water demand.

• Incoming water pressure and flow.

• Gas-supply pipe size.

• Available gas pressure.

• Boiler and flue position.

• Condensate drainage.

• Radiator sizes.

• Heating controls.

• Future extensions or alterations.

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Energy Saving Trust states that, where mains gas is available, a modern condensing gas boiler will usually be cheaper to run than an oil, LPG or direct-electric boiler, although actual costs depend on tariffs, property heat loss and system use.

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Condensing Combi Boilers

 

A condensing combi boiler provides central heating and domestic hot water from one appliance.

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Hot water is produced when a tap or shower is opened, so a separate hot-water cylinder is not normally required.

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A condensing combi boiler may suit:

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• Flats and apartments.

• Smaller or medium-sized houses.

• Properties with one principal bathroom.

• Households with moderate hot-water demand.

• Homes where cupboard space is limited.

• Properties with good incoming water pressure and flow.

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A combi may be less suitable where several bathrooms or hot-water outlets are regularly used at the same time. Energy Saving Trust identifies combi boilers as a good space-saving option but notes that they can be less suitable for homes with high simultaneous hot-water demand or low water pressure.

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Before recommending a combi boiler, the installer should test:

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• Incoming water flow.

• Static water pressure.

• Working water pressure.

• Likely simultaneous outlet use.

• Pipe sizes between the boiler and bathrooms.

• The expected shower and bath performance.

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A larger boiler cannot create more water than the incoming mains supply can deliver.

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Condensing System Boilers

 

A condensing system boiler provides central heating and heats water stored within a separate cylinder.

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This arrangement can be suitable for:

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• Larger family homes.

• Properties with two or more bathrooms.

• Homes with high hot-water demand.

• Households where several outlets may operate together.

• Properties already fitted with a suitable modern cylinder.

• Homes that may later incorporate solar thermal or another heat source.

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System boilers can deliver hot water to several outlets from the stored supply. The cylinder must nevertheless be correctly sized for the number of occupants and the way the household uses hot water.

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Condensing Regular Boilers

 

A condensing regular boiler may also be called a heat-only or conventional boiler.

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It usually works with a separate hot-water cylinder and may also use storage tanks within the roof space.

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A regular boiler can remain suitable where:

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• The property already has a traditional heating system.

• Existing pipework is designed for an open-vent arrangement.

• Mains water pressure is limited.

• The property has several bathrooms.

• A large supply of stored hot water is required.

• A like-for-like replacement would avoid unnecessary disruption.

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Replacing an old regular boiler does not automatically mean that the system should be converted to a combi boiler. Retaining stored hot water may provide better performance in a larger property.

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Condensing Oil Boilers

 

A condensing oil boiler can provide efficient heating for rural and off-grid homes that do not have access to mains gas.

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Oil boilers are available as:

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• Oil combi boilers.

• Oil system boilers.

• Regular oil boilers.

• Internal boilers.

• External boilers.

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The installation also requires an assessment of:

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• The oil-storage tank.

• Fuel-supply pipework.

• Filters and fire valves.

• Delivery access.

• Flue position.

• Condensate drainage.

• Boiler servicing access.

• Fire and environmental protection.

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The oil tank and fuel supply form part of the heating installation and should be inspected when a new boiler is proposed.

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A replacement boiler does not automatically require a replacement oil tank, but an old, damaged or poorly positioned tank may need remedial work.

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Why Condensing Boilers Need a Drain

 

Condensing boilers produce water as part of their normal operation.

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This condensate must be discharged safely through an appropriate drainage arrangement. Depending on the boiler position, the installation may use:

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• An internal waste connection.

• An external condensate pipe.

• A condensate pump.

• A purpose-designed soakaway.

• A neutralisation unit where required.

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External condensate pipes should be kept as short and well protected as reasonably possible to reduce the risk of freezing during cold weather.

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The condensate route should be planned before the boiler position is agreed. A convenient flue location does not necessarily mean that suitable drainage is available nearby.

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Condensing Boiler Flues

 

The boiler flue carries combustion gases safely outside the property.

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Flue positioning should consider:

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• Windows and doors.

• Air vents.

• Boundaries.

• Neighbouring properties.

• Roofs and eaves.

• Extensions and conservatories.

• Passageways.

• External seating areas.

• Boiler-manufacturer clearances.

• Access for inspection and servicing.

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A visible plume may occasionally be produced when water vapour within the flue gases condenses in cold outside air. This is a normal feature of many condensing boilers, but the flue should be positioned to avoid causing a nuisance or affecting neighbouring property.

A plume-management kit or alternative flue route may sometimes be required.

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What Are the Best Condensing Boilers?

 

There is no single boiler that can accurately be described as the best condensing boiler for every home.

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The best condensing boilers are those that are correctly selected for the property, heating system and household.

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Important comparison points include:

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• Appropriate minimum and maximum output.

• Modulation range.

• Hot-water performance.

• Compatibility with heating controls.

• Flue options.

• Appliance dimensions.

• Noise levels.

• Manufacturer warranty.

• Availability of replacement parts.

• Local servicing support.

• Installer familiarity with the product.

• Ease of maintenance.

• System and water-treatment requirements.

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A well-known premium boiler that is oversized or badly installed may perform less effectively than a more modest appliance that has been correctly sized, controlled and commissioned.

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Homeowners should compare the proposed installation specification rather than relying solely on brand reputation or online boiler rankings.

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Boiler Modulation

 

Modern boilers can reduce or modulate their output when the property requires less heat.

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A wide modulation range can help the appliance operate steadily during milder weather rather than repeatedly turning on and off.

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This can support:

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• More consistent room temperatures.

• Longer heating cycles.

• Reduced boiler cycling.

• Lower return temperatures.

• Improved condensing operation.

• Reduced component wear.

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The installer should consider the boiler’s minimum output as well as its headline maximum output.

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A very high-output combi may be selected to provide fast domestic hot water, but its minimum central-heating output may still be greater than a small, well-insulated property requires.

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Flow and Return Temperatures

 

A condensing boiler achieves its best efficiency when the water returning from the heating system is sufficiently cool.

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Reducing the heating flow temperature also reduces the return temperature, allowing the boiler to recover more heat from the flue gases.

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Energy Saving Trust confirms that boilers operate more efficiently with a lower return temperature.

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The practical flow temperature will depend on:

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• Outdoor temperature.

• Property heat loss.

• Radiator sizes.

• Heating hours.

• Required room temperatures.

• Whether a hot-water cylinder is connected.

• Heating-control strategy.

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A combi boiler’s radiator temperature can usually be adjusted separately from its domestic hot-water setting.

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A conventional boiler that also heats a cylinder needs to maintain a suitable cylinder temperature. Energy Saving Trust advises against reducing the shared boiler setting below 65°C where it heats stored domestic hot water because of the need to control legionella risk.

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Homeowners should not apply generic internet settings without understanding their particular system.

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Radiator Sizes and Low-Temperature Heating

 

A condensing boiler can operate more effectively at lower water temperatures where the radiators are large enough to heat the rooms under those conditions.

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Older radiators may have been sized around relatively high flow temperatures. In some properties, lowering the boiler temperature may mean that:

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• Rooms take longer to warm.

• Some radiators need upgrading.

• Heating needs to operate for longer periods.

• System balancing becomes more important.

• Insulation improvements are required.

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Where a property is being extended or renovated, the heating design should consider lower-temperature operation rather than automatically selecting the smallest possible radiators.

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This may also make the property easier to convert to a heat pump in the future.

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Heating Controls for Condensing Boilers

 

Suitable controls help a condensing boiler adjust its output to the needs of the home.

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Options can include:

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• Programmable room thermostats.

• Smart thermostats.

• Thermostatic radiator valves.

• Separate heating zones.

• Load-compensation controls.

• Weather-compensation controls.

• Hot-water programmers.

• Cylinder thermostats.

• Individual room controls.

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Load and weather compensation can alter the boiler’s flow temperature according to indoor or outdoor conditions.

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This can allow the appliance to operate at lower temperatures during milder weather while increasing output when heating demand rises.

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The controls should be compatible with the boiler and designed as part of the complete installation.

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Condensing Boiler Cost

 

The condensing boiler cost will depend on the appliance and the complexity of the installation.

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Energy Saving Trust currently gives a broad average of approximately £3,700 for replacing a G-rated gas boiler with an A-rated boiler and thermostatic radiator valves in Great Britain. Its published oil-boiler benchmark is approximately £3,200 in Northern Ireland. These are general averages rather than fixed quotations for every property.

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The final price may be affected by:

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• Boiler fuel and type.

• Manufacturer and model.

• Required output.

• Boiler relocation.

• Flue alterations.

• Condensate drainage.

• Gas or oil supply work.

• Hot-water cylinder replacement.

• Removal of tanks.

• Heating-control upgrades.

• Radiator and valve work.

• System cleaning.

• Magnetic filtration.

• Scaffolding or specialist access.

• Electrical work.

• Making good.

• Warranty arrangements.

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A basic like-for-like boiler swap will normally cost less than relocating the appliance or converting from a regular system to a combi boiler.

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Comparing Condensing Boiler Quotations

 

A clear quotation should identify:

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• Boiler manufacturer and model.

• Boiler type and output.

• Modulation range.

• Proposed location.

• Flue arrangement.

• Condensate route.

• Fuel-supply alterations.

• Heating controls.

• System cleaning and treatment.

• Magnetic filter.

• Cylinder, radiator or valve work.

• Removal of the old boiler.

• Commissioning.

• Building Regulations notification.

• Manufacturer warranty.

• Making good.

• Waste disposal.

• VAT and exclusions.

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Energy Saving Trust recommends obtaining at least three quotations when arranging a new or replacement boiler.

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A cheap quotation may cover only the appliance swap and exclude work needed to make the heating system operate correctly.

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Cleaning and Protecting the Heating System

 

A new condensing boiler should not be connected to a dirty heating system without suitable assessment and treatment.

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Corrosion, scale and sludge can:

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• Restrict circulation.

• Cause cold radiator areas.

• Damage pumps and valves.

• Block narrow boiler waterways.

• Reduce heating performance.

• Increase noise.

• Affect warranty protection.

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Energy Saving Trust advises that corrosion inhibitors help control scale and sludge and should be renewed when a heating system is drained during replacement or maintenance. It also notes that magnetic filtration is encouraged when a boiler is replaced.

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The cleaning method should reflect the condition of the system rather than applying the same process to every installation.

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Qualified Boiler Installers

 

A condensing gas or LPG boiler must be installed by a business and engineer holding the appropriate Gas Safe registration.

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For oil boilers, homeowners should use a technician with suitable oil-heating competence, such as an appropriately registered OFTEC technician.

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OFTEC-registered installers can self-certify qualifying heating work in England and Wales, avoiding the need for a separate inspection by Building Control where the work falls within the scheme.

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Homeowners should check:

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• Registration status.

• The individual engineer’s qualifications.

• Experience with the proposed boiler.

• Public liability insurance.

• Manufacturer accreditation where relevant.

• Warranty arrangements.

• What certification will be provided.

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Building Regulations and Certification

 

New and replacement boiler installations must comply with the applicable Building Regulations, including energy-efficiency requirements under Part L. The precise guidance will depend on the location of the property and which regulatory edition applies to the work.

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In England and Wales, a Gas Safe registered business must notify the local authority when it installs a heat-producing gas appliance. The homeowner should then receive a Building Regulations Compliance Certificate, which should be retained with the property records.

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The installer should also provide:

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• Boiler commissioning documentation.

• Manufacturer warranty information.

• Operating instructions.

• Heating-control instructions.

• Details of system cleaning and treatment.

• Servicing requirements.

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Condensing Boiler or Heat Pump?

 

A heat pump may be worth considering before replacing an old gas, oil or LPG boiler.

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A heat pump is not normally a direct appliance swap and may require:

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• A room-by-room heat-loss calculation.

• Larger radiators.

• A hot-water cylinder.

• Improved insulation.

• External equipment.

• Pipework alterations.

• Electrical-supply work.

• Different heating controls.

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A condensing boiler may remain the practical option where the existing heating system is suitable, installation disruption needs to be limited or the property cannot easily accommodate heat-pump equipment.

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The decision should consider installation cost, running costs, property suitability and the homeowner’s long-term plans.

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Find a Condensing Boiler Installer Through APN Boilers

 

APN Boilers brings together experienced heating professionals serving homeowners across the UK.

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A good condensing-boiler installation should provide more than a high-efficiency appliance. It should include:

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• The correct boiler type.

• An output matched to the property.

• Suitable hot-water performance.

• Correct flue and condensate arrangements.

• Effective heating controls.

• Clean and balanced heating circuits.

• Proper commissioning.

• Building Regulations certification.

• Reliable servicing and aftercare.

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Whether you are considering a condensing gas boiler, condensing oil boiler or condensing combi boiler, the complete heating system should be assessed before the appliance is selected.

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Tell APN Boilers about your existing boiler, property, number of bathrooms and heating requirements. We will help you find an appropriate local professional to advise on and install your new condensing boiler.

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