• Incoming water pressure and flow.

• The number of occupants.

• Hot-water demand.

• Existing heating pipework.

• Boiler and flue position.

• Heating controls.

• Planned extensions or future alterations.

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APN Boilers helps homeowners find experienced Gas Safe registered professionals who can assess the complete heating system and recommend an appropriate gas boiler for the property.

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What Is a Gas Boiler?

 

A gas boiler burns natural gas to heat water.

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That heated water is circulated through radiators or wet underfloor heating to warm the property. Depending on the type of boiler, it may also provide hot water directly to taps and showers or heat water stored within a separate cylinder.

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The three main types of domestic gas boiler are:

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• Gas combi boilers.

• Gas system boilers.

• Regular or heat-only gas boilers.

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Energy Saving Trust identifies combi, system and regular boilers as the principal domestic boiler arrangements, with each suited to different property sizes and hot-water requirements.

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Gas Combi Boilers

 

A gas combination boiler, normally shortened to gas combi boiler, provides central heating and hot water from one appliance.

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It heats domestic hot water directly from the mains supply when a tap or shower is opened. A separate hot-water cylinder and cold-water storage tank are not usually required.

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Benefits of Gas Combi Boilers

Gas combi boilers can offer:

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• Heating and hot water from one compact appliance.

• No separate hot-water cylinder.

• No traditional cold-water storage tank.

• More available cupboard and roof space.

• Hot water produced when required.

• A comparatively straightforward heating arrangement.

• Good suitability for flats and smaller homes.

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Combi boilers are often attractive where space is limited because the boiler provides both heating and hot water without requiring a separate cylinder.

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When Is a Gas Combi Boiler Suitable?

A gas combi boiler may suit:

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• A flat or apartment.

• A smaller or medium-sized house.

• A property with one main bathroom.

• A household with moderate hot-water demand.

• A home with good incoming water pressure and flow.

• A property where storage space is limited.

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However, property size alone should not determine whether a combi boiler is suitable.

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The installer must also consider how many taps, showers and baths may be used at the same time.

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Water Pressure and Flow Rates

The hot-water performance of a gas combi boiler depends on the incoming mains water supply.

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A powerful boiler cannot provide a greater flow of water than the incoming main can deliver. The installer should therefore test the static pressure, working pressure and flow rate before selecting a combi boiler.

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This becomes particularly important where:

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• The property has two or more bathrooms.

• Several occupants use hot water at similar times.

• A large rainfall shower is proposed.

• The incoming main is old or restricted.

• The boiler is located a considerable distance from the bathrooms.

• Several outlets may operate simultaneously.

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Installing an oversized combi boiler will not correct poor incoming water flow.

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Gas System Boilers

 

A gas system boiler provides central heating and heats water stored within a separate hot-water cylinder.

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Unlike many traditional regular boiler installations, several of the principal heating components are incorporated within the boiler itself.

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Gas system boilers can be particularly suitable for larger homes and households with higher hot-water requirements.

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Benefits of Gas System Boilers

A gas system boiler can provide:

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• Stored hot water for larger households.

• Better support for simultaneous bathroom use.

• Compatibility with modern unvented cylinders.

• Good hot-water flow where the incoming supply is suitable.

• A neater arrangement than some traditional tank-fed systems.

• Potential compatibility with solar thermal and other heat sources.

• Greater flexibility for homes with several bathrooms.

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Energy Saving Trust notes that system boilers use a hot-water cylinder and can be better suited to larger households where several people may require hot water.

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When Is a Gas System Boiler Suitable?

A gas system boiler may be appropriate for:

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• A larger family home.

• A property with two or more bathrooms.

• A household with high hot-water demand.

• A home where several outlets may be used together.

• A property already fitted with a suitable cylinder.

• A planned extension that will add bedrooms or bathrooms.

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The cylinder must be correctly sized for the number of occupants and expected hot-water use.

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A large boiler connected to an undersized cylinder will not provide the expected improvement in hot-water performance.

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Regular or Heat-Only Gas Boilers

 

A regular gas boiler may also be described as:

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• A conventional boiler.

• A traditional boiler.

• A heat-only boiler.

• An open-vent boiler.

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

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When Is a Regular Gas Boiler Suitable?

A regular boiler can remain suitable where:

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• The property already has a traditional heating system.

• The existing cylinder and tanks are in good condition.

• The incoming mains water pressure is limited.

• The property has several bathrooms.

• The existing pipework is designed for an open-vented system.

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

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Replacing a regular boiler does not automatically mean that the complete system should be converted to a combi boiler.

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In many older or larger homes, retaining stored hot water may provide better performance and involve fewer alterations.

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Gas Combi Boiler or Gas System Boiler?

 

The choice between gas combi boilers and gas system boilers normally comes down to space, water performance and hot-water demand.

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A Gas Combi Boiler May Be Better When:

• The property has one principal bathroom.

• Cupboard space is limited.

• The incoming water supply is strong.

• Hot-water outlets are not normally used together.

• The homeowner wants to remove an old cylinder and tanks.

• A compact installation is preferred.

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A Gas System Boiler May Be Better When:

• The property has several bathrooms.

• Several people may use hot water simultaneously.

• A suitable hot-water cylinder can be accommodated.

• Stored hot water is preferred.

• The property already has a modern cylinder.

• Future extensions or additional bathrooms are planned.

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A professional installer should test the water supply and discuss the household’s daily hot-water use before recommending either system.

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Choosing the Correct Gas Boiler Output

 

Boiler output should not be chosen using bedroom numbers alone.

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Two houses with the same number of bedrooms can have very different heating requirements because of differences in:

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• Floor area.

• Construction.

• Insulation.

• Ceiling heights.

• Window sizes.

• Exposed walls.

• Air leakage.

• Extensions.

• Radiator sizes.

• Occupancy.

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The installer should assess the property’s heat loss and calculate the output required for space heating.

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For combi boilers, domestic hot-water performance must also be assessed separately. A boiler may require a higher output to deliver hot water quickly than it requires to heat the radiators.

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Correct sizing helps the boiler operate efficiently and reduces excessive cycling, where the appliance repeatedly turns on and off because its output is greater than the heating system requires.

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Replacing an Old Gas Boiler

 

A replacement gas boiler should not simply be selected because it is physically similar to the existing appliance.

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Before recommending a new boiler, the installer should inspect:

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• The gas supply pipe.

• Boiler location.

• Flue arrangement.

• Condensate drainage.

• Heating flow and return pipes.

• Radiators.

• Pumps and valves.

• Hot-water cylinder.

• Existing controls.

• System pressure.

• Water quality.

• Signs of corrosion or sludge.

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The old boiler may have been oversized, or the property may have changed since it was installed.

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Extensions, loft conversions, insulation upgrades, replacement windows and additional bathrooms can all alter the requirements of the heating system.

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Converting to a Gas Combi Boiler

 

Homeowners with a regular or system boiler may consider changing to a gas combi boiler when replacing the appliance.

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This can free up cupboard and roof space, but the conversion may require substantial work.

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The installation could involve:

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• Removing the hot-water cylinder.

• Removing roof-space tanks.

• Altering heating flow and return pipework.

• Installing new hot- and cold-water connections.

• Upgrading the gas supply.

• Providing a new condensate drain.

• Altering the boiler flue.

• Removing pumps and motorised valves.

• Installing new heating controls.

• Making good where equipment has been removed.

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The incoming water supply must be tested before the cylinder is removed.

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Where the property has several bathrooms or poor mains water flow, retaining a stored hot-water system may provide a better result.

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Gas Boiler Installation and System Design

 

A new gas boiler is only one part of the heating installation.

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A high-quality boiler connected to undersized pipework, poorly balanced radiators or ineffective controls may not perform properly.

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A professional gas boiler installation should consider:

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• Room-by-room heating demand.

• Boiler output.

• Pipe diameters.

• Radiator outputs.

• Heating zones.

• Pump performance.

• Hot-water demand.

• Water pressure and flow.

• Gas supply capacity.

• Flue positioning.

• Condensate drainage.

• Heating controls.

• System-water quality.

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Where the property is being extended, the heating design should take account of the additional rooms rather than simply adding more radiators to the existing system.

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Does the Gas Supply Pipe Need Upgrading?

 

A replacement boiler may require a different gas flow from the old appliance.

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The Gas Safe engineer must confirm that the supply pipe can provide the necessary pressure when the boiler and any other gas appliances are operating.

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An upgrade may be required where:

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• The existing pipe is undersized.

• The boiler is being moved.

• The new boiler has a higher input.

• The route from the meter is unusually long.

• Additional gas appliances have been installed.

• Pressure loss is excessive.

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The quotation should clearly identify whether gas-pipe alterations are included.

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Boiler Flues and Condensate Drainage

 

Modern condensing gas boilers require:

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• A compliant combustion flue.

• A suitable condensate drain.

• Adequate access for servicing.

• Installation clearances specified by the manufacturer.

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The flue position must be assessed in relation to:

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• Windows and doors.

• Boundaries.

• Neighbouring properties.

• Passageways.

• Roofs and eaves.

• Extensions.

• Air vents.

• External seating areas.

• Future building work.

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The condensate pipe should be routed and protected appropriately, particularly where any part of it is external.

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Relocating the boiler may create a better internal layout, but it can increase the amount of gas, heating, water, electrical and drainage work required.

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Heating Controls for Gas Boilers

 

Modern controls can improve comfort and reduce unnecessary heating.

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Suitable controls may include:

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• Programmable room thermostats.

• Smart thermostats.

• Thermostatic radiator valves.

• Separate heating zones.

• Independent hot-water timing.

• Load-compensation controls.

• Weather-compensation controls.

• Individual room controls.

• App-based heating management.

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Combi boilers commonly use a single heating-control channel because hot water is produced on demand. System and regular boilers normally require separate control of heating and stored hot water.

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

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Improving the Efficiency of Gas Boilers

 

Modern gas boilers are condensing appliances designed to recover heat that would otherwise be lost through the flue.

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However, installing a new boiler does not automatically guarantee that the complete heating system will work efficiently.

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Performance also depends on:

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• Correct boiler sizing.

• Appropriate flow temperatures.

• Correctly sized radiators.

• Balanced heating circuits.

• Effective thermostats.

• Suitable zoning.

• Clean system water.

• Adequate insulation.

• Regular servicing.

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Reducing the central-heating flow temperature can help a condensing boiler operate more efficiently, although any adjustment must remain suitable for the property and heating system.

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Cleaning and Protecting the Heating System

 

Old heating systems can contain corrosion products, sludge and other debris.

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This contamination can:

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• Restrict water circulation.

• Cause cold areas within radiators.

• Damage pumps and valves.

• Block boiler components.

• Increase noise.

• Reduce heating performance.

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Before commissioning the new gas boiler, the installer should assess the water quality and recommend an appropriate cleaning process.

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The completed system may also require:

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• Chemical inhibitor.

• A magnetic system filter.

• Radiator balancing.

• Replacement thermostatic valves.

• Further investigation of persistent circulation problems.

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The boiler warranty may also contain requirements relating to system cleaning, water treatment and servicing.

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British Gas Boiler or an Independent Installer?

 

People searching for a British Gas boiler are often looking for a replacement boiler supplied and installed by a large national company.

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British Gas offers new gas boilers with installation included in its quoted package and provides options for combi and system boiler installations.

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APN Boilers is independent and is not affiliated with British Gas.

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When comparing a British Gas boiler quotation with one from an independent local installer or APN professional, homeowners should compare:

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• Boiler make and model.

• Boiler output.

• Warranty length and conditions.

• Heating controls.

• System cleaning.

• Magnetic filters.

• Gas-pipe alterations.

• Flue and condensate work.

• Cylinder or tank removal.

• Radiator and valve work.

• Building Regulations notification.

• Servicing and aftercare.

• Making good.

• VAT and exclusions.

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The lowest headline figure may not represent the same scope of work.

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The most useful comparison is between detailed, like-for-like specifications that explain why the proposed boiler is suitable for the property.

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Use a Gas Safe Registered Engineer

 

Anyone fitting, repairing or servicing a gas boiler as part of a business must be appropriately registered with Gas Safe Register.

Homeowners should check:

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• The business is registered.

• The individual engineer has a valid Gas Safe ID card.

• The engineer is qualified for boiler work.

• The categories on the card cover the proposed installation.

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Gas Safe Register confirms that anyone fitting or working on a gas boiler must be Gas Safe registered.

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Registration should be checked before work begins, not simply assumed because the installer describes themselves as a heating engineer.

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Building Regulations and Boiler Certification

 

A new or replacement gas boiler must comply with the applicable Building Regulations, including energy-efficiency requirements under Part L.

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Following a qualifying installation, the Gas Safe registered business should notify the work to the relevant local authority. The homeowner should receive a Building Regulations Compliance Certificate and retain it with the property records.

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The installer should also provide:

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• Completed commissioning documentation.

• Boiler operating instructions.

• Control instructions.

• Manufacturer warranty details.

• Details of system treatment.

• Service requirements.

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These documents may be important when the property is sold or the boiler warranty is used.

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Can Gas Boilers Still Be Installed?

 

Gas boiler replacement remains relevant for existing homes connected to the gas network.

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The Future Homes and Buildings Standards introduced in England focus on higher energy-efficiency and low-carbon heating standards for new homes. The government states that gas boilers will not meet the required performance standard for new homes under those standards.

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That distinction is important: rules applying to newly constructed homes should not be confused with the current replacement of an existing boiler in an established property.

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Homeowners considering a new gas boiler may nevertheless wish to compare it with a heat pump, particularly where the property is being extensively renovated.

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Gas Boiler or Heat Pump?

 

A heat pump may be worth considering before replacing an old gas boiler, but it is not always a direct appliance swap.

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A heat-pump project may require:

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

• Larger radiators.

• Improved insulation.

• Changes to heating pipework.

• A compatible hot-water cylinder.

• Space for an external unit.

• Electrical-supply work.

• Different heating controls.

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The Boiler Upgrade Scheme provides financial support for eligible property owners in England and Wales replacing fossil-fuel heating with a qualifying heat pump or biomass boiler.

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The choice should be based on installation cost, expected running costs, property suitability, disruption and the homeowner’s long-term plans.

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What Should a Gas Boiler Quotation Include?

 

A professional gas boiler quotation should identify:

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• Boiler manufacturer and model.

• Boiler type and output.

• Proposed boiler position.

• Manufacturer warranty.

• Gas-supply alterations.

• Flue arrangement.

• Condensate route.

• Heating controls.

• System cleaning and treatment.

• Magnetic filter.

• Cylinder or tank alterations.

• Radiator and valve work.

• Electrical work.

• Removal of the existing appliance.

• Commissioning.

• Building Regulations notification.

• Making good.

• Waste disposal.

• VAT.

• Any exclusions or provisional costs.

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The installer should explain why the boiler has been selected and whether the existing heating system requires further improvement.

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Find a Gas Boiler Installer Through APN Boilers

 

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

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The APN network focuses on more than supplying an appliance. A good installation should provide:

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• The correct type of boiler.

• An output matched to the property.

• Reliable heating.

• Suitable hot-water performance.

• Effective controls.

• Clean and balanced heating circuits.

• Proper certification.

• Accessible local support.

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Whether you are considering a gas combi boiler, gas system boiler or a like-for-like regular boiler replacement, the property should be properly 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 gas boiler.

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