Boilers can then be powered by mains gas, oil, LPG or electricity. Biomass boilers and hybrid heating systems provide additional options for some properties.

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APN Boilers helps homeowners understand the available boiler types and find experienced local professionals capable of designing and installing the right system for the property.

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What Is a Boiler?

 

A boiler heats water for circulation through a wet central-heating system.

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The heated water normally travels through radiators, underfloor-heating pipes or a combination of both. Depending on the boiler type, the appliance may also provide hot water directly or work with a separate hot-water cylinder.

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Boilers remain one of the most common forms of domestic heating in UK homes. They are generally powered by gas or oil, although LPG, electricity and biomass are also used.

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The term boiler type can refer to two different things:

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• The heating-system arrangement, such as combi, system or regular.

• The fuel or energy source, such as gas, oil, LPG or electricity.

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Both need to be considered before a replacement boiler is selected.

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Combi Boilers

 

A combination boiler, usually called a combi boiler, provides central heating and hot water from one compact appliance.

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When a hot tap or shower is opened, the boiler heats water directly from the incoming mains supply. A separate hot-water cylinder and cold-water storage tank are not normally required.

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Advantages of a Combi Boiler

A combi boiler can provide:

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• Heating and hot water from one appliance.

• No separate hot-water cylinder.

• No cold-water storage tank in the roof.

• A compact installation.

• Hot water without waiting for a cylinder to reheat.

• Reduced risk of stored hot water running out.

• More usable cupboard and roof space.

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When Is a Combi Boiler Suitable?

A combi boiler may suit:

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• Flats and apartments.

• Smaller and medium-sized houses.

• Homes with one principal bathroom.

• Properties with limited storage space.

• Households that do not regularly use several hot-water outlets together.

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Limitations of a Combi Boiler

The hot-water performance of a combi boiler depends on the incoming mains water pressure and flow rate.

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Installing a more powerful boiler cannot create more water than the incoming supply can provide. The installer should therefore test the water supply before recommending a particular model.

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A combi may be less suitable where:

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• Several bathrooms may be used simultaneously.

• The incoming water flow is poor.

• The property has a large family.

• Very high hot-water demand is expected.

• The homeowner wants to combine the system with solar water heating.

• The property already has an effective modern hot-water cylinder.

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The decision to install a combi boiler should be based on water performance and household demand rather than simply the desire to remove the cylinder.

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System Boilers

 

A system boiler provides central heating and works with a separate hot-water cylinder.

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Many of the principal heating components are built into the boiler, but hot water is stored within the cylinder until it is needed.

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System boilers can provide hot water to several taps and showers at the same time, making them well suited to larger families and properties with higher hot-water requirements.

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Advantages of a System Boiler

A system boiler can offer:

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• Stored hot water for larger households.

• Better support for simultaneous bathroom use.

• No traditional cold-water feed and expansion tank in many installations.

• Compatibility with modern unvented cylinders.

• Suitability for properties with more than one bathroom.

• Potential integration with solar thermal or other heat sources.

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When Is a System Boiler Suitable?

A system boiler may be appropriate for:

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• Larger houses.

• Homes with two or more bathrooms.

• Families with high hot-water demand.

• Properties already fitted with a suitable cylinder.

• Homes where several showers or taps may operate together.

• Heating systems that may later work with alternative energy sources.

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Limitations of a System Boiler

A system boiler requires space for a hot-water cylinder.

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Stored hot water is finite, so the cylinder must be correctly sized. Once the stored supply has been used, it will need time to reheat.

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There will also be some heat loss from the cylinder, although modern insulated cylinders are designed to limit this.

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Regular or Heat-Only Boilers

 

A regular boiler may also be described as a conventional, traditional or heat-only boiler.

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It normally works with a separate hot-water cylinder and, in many older systems, cold-water storage and feed-and-expansion tanks within the roof space.

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Advantages of a Regular Boiler

A regular boiler can be useful where:

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• The property already has a traditional heating system.

• Existing pipework is designed for a lower-pressure arrangement.

• There are several bathrooms.

• The incoming mains water pressure is limited.

• A large volume of stored hot water is required.

• Replacing the complete system would cause unnecessary disruption.

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When Is a Regular Boiler Suitable?

A like-for-like regular boiler replacement may be the most practical solution in a larger or older property where the cylinder, tanks and pipework remain suitable.

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Changing automatically to a combi boiler could require extensive alterations and may not improve hot-water performance.

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Limitations of a Regular Boiler

A traditional system can require:

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• A boiler.

• A hot-water cylinder.

• Cold-water storage tanks.

• Pumps and motorised valves.

• More associated pipework.

• Additional cupboard and roof-space storage.

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When replacing a regular boiler, the installer should inspect the cylinder, tanks, pumps, controls and pipework rather than assessing the boiler in isolation.

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Condensing Boilers

 

The term condensing boiler describes how a boiler uses heat rather than whether it is a combi, system or regular boiler.

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Modern gas and oil boilers are generally condensing appliances. They recover additional heat from the combustion gases before those gases leave through the flue.

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A condensing boiler can therefore be:

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• A condensing combi boiler.

• A condensing system boiler.

• A condensing regular boiler.

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The heating system should be designed and controlled to help the boiler operate in condensing mode for as much of the heating cycle as reasonably possible.

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Correct boiler sizing, suitable radiator outputs, balanced pipework and appropriate flow temperatures all contribute to the performance of the complete system.

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Gas Boilers

 

Gas boilers are widely used in properties connected to the mains gas network.

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They are available as:

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• Gas combi boilers.

• Gas system boilers.

• Gas regular boilers.

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Gas boilers are compact, widely available and compatible with conventional radiators and wet underfloor heating.

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The installer must check:

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• The required boiler output.

• The existing gas supply pipe.

• Available gas pressure.

• Boiler location.

• Flue clearances.

• Condensate drainage.

• Ventilation where required.

• Heating controls.

• Existing system-water condition.

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Anyone carrying out gas work as part of a business must be appropriately qualified and registered with Gas Safe Register. The engineer’s Gas Safe ID card should show the categories of work they are permitted to undertake.

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Oil Boilers

 

Oil boilers are commonly installed in rural and off-grid homes without access to mains gas.

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They can be supplied as:

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• Oil combi boilers.

• Oil system boilers.

• Oil regular boilers.

• Internal boilers.

• External boilers.

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An oil heating installation also requires a suitable fuel-storage tank and supply pipe.

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The installer should assess:

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• The oil tank.

• Tank capacity and location.

• Fuel line.

• Fire valve.

• Boiler output.

• Flue position.

• Condensate disposal.

• Existing cylinder and radiators.

• Heating controls.

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OFTEC operates a competent-person scheme for technicians working with oil and other off-grid heating technologies. Appropriately registered technicians can self-certify qualifying work in the areas covered by the scheme.

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LPG Boilers

 

LPG boilers operate in a similar way to mains-gas boilers but use liquefied petroleum gas stored in a bulk tank or cylinders.

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They can be suitable for:

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• Rural properties.

• Park homes.

• Holiday accommodation.

• Properties without mains gas.

• Homes where oil storage is unsuitable.

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LPG boilers are available in combi, system and regular arrangements.

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The installation should consider:

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• LPG tank or cylinder storage.

• Fuel-delivery access.

• Pressure regulators.

• LPG supply pipework.

• Boiler conversion or manufacturer approval.

• Flue and condensate routes.

• The household’s heating and hot-water demand.

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The installer must be Gas Safe registered and specifically qualified for the relevant LPG work. A qualification for mains-gas work does not automatically cover every LPG installation.

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Electric Boilers

 

Electric boilers use electricity to heat water for circulation through radiators or underfloor heating.

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They do not require:

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• A gas connection.

• An oil or LPG tank.

• A combustion flue.

• On-site fuel deliveries.

• Combustion ventilation.

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Electric boilers can be helpful in flats, smaller homes, internal boiler locations and properties where a conventional boiler flue cannot be provided.

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Electric flow boilers are designed to provide wet central heating in properties where mains gas is unavailable or flue restrictions prevent a gas boiler installation.

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Points to Consider With an Electric Boiler

The proposed installation should be assessed carefully because an electric boiler can place a substantial load on the electrical supply.

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The installer and electrician may need to check:

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• The incoming electrical capacity.

• Consumer-unit capacity.

• Dedicated electrical circuits.

• Cable sizes.

• Protective devices.

• Boiler output.

• Property heat loss.

• Radiator sizes.

• Hot-water storage.

• Electricity tariffs and expected running costs.

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An electric boiler may be straightforward to install in the right property, but it should not be selected without considering long-term electricity use.

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What Is an Electric Hot Water Boiler?

 

The term electric hot water boiler can describe several different products.

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It may refer to:

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• An electric boiler supplying central heating.

• An electric boiler working with a hot-water cylinder.

• A combined electric boiler and cylinder unit.

• An electric storage water heater.

• A small electric water boiler supplying sinks or basins.

• An electric water heater that does not provide central heating.

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This distinction is important because heating the property and providing domestic hot water are separate functions.

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An electric flow boiler may heat water for radiators without producing domestic hot water directly. A separate cylinder or water heater would then provide water for baths, showers and taps.

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Combined units are also available that incorporate an electric central-heating boiler and an unvented hot-water cylinder within one packaged system.

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Electric storage water heaters can provide a stored supply to multiple sinks or basins but may not provide central heating.

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The correct electric hot-water system will depend on:

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• The number of occupants.

• The number of bathrooms.

• Bath and shower use.

• Available cupboard space.

• Incoming water pressure.

• Electrical capacity.

• Available electricity tariffs.

• Whether central heating is also required.

• The time available for stored water to reheat.

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A dedicated APN Boilers page should target electric hot water boiler separately and explain the differences between electric boilers, cylinders and standalone water heaters.

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Biomass Boilers

 

Biomass boilers burn organic fuel, most commonly wood pellets, to provide central heating and hot water.

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They usually require:

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• Space for the boiler.

• Fuel-storage space.

• Access for fuel deliveries.

• A suitable flue.

• Regular cleaning and maintenance.

• Ash removal.

• A hot-water cylinder or thermal store.

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Biomass boilers may suit some larger rural properties, but the space, fuel-handling requirements and installation costs should be considered carefully.

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Boiler or Heat Pump?

 

A heat pump is not technically a boiler, although it can perform a similar role by heating water for radiators, underfloor heating and a hot-water cylinder.

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Heat pumps generally operate at lower water temperatures than traditional boilers. A suitable installation may require:

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

• Larger radiators.

• Changes to heating pipework.

• A compatible hot-water cylinder.

• External space.

• Electrical-supply work.

• Improved insulation.

• Different heating controls.

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A heat pump may be worth considering when replacing a gas, oil, LPG or electric heating system, but it should be assessed as a complete property and heating-system project rather than as a simple appliance swap.

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The Boiler Upgrade Scheme provides grants towards eligible heat-pump and biomass installations in England and Wales.

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Which Boiler Type Is Best for a Flat?

 

A combi boiler is often appropriate for a flat because it does not normally require a separate cylinder or roof-space tanks.

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An electric boiler may be considered where:

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• There is no gas supply.

• A compliant flue cannot be installed.

• The property is relatively small.

• The electrical supply is suitable.

• The likely running costs have been assessed.

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The final choice should consider water pressure, available space, lease restrictions, flue routes and the number of hot-water outlets.

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Which Boiler Type Is Best for a Small House?

 

A combi boiler may suit a small house with one bathroom and good incoming mains water flow.

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A system boiler may be better where:

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• The household has high hot-water demand.

• More than one outlet is frequently used.

• A suitable cylinder already exists.

• The homeowner wants stored hot water.

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Property size alone should not determine the boiler type. The way the household uses hot water is equally important.

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Which Boiler Type Is Best for a Large House?

 

A system or regular boiler with a correctly sized hot-water cylinder will often provide better performance in a large house with several bathrooms.

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The system should be designed around:

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• Peak hot-water demand.

• Number of occupants.

• Simultaneous shower use.

• Cylinder recovery time.

• Mains water pressure.

• Pipe sizes.

• Radiator outputs.

• Heating zones.

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A high-output combi boiler may still be unsuitable if the incoming water supply cannot provide the required flow.

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Choosing a Boiler by Property Size

 

Boiler output should not be selected using bedroom numbers alone.

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

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• Age and construction.

• Insulation.

• Ceiling heights.

• Window sizes.

• Air leakage.

• Exposed walls.

• Extensions.

• Room use.

• Desired temperatures.

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A proper heating assessment should calculate or estimate the property’s heat loss and consider the required hot-water performance separately.

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Heating-System Design Matters

 

The boiler is only one part of the heating installation.

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A high-quality boiler connected to undersized pipes, poorly balanced radiators or ineffective controls may perform badly. Equally, replacing an old boiler without investigating existing circulation problems may not resolve the homeowner’s concerns.

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A professional assessment should consider:

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• Boiler size.

• Heating flow and return pipework.

• Radiator outputs.

• Underfloor-heating zones.

• Pumps and valves.

• System-water quality.

• Hot-water cylinder capacity.

• Water pressure and flow.

• Fuel or electrical supply.

• Flue and condensate arrangements.

• Heating controls.

• Future property alterations.

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The best boiler type is the one that forms part of a properly designed heating and hot-water system.

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Boiler Controls and Zoning

 

Modern heating controls may include:

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• Programmable room thermostats.

• Smart thermostats.

• Thermostatic radiator valves.

• Separate upstairs and downstairs zones.

• Independent hot-water timing.

• Load-compensation controls.

• Weather-compensation controls.

• Individual room controls.

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System boilers normally require controls for both heating and stored hot water, while combi boiler controls can manage the heating and, in some systems, multiple heating zones.

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Good controls can improve comfort and reduce unnecessary heating, but they must be compatible with the selected boiler and system design.

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Boiler Installation and Building Regulations

 

New and replacement heating systems must comply with the applicable Building Regulations, including energy-efficiency requirements under Part L.

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For a gas boiler installation in England and Wales, the Gas Safe registered business should notify the local authority so that a Building Regulations Compliance Certificate can be issued.

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Homeowners should retain:

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• Building Regulations certification.

• Boiler commissioning documents.

• Benchmark documentation where applicable.

• Manufacturer warranty information.

• Control instructions.

• Service records.

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The installer should also explain how to operate the boiler, thermostat, hot-water system and any zoning controls.

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Find the Right Boiler 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 selecting the right boiler type rather than simply fitting the nearest replacement appliance.

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Whether you are considering a combi, system, regular, gas, oil, LPG or electric boiler, the property and existing heating system should be properly assessed first.

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Tell APN Boilers about your property, current boiler, number of bathrooms and heating requirements. We will help you find an appropriate local professional to advise on the best boiler type for your home.

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