The correct system depends on the size of the property, number of occupants, available fuel, incoming water pressure, electrical capacity, available storage space and the amount of hot water required.

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APN Boilers helps homeowners find experienced professionals who can assess the property and recommend an appropriate hot-water system rather than selecting an appliance based solely on its advertised output.

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What Is a Hot Water Boiler?

 

A hot water boiler is an appliance that uses gas, oil, LPG, electricity or another energy source to heat water.

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Some boilers provide both central heating and domestic hot water. Others provide hot water only.

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The distinction is important because water used within radiators is normally contained within a closed heating circuit. Domestic hot water is fresh water supplied to baths, showers and taps.

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A central-heating flow boiler may therefore heat the radiators without directly producing domestic hot water. A separate cylinder, immersion heater or water heater may still be required.

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What Does “Water Boiler” Mean?

 

The search term water boiler is unusually broad.

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In domestic heating, it may describe:

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• A gas or electric boiler.

• A hot-water cylinder.

• A storage water heater.

• An instantaneous water heater.

• A boiler serving central heating and hot water.

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In commercial kitchens and offices, the same term is often used for wall-mounted boiling-water dispensers supplying water for tea and coffee. Manufacturers classify these separately from domestic hot-water cylinders and heating boilers.

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This APN Boilers page focuses on systems providing domestic hot water for washing, bathing, showering and household use.

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Combi Boilers for Hot Water

 

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

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

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Benefits of a Combi Boiler

A combi boiler may provide:

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• Heating and hot water from one appliance.

• No separate cylinder.

• No traditional roof-space water tank.

• Hot water without waiting for a cylinder to reheat.

• A compact installation.

• More available cupboard space.

• Good performance for smaller households.

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Combi boilers are generally suited to smaller or medium-sized properties where hot-water demand is moderate and the incoming water supply is strong enough.

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Combi Boiler Limitations

The hot-water performance of a combi boiler depends on:

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• Incoming mains flow rate.

• Static and working water pressure.

• Boiler hot-water output.

• Pipe sizes.

• Distance from the boiler to the outlet.

• The number of taps and showers used simultaneously.

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A larger combi boiler cannot deliver more water than the incoming main supplies.

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Where two showers are used at once, the available flow is divided between them. A stored hot-water system may therefore provide better performance for a larger family home.

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System Boilers and Hot-Water Cylinders

 

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

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Stored water can supply several baths, showers and taps simultaneously, subject to the cylinder capacity, pipework and incoming water supply.

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System boilers are often well suited to larger homes and households with higher hot-water demand. The cylinder also allows heat from other sources, including solar thermal, to be stored where the system is designed for it.

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The cylinder should be selected around:

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• The number of occupants.

• Number of bathrooms.

• Bath and shower use.

• Simultaneous demand.

• Required flow rates.

• Reheat time.

• Available cupboard space.

• The heat source connected to it.

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A high-output boiler connected to an undersized cylinder may still run out of stored hot water during periods of heavy use.

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Regular Boilers and Traditional Hot-Water Systems

 

A regular or heat-only boiler usually works with a separate hot-water cylinder and may also use cold-water storage tanks within the roof space.

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This arrangement is commonly found in older and larger properties.

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A traditional system may remain appropriate where:

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• Existing tanks and cylinders are serviceable.

• The incoming mains pressure is limited.

• Several bathrooms require hot water.

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

• A like-for-like replacement would reduce disruption.

• A large volume of stored water is required.

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Removing a cylinder and converting automatically to a combi boiler will not necessarily improve hot-water performance. Water pressure, flow and simultaneous demand should be tested first.

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Gas Hot Water Boilers

 

Gas hot water boilers can include:

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• Gas combi boilers.

• Gas system boilers connected to a cylinder.

• Regular gas boilers.

• Storage-combination boilers.

• Instantaneous gas water heaters.

• Gas-fired hot-water-only appliances.

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Gas Safe Register separately identifies boiler and water-heater work categories. An engineer qualified for one category of gas work may not automatically be qualified for every gas appliance, so the individual engineer’s ID card should be checked.

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Gas Water Heaters

A gas water heater provides domestic hot water without operating a complete central-heating system.

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These appliances may be useful where:

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• Space heating is provided separately.

• A flat or small property needs hot water only.

• A commercial or ancillary space has limited demand.

• An existing hot-water-only appliance is being replaced.

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The installer must consider:

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• Gas-supply capacity.

• Flue and combustion requirements.

• Ventilation.

• Water pressure and flow.

• Outlet demand.

• Appliance location.

• Condensate drainage where applicable.

• Access for servicing.

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Approved Document J provides guidance for domestic combustion installations, including space- and water-heating appliances and their flues.

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Electric Hot Water Boilers

 

An electric hot water boiler can describe several different systems.

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It may be:

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• An electric boiler connected to a cylinder.

• A combined electric boiler and cylinder.

• A direct electric hot-water cylinder.

• An electric storage water heater.

• An instantaneous electric water heater.

• A point-of-use heater.

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The specification should clearly state whether the appliance:

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• Provides central heating.

• Provides domestic hot water.

• Provides both services.

• Heats water instantaneously.

• Stores water for later use.

• Requires one or several electrical circuits.

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Electric boilers can heat water for radiators or underfloor heating, but many installations also need separate hot-water storage.

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Electric Boilers With Integrated Hot-Water Storage

 

Some products combine an electric central-heating boiler and an unvented hot-water cylinder within one enclosure.

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These systems can provide:

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• Wet central heating.

• Stored mains-pressure hot water.

• Factory-fitted pumps.

• Integrated controls.

• A compact plant arrangement.

• Heating and hot-water operation from electricity alone.

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For example, packaged systems are available that combine an electric flow boiler, storage cylinder and circulating pump. This should not be confused with a gas combi boiler producing high-flow hot water instantaneously.

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The installer should confirm:

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• Cylinder capacity.

• Heating output.

• Hot-water recovery time.

• Electrical circuits required.

• Whether heating and hot water can operate together.

• Peak and off-peak electricity requirements.

• The physical space required.

• Available servicing clearances.

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Electric Boiler for Hot Water Only

 

An electric boiler for hot water only may be appropriate where space heating is supplied separately.

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Examples include properties using:

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• Electric panel heaters.

• Storage heaters.

• Air-conditioning units providing heating.

• A separate heat pump.

• Communal central heating.

• No permanent space-heating system in the area concerned.

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Hot water may then be provided by:

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• A direct electric cylinder.

• A small storage heater.

• An instantaneous electric heater.

• An unvented electric water heater.

• Several point-of-use heaters.

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The best arrangement depends on how much hot water is required and whether several outlets need to operate simultaneously.

A small heater may serve a basin or kitchen sink but will not normally provide the storage or flow needed for a family bathroom.

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Electric Water Boilers

 

The phrase electric water boiler can refer to either domestic hot-water equipment or a drinking-water appliance.

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For household washing and bathing, the principal options are:

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Electric Storage Water Heaters

A storage water heater heats and stores a set quantity of water.

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It may serve:

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• A kitchen sink.

• Several basins.

• A shower.

• A small flat.

• A larger household where a full cylinder is installed.

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Storage units are available in small point-of-use capacities and much larger cylinder sizes. Manufacturer ranges include unvented units designed to supply multiple outlets from the cold-water main.

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Instantaneous Electric Water Heaters

An instantaneous water heater heats water as it passes through the appliance.

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This can avoid standing heat loss from stored water and may be useful for:

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• Remote sinks.

• Bedsits.

• Student accommodation.

• Small washrooms.

• Occasional-use outlets.

• Areas without a central hot-water supply.

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Higher electrical output is generally required to produce a useful flow of instantaneous hot water. Available domestic and light-commercial units commonly require dedicated electrical circuits.

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Point-of-Use Water Heaters

Point-of-use heaters are installed close to the tap or basin they serve.

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They can reduce the time spent waiting for hot water and avoid running long hot-water pipes through the property.

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They may be useful for:

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• Garden offices.

• Garages.

• Utility rooms.

• Cloakrooms.

• Small commercial kitchens.

• Remote extensions.

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The heater capacity and output must be matched to the intended outlet.

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Vented and Unvented Hot-Water Storage

 

Hot-water storage systems may be vented or unvented.

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Vented Water Heaters

A vented heater normally works with a special open-outlet tap or receives water from a storage cistern.

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Small vented heaters can be compact and suitable for individual sinks. Products are available in small capacities with relatively modest electrical ratings.

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Unvented Water Heaters

An unvented heater is supplied directly from the cold-water main.

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It can provide mains-pressure hot water, subject to the available pressure and flow. Unvented systems incorporate safety controls to protect against excessive pressure and temperature.

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Approved Document G requires unvented hot-water storage systems to incorporate independent safety devices and appropriate discharge arrangements.

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Installation and commissioning should be completed by a person competent to work on unvented hot-water systems.

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Choosing the Correct Hot-Water Cylinder

 

The cylinder should be selected around the household rather than the space available in the cupboard alone.

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The assessment should consider:

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• Number of occupants.

• Number of bathrooms.

• Shower flow rates.

• Bath sizes.

• Simultaneous use.

• Typical morning and evening demand.

• Cylinder recovery time.

• Boiler or immersion output.

• Available electricity tariffs.

• Heat-pump or solar compatibility.

• Standing heat losses.

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A small cylinder can run out during peak use. A cylinder that is unnecessarily large may occupy extra space and store more water than the household regularly requires.

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Water Pressure and Flow Rate

 

Pressure and flow are different measurements.

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Water pressure describes the force available within the supply. Flow rate describes the quantity of water delivered over time.

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A property may have:

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• Good static pressure but poor flow.

• High flow but excessive pressure.

• Variable pressure at busy times.

• Restricted pipework.

• An undersized incoming main.

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The installer should test the supply before recommending:

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• A combi boiler.

• An unvented cylinder.

• An instantaneous heater.

• A high-flow shower.

• Several simultaneous bathroom outlets.

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A high-performance hot-water appliance cannot overcome a seriously restricted incoming water supply.

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Electrical Supply Requirements

 

Electric water-heating equipment can create a significant electrical load.

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The electrician should consider:

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• Appliance output.

• Dedicated circuits.

• Cable size.

• Consumer-unit capacity.

• Circuit protection.

• Local isolation.

• Main-fuse rating.

• Existing immersion heaters.

• Electric showers.

• Electric vehicle charging.

• Other major household loads.

• Single- or three-phase supply.

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An instantaneous electric heater may require considerably more electrical power than a small storage unit because it must raise the water temperature while the water is flowing.

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The electrical installation should be checked before equipment is purchased.

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Hot-Water Controls and Electricity Tariffs

 

Electric storage systems can sometimes be heated during lower-cost tariff periods.

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Possible controls include:

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• Off-peak immersion heaters.

• Daytime boost elements.

• Cylinder thermostats.

• Time switches.

• Smart controls.

• Solar-diversion controls.

• Separate heating and hot-water programmers.

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Energy Saving Trust explains that some electric boiler systems use large hot-water stores and are paired with electricity meters offering cheaper electricity during particular periods.

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The complete tariff must be considered because lower off-peak rates may be accompanied by higher daytime prices.

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Solar Panels and Hot-Water Storage

 

A hot-water cylinder can sometimes store energy produced by solar technology.

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Solar thermal panels heat water directly within a cylinder or thermal store. Solar photovoltaic panels can also power an immersion heater through a suitable diversion control when surplus electricity is available.

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The cylinder must be designed with the appropriate coils, immersion positions and controls for the intended system.

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Hot-Water Safety

 

Domestic hot-water systems must be designed to control the risks associated with:

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• Excessive temperature.

• Excessive pressure.

• Stored-water expansion.

• Safety-valve discharge.

• Legionella.

• Scalding.

• Electrical faults.

• Combustion gases.

• Carbon monoxide.

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Approved Document G covers hot-water safety and unvented storage in England. It should be considered alongside the requirements dealing with structure, fire safety, combustion appliances, energy efficiency and electrical safety.

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Thermostatic mixing valves may be appropriate where additional scald protection is required, particularly for baths used by children, older people or vulnerable occupants.

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Gas Safe Registration

 

Gas boilers and gas-fired water heaters must be installed, serviced and repaired by appropriately qualified Gas Safe registered engineers.

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Gas Safe Register is the official registration body, and businesses carrying out gas work must be registered by law. The individual engineer’s card identifies the appliances they are qualified to work on.

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For gas water-heater work, homeowners should confirm that the engineer holds the relevant boiler or water-heater category.

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Building Regulations and Certification

 

Hot-water installations may involve several areas of Building Regulations, including:

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• Part G for hot-water safety.

• Part J for gas, oil, LPG and other combustion appliances.

• Part L for energy efficiency.

• Part P for electrical work.

• Part H for drainage and safety discharges.

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In England and Wales, the local authority must be informed when a heat-producing gas appliance is installed so that a Building Regulations Compliance Certificate can be issued.

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Homeowners should retain:

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• Building Regulations certification.

• Gas Safe notification certificate.

• Electrical installation certificates.

• Unvented-cylinder commissioning records.

• Manufacturer warranties.

• Operating instructions.

• Service records.

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What Should a Hot-Water Quotation Include?

 

A professional quotation should identify:

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• Appliance manufacturer and model.

• Gas or electrical energy source.

• Heating-only, hot-water-only or combined operation.

• Instantaneous or stored hot water.

• Cylinder capacity.

• Hot-water flow rate.

• Recovery time.

• Proposed appliance and cylinder positions.

• Incoming water requirements.

• Gas or electrical supply alterations.

• Flue and condensate work.

• Safety-valve discharge route.

• Heating and hot-water controls.

• Pipework alterations.

• Removal of old equipment.

• Commissioning and certification.

• Making good.

• VAT.

• Exclusions and provisional costs.

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The installer should explain how the proposed system will perform when the household’s busiest hot-water outlets are used.

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Which Hot-Water Boiler Is Right for Your Home?

 

A combi boiler may suit a smaller household with one bathroom and a strong incoming water supply.

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A system boiler and cylinder may be better where:

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• The property has several bathrooms.

• High-flow showers are installed.

• Several outlets may be used together.

• Solar or another heat source may be incorporated.

• Stored hot water is preferred.

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An electric hot-water system may be suitable where:

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• There is no gas supply.

• A combustion flue cannot be installed.

• The property is small or well insulated.

• Space heating is provided separately.

• The electrical supply is adequate.

• Hot-water demand is moderate.

• Off-peak electricity can be used effectively.

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A professional survey should consider the complete household requirement rather than recommending the same system for every property.

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Find a Hot Water Boiler Installer Through APN Boilers

 

APN Boilers brings together experienced heating, plumbing, gas and electrical professionals serving homeowners across the UK.

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A successful hot-water installation should provide:

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• Enough hot water for the household.

• Suitable pressure and flow.

• An appliance matched to the energy supply.

• Correctly sized storage where required.

• Safe pressure and temperature controls.

• Efficient pipework and insulation.

• Proper commissioning.

• Building Regulations certification.

• Reliable local servicing support.

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Whether you need a gas hot water boiler, electric hot water boiler, electric water boiler or an electric boiler for hot water only, the system should be assessed before the appliance is selected.

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Tell APN Boilers about your property, number of occupants, bathrooms, current system and hot-water problems. We will help you find an appropriate local professional to advise on and install your new hot-water system.

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