DBSP Factsheets

a.   A room-by-room heat loss assessment should be done prior to your new system being designed. This should include heat lost through your walls, floors, ceilings, windows, etc. (fabric heat losses), heat lost through draughts and other air escaping your house (ventilation heat loss), and recommendations for improvements to reduce these losses. External design conditions, such as temperature, altitude, and exposure, will also be established. The CIBSE Domestic Heating Design Guide sets out the process for designers undertaking this assessment, and it is recommended to be followed.

b.    It is important that a full room-by-room heat loss assessment is done, rather than a ‘whole house’ assessment, rules of thumb, or sizing based on what already exists. This is to prevent oversizing of the heat generator and the heat emitters, ensuring your system operates as efficiently as possible and costs as little to run as possible.

c.    Your preferences and requirements will also be identified, around hot water provision and room temperature settings, etc.

d.    The CIBSE Domestic Heating Design Guide recommends that the full heat loss calculation is provided to you as part of the handover pack following the installation, but the key points will also be included in the System Criteria document (see cibse.org/dbsp-systemcriteria), which you should agree with the designer before installation.

e.    The installer should also assess the existing system if relevant. They should identify any parts of the existing system that might be reused and ensure they are in good condition and suitable for the new system. They should also be looking at where the new equipment will be located and how to link it all together.

This depends on whether an entirely new system is being installed or if an existing system is being replaced, the type of heat generator being installed, and whether any grants or incentives are being claimed. Often, your installer can notify building control through their competent person scheme and any assurance schemes, such as the Microgeneration Certification Scheme, if relevant. You should discuss what schemes your installer is a member of with them.

a.    By taking a fabric-first approach, like upgrading insulation and draughtproofing.

b.    Any structural defects should be repaired, and any unwanted wall vents should be sealed, including redundant extractor vents or permanent ventilation openings serving unused open flued appliances (i.e. gas fires, open fires and wood burning stoves).

c.    Attention should also be paid to minimum ventilation rates to ensure indoor air quality is maintained.

d.    Good design ensures your system also operates as efficiently as possible.

This depends firstly on whether the installation requires any external changes to your property, such as an external heat pump unit or a new flue. If your property is listed or in a conservation area, installing anything external to the property will require Listed Building Consent and/or planning permission, depending on the visual impact. If not, with certain other caveats, you may be able to proceed with the installation under permitted development. Consult your local authority and use an installer experienced with historic properties. 

The designer can often work with your existing radiators, but this will need to be assessed on a case-by-case basis. The only accurate way to determine which radiators are suitable is to conduct an initial system design based on a room-by-room heat-loss calculation and the proposed system temperatures. This will determine whether a radiator needs to be replaced to meet this heat load and ensure even heat distribution throughout the property. Radiators may also need to be replaced due to poor condition. 

The CIBSE Domestic Heating Design Guide sets out the process for designers undertaking this assessment and design, and it is recommended that these processes be followed.

This depends on the appliance. For all systems, the property's electrical system must be checked by a competent, qualified electrician to ensure it is suitable for use with the new system. This should also include assessing any existing meters or fuses, as some types of heat generators may draw more electrical load than older equipment can supply. A 60 amp (A) main fuse was previously common, but this may not be sufficient. Newer Homes typically have a 100A main fuse, which is usually sufficient to meet the higher demand.

For heat pumps (and possibly other electrical heat generators), the supply to the property will also need to be assessed by the local Distribution Network Operator. This will usually be requested by your electrician or installer, via an electricity supply assessment from the Energy Networks Association. This will then inform you whether a supply upgrade is needed.

The heating system should be designed for a representative coldest day. This is defined as the coldest it is likely to get in your location in winter, excluding rare, extreme cold snaps. This is termed as the ‘outdoor design temperature’ and is further explained in the System Criteria document, and its supporting guide (available at cibse.org/dbsp-systemcriteria). Designing for the absolute coldest day is not recommended, as it will result in an oversized system and may be less efficient for most of the time.

Your designer should discuss other factors with you, such as your property's insulation level and how important it is to you to maintain the exact desired indoor temperature at all times, as these will affect how much safety margin is built into the design.

With gas combination boilers widely used across the UK, many homes do not require a hot water cylinder.  While gas combination boilers are still a common way to provide hot water and space heating for your home, low-temperature, low-carbon solutions such as heat pumps generally require a hot water cylinder.

Your designer will size the cylinder based on the number of people living in your home, the selected heat generator's requirements, and the temperature the cylinder will be heated to.  Lower storage temperatures will require a larger cylinder and may take longer to reheat. Larger households or those with multiple bathrooms may also need a larger cylinder to ensure there is enough hot water during busy times.

If space is limited in your property, your designer should also advise you on minimising the size of the required cylinder or alternative options for supplying domestic hot water.

The CIBSE Domestic Heating Design Guide sets out the process for designers to assess a suitable cylinder and heat exchanger for your property, and it is recommended that this be followed.

This largely depends on the occupier and the heat generator selected. In general, the temperature for daily use typically ranges from 50–60 °C.

A controlled weekly thermal disinfection cycle will also be required. This is to minimise the risk of bacterial contamination, such as Legionella. This will likely involve configuring your system to heat all stored water to at least 60 °C for at least 1 hour, at least once per week.

The System Criteria document and its supporting guide (available at cibse.org/dbsp-systemcriteria), provides more guidance on these temperatures.

If particularly vulnerable people reside in the property, your designer may also arrange for a Legionella risk assessment to be conducted, providing guidance on any additional measures that need to be taken. 

Most heat pump manufacturers require the use of a cylinder, as it enables the heat pump to run more efficiently by heating the water over a longer period. 

Modern cylinders are factory‑insulated to a high standard. Better insulation means less wasted heat and lower running costs. In England and Wales, the cylinder should at least meet the minimum requirements of Approved Document L. 

Many existing cylinders are not suitable for low-temperature systems. Modern cylinders designed for heat pumps feature larger, high-efficiency heat exchangers that can effectively heat water at lower system temperatures.

Modern cylinders are heavy when full. Your installer may need to consult a structural engineer to assess the load and determine whether the floor requires strengthening, particularly in older homes or when installing a cylinder on an upper floor.

Unvented cylinders need good water pressure and flow. Your installer should test this during the survey. If the supply isn’t strong enough, upgrades should be recommended

An inline blending valve will likely be installed to at least limit the bath water temperature to 48 °C by mixing hot and cold water. It is recommended that similar valves be also used on all outlets throughout the house, to ensure hot taps and thermostatic showers deliver water at a safe temperature. 

This should be discussed with the installation company, requesting the safe removal and disposal of any remaining fuel oil, sludge, and tank. This specialist work is regulated under The Control of Pollution (Oil Storage) (England) Regulations 2001

Monoblock heat pumps are self-contained units that a heating engineer can partially decommission on-site by disconnecting the water pipework, then transport to a refrigerant disposal site for safe recovery. 

Split-type heat pumps, with on-site refrigerant pipework, must be fully decommissioned in situ by a qualified operative from a business registered with the F-Gas Register (or equivalent).

Ask the designer if the installation company can remove or contact the supplier or a qualified Gas Safe registered engineer who will ensure the tank is safely emptied, purged and taken away from the property.  

Any work on gas appliances, including decommissioning, must only be undertaken by a competent Gas Safe registered operative

Eligibility varies by scheme and region. Installers must usually be certified with the Microgeneration Certification Scheme (MCS) to claim incentives. They can confirm eligibility and manage your applications.
For more information, see Clean energy - Clean Energy Homepage

Ask your installer to confirm any potential extra costs in advance. These may include radiator or pipework upgrades, electrical work, scaffolding, waste disposal, or the removal of your old boiler and oil tank. Ensure these are stated clearly to avoid surprises.

Installers may ask for a deposit to secure the booking and help cover the cost of the equipment, but this should be clearly stated in the terms and conditions of your contract. Avoid paying high upfront costs before any work has started, and a contract has been entered into.

An installer should always provide a clear, itemised quotation. This should show equipment cost, labour, and commissioning so you can understand exactly what you are paying for. 

Some installers may offer finance options. Check interest rates, repayment terms, early‑repayment options, and whether the finance is unsecured or secured against your home.

Most installers offer workmanship guarantees, and the heat generator usually comes with a manufacturer's warranty. If your installer is MCS‑certified, you should also receive consumer protection through an approved insurance‑backed guarantee.

Commissioning a new system is critical to ensure it performs as designed. This is often overlooked because it is the last step in the installation process. The installer, a commissioning specialist, or the heat generator manufacturer will review and adjust all settings on your heat generator and system to ensure everything operates as a single, efficient system. This process can take time and require multiple visits but should not be overlooked or rushed.

On completion of your installation, you should receive handover documentation confirming commissioning and system criteria, along with operating instructions and full details of guarantees and warranties. Depending on the type of appliance and system design, you may also receive various Benchmark certificates, an MCS certificate, and a Compliance Certificate (confirming that your installation has been notified to building control).

The System Criteria document (see cibse.org/dbsp-systemcriteria), which you should have agreed with the designer before installation, will also be included in the handover pack.

Your designer or installer may provide an estimate of energy consumption or running costs. These numbers are only estimates, so they should be treated as a guide. Actual usage will depend on how you operate the system, your energy tariff, and weather conditions.

The installer and/or manufacturer will likely offer an annual service and maintenance package for major work, but they should also guide you on any smaller maintenance you can do yourself and, on any issues, to watch for.

The installer should have explained how to operate the heat generator and the system at handover, and the operating instructions should have been included in the handover pack.

The installation company is generally responsible for the initial product registration, which often includes a limited warranty period that you may be invited to extend. You should check with the installer that they will register the installation with all relevant manufacturers and what parts of the registration process will require your input.

As part of the handover pack and process, your installer should explain to you what to do if something goes wrong. Generally, the first point of contact will be the installer, but specific components in the system may be covered by others.

Excessive noise or vibration from any part of the system is generally a sign of an issue. Also, any loss of water pressure is not normal and could indicate a leak - constantly topping up the system with more water introduces oxygen to the system and increases the likelihood of corrosion. Your installer should also advise you of other things to look out for with your specific system.

Manufacturers generally require that the heat generator be serviced annually; failure to do so will often void the warranty. Other pieces of equipment may also require service. Your installer should advise you of these as part of the handover process and may offer you a service plan to undertake them.

Any moving parts of the system, a fan, a circulation pump, or an electronic valve, and any parts involved in combustion, will, depending on operation, be serviceable parts with a limited life. You should check with your designer and installer what the life expectancy of the components they specify is, and if they are covered by any warranties. You should also check with your installer whether any measures will be taken to prolong your system's life, such as using a high-quality inhibitor in the system water.

The person commissioning the installation will have set the main controls to operate efficiently, with everything recorded in the handover pack and the System Criteria document (see cibse.org/dbsp-systemcriteria). Generally, apart from adjusting room thermostats and programmed times as required, there should be little to do.