Replacing a regular boiler with a system boiler: what stays?

You can consider replacing a regular boiler with a system boiler while keeping stored hot water. Whether your existing cylinder, radiators and pipework can stay depends on their condition and compatibility with the proposed installation. Ask for a surveyed comparison with a replacement regular boiler before choosing the conversion.

A system boiler does not automatically mean every tank in the loft can be removed. The heating circuit and the water feeding your taps do different jobs. That distinction belongs in the quote, particularly if freeing loft space is your reason for changing.

What changes when you choose a system boiler?

A system boiler has key heating components, including the pump and expansion vessel, inside the boiler casing. A typical older regular-boiler installation has more of those components elsewhere. Both types can heat a separate hot-water cylinder; choosing a system boiler is not the same as converting to a combi and removing stored hot water.

Ask the installer to identify your existing arrangement rather than diagnose it from the brand name or the number of visible tanks. The quote should name the new boiler and explain which external components will be retained, replaced or removed.

Source: Baxi: differences between boiler types

Can the existing hot-water cylinder stay?

Possibly. Vaillant confirms that system boilers can work with either a vented or an unvented cylinder. That describes the available arrangements, not approval to reuse any particular old cylinder. Ask the installer to check yours against the proposed boiler and controls, its condition and your household's hot-water needs.

If you are happy with the existing hot-water service, request a quote that explains whether retaining it is viable. If you want better shower performance or more stored water, describe the problem first. The installer can then explain whether the change involves the cylinder, water supply, controls or something else. Replacing only the boiler is not evidence that every hot-water problem will disappear.

Source: Vaillant: system boilers and vented or unvented cylinders

Which loft tank can actually be removed?

In a typical older installation, the small feed-and-expansion tank serves the heating system. The larger cold-water storage tank supplies domestic water, including the hot-water cylinder. Baxi's glossary distinguishes these two roles.

Changing the heating system to a sealed arrangement can remove the need for its feed-and-expansion tank. But if the proposed hot-water arrangement still uses a vented cylinder supplied from a cold-water storage tank, that supply still has a job. Ask the installer to state what feeds the cylinder afterwards and whether the larger tank supplies any other outlets.

If removing all loft tanks is essential, say so before the survey. Do not accept 'system boiler, tanks removed' as a complete design. Get the proposed cylinder type and water-supply assessment written into the scope. Do not disconnect or alter tanks yourself.

Source: Baxi: heating and domestic water tank terminology; Vaillant: cylinder options with a system boiler

Will old radiators and pipes cope?

This needs assessment, not an age-based promise. Worcester Bosch notes that an older radiator system may not cope with the higher pressure associated with a system or combi installation, so a regular replacement can remain the appropriate option. Ask how the installer will assess the retained heating circuit and deal with any problems found.

Request separate details for necessary repairs or alterations. Keeping existing radiators should mean that they have been assessed for the proposed installation, not that their suitability has been assumed. These are professional survey questions, not instructions to pressure-test the system yourself.

Source: Worcester Bosch: boiler types and older heating systems

Ask for two quotes with the same boundaries

Where both options are suitable, ask for a regular-boiler replacement and a system-boiler conversion. Give each installer the same priorities and use this worksheet. Mark missing information as 'not specified' rather than assuming it is included.

When is conversion worth taking forward?

Take it forward when the assessed design solves a specific problem you care about and the extra work is clear. Retaining a sound cylinder may be useful; freeing loft space may require a more extensive change. If a regular replacement already meets your needs, ask what practical benefit the conversion adds before comparing the totals.

Boiler Comparisons introduces homeowners to independent installers; it does not install boilers or design the heating system. Request replacement quotes and ask each installer to explain the appropriate options for your home. Availability depends on your postcode and the installers able to take the work.