Boiler Bolt-On · System conversion
Heat-only to system boiler conversion
A heat-only (regular) boiler can be part of an open-vented heating system with a small feed-and-expansion tank in the loft. A system boiler uses a sealed, pressurised heating circuit and still works with a separate hot-water cylinder. We check the existing pipework, radiators, cylinder and water supply before recommending the change.
Illustrative layout
What changes in the heating circuit?
In this example, the open-vented heating feed becomes a sealed circuit. Hot water is still stored in a separate cylinder. The cylinder feed, controls and exact pipework are assessed independently.
Before · open-vented heat-only
After · sealed system boiler
How we plan the conversion
Map the existing setup
Confirm whether the heating circuit is open vented or already sealed, and identify the cylinder, loft tanks and controls.
Test suitability
Assess the older pipework, radiators and fittings for a pressurised circuit, plus hot-water demand and incoming mains performance.
Specify the work
Itemise the boiler, cylinder decision, any heating changes, tank disconnection/removal and the finishing work in the written quote.
What is involved
- Replacing the old appliance with a suitable system boiler in the agreed position.
- Converting the heating circuit to a sealed system where required and appropriate.
- Adapting connections and controls for the separate hot-water cylinder.
- Decommissioning or removing redundant heating feed equipment as quoted.
What we check at survey
- Existing heating pressure arrangement, radiator and pipework condition.
- Cylinder type, age, location and compatibility with the proposed system.
- Loft feed-and-expansion tank versus any separate domestic cold-water storage.
- Incoming water supply, flue and condensate routes, controls and access.
What affects the quote
- Whether the heating circuit is already sealed or needs conversion.
- Condition of old pipework and components when operated at sealed-system pressure.
- Whether the cylinder can remain or needs a separately specified replacement.
- Tank access and removal, controls, valves, pipe routes and making good.
What your written quote includes
- The chosen system boiler and installation position.
- Sealed-system conversion components and any cylinder or control work.
- Which tanks and old components are disconnected, removed or retained.
- Water-supply work, system cleaning, finishing, exclusions and survey conditions.
Heat-only to system boiler questions
Will I still have a hot-water cylinder?
Yes. A system boiler works with a separate hot-water storage cylinder. We check whether the existing cylinder is suitable or whether replacement should be included in the quote.
Will the loft tanks be removed?
The heating feed-and-expansion tank may become redundant after a suitable sealed-system conversion. A separate cold-water storage tank may still serve the cylinder or other outlets; its future depends on the chosen hot-water arrangement. The quote lists exactly what is disconnected, removed or retained.
Is a system boiler the same as a combi?
No. A combi generally heats domestic hot water on demand without a separate cylinder. A system boiler heats and stores hot water in a cylinder.
Can my existing radiators and pipes take the new pressure?
They need to be assessed. Older fittings or hidden weak points can show leaks when the heating circuit is pressurised. We explain findings and any remedial work before agreeing the conversion.
Will the hot-water pressure improve?
Not automatically. That depends on the cylinder type, incoming mains supply and plumbing arrangement. We check flow and pressure at survey rather than promise a particular result.
Can I keep my current controls?
Possibly. We review the existing programmer, thermostat and valves against the new boiler and cylinder controls, then specify what is retained or changed.
Get the right conversion scoped
Send your postcode and photos of the current boiler, hot-water cylinder, loft tanks and controls if available. We will confirm the suitable system and exact work at survey.
Prefer the website form? Contact MPH Boilers and mention “Heat-only to system boiler”.