System Boiler FAQs
FAQs about system boilers and hot water cylinders.
When should I choose a system boiler over a combi?
Choose a system boiler if your home has multiple bathrooms and a household that regularly uses hot water simultaneously; if your mains water pressure is low or variable; if you want solar thermal or a heat pump to contribute to your hot water; or if you're converting an existing regular boiler system and want to retain the cylinder setup. System boilers offer higher peak hot water flow rates than combi boilers.
What size hot water cylinder do I need?
Cylinder sizing depends on the number of occupants and peak hot water demand. As a guide: 120–150 litres for 1–2 people; 180–210 litres for 3–4 people; 250–300 litres for 5+ people or homes with high demand. An undersized cylinder will run out of hot water before demand is met; an oversized one costs more to heat. We'll recommend the correct size during your free survey.
What is the difference between an unvented and vented cylinder?
An unvented (pressurised) cylinder connects directly to the mains cold water supply and delivers hot water at mains pressure — giving powerful, consistent flow to all outlets. A vented cylinder uses a cold water storage tank in the loft and delivers hot water at lower pressure. Unvented cylinders provide a much better shower experience and are now the standard choice for new installations. They must be installed by a qualified G3 engineer.
What is a pressure vessel and why does my system boiler have one?
A sealed central heating system — as used by system boilers — expands as the water heats up. The pressure vessel (or expansion vessel) absorbs this expansion, preventing pressure from rising dangerously. System boilers have the pressure vessel built in, whereas regular (heat-only) boilers rely on an open feed-and-expansion tank in the loft. If the expansion vessel fails, pressure will rise on every heating cycle.
How does a system boiler compare in cost to a combi?
A system boiler installation is typically more expensive than a combi installation due to the additional cost of the hot water cylinder and its installation. However, for homes where a system boiler is the right choice, the improved hot water performance and reliability often justify the extra investment. Cylinder costs range from £500 to £1,500+ depending on size and whether it's a standard or unvented model.
How much space does a system boiler installation require?
A system boiler requires space for the boiler unit itself (similar dimensions to a combi) plus a hot water cylinder. Cylinders are typically installed in an airing cupboard and range from approximately 45cm in diameter and 1m tall for smaller models up to 60cm × 1.6m for larger units. The cylinder can be located in a different room from the boiler.
Do system boilers need a cold water tank in the loft?
No. System boilers are sealed systems that connect directly to the mains water supply. They do not require a feed-and-expansion tank in the loft, which is one of their advantages over older regular (heat-only) boiler systems. Removing the loft tank and sealing the system is often part of a system boiler installation.
Can I add solar thermal to a system boiler setup?
Yes, a system boiler with a twin-coil hot water cylinder is ideal for incorporating solar thermal collectors. The solar coil in the lower part of the cylinder is heated by the solar panels; the upper coil is connected to the boiler for top-up heating when solar input is insufficient. This can reduce your hot water heating bills by 50–70% in summer months.
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