System Boiler: What It Is, How It Works and a Simple Diagram

What is a system boiler?

A system boiler is one of the three main types of boiler in UK homes, alongside the combi and the heat-only (or regular) boiler. It gets its name because many of the parts that used to be separate items around the heating system are built into the boiler casing: the circulating pump, the expansion vessel, the pressure relief valve and the pressure gauge.

Unlike a combi, a system boiler does not heat water for the taps on demand. It heats a store of water in a separate hot water cylinder, usually in an airing cupboard. Unlike a heat-only boiler, it does not need a feed and expansion tank or a cold water storage tank in the loft, because the heating circuit is sealed and topped up from the mains, and the cylinder is normally fed straight from the mains as well.

How does a system boiler work?

The boiler has one job: heating water in the primary circuit. Where that hot water goes is decided by the controls and a set of motorised valves.

1. Heating the radiators. When the room thermostat calls for heat, the boiler fires and its built-in pump sends hot water through the heating zone valve to the radiators and back.
2. Heating the cylinder. When the cylinder thermostat calls for hot water, the hot water zone valve opens and the same boiler water flows through a coil inside the cylinder. The coil heats the stored water without mixing with it, then returns to the boiler.
3. Both at once. With both valves open, the boiler heats radiators and cylinder together, sharing its output.
4. Using hot water. When you turn on a tap, cold mains water enters the bottom of the cylinder and pushes hot water out of the top to the tap. Because the cylinder is at mains pressure, the flow is strong.

Most systems use two separate 2-port zone valves, known as an S-plan, or a single 3-port mid-position valve, known as a Y-plan. Honeywell and Drayton valves are common in both. If a heating zone valve fails, you get hot water but no heating, which is covered in our guide to heating not working but hot water is.

System boiler diagram

The diagram below shows a typical S-plan layout with an unvented cylinder. Hot water from the boiler (flow) goes out through the zone valves and comes back cooler (return).

Does a system boiler need a cold water tank?

Usually not. With an unvented cylinder, both the heating and the hot water are fed from the mains, so there is no cold water storage tank and no feed and expansion tank in the loft. That is one of the main differences from a heat-only boiler, which relies on both.

There is one exception. A system boiler can be paired with an older vented cylinder that is fed from a cold water tank in the loft. The heating side is still sealed, but the hot water comes from the tank, so the flow is gravity-fed and usually weaker. Many people switching from a regular boiler do this first and change the cylinder later.

What an unvented cylinder needs

An unvented cylinder stores hot water at mains pressure, so it has its own safety devices: a pressure reducing valve on the incoming mains, an expansion vessel or air bubble for the hot water, and temperature and pressure relief valves. Under Part G3 of the Building Regulations, an unvented cylinder must be installed by someone qualified to do so, and the installation must be notified to building control or self-certified. Manufacturers also require an annual check of the safety devices to keep the cylinder's guarantee.

Most cylinders also have an electric immersion heater. That gives you a backup for hot water if the boiler breaks down, something a combi cannot offer.

System boiler vs combi boiler

A combi suits most flats and homes with one bathroom. A system boiler suits homes where two or more people regularly shower at the same time, or where you want to keep a cylinder for solar water heating or a future heat pump. Our combi boiler guide explains the flow limits of a combi in more detail.

Pros and cons of a system boiler

• Strong flow to several outlets. An unvented cylinder supplies showers and taps at mains pressure at the same time.
• No loft tanks. That frees the loft, removes the risk of a frozen or overflowing tank and suits loft conversions.
• Backup and flexibility. The immersion heater gives hot water if the boiler fails, and the cylinder can later work with solar panels or a heat pump.
• Needs space. The cylinder takes up a cupboard, which matters in smaller homes.
• Heat loss from the store. A cylinder loses some heat all day, although modern factory-insulated cylinders lose far less than old lagged ones.
• More parts to maintain. Zone valves, the cylinder thermostat and the unvented safety devices are extra things that can fail or need checking.

Switching from a regular boiler to a system boiler

Many older homes have a heat-only boiler with a cold water tank and a small feed and expansion tank in the loft. Switching to a system boiler with an unvented cylinder removes both tanks and seals the heating system. It is a bigger job than a straight boiler swap: the pipework to the loft is capped off, the old cylinder is replaced, and the system is flushed, filled and treated with inhibitor. Older radiators and joints that were fine on a low-pressure open system can weep once the system is pressurised, so a good installer checks them first. Our guide to new boiler costs covers how a change of boiler type affects the price.

Getting the most from the cylinder

Set the hot water programmer to heat the cylinder shortly before you usually need it, rather than all day. The Health and Safety Executive's guidance on controlling legionella is that stored hot water should be kept at 60°C or above, so do not turn the cylinder thermostat right down to save money. Check that the cylinder has a factory-fitted foam jacket rather than a loose lagging jacket, and that the hot pipes near it are insulated.

Sizing a system boiler and cylinder

A system boiler only has to heat the radiators and reheat the cylinder, not heat water instantly, so it is usually a smaller output than a combi for the same home. The boiler size comes from the home's heat loss, and the cylinder size from how many people and bathrooms it serves. Your installer should work out both, rather than simply matching what was there before.

Problems with system boilers

Because the pump, expansion vessel and relief valve are inside the boiler, the pressure faults are the same as on a combi. If the pressure drops, see our guide to boiler pressure too low. If it climbs near 3 bar when hot, the expansion vessel is a common cause. Faults with zone valves, cylinder thermostats and the cylinder's own safety valves are specific to this type of system.