What Is a System Boiler? How It Works With a Cylinder

System, combi or regular: the short version

There are three common types of gas boiler in British homes, and the difference is really about how each one gives you hot water.

• Combi boiler. Heats the radiators and heats water instantly as you open a tap. No cylinder, no tanks. Our guide on how does a combi boiler work covers it in detail.
• System boiler. Heats the radiators and a hot water cylinder. The heating side is sealed and pressurised, with the pump and expansion vessel inside the boiler. No tanks in the loft for the heating.
• Regular boiler, also called a heat only or conventional boiler. Heats the radiators and a cylinder, but the pump and controls are separate, and there is usually a cold water storage tank and a small feed and expansion tank in the loft.

How a system boiler works

The boiler burns gas to heat water in its heat exchanger, and the built in pump pushes that hot water around a sealed loop of pipework. From there, a motorised valve, or two of them, decides where it goes: to the radiators, to a coil inside the hot water cylinder, or to both.

Most system boilers are wired as an S plan, with one two port valve for the heating and another for the hot water, each switched by its own thermostat. Some older installations use a Y plan with a single three port valve. Our guide to Y plan and S plan heating systems shows how to tell which you have from the valves near the cylinder.

When the programmer calls for hot water, the cylinder valve opens and water from the boiler flows through the coil inside the cylinder. It heats the stored water without ever mixing with it. When the cylinder thermostat reaches its setting, usually around 60 degrees, the valve closes and the boiler stops, unless the heating is also calling. The stored hot water then sits there, insulated, until you use it. Cold water refills the cylinder from the bottom as hot water leaves from the top.

Because the heating circuit is sealed, it runs under pressure, usually between 1 and 1.5 bar when cold on the boiler gauge. The expansion vessel inside the boiler absorbs the water as it expands with heat, and a pressure relief valve opens if the pressure climbs too far. If your gauge keeps creeping up, see our guide on boiler pressure too high.

Why a system boiler usually means an unvented cylinder

A system boiler can heat either a vented cylinder, fed from a cold tank in the loft, or an unvented cylinder, fed straight from the mains. In most London homes fitted in the last twenty years, it is paired with an unvented cylinder such as a Megaflo, and that pairing is where the real advantage lies: mains pressure hot water at every tap, with no loft tanks at all.

That also means the cylinder, not the boiler, is where most of the safety equipment sits. An unvented cylinder carries a pressure reducing valve, an expansion vessel of its own, a temperature and pressure relief valve, a tundish and a discharge pipe to outside. Fitting or working on it needs an engineer with the unvented hot water qualification, often called G3. Our guide on unvented vs vented cylinder explains how to tell which type you have.

How to tell if you have a system boiler

You can usually work it out in a couple of minutes without taking any covers off.

• There is a hot water cylinder somewhere in the home, often in an airing cupboard.
• There is a pressure gauge on the front or underneath the boiler, and a filling loop, a braided hose or a lever key, near it or under it.
• There is no small feed and expansion tank in the loft for the heating.
• There is no separate pump next to the cylinder. On a regular boiler you would usually find one there, often with a pair of valves either side.
• The boiler has two sets of controls or one programmer with separate heating and hot water channels, and hot water does not come instantly from the boiler when you open a tap.

If there is no cylinder anywhere and the hot water comes on the moment you open a tap, you have a combi. If there are tanks in the loft and a pump in the airing cupboard, you most likely have a regular boiler.

System boiler pros and cons

A system boiler with a cylinder suits homes that use a lot of hot water at once. Two showers can run together without one going cold, a bath fills quickly, and a household of four or five does not have to queue. Many cylinders also carry an immersion heater, which gives you a backup if the boiler fails. Our guide on what is an immersion heater explains how the two work together.

The downsides are space and waiting. The cylinder needs a cupboard, and once the stored hot water is used it takes time to heat again, commonly half an hour to an hour. Heat is also lost slowly from the stored water, though a modern factory insulated cylinder loses very little. For a one or two person flat with one bathroom, a combi is often simpler. We compare the two properly in our guide on combi or system boiler with a cylinder.

What usually goes wrong

When a system boiler setup gives trouble, the boiler itself is only one of the suspects. The components around it fail just as often.

• Heating works but no hot water: usually the hot water motorised valve, the cylinder thermostat or the programmer. Our guide on no hot water but heating works goes through the checks in order.
• Radiators hot when hot water is on and the heating is off: often the heating valve stuck open.
• Pressure keeps dropping: a small leak on the heating, or the expansion vessel inside the boiler has lost its charge and the relief valve is letting water out.
• Pressure keeps rising: a filling loop left partly open, or a failed expansion vessel.
• Water from the tundish by the cylinder: an unvented cylinder safety valve or cylinder expansion vessel problem, not the boiler. See pressure relief valve leaking.
• Boiler locks out: see how to reset a boiler for the safe way to try once, and when to stop.

Who should work on it

Anything on the gas side of the boiler needs a Gas Safe registered engineer, and you can check that on the register before anyone arrives. Anything on the safety side of an unvented cylinder needs someone with the unvented qualification. The motorised valves, thermostats and wiring in between are where a lot of faults actually sit, and they are where a proper diagnosis saves you from paying for parts that were never the problem.

If your hot water or heating has stopped and you are not sure whether the boiler or the cylinder is to blame, call us on 020 4652 5111 or send an enquiry. Tell us the make of boiler, whether there is a cylinder, and what the boiler display is showing. We have 20 engineers across London who work on system boilers and unvented cylinders every day, and our rate is on the pricing page, agreed before anyone attends.