System boilers explained
A sealed boiler that works with a hot water cylinder, with the pump and valves built into the case. The usual answer for family houses with more than one bathroom.
Hot water
Stored in a cylinder
Space needed
Cylinder cupboard
Best for
Two or more bathrooms
Depends on
Cylinder sized correctly
The short answer
A system boiler heats the radiators and a hot water cylinder, and stores the hot water until it is used. Because the water is already hot and sits under mains pressure in an unvented cylinder, two showers can run at the same time without either one weakening. The trade-off is that the cylinder holds a finite amount and takes twenty to forty minutes to reheat once emptied.
How it works
The boiler heats primary water
A sealed, pressurised circuit runs from the boiler to the radiators and to a coil inside the cylinder.
A motorised valve chooses the destination
Heating, hot water or both. The cylinder thermostat and the programmer decide when each one calls.
The cylinder stores the result
An unvented cylinder holds hot water at mains pressure, so every tap and shower in the property runs at full pressure.
The pump and valves live in the boiler
Unlike a regular system there is no separate pump or expansion tank in the airing cupboard, which makes for a tidier installation.
What it needs in the property
Cupboard space for a cylinder — typically 500 mm across and 1.5 to 2 metres tall.
An unvented cylinder installation carried out by a qualified engineer, with a discharge pipe to outside.
A gas supply and flue position as with any gas boiler.
An expansion vessel and a pressure relief route for the cylinder, checked annually.
A cylinder thermostat and a two or three-channel programmer.
Advantages and trade-offs
Advantages
Several outlets at once with no drop in flow or temperature.
Full mains pressure at every tap and shower with an unvented cylinder.
An immersion heater in the cylinder means hot water even if the boiler fails.
No loft tanks, so nothing to freeze or overflow above the ceiling.
Works well with solar thermal, a heat pump or a future low-carbon system.
Trade-offs
A cupboard is permanently given up to the cylinder.
Hot water is finite and takes time to reheat once used up.
Standing heat loss from the cylinder, though modern insulation makes this small.
More expensive to install than a combi swap.
Unvented cylinders need an annual check to stay compliant and safe.
Is it right for your home?
Suits
Houses with two or more bathrooms, households where showers overlap in the morning, and properties where the mains supply is not strong enough for a combi to perform.
Less suitable
One-bathroom flats where cupboard space is scarce, and properties where the only realistic cylinder position is a long pipe run from the bathrooms.
Sizing it properly
The boiler is sized from the heat loss of the property and the number of radiators, and the cylinder from the number of people using hot water. Getting the cylinder right matters more day to day than the boiler output.
150 litres
Two to three people, one bathroom plus en-suite
180–210 litres
Three to four people, two bathrooms
250–300 litres
Five or more people, or three bathrooms
We size from a heat loss calculation and a measured mains flow rate, not from the output of the old boiler.
Book a surveyWhat goes wrong
Hot water not heating, radiators fine
Usually a failed motorised valve head, the cylinder thermostat or a wiring centre fault.
Hot water runs out quickly
A scaled or leaking cylinder coil, or a cylinder that was undersized for the household in the first place.
Water discharging outside the property
The cylinder expansion vessel has lost its charge or the temperature relief valve is passing. This needs attention promptly.
Noisy circulation
Air in the system, a failing internal pump or debris from an uncleaned older system.
System boiler questions
Get a fixed price
Send the property details and a photo of the existing boiler and we will quote the whole job in writing, itemised.