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Combi, system and regular boilers: how your hot water works
A plain explainer of the three common UK home setups (combi, system boiler with a cylinder, and regular boiler with tanks), what each means for hot water and showers, and when a shower pump is even relevant.
Most UK homes heated by a boiler have one of three setups. Knowing which one you have explains a lot: why the shower behaves as it does, whether there is a tank in the loft, what a shower pump would do, and who to call when something goes wrong.
This guide only explains how the systems differ. It is not a guide to choosing, sizing, fitting or working on a boiler. Work on a gas boiler is for a Gas Safe registered engineer, and choosing a new system is a conversation to have with a qualified heating installer who has seen your home.
The three setups at a glance
| Setup | Hot water cylinder? | Cold tank in the loft? | Hot water pressure |
|---|---|---|---|
| Combination (combi) boiler | No | Usually no | Mains pressure, heated as you use it |
| System boiler with a cylinder | Yes, usually unvented | Usually no | Mains pressure, from stored hot water |
| Regular (heat-only) boiler | Yes, vented | Yes, usually two tanks | Gravity-fed, from stored hot water |
Homes vary. Older systems get altered over the years, so treat the table as a starting point, not a diagnosis.
Combination (combi) boiler
A combi does two jobs in one box. It heats the radiators, and when you turn on a hot tap it heats mains cold water as it flows through. There is no hot water cylinder and usually no cold water tank in the loft.
What that means day to day:
- Hot water on demand. There is no stored hot water to run out, but the flow is limited by how fast the boiler can heat water and by the incoming mains. Running a shower and a kitchen tap at once can make both weaker or cooler.
- Mains pressure. Showers run at whatever pressure and flow the mains provides. A strong mains supply usually means a decent shower. A weak one means a weak shower.
- No backup heat source. If the boiler stops working, there is usually no hot water at all, because there is no cylinder with an immersion heater to fall back on.
Shower pumps: a standard gravity shower pump is the wrong product for a combi. See shower pump sizing and low water pressure.
System boiler with a hot water cylinder
A system boiler heats the radiators and heats a separate hot water cylinder through a coil inside it. The boiler and cylinder are linked by the heating pipework and controlled by a programmer and valves.
In most modern installs the cylinder is unvented. It is fed straight from the mains, with no cold tank in the loft, and stores hot water at mains pressure. It has its own safety devices and a discharge pipe, and work on it is specialised. Our unvented cylinder basics guide explains what that means.
What that means day to day:
- Stored hot water. Several outlets can usually run at once, because hot water is already waiting. When the cylinder is used up, it needs time to reheat.
- Mains pressure. Like a combi, shower pressure depends on the incoming mains.
- A backup. Many cylinders have an electric immersion heater as well, so there may still be hot water if the boiler is off. See hot water cylinders and immersion heaters.
Shower pumps: with an unvented cylinder, a standard gravity shower pump is the wrong product. Some system boilers are paired with a vented cylinder and a loft tank instead, and those behave like the next setup.
Regular (heat-only) boiler with tanks and a vented cylinder
This is the older layout, sometimes called a conventional, traditional or open-vented system. The boiler heats the radiators and a vented hot water cylinder. In the loft there is usually a large cold water storage cistern, which feeds the cylinder, and often a smaller feed and expansion tank for the heating.
What that means day to day:
- Gravity pressure. Hot water, and often some cold taps upstairs, is pushed only by the height of the loft tank above the outlet. Showers close to the loft can be weak. That is how the system is designed, not a fault.
- Stored hot water, with the same "it can run out" limit as any cylinder, and usually an immersion heater as a backup.
- Tanks to look after. A loft cistern needs a lid and insulation, and the pipes up there need lagging. See pipe lagging.
Shower pumps: this is the system most shower pumps are designed for. Whether you need a positive or negative head pump, and a single or twin ended one, is covered in shower pump sizing.
How to tell which one you have
You can usually tell by looking, without opening anything:
- Is there a hot water cylinder? Look in airing cupboards and near the boiler. No cylinder at all usually means a combi.
- If there is a cylinder, is there a cold water tank in the loft feeding it? A loft tank and a copper cylinder, often with a foam jacket or sprayed foam, usually mean a vented, gravity-fed system.
- No loft tank, and the cylinder has a label, pressure safety valves and a short pipe into an open funnel (a tundish)? That usually means an unvented cylinder.
- Still unsure? The boiler and cylinder labels and the installation paperwork usually say. Your installer or a heating engineer can confirm it.
Do not remove panels, covers or casings to find out. Labels, tanks and paperwork are enough.
Who to call
- Anything inside the boiler, the gas supply or the boiler's casing: a Gas Safe registered engineer. Our when to call a Gas Safe engineer guide explains the warning signs and how to check registration.
- An unvented cylinder: someone competent in unvented hot water storage systems. See unvented cylinder basics.
- An immersion heater or any wiring: a qualified electrician.
- Taps, toilets, tanks and general pipework: a plumber, or yourself for the jobs our other guides cover.
Changing from one setup to another involves Building Regulations, and work on gas appliances and unvented cylinders has to be done by suitably qualified people. Any decision about that belongs with an installer who has surveyed your home. This page does not tell you which system to choose.
Is a combi better than a system boiler?
Neither is better in every home. A combi saves space and gives hot water on demand. A cylinder system can supply several outlets at once and may give you a backup heat source. Which suits you depends on your home, your mains supply and how you use hot water. Ask a qualified installer.
Why does my shower go cold when someone runs a tap?
On a combi, two outlets share the boiler's heating capacity and the incoming mains, so both can lose flow or temperature. On a stored system, the cause is more often the pipework layout or the cylinder running low. Start with our low water pressure guide.
Can I add a shower pump to any of these systems?
A standard shower pump is made for a gravity-fed system with a loft tank and a vented cylinder. It is the wrong product for a combi or an unvented cylinder. See shower pump sizing.