Quick answer
A combi boiler heats water on demand with no hot water cylinder, which suits most flats and smaller homes with one bathroom. A system boiler stores hot water in a cylinder, so it suits homes with two or more bathrooms used at the same time. The right choice depends on hot water demand, mains pressure and space.
How a combi boiler works
A combination (combi) boiler provides heating and hot water from one compact unit. It heats water directly from the cold mains when you turn on a hot tap, so there is no hot water cylinder and usually no tanks in the loft.
Good points: saves space, hot water on demand, no waiting for a cylinder to reheat, and usually a simpler, lower-cost installation.
Limits: flow is shared between outlets, so running a shower and a hot tap together reduces flow to both. Performance depends on your incoming mains pressure and flow rate.
How a system boiler works
A system boiler heats water which is stored in a hot water cylinder, often an unvented cylinder fed at mains pressure. Key components such as the pump and expansion vessel are built into the boiler, so there is usually no tank in the loft.
Good points: several outlets can run at once, strong showers from a mains-pressure cylinder, and the cylinder can be paired with other heat sources such as solar thermal or a future heat pump.
Limits: needs space for a cylinder, stored hot water can run out and must reheat, and installation is usually more involved. Unvented cylinders also need annual servicing.
Quick comparison
| Combi | System | |
|---|---|---|
| Hot water cylinder | No | Yes |
| Space needed | Least | Boiler plus cylinder cupboard |
| Typical home | Flats and homes with one bathroom | Homes with two or more bathrooms |
| Several taps at once | Flow is shared | Handles simultaneous use well |
| Hot water availability | On demand, no reheating wait | Limited by cylinder size; reheats |
| Works with solar or heat pump later | Limited | Cylinder can be adapted |
Questions to ask before choosing
- How many bathrooms, and are they used at the same time? Morning rushes with two showers usually favour a system boiler.
- What is your mains water pressure and flow? A combi relies on it. Your engineer should measure it during the survey.
- Do you have, or want, a cylinder cupboard? Swapping to a combi frees space; keeping a cylinder keeps stored hot water.
- What system do you have now? Converting from a regular (heat-only) or system boiler to a combi involves removing tanks and cylinders and altering pipework, which adds to the job.
What about regular (heat-only) boilers?
Regular boilers use a cylinder plus cold water storage and feed-and-expansion tanks in the loft. They are common in older homes and can suit properties with low mains pressure or where existing pipework is designed for them.
Sizing matters as much as type
A combi's output affects hot water flow rate, while a system boiler's output affects heating and how quickly the cylinder recovers. A proper survey looks at heat loss, hot water demand and the existing pipework, rather than simply matching the old boiler's size.
CombiCert in Telford
CombiCert Ltd is a Gas Safe registered (621146) gas and heating company based at The Old School House, Park Lane, Telford TF3 4TG, working across Telford and Shropshire for homes and businesses. We are open Monday to Saturday, 08:00 to 19:45 (closed Sunday).
New boilers from £1,750 inc VAT. This is a headline 'from' price; most quotes are higher, and all quotes are subject to a site inspection.
See New boiler Telford for details, or call or WhatsApp 07393 220694, or email hello@combicert.co.uk.
Frequently asked questions
Does a combi boiler cost less to install than a system boiler?
Often, yes, when replacing like for like, because there is no cylinder to supply and fit. Converting from a system or regular boiler to a combi adds work, such as removing tanks and altering pipework. Costs depend on the property, so any accurate price needs a site inspection.
Can a combi boiler run two showers at once?
It can supply two outlets, but the flow is shared, so both showers will be weaker. Larger combis and good mains pressure help, but homes that regularly run two showers together are usually better suited to a system boiler with a mains-pressure unvented cylinder.
Which is more efficient, a combi or a system boiler?
Modern condensing combi and system boilers have similar efficiency ratings. A combi avoids heat lost from a stored cylinder, while a system boiler can heat water in larger batches. Real-world running costs depend more on correct sizing, controls, system condition and how the household uses hot water.
Do I need a hot water cylinder with a system boiler?
Yes. A system boiler heats the central heating and a separate hot water cylinder, usually unvented and fed at mains pressure. The cylinder needs a cupboard and, if unvented, an annual check. A combi boiler does not need a cylinder because it heats water on demand.
Can I switch from a system boiler to a combi?
Usually, yes. The cylinder and any loft tanks are removed and pipework is altered so the combi takes cold water directly from the mains. Your engineer should first check your mains pressure and flow rate, and confirm a combi can meet your household's hot water needs.
This guide is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.