Combi Boiler or Hot Water Tank?
Officially RegisteredA combi boiler heats water instantly from the mains and needs no tank, making it ideal for smaller homes with one bathroom. A system or regular boiler with a hot water tank stores heated water, which suits larger homes where several showers and taps run at once. The right choice depends on your space, water pressure, bathrooms and energy plans.
How a Combi Boiler Works
A combination (combi) boiler provides both central heating and hot water from one compact unit. When you turn on a hot tap, cold water from the mains passes through a heat exchanger inside the boiler and is heated instantly. There is no hot water cylinder and usually no cold water tank in the loft, so the whole system can fit in a kitchen cupboard.
Because water is heated on demand, you never wait for a tank to reheat — but the boiler can only heat a certain flow of water at a time, and it relies on the strength of your incoming mains supply.
How a Hot Water Tank System Works
With a system boiler or regular (heat-only) boiler, hot water is heated in advance and stored in a cylinder, ready to use. A system boiler works with a sealed, pressurised heating circuit and is usually paired with an unvented cylinder fed directly from the mains, giving strong pressure at every tap. A regular boiler uses a vented cylinder plus feed and expansion tanks in the loft, with pressure created by gravity.
Stored hot water can supply several outlets at once, but once the cylinder is used up it takes time to reheat.
Pros and Cons at a Glance
| Factor | Combi boiler | Hot water tank |
|---|---|---|
| Space needed | Compact — no cylinder or loft tanks | Needs room for a cylinder (and loft tanks for regular boilers) |
| Hot water supply | On demand, never runs out | Stored — can run out until reheated |
| Several taps at once | Flow shared, weaker with two outlets | Strong supply to multiple outlets |
| Water pressure | Depends on mains pressure | Unvented cylinders give mains-pressure flow |
| Off-peak / solar / heat pump | Not compatible on its own | Compatible via immersion or coils |
| Best suited to | Flats and 1-bathroom homes | Larger homes, 2+ bathrooms |
Space, Water Pressure and Multiple Bathrooms
Space: a combi frees up the airing cupboard and loft, which is valuable in flats and smaller houses. A cylinder takes up a cupboard, and a regular boiler also needs loft tanks.
Water pressure: a combi can only deliver what the mains provides, so homes with low incoming pressure or flow may get a weak shower. Unvented cylinders also rely on mains pressure but deliver it to several outlets at once; gravity-fed vented systems typically give lower pressure unless a pump is fitted.
Multiple bathrooms: this is often the deciding factor. If two showers regularly run at the same time, a cylinder-based system will usually give a far better experience than a combi.
Off-Peak Tariffs and Future Technologies
A hot water cylinder can store energy. With an immersion heater, it can heat water using cheaper off-peak electricity overnight, and it can be fed by solar thermal panels or a heat pump. A combi heats water only when you use it, so it cannot take advantage of time-of-use tariffs or renewable heat sources on its own. If you plan to add a heat pump or solar in future, keeping a cylinder can make that upgrade simpler.
Running-Cost Considerations
As a general industry rule, combis tend to be efficient for lower hot water demand because they do not lose heat from stored water. Cylinders lose some standing heat, though modern factory-insulated cylinders keep this low. In high-demand households, a cylinder heated efficiently — or on off-peak electricity — can be just as economical. Whatever you choose, an annual service keeps the boiler burning efficiently.
Which Property Suits Which System?
- Flats and 1–2 bedroom homes with one bathroom: a combi is usually the best fit.
- Family homes with two or more bathrooms: a system boiler with an unvented cylinder.
- Older homes with low mains pressure or existing gravity systems: a regular boiler and vented cylinder may be the practical choice.
- Homes planning solar or a heat pump: keep or add a cylinder.
Getting Expert Advice
A Gas Safe registered engineer can test your mains flow, check gas pipe sizing and recommend the right setup. Our Gas Safe boiler services cover installation, replacement and servicing, and our Gas Safe installers handle combi and cylinder systems. Leaning towards a combi? See our full guide to combi boiler benefits. Already have a boiler? Read what a boiler service includes, book boiler servicing in London, or see how often you should replace your boiler. Contact us for advice.
Frequently Asked Questions
Is a combi boiler better than a hot water tank?
Neither is better for every home. A combi suits smaller homes with one bathroom and good mains pressure. A system or regular boiler with a hot water cylinder suits larger homes where several taps or showers run at the same time.
Can a combi boiler run two showers at once?
Most combis struggle to supply two showers at once because they heat water on demand and flow is shared. Larger combis and good mains flow help, but homes with two or more bathrooms used together usually do better with a cylinder.
Is a hot water tank cheaper to run than a combi?
Combis avoid heat loss from stored water, so they are often slightly cheaper for low hot water use. A well-insulated cylinder can be cost-effective for high use, and can be heated with cheaper off-peak electricity or solar where available.
Do I need a hot water tank with a heat pump or solar?
Yes, in most cases. Heat pumps and solar thermal systems heat water gradually, so they need a cylinder to store it. A combi is not compatible with these technologies on its own.
Can I switch from a hot water tank to a combi boiler?
Yes. Converting removes the cylinder and often loft tanks, freeing space. A Gas Safe engineer must check the gas supply, pipe sizing and mains water flow, because a combi relies on adequate mains pressure.
Who can install or replace a gas boiler?
Only a Gas Safe registered engineer can legally install, replace or service a gas boiler in the UK, under the Gas Safety (Installation and Use) Regulations 1998.