Guide
Combi vs system boiler
Hot water on demand, or a cylinder of stored hot water? The right choice depends on your bathrooms and your mains pressure. Fitted prices are close: median €2,600 vs €2,500.
A combi heats water straight from the mains when you open a tap, with no cylinder or attic tank. A system boiler heats a cylinder of stored hot water. On published Irish prices, a combi swap has a median of €2,600 and a system or regular boiler €2,500.
A combi usually suits a home with one bathroom. A system boiler usually suits a busy house where two showers run at once.
How each one works
A combi boiler is a heating boiler and a water heater in one box. When you turn on a hot tap, it fires up and heats mains water as it flows through. There's no hot water cylinder in the hot press and no cold water tank in the attic.
A system boiler heats the radiators and a hot water cylinder, which stores hot water for later. Most of the pumps and valves are built into the boiler, and the cylinder can be fed from the mains or from a tank. A regular (heat-only) boiler is the older set-up: boiler, cylinder and a cold water tank in the attic, with more of the parts outside the boiler.
Combi vs system boiler: the comparison
| Combi boiler | System boiler | |
|---|---|---|
| Hot water | On demand, from the mains | Stored in a cylinder |
| Cylinder and attic tank | None | Cylinder, sometimes a tank |
| Two showers at once | They share the flow | Usually fine, until the cylinder runs low |
| Mains pressure matters? | Yes, a lot | Less, if the cylinder is tank-fed |
| Space | Frees up the hot press and attic | Needs room for the cylinder |
| Backup if the boiler fails | None, unless you have an electric shower | An immersion in the cylinder |
| Median published price | €2,600 | €2,500 |
| Installers in our survey | 12 | 32 |
Which suits your house?
Go combi if you have one bathroom, or two where showers rarely run at the same time, your mains pressure is decent and you'd like the hot press and attic space back. Apartments and smaller houses are the classic combi homes.
Keep a system boiler if two or more showers often run together, the mains pressure is weak, or you want the immersion as a backup. Bigger houses with several bathrooms usually do better with stored hot water.
Thinking about a heat pump later? A heat pump also heats a cylinder of stored water. If one is likely in your future, the cylinder you'd remove for a combi is the one you'd need again. SEAI's heat pump grant now goes up to €14,500 for a house (SEAI). See boiler grants.
Price: combi vs system boiler
Like for like, the two cost about the same to fit: the median published price is €2,600 for a combi swap and €2,500 for a system or regular boiler. The real cost difference comes when you switch from one to the other. Converting a system or regular boiler to a combi has a median published price of €3,400, because the cylinder and tank come out and the hot water pipes change.
Questions to ask the installer
- What's my mains flow rate, and is it enough for a combi?
- How many showers can run at once with this boiler?
- What output (kW) does my house need, and why?
- Does the price include moving or removing the cylinder and tank?
- What warranty comes with it, and what does it need to stay valid?
Questions people ask
Is a combi or system boiler better for a three-bed house?
It depends on bathrooms more than bedrooms. With one main bathroom and decent mains pressure, a combi usually works well. With two bathrooms in daily use at the same time, a system boiler with a cylinder is usually the safer choice.
Is a combi boiler cheaper to run?
Often a little, because there's no cylinder of hot water cooling down while you're not using it. How much depends on how much hot water your household uses.
How much more does a combi cost than a system boiler?
Like for like, about the same: median €2,600 for a combi swap and €2,500 for a system or regular boiler, from installers' published prices. Switching from a system boiler to a combi costs more, median €3,400.
Can a combi boiler work with a heat pump later?
Not directly. A heat pump heats a cylinder of stored water, so going combi now means fitting a cylinder again if you switch to a heat pump later.
Want a fixed price for a new boiler?
Call 087 162 1846 between 8am and 8pm, or leave your number and one RGI-registered installer who covers your county rings you back.