Guide · larger homes
Combi boilers for large houses
Heating a big house is easy for a combi. Hot water for a big household is the test. Here are the makers' flow rates, an installer's sizing guide, and the point at which a cylinder wins.
A combi can heat a large house. What it struggles with is hot water for a large household, because every tap shares one flow. The honest test is not bedrooms but showers: if two run at once most mornings, a system boiler with a cylinder, or a big combi with a flow rate to match, is the answer.
Installers price the bigger combis a little higher: General Gas lists a 35 kW Ideal Logic Max from €2,470 fitted and a 40 kW Vogue Max from €2,790, and a Dublin sizing guide puts a 5-bedroom house at 35 kW or more from €2,795 plus VAT.
Heating a big house is the easy part
Combi output runs to 40 kW and beyond, and the heating side of a combi works like any boiler: it heats the radiators through the same pipework, in zones if the house is zoned. A Dublin installer's own sizing guide gives the usual shape of it (gasboiler.ie, read 29 October 2026):
| House | Suggested output | Installer's price from |
|---|---|---|
| 2 bedrooms, up to about 10 radiators | 9–18 kW | €2,095 + VAT |
| 3 bedrooms, 10–15 radiators | 18–26 kW | €2,195 + VAT |
| 4 bedrooms, 15–20 radiators, more than one bathroom | 25–30 kW or more | €2,249 + VAT |
| 5 or more bedrooms | 35 kW or more | €2,795 + VAT |
Source: gasboiler.ie, read 29 October 2026. Prices are "from", plus VAT at 13.5%. The guide itself says there is no quick rule and the number of radiators and bathrooms decides it.
Hot water is the hard part
A combi heats water as it passes through, so its hot water is a flow rate, in litres a minute, not a stored volume. Open a second tap and the flow is shared. Vaillant publishes the figures for its ecoTEC plus combis at a 35 °C rise, roughly cold mains to a warm shower (Vaillant technical sheet, read 29 October 2026):
| Combi | Output | Hot water, litres a minute (35 °C rise) |
|---|---|---|
| ecoTEC plus 825 | 25 kW | 10.5 l/min |
| ecoTEC plus 832 | 32 kW | 13.0 l/min |
| ecoTEC plus 835 | 35 kW | 14.3 l/min |
| ecoTEC plus 838 | 38 kW | 15.9 l/min |
Source: Vaillant ecoTEC plus combination boiler technical sheet, domestic hot water flow rate at Δt 35 K. At a 45 K rise, for a hotter tap in winter, the same boilers give 8.8, 10.9, 11.9 and 13.2 litres a minute.
A shower uses something like 8 to 12 litres a minute depending on the head, so a 25 kW combi runs one shower well and two badly. A 38 kW combi at 15.9 litres a minute can run two modest showers at once, but not two power showers and a bath. The bigger the combi, the more it depends on the mains being able to deliver that flow in the first place, which an installer measures at the tap before quoting.
The big combis installers sell
General Gas, a North Dublin installer, labels its larger combis for larger homes on its own price cards (General Gas, read 29 October 2026; "from" prices, fitted, VAT not stated):
| Combi | Output | Warranty | From, fitted | The installer's own label |
|---|---|---|---|---|
| Ideal Logic Max C35 | 35 kW | 10 years | €2,470 | 4-bed homes with high demand |
| Ideal Vogue Max C40 | 40 kW | 12 years | €2,790 | Large homes & busy households |
| Worcester Bosch Greenstar 8000 Style | 35 kW | 12 years | €3,390 | Large family homes |
Across all the installers in our survey, a like-for-like combi swap has a median of €2,600 and the middle half of prices runs €2,199–€2,800. A 35 or 40 kW model sits towards the top of that range. See every published price.
When a large house should keep a cylinder
| Go combi | Keep a cylinder (system boiler) |
|---|---|
| Two bathrooms, rarely used at the same time | Two or more showers running together most mornings |
| Good mains pressure and flow, measured | Weak mains, or a long thin supply pipe |
| You want the hot press and attic tank gone | You want an immersion as a backup, or solar water heating |
| No heat pump planned | A heat pump likely in the next few years: it needs the cylinder |
| Median combi swap €2,600 | Median system or regular boiler €2,500 |
Like for like, the two cost about the same to fit. The real cost is switching from one to the other: a system-to-combi conversion has a median of €3,400 in our survey, because the cylinder and tank come out and the hot water pipes change. The full comparison is on combi vs system boiler.
Two things that make a combi work in a big house
- Measure the mains. A 38 kW combi needs the water to arrive at 15 litres a minute to deliver 15 litres a minute. Ask the installer for the measured flow and pressure, not a guess.
- Zone the heating. A big house on one zone runs the boiler for rooms nobody is in. Zoning downstairs and upstairs, with a thermostat on each, is also the shape of SEAI's €700 heating controls grant, which on a combi requires a second zone. See combi boiler grants.
Questions people ask
Can a combi boiler heat a 4 or 5 bedroom house?
Yes. Heating is the easy part: 30 to 40 kW combis exist, and one Dublin installer's guide puts a 4-bedroom house at 25–30 kW or more and a 5-bedroom house at 35 kW or more. The question is hot water for the number of showers in use at once.
How many showers can a combi run at once?
One well, on a 25 kW combi giving about 10.5 litres a minute at a 35 °C rise. A 38 kW combi gives 15.9 litres a minute, enough for two modest showers together. Two power showers and a bath need a cylinder.
Is a system boiler better for a big house?
Usually, if two or more showers run together most mornings, or a heat pump is planned. A combi suits a big house with one busy bathroom and good mains pressure.
How much is a large combi boiler installed?
General Gas lists a 35 kW Ideal Logic Max from €2,470 fitted, a 40 kW Vogue Max from €2,790 and a 35 kW Worcester Bosch 8000 Style from €3,390 (VAT not stated). A Dublin sizing guide starts a 5-bedroom house at €2,795 plus VAT.
What kW combi do I need for a 4 bed house?
One installer's guide suggests 25–30 kW or more for 15 to 20 radiators and more than one bathroom. Size on the hot water flow you need, and have the installer measure the mains.
Does a bigger combi cost more to run?
Not for heating: it modulates down to what the house needs. It burns more gas per minute while a tap runs, for the same hot water. See combi boiler running costs.
Sources (3)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- gasboiler.ie: gasboiler.ie
- Vaillant technical sheet: professional.vaillant.co.uk
- General Gas: generalgas.ie
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