What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
No local equivalentcombi boiler
also combination boiler, combi, instantaneous boiler
The dominant arrangement in British housing. The alternatives are named against it — a system boiler and a heat-only boiler both keep a cylinder, and a combi is defined by not having one.
Australia
No local equivalentcombi boiler
also continuous flow hot water, instantaneous gas
Hydronic heating is uncommon, so the hot-water half of the idea exists widely as continuous-flow gas and the heating half rarely does.
United Statesyours
Same word, different thingcombi boiler
also combination boiler, tankless water heater, boiler
A TANKLESS WATER HEATER does the hot water only and does not heat the house. American combi boilers exist and are a specialist product; the default is a furnace or heat pump for heating plus a separate water heater.
Canada
Same word, different thingcombi boiler
also combination boiler, tankless coil, on-demand
As the United States, with hydronic heating more common, so a true combi is less unusual than south of the border.
The governing standard, by market
United Kingdom
BS EN 15502 / Gas Safe requirements
Gas-fired central heating boilers, and who may lawfully install or work on one.
United States
ASME CSD-1 / NFPA 54
Controls and safety devices for automatically fired boilers, and the national fuel gas code.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is a combi boiler the same as a tankless water heater?
- It contains one and is not one. A tankless water heater heats domestic hot water on demand and does nothing else; a combi does that AND runs the radiators, switching between the two with a diverter valve that gives hot water priority — which is why the heating drops out briefly while somebody runs a bath and then comes back. Specifying one against the other across markets leaves either a house with hot taps and no heating, or a duplicated appliance. The second difference matters for sizing: a tankless unit is sized on hot water flow alone, while a combi is sized on the LARGER of its two duties, and for most British houses that is the hot water rather than the heat loss — which is why combis are routinely far more powerful than the house needs for heating.
- Why does a combi struggle with two showers at once?
- Because it heats water as it flows, so its output is a flow RATE at a temperature rise, not a quantity. A combi rated at 30 kW can lift roughly twelve litres a minute by about 35 degrees — enough for one good shower. Open a second outlet and the same energy is spread across twice the flow, so both run cooler, or the flow regulator holds the temperature and halves the flow instead. A stored-cylinder system behaves completely differently: it has a reservoir, so it will serve several outlets at full flow until the reservoir runs out and then serve none. That is the real trade between the two, and it is why a house with two bathrooms in simultaneous use is frequently the wrong house for a combi regardless of the boiler's rating.
- Why does the incoming water temperature change what a combi can do?
- Because the boiler's job is the temperature RISE, and the rise it has to deliver changes with the season. Mains water arrives at perhaps 20 degrees in late summer and perhaps 5 in February, and a shower wants around 40 — so the same appliance is being asked for a 20-degree lift in one season and a 35-degree lift in the other, which is three quarters more energy for the identical shower. That is why a combi that was adequate in September feels weak in January, why manufacturers quote flow rate at a stated rise and the quoted figure is usually the generous one, and why a specification should state the rise it assumes. A flow rate quoted without a temperature rise is not a specification.