A combi heats the taps from the cold main as you run them, and heats the radiators or underfloor from the same box. There is no cylinder and no immersion backup. If the boiler stops, heating and hot water stop together. An electric shower is the usual fallback, not a stored tank. Most UK houses already have a combi — commonly quoted in the 70–80% range — because it saves space. That popularity is not proof it is right for this house.
If you are still choosing a boiler type, stop here and read that first. The rest of this page assumes you have accepted the combi trade-off and need a size and a brand. What the install should include is on new combi boiler. Fitted prices sit on combi boiler cost.
Hot water first
On a combi the badge kW is usually set by domestic hot water, not by the radiators. Heating load for a typical insulated house is often 6–12 kW. The combi is 24–37 kW because of the taps.
Makers quote a flow at a temperature rise — often 35 °C (about 10 °C in, 45 °C out). That is a calculation assumption, not the temperature at your shower. Roughly:
| Hot-water DHW rating | About this flow at 35 °C rise | Typical use |
|---|---|---|
| Around 24 kW | About 9 litres a minute | One bathroom, modest shower |
| Around 30 kW | About 12 litres a minute | One good shower, or bath plus basin if you are careful |
| Around 37 kW | About 15 litres a minute | Stronger single outlet — still not two showers at once unless the main can feed both |
A larger combi does not create flow. The cold main has to supply it. The kitchen cold tap, everything else shut, filling a 4.5 litre bucket in about 30 seconds is 9 litres a minute. That is enough for a small combi and short of a large one. Low mains water pressure explains the Ofwat 0.7 bar (7 metres) duty at the boundary — the hub’s “1 bar and 9 litres, legally” mixed a marketing line with the guaranteed standard. Use what size boiler as a starting band, then a heat-loss and a measured flow on site.
Heating still has to be sized
Radiators and underfloor need a heat-loss, not a bedroom count. A combi that is large for hot water can still be range-rated down for heating so it condenses. Turning the flow temperature down is usually worth more than chasing a brand slogan.
Brand is second to the engineer
The hub’s “top brands” table listed annual volumes (Ideal ~25%, Worcester ~30%, Vaillant ~20–23%) and a High/Medium quality score. Those shipment numbers are years old and we are not republishing them as current. Worcester Bosch, Ideal, and Vaillant have long been the volume names in the UK. That does not make them the right box if nobody nearby will service them in February.
Pick the engineer first. Annual service and parts access matter more than a brochure guarantee. Then look at what that engineer knows well.
| Brand | OpenTherm / modulation | What is actually distinctive |
|---|---|---|
| Atag | Yes on the models we care about | Stainless heat exchanger. Smaller UK support footprint — check who will service it. |
| Baxi, Main, Potterton | Selected models — check the exact badge | Same wider group. Main is usually the keener price. Guarantee length is not the same across the three names. |
| Glow-worm | Vaillant Group eBUS, not a simple “yes OT” | Shares group parts and service. Confirm the control bus before you buy a third-party OT stat. |
| Ideal | Yes on current ranges we see | UK-made, simple layouts. Hub “12 years” was a then-current max — read today’s terms. |
| Intergas | Yes | Two-in-one heat exchanger, few moving parts. We use them. Guarantee is typically shorter unless extended. |
| Vaillant | Not native OT. eBUS, or a VR33 on many ecoTEC boilers | The hub “OpenTherm: No” was too blunt. You can modulate; you may need Vaillant’s bus or the interface board. |
| Viessmann | Some models / limited OT | Stainless Inox heat exchanger on the Vitodens we specify. Confirm the exact control. |
| Worcester Bosch | EMS, not native OT. See the adaptor note | Large UK service network. Guarantee length depends on filter and registration — current lengths. |
The full model list lives on OpenTherm boilers and controls. Do not treat a brand-level “yes” as a promise for the 28 kW box in the van.
Other constraints that pick the boiler for you
- Cupboard and flue. A bigger output is often a bigger case. Flue routing (and plume) can rule out a model before you like its spec sheet.
- Guarantee. Statutory minimum is one year. Maker packages run much longer if you register, fit their filter, and service every year. A long warranty from a thin UK network is a paper comfort.
- OpenTherm or equivalent modulation. Worth having for the next decade of controls — Nest, Honeywell/Resideo, Tado, Wiser, and the rest. See the Nest Thermostat E page if that is the stat you already own.
- Parts and merchant stock. If the local merchant does not hold the heat exchanger, winter waits get long.
- Gas pipe. A 37 kW combi on a long, hidden, undersized supply will not run at the badge. That is a Gas Safe pipe-sizing job, not a bigger boiler on the same 15 mm.
- Water pipe. Lead or 15 mm from the boundary will starve a large combi. Upgrading the private supply is plumbing, not a heating-only quote.
A combi is a poor fit if you want two showers at once, have a weak main you cannot upgrade, or you are already heading for a heat pump (you will want a cylinder). In those cases a system boiler and store, or a heat-pump design, is the honest conversation.
Related
- New combi boiler — what a modern condensing combi actually is
- Combi boiler cost
- What size boiler
- Boiler types
- OpenTherm boilers and controls
- Low mains water pressure
- Quote
Guidance only. Gas work in the UK needs a Gas Safe registered engineer. Guarantee lengths, OpenTherm support, and model names change — check the current installer pack, not this table alone.