Gas Combi Boilers

Our gas combi boiler range covers A-rated, 24–30kW instant hot water and heating units from Baxi, Vaillant, Ariston, Biasi, Glow-Worm and Viessmann, with warranties from 7 to 10 years depending on model. Combi boilers heat water on demand directly from the mains, combining central heating and hot water in a single wall-mounted unit with no cold water tank or hot water cylinder required — making them the standard specification for flats, new builds, and 1–4 bedroom properties with a single bathroom.

Sizing: kW rating vs actual hot water delivery

A combi's headline kW figure is its domestic hot water (DHW) output, not its central heating output — a "30kW combi" typically delivers only 18–24kW to the heating circuit, with the remainder going to instantaneous water heating. This is a common point of confusion: buyers often assume a higher kW figure guarantees stronger showers, but flow rate is what determines that. At a standard 35°C temperature rise, every roughly 2.5kW of DHW output delivers about 1 litre per minute, so a 24kW combi produces around 9.8 l/min while a 30kW unit delivers closer to 12 l/min. Boiler Plus sets a minimum flow rate standard of 10 l/min for new combi installations, and flow can only be as good as the incoming mains supply allows — a strong boiler paired with a weak mains feed won't outperform the pipe.

When a combi is — and isn't — the right choice

Combis suit intermittent, single-outlet demand: one shower or bath running at a time. Where two bathrooms may be used simultaneously, splitting the available flow rate reduces performance at each outlet, and a system boiler with an unvented cylinder is usually the better specification. For higher-demand households, see our system boiler and unvented cylinder ranges.

What this collection includes

24–30kW A-rated ErP models from Baxi, Vaillant, Glow-Worm and Viessmann, covering entry-level and premium tiers, with manufacturer warranties from 7 to 10 years. As the range expands, additional brands and outputs will be added here.

Specifying and installation notes

Under Building Regulations Part L, a room-by-room heat loss calculation is mandatory before any boiler replacement to prevent oversizing — oversized combis cycle more frequently, spend less time condensing, and run less efficiently than a correctly sized, range-rated unit. Boiler Plus also requires a system filter to be fitted on all new gas boiler installations to protect against magnetite sludge; see our magnetic filter and scale inhibitor ranges for compatible products. All gas combi boiler installation and commissioning must be carried out by a Gas Safe registered engineer.