Condensing boiler

Checked against its sources: Updated 3 November 2026

A condensing boiler pulls extra heat from its flue gases before they leave, and drains away the water that condenses out.¹ All domestic boilers sold today are condensing, whether combi, system or heat only, according to Baxi.¹

Condensing boiler: key facts

A boiler fitted in a kitchen wall cupboard. Maker's badge blurred.

Not the same as	A combi: one describes the heat recovery, the other the layout²
By-product	Condensate, carried off in a narrow plastic pipe¹
Visible sign	A plume of water vapour at the flue, at times³
Required for gas	New gas boilers since 1 April 2005, per Vaillant²
Required for oil	New oil boilers since 1 April 2007, per Vaillant²
England minimum	92% ErP for gas in existing homes⁴

Covers: Gas, LPG and oil boilers, UK. Efficiency rules differ by country and are labelled.
You: Check the drain for blockages where your manual says to; thaw a frozen pipe only as that manual allows.
A professional: Altering or re-routing the condensate pipe, the flue and all internal work. Gas Safe registered engineer for gas.

1 How it works

Burning fuel makes hot gases. In a non-condensing boiler those gases and their heat go up the flue. Vaillant explains that a condensing boiler sends them through a heat exchanger, recovering heat into the primary circuit before they leave². Baxi says so much heat comes out that water in the gas sometimes condenses, and this condensate runs away through the condensate pipe¹.

The Baxi 600 Heat guide notes that the cooled flue gases can look like a cloud of steam, which is normal, and that only a qualified professional should change the route of the drain⁵. Worcester says the same of the plume on its i ErP combi³. For what that cloud does and does not mean, see flue and plume.

2 Condensing and combi are separate words

They get used as if they meant the same thing, and Vaillant suggests that is down to timing, with combis growing popular around when condensing became mandatory². A combi boiler can be condensing, and so can system and heat-only ones¹⁶.

3 Efficiency, and whose figures they are

Vaillant says newer condensing models can reach over 90% efficiency and puts non-condensing boilers at 70-80%². Those are a maker's general figures, not a test of any particular boiler.

The minimums in building guidance are firmer, and each is a separate rule:

Minimum efficiency for a replacement gas boiler, by country
Where	Figure	Source
England	92% ErP; 78% SEDBUK 2009 natural gas in exceptional circumstances only	Approved Document L 2021, Table 6.2⁴
Wales	92% ErP	Approved Document L 2022⁷
Scotland	Not less than 92% ErP, per its 2022 compliance guide	Read from a consultation copy⁸
Northern Ireland	88% SEDBUK 2009 or 90% SEDBUK 2005	Technical Booklet F1, June 2022⁹

Replacement rules in more detail are under boiler building regulations.

4 Flow temperature

Flow temperature affects how a boiler runs, and the official running advice and the design rules are different things; both are set out under flow temperature.

Sources

Baxi heating terms glossary read 3 November 2026.
What is a condensing boiler, Vaillant read 3 November 2026.
Worcester Greenstar i ErP instructions 6720806946, p29, 2015/03, read 3 November 2026.
Table 6.2, Approved Document L Volume 1 for England, 2021 with 2023 changes read 3 November 2026.
Heat-only guide for the Baxi 600 Heat 7726861-01, p13, 1/19, read 3 November 2026.
Baxi FAQ on condensing and combi boilers read 3 November 2026.
Part L guidance, Welsh Government read 3 November 2026.
Scottish compliance guide 2022, on the Scottish Government's consultation site read 3 November 2026. Its status as the final edition was not separately confirmed.
Technical Booklet F1, Department of Finance NI, para 3.32 read 3 November 2026.