Combination (combi) boilers that primarily circulate hot water for space heating — with domestic hot water as a secondary function — generally classify under 8403 as central heating boilers. The essential character test applies: if space heating is the principal design purpose, 8403 is the correct heading. Consult a licensed customs broker or request a binding ruling if the domestic hot water function is equally prominent.

Heat exchangers specifically designed for use in central heating boilers of heading 8403 classify under 840390 as parts. However, if a heat exchanger is a general-purpose industrial type not specifically designed for central heating boilers, it may classify under the heat-exchanger heading in Chapter 84 instead. The 'specifically designed for' criterion is the key test.