Overview
A heat-only (regular) boiler works with a hot water cylinder and cold water header tank in the loft. A combi eliminates both — heating water directly from the mains on demand.
Installation cost
Like-for-like heat-only replacement: from £1,299. Converting to combi (removing cylinder and tank): from £1,499. Heat-only wins if you're replacing like-for-like.
Space
Converting to combi removes the loft tank and frees up the airing cupboard — two significant space gains. Combi wins on space.
Hot water
Heat-only with a large cylinder delivers high-volume hot water. Combi is limited by flow rate. For large families: heat-only may be better.
Complexity
Heat-only systems are more complex (more components) but very repairable. Combis are simpler overall.
Verdict
Converting to combi is worthwhile if you want to free up space and the existing cylinder is aging anyway. For large families with high hot water demand, replacing like-for-like (heat-only) is often better value.