Are there any downfalls to combi boilers? There's no need for a separate hot water storage cylinder or cold water feed cistern, so they take up much less room than heat-only boilers (below). Call us today at -- for a free estimate! The tank must be completely full before connecting the wiring to avoid dry firing – which ruins the heating element. Ap provides around-the-clock plumbing services when you need us we’ll be there with a fully stocked truck hours.