For Opus wall-hung gas combination boiler range, the first channel regulates the times of operation for your central heating. The second channel controls the time the pre-heat function takes place. The hot water pre-heat function enables a vastly better response to the hot water tap. This prevents the boiler from firing up out of hours to pre-heat the boiler's domestic hot water heat exchanger, perfectly balancing hot water availability with economical operation. Hot water is always available, even if pre-heat is timed to be off, however, the response may just be a few seconds longer. Please note: Our floor-standing gas and oil combination boiler ranges incorporate a 'heat-bank' which stores heat to enable the boiler to produce the high-performance hot water you require. This heat-bank always needs to be hot to satisfy maximum hot water demands. Just as with our wall-hung gas combination boilers, using a twin channel programmer ensures the central heating and heat-bank are always on when required and avoids unnecessary firing of the boiler.