Combination Boilers (combis for short) heat water instaneously and require no external hot water storage cylinders. Howeve, with a solar system, a hot water cylinder is required as a stoe for free solar energy collected during the day. The cylinder insulation will keep the water hot ready for use.

Therefore to benefit from a solar system the combi boiler must be able to accept mains pressure hot water from a storage vessell. Unfortunately many new combi's and most old combi's will only work with a cold mains supply. These boilers may shut down if their cld feed is higher than 20C.

There are some solutions:-

Replace the combi boiler with a pre heat compatible model. The most common types have a small store of hot water, which is thermostatically controlled. A thermal store or mains pressure solar hot water cylinder is required to accompany the new boiler, so that pre heated water will be at mains pressure. This solution is only economically viable if the boiler is going to be upgrade or changed.