Many people would agree that the most important requirement of a solar water heater is not an exceptionally high thermal efficiency, but rather a good long-term reliability. How one can ensure that a solar heating system will give a long and trouble free service, however, is not obvious. One move in the right direction has been the issuing of a British Standard Code of Practice, but because it should not inhibit development in a new industry this document cannot be sufficiently restrictive to preclude the installation of unreliable systems. Another possibility would be the introduction of durability tests, but these must be simple and cheap or their very existence might have an adverse effect in the market place.