It appears that my brother has bought a bungalow with an unconventional heating system, that doesn't fall into any category (eg. Y, C, S plan) Its like a C plan, but the single 2 port valve is on the heating flow. The HW has no control valve, so whenever the heating is on, the primaries on the cylinder are also being pumped from the boiler too. He wants to replace the ancient boiler. Can we convert to S plan, by adding a 2 port valve to the cylinder primaries? Obviously the wiring would need to change too, but what other changes are necessary?