I have an existing pressurised Megaflo (170 litres) heated via an indirect coil fed from a Johnson and Starley Janus water heater via a pump and motorised stop valve. The Megaflo temperature is controlled by a thermostat which controls the pump and valve. The primary circuit is served by a small gravity feed and expansion tank which is (correctly in my opinion) on the suction (return) side of the pump. This system has worked very well for 20 years. But I now have to have a new warm air system and the primary circuit will now feed both the primary coil in the Megaflo AND the heat exchanger in the warm air system via a diverter valve which will send the water either one way or the other - I am not bothered about losing heat to the house for brief periods likely to be between 5 and 10 minutes or less. My concern is limescale. The feed and expansion tank presumably sees a small amount of evaporation and will thus receive new water - I reckon about 1 litre per week. The tubes in the Johnson and Starley heat exchanger I think are smaller bore then the Megaflo coil. Should I fit a sealed system including an expansion vessel with a diaphragm and a means of topping up if necessary? If so should the expansion tank be on the flow or return side. Alternatively I could arrange for the feed and expansion tank to be fed with soft water from my softener. I would be grateful for some thoughts on this.
John Spiers