The Heating and Hotwater Industry Council (HHIC) have a Q&A in its consumer guide about protecting your boiler and heating system. Here’s what is said regarding limescale, softened water and boilers:
“What about limescale?
Limescale is particularly problematic in hard water areas, and for combi boilers, which frequently take in new, “fresh” water and heat it directly. Part L of The Building Regulations recommends that in hard water areas the cold mains supply to combi boilers is treated with a scale prevention device. This ensures that lime-scale does not block the heat exchanger in your combi boiler, reducing ow rates and the temperature of the hot-water delivered.
Water softeners are a way to offer “whole house” protection against limescale. Some boilers are not recommended for use with softened water (ion-exchange) in the primary circuit, so, if in doubt, always fill the heating system using un-softened mains water. This can be done by simply opening the bypass valve on the water softener installation plumbing temporarily whilst filling, or by having a separate mains water feed directly to the heating system filling loop, or feed and expansion tank. The boiler manufacturer may give more detailed advice in their appliance instructions, which must always be complied with to ensure that your boiler warranty is protected.”
You can find all the details here: http://www.hhic.org.uk/uploads/5ADD9CC24F6B6.pdf