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Doctor Drivel
Does anyone else find Agile more irritating than informative!
Half the information.......everyone else seemed to manage
sturharv, I would seriously consider a thermal storage system. The store is a buffer reserve of heat to use a peak times. The state of the existing boiler you don't say. Assuming all is OK with it, thermal storage/heat buffering is the most cost effective approach to look at
Calculate a store that holds 65kW of heat plus 50% more then add on top of this DHW needs, then that is approx your size for normal system - to be sure you could go well over 100%, so 130kW or more of CH heat stored. Remember the store and boiler will be working together to heat the place. In less than 100% peak times the store will be topped up by the boiler and both working when at full demand. In the morning a full cylinder may dump half or less of its heat into the rads super quick bringing the house up to temp, and the boiler kicks into re-heat at full temp. At peak demand times (when -1C outside), the boiler will be working flat out to cope, but the heat buffer will smooth it out, always having that reserve of heat to draw off.
Take the CH circuits off the store cylinder. You have to zone the CH in a house that big anyhow and take the two or more CH zones directly off the store. Then TRVs can be fitted on all rads using an Alpha or Smart pump and no unrepresentative nuisance wall stats.