I had the whole system upgraded around 10 years ago. New double radiators everywhere except bathroom, hallway and one in the front room. Two doubles are not allowed in the same room. New manual trvs, valves.
Do you have a delay between when your DT4 calls for heat and your boiler fires up? Also does your boiler temperature reach high levels? Mine is acting strange, a delay and boiler temp is in the 40s
