There is really no excuse for a sealed combustion non-condensing boiler and micro-zones are not a sin unless they are in high load/sq.ft. areas and allowed to call the boiler independently. Still the answer is more radiation. European wall-hung panel radiators are the thing for sure, easily designed to 140° AWT and assured full-time condensing mode. 3 minutes run time is acceptable but if this only occurs under design conditions, I would not tolerate it.