I have a Vaillant ecotec condensing boiler and we seem to be getting variable heating results on both central heating and hot water tank recently. The boiler will switch on start heating the water, get up to 65deg target temperature, then immediately backs off and the water temp drops to about 45deg, the boiler then is content to keep heating the water at this lower value. To a layman, I would expect the boiler to run continuously at 65deg until a control like the hall thermostat tells it to turn off cos the house is warm enough.