All oil fired combi boilers operate this way.
It is not possible to have a low water content heat exchanger and modulating burner with oil.
Therefore a buffer tank is heated to transfer the heat via a plate heat exchanger when demanded.
O.K. Terrywookfit and than you for answering quickly and straight to the point with a simple easy to understand answer.
I sort of new the basic as per your explanation but did not know if this applied to all condensing boilers, irrespective of make.
To me then, this is just one more reason why condensing boilers should be avoided at all costs......if one can.....which is difficult because of the likes of John Prescott and useless EU laws which pander to the Carbon Emissions Save The planet (I respect the point of view though) Crowd.
Though a Boiler which is designed to keep the Hot Water tank full of Hot Water if one wants it or not flies in the face of that particular aim.
I mean, the Wife and I go on holidays during the Winter and leave the heating on a target temperature of 12 degrees. So invariably the heating comes on while we are away....and of course....the lovely 10 to 15 gallons of Hot Water the tank holds is continually being provided as well!
The same can happen overnight after leaving the Target Temp. of 12 Degrees.
This practice not only puts extra usage on the boiler but an extra strain on my pocket with the price of oil nowadays.