Yup, that is what I meant by "every half hour indefinitely, or faster for a limited time" ;-)
Although looking again at the figures - you probably can't draw that much energy out of the store without it impacting the final water temp a little toward the end...
that 33.5MJ would represent a 26.5 degree store temperature drop without any replenishment. With 20kW going back in that would be a 22 degree drop. So the last bit of the bath fill would start to cool very slightly. If you were sticking 35kW back in then you would only see a 16 degree drop in the store and output temp would be unaffected.
Well if you allow a couple of degrees from loss on the PHE, then that
18 degree difference represents (so 300 * 4200 * 18 = 22.7MJ) of energy. Once you have drawn that and without any replenishment then the output temp from the PHE will start to fall below 60. That equates to about 100l of drawn water.Not quite - you can't usefully get all of the energy out of the store unless you can accept a lower final water temperature. If you say that the minimum useful temperature of the store water is 50 degrees (to allow draw off at say 48 deg), and the maximum store temp is 80, then you have 30 x 4200 x 300 = 37.8 MJ for water heating with a 300l store.
With ground water at 5 degs (i.e. worst case in the winter) and a bath at 45 final temperature, that gives you no more than 300 / 40 x 30 = 225 l of hot water without replenishment. So with the boiler running, you can probably just squeeze two deep baths in quick succession out of the store. You would also need to adjust taps from time to time to allow for the falling hot water temperature.