It is the total time in filling two averaged sized baths that matters. It
may spurt out 20 litres/min until half full then drop to 11 litres/min for
the rest for the fill still that is not a slow bath fill in total. In all
with 2/3 to 3/4 of the bath being cold water a 100 litre bath will fill to
half full in about 2 minutes the 4 minutes for the remain half giving
approx
a 6 minute bath fill filling two baths. Not bad at all.

Complete rubbish.

Assuming small 100 litre baths and that the store is designed to fill that
bath at 40C, then the total period required is the 5 minutes to fill the
first bath, plus the 11 lpm to fill the other bath, which is 9 minutes,
giving a total of 14 minutes. Your calculation breaks the law of
conservation of energy. The energy supplied to the baths must either be
stored or converted by the boiler during the period in question.

Your incorrect assumptions are
(1) Each bath would get 20lpm until half full.
- incorrect. Each bath would get 10lpm until half full.
(2) Each bath would subsequently get 11lpm until full
- incorrect. Each bath would get 5.5lpm until full.

Assuming that the bath is again a miserly 100l, this gives:
Until half full 5 minutes
Until full, a further 9 minutes.

Oh look, you get exactly the same time as my previous calculation using a
different method. It basically doesn't matter how you do the calculation, or
how you fill the baths, simultaneously or in sequence. It will still take 14
minutes to fill both simultaneously, unless you allow the store to fully
recover and don't count the time that recovery takes.

As the store is capable of providing an additional 9lpm for 5 minutes, the
boiler at full power will recover in just over 4 minutes. You could fill the
first bath in 5 minutes, wait 4 minutes for full recovery and then fill the
second bath in 5 minutes. Shock horror! That also takes 14 minutes!

What is not is dispute is that a boiler designed this way could be an
extremely useful product that would satisfy the needs of a large number of
domestic users, far more than an instantaneous combi of the same burner
power. However, fill 2 baths in 6 minutes? IT WILL NOT!

Christian.