Hi Troepadoer
The heat exchanger , must be bought , mine is a 6 plate SS/copper , rather small , because I don't have too much space .
But I believe it should be enough.
After all we just have to heat up the water From 15 C to 37C approx.
If that will take 15 min.instead of 10 , I am still happy

.
But change the HE , is a gift from a friend of mine that works in the house heaters industry.
I bet you can find them any were .
They should not be too expensive either .
Will advise when we come to South Africa .
Where are you based in ZA ?