Legionella

http://www.whtlimited.com/doc/lib/212/all-about-legionella.pdf

HSG 274 discusses low volume/low risk systems. As soon as you put a shower into the system you obviously increase the potential risk due to aerosol. As for 38°C - you are asking for problems. If you can guarantee (which you can't) that the water will be flushed daily, then you could argue that the system throughput would help prevent proliferation. The trouble is that people go on holiday etc, therefore units may not be used for weeks, at 38°C you'd have a lovely biofilm! With regards to cases of LD - 50% are source unknown, a once-through systems that has stagnated over a holiday then been flushed clear whilst infecting someone is a perfect example of why the source would be unknown.