Legionella

http://www.hse.gov.uk/pubns/priced/hsg274part2.pdf

From LinkedIn Legionella Risk Group

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.

Note on Biofilm

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

Biofilm forms when bacteria adhere to surfaces in aqueous environments and begin to secrete a slimy, glue-like substance that can anchor them to many materials such as metals and plastics. A biofilm can be formed by a single bacterial species, but more often biofilms consist of many species of bacteria as well as fungi, algae, protozoa, debris and corrosion prod ucts. Essentially, biofilm may form on any wetted surface exposed to bacteria. Biofilm develops where the temperature is right for growth and where there is a nutrient source. Nutrients can be scale, sediment, corrosion products, or trapped organic and inorganic molecules supplied by the flowing water.