Secondary return on the hot water?

[John Rumm]

Higher install cost, and you are shifting your hot water heating requirements from cheap centrally heated water, to that done with peak rate electricity in most cases.

[Fredxx]

Agreed, but it depends on what pipe length and corresponding heatloss, together with a pump running for most of the day, compared to actual water usage.

[YAPH]

According to sources cited in the Wikipedia article

tends to die above 50 - 55C, and absolutely dies at 70C. But maybe harry knows something the World Health Organisation, Chartered Institute of Plumbing ad Heating Engineering and Health and Safety Executive don't?

[John Rumm]

Indeed - as with all these things, the devil is in the detail, and there is no substitute for actually doing the numbers to see what really makes sense.