After having secondary circulation in my own dwelling for many years I can give some anecdotal evidence on performance, which repeat some of the points already made:
1. Bronze circulators are expensive and last about five years, due to soft water area.
2. Bronze circulators run on electricity further increasing expense. Noise and rattle, especially on mega-flow systems can be a problem.
3. Insulation needs to installed to a very high standard (the 9mm I fitted on mine 20 years ago is now costing me.
4. Brass Towel rails may be incorporated within the loop which gives summer warmth to towels, and heat in spring and autumn when heating is off.
5. Saves water, but questionable savings on energy for domestic systems.
6. Reduces risk of legionella but questionable on domestic systems.
7. Risk of scalding at tap, because heat is there instantly, so increase need for blending valves in public sector, and caution in domestic dwellings.
8. Increased recovery time for cylinder when drawing say one shower (whole cylinder dilutes to say 40 degrees c, needing whole volume to be recovered, because circulator running). This can be a real pain in mornings when hot water is needed.
In all it is a luxery to have secondary circulation, and I would say it costs me to heat the towel rail in my en-suite as much as the heating costs to run. So I consider it a technology of yesteryear for existing domestic dwellings (new build with solar options may be different).
Instead I would recommend point of use heaters where possible such as undersink displacement heaters with flow restrictors in the hot tap. They use this in Holland and have done for the last three decades, rather than having long dead legs for hot water. These also carry far less Legionella risk if you are specifying for public sector applications. In addition temperature can be more accurately set.
