2-1/2" DHW return loop piping? Minimum velocity requirements?

Thanks for the feed back. The city water system is chlorinated and there's frequent water use. I know that doesn't eliminate risk, but many millions of water heaters are in this same condition daily and the cases aren't there. I find a lot of public hot water systems operating at 125-127F. Not my design! It's hard to know what to do, where liability starts and stops. I've never seen any domestic water line look like that scaled pipe in these parts, municipal water system or well.

There's also a high velocity problem. This existing series 2-1/2" B&G is likely on the curve above 35gpm continuously. The water inlets for the WH tanks are 1-1/4 balanced. The 2-1/2" cold and the return water share them. They see 17 or so GPM each from recirc. With 60 rooms at peak use, those cold inlet pipes must have very high velocity. It might be causing the recirc. head pressure to spike at those times, pushing the pump off the curve. Now consider, at one point there was a second functional 2-1/2" B&G with a very low head loop periodically competing for flow on these same two 1-1/4" cold tank drops. What a mess. It's troublesome to see the faults. The manager and lead maintenance are both new so they have no history or understanding of the building or water systems at all. I've found if you try to inform them and their higher up's of the situation, they will promptly start looking for someone without all the complexities and strange things to consider.

Bear traps, landmines, piles and pits. Maybe I should just rebuild the old pump and walk.

I have written out an explanation to the manager detailing the concerns, including my duty to inform and imperative to avoid liability. Fixing the B&G doesn't achieve high velocity or eliminate the risk from sub 140F water. It does keep it moving. Right now, there is a stagnant recirc. pipe and it is in the peak temperature danger zone. It is very short, the last room on the loop is less than 20' feet away. I'm recommending to repair existing pump and for the owners to engage with the problems. @Larry Weingarten I will recommend, in writing, that they seek expert Legionella advice . I don't think there's much else I can rationally do. I don't want to be involved in making it worse, and I'm not going to become a crusader and stop the line. These are the owners' problems.

@mattmia2 I will mention periodic high flow as an option, but I think letting an expert spec a solution is best for me. The potential for being the cause of much money spent and someone still gets sick is a trap I want to avoid. One idea I had is to reconnect the 2-1/2" low head hot water loop to be able to hit high velocity and injecting mixed down 140F from the tanks into the loop with close tees to maintain the temp. Then both velocity and loop temp could be easily periodically raised if a safe window for that could be created.

I know there's good reason for the ASHRAE guidance. I also know there is a vast disconnect between best practices and real world happenings. It would seem the majority of today's hot water systems are on the verge of killing hundreds of millions this morning, but they don't. I understand there are some cases, and they are often from very specific conditions that are rarely related to domestic water heaters and piping. Spa's! Lawyers would build an empire to profit from the damages due to hen's teeth if they could. That's not to say there's no harm potential from Legionella, it is not hen's teeth or a very common killer.

Thanks for all the guidance. I'm lucky to be able to consult with the wise here.

@hot_rod in a book I just read, The Arrogant Ape, a biologist calls the human a bacterial life support system invention. Each of us has about three pounds of gray matter and three pound of non-human DNA life on broad at all times. The bacteria in and on us out numbers our own cell count by a lot. Bacteria had already done much of the inventing in life systems long before we were around. The Mitochondria structure is a bacteria symbiotic invention. The long history of life, in a way, is really their show. We just recently grasped CRISPR-Cas9 the very old bacterial invented gene editing tool.

Nerdy Harvard professor book, but It's really not that off beat. "The Arrogant Ape is #2 for non-fiction books New York Times readers say they're most eager to dive into this season." I would not have been accepted into Harvard if I had tried and could not have afforded it, but I can read the professor's books. The title is the same as a course offered there for a while. There are few books I regret reading, even ones I didn't really like.