Controlling legionella is pretty simple; 0.5 ppm chlorine added to the water - which most municipalities in the US already do, although Canada may be different - or maintaining hot water temperatures that are 125░ or above; either will not only prevent any active bacteria from multiplying, but will also kill off any new entrants into the system in relatively short order. This is one of the reasons that we set any heater we install to 125░; the other is that 140░ is hot enough to cause severe discomfort. Personally, I'd put in the conventional tank-style heater, set it at whatever temperature I wanted that was 125░ or above, and invest the significant amount of money that I'd saved from NOT buying a tankless heater into a nice tempering faucet.
Also: periodic maintenance on a water heater will greatly extend its life. Flush the tank once or twice a year, change the elements as needed and check the anode, and you can get a LOT of life out of a modestly-priced unit.
Well the the trigger's been pulled and a 67 litre (~17.7 US Gal, or 14.7 Imp Gal.) tank was ordered - (or back-ordered actually) electric as there is already service to the space in question. Gas was run in last winter so we could heat at least part of the house with a gas fireplace, but was ruled out for the back garage.
We're chlorinated here so I'll go with the 125F setting. The money saved not going tankless has already been spent if you talk to SWMBO. (drainage and water so I would need a water heater...)
Got the sleepers down (level floor, yay), the subfloor down and about to start the partition wall...
So much closer to having a darkroom again after two years of NOT.
I have a 40 gal gas-fired water heater in my garage...where the darkroom (or dimroom as some alt. process say) will be going in soon (hopefully!) If the water heater fails on me, I would consider building it a little insulated shed attached to the outside wall of the garage for the new one...get a little more floor space (also have the washer and drier in the garage) and no worries about leaks from the water heater. I use acetone in carbon printing, so not having a flame source in my dimroom would be nice. I could use straight grain alcohol (Everclear) instead of acetone...a little for the tissue, a little for me...But I think I'll keep to the acetone and just use a different room for it, or install sort of an exhaust hood that exhausts outside.
At least with LF landscape, a bad day of photography can still be a good day of exercise.