Heat accumulator and Legionellas?
 

Greetings! I would like to preheat water with sun or attic heat before letting it into my electric heater. I imagine an additional tank with heat exchanger could easily do that. But there is the question-room temperature water is a great place for Legionella bacteriia to grow. If I preheat water with my additional tank, that will be a home for bacteria. Would you see that a concern or its just me exaggerating?

If it's city water, it will have chlorine added specifically to prevent that problem.

Chlorine does not kill all legionella.

If your electric hot water heater is set to 140F or higher you will be fine.

FYI: the optimal reproduction temperature for legionella bacteria is about 110 to 115 F.

That could be a problem.. https://en.wikipedia.org/wiki/Legion...onella_control

However, if the temperature is below freezing, your outdoor or attic Collector array can be frozen. That can cause damage and leaking. So, you need to use one of the standard methods to prevent that damage.

Use a drain-down system with a smart controller, or use a closed loop system, that uses water with non-toxic antifreeze as a transfer medium between collectors and the indoor (Double-Wall) heat exchanger that heats your storage tank.
The HX must be double wall to prevent any antifreeze contaminating the potable hot water.

Legionella is definitely a concern but I would assume if your water heater is hot enough like mentioned you should be Ok but I worry if water flow is high enough at times that some could get through as to not being heated long enough.

Chlorine doesn't stop it and many citys in the US have stopped using it and now use other methods. We have issues here outside of Fort Worth with legionella in the summer time as the incoming water from the city will get up to 80* in the summer. This is a really good reason not to use those water misters here as they are a perfect delivery system.
We really need a water softener here as out hardness is way up there but since it would be in the garage and already being filled with 80* water and then sitting in the garage it will heat up even more... Not to mention the city water often stinks like the lake. It's safe but just nasty water and we don't drink it. My R/O has a house filter for a prefilter and plugs up in 2 months with crap from the old 1.5" water main.

Nowadays, it's common to use chloramine (chlorine + ammonia) rather than chlorine alone.

Old person posting while half asleep..?.
 

With a solar hot water system, you can only control the maximum temp.
The minimum temperature is controlled by the weather and the sun.
If you have heavy snow covering your panels or it's heavily overcast all week.
Your storage tank is going to well below 140F.
When I used a solar hot water system, it was only used as a pre-heater.
It warmed up the water before it went to the real hot water heater (oil burner).

The last solar hot water system that I used had an air source heat pump as the main heat source.
It was solar PV assisted by 600 watts of solar panels connected to heating elements in the storage tank..
(600Wh = 2047 BTUh) It worked pretty well, until the tank started leaking.
It was 800 watts of PV, but a branch fell on the array and killed one of the panels.
One little crack in the glass was all it took. Hope it never hails here!

I'm back to burning oil for hot water again. No more solar and hopefully no more water leaks in the basement.

Cheers,
Rich

I occasionally throw a gallon of bleach in the tank and call it good enough. Both of my hot water systems I've built have a pex coil type heat exchanger and I don't think the water going through the HX is in it long enough to grow anything. Open to better chemical suggestions!

if you're using a preheat tank with an HX, use the tank water as part of your thermal system, and the HX for potable water, instead of the other way around. that way you don't have 40 gallons of warm-ish water sitting there. it will likely have a small efficiency penalty, but if set up correctly that can be minimized (obviously doesn't work with an internal HX at the bottom only).