I've been scared out of solar hot-water by the threat of legionella and the complexity of a necessary heat exchanger in the working fluid w/ evaporation, antifreeze/corrosive, maintenance, etc... Won't a desuperheater have the same issues without a complex separate tank and heat exchanger arrangement? Will this expensive DSH option then result in even greater expense when the contractor decides that to 'be safe' a more complex (and expensive) system is needed? Is this whole legionella thing overstated? Is it more a function of stagnation than just hot water in a tank? If it's such a threat why aren't there more problems with conventional WHs, either gas or electric? I've got an LPG on demand WH now that works great but it seems pre-heating the water would result in great savings, using a DSH seems like a good way to do it...

Is this whole legionella thing overstated?

Usually yes, unless there are some high risk people present (for example, a hospital).

If using solar or a DSH, or a wood boiler, or any other system to preheat your DHW, and are concerned about legionella, make sure you have a system that will heat the water to 140 degrees once a day. There are solar tanks that have an electric element in them, or you can use the preheated water to feed an electric tank on a timer that heats once a day. Probably even 120 degrees F would probably do it since legionella seems to be a problem at around 80 degrees - if it is really a problem at all.
-Rosalinda

Say you have high risk residents or some type of system that is more prone to Legionella. Or you just want to avoid a small risk, even if it costs more. What can you do?

My guess is to design the hot water system to have minimize dead legs and then add a re-circulation pump plus returns and some type of biocide (UV, ozone, copper ions, etc). Other less effective options (piping can still harbor it) - keep the hot water tank at 160F and use a mixing valve near the shower or use a tankless heater.


http://www.osha.gov/dts/osta/otm/legionnaires/hotwater.html

Can you direct me to one case of legionella disease secondary to hot water tank running too low of a temperature? Just one reported case?!?

Gotta step in here. There are very few cases of legionella around and almost all of them deal with large building HVAC systems with cooling towers/humidification systems. It takes days of dead water, in contact with the legionella virus, to become a problem and that is rare. Then it must be transferred as an aerosol.

Every European solar controller allows the tank to rise in temp above 60C for an hour or two, every couple of days. This will kill anything but you must remember that every solar system has a back up and most of those back up tanks (or boiler) already heats to the needed temps anyway. You do NOT need to keep a tank at 160F, it is just a waste of energy, 130-135F will do.

There is a paranoia about propylene glycol and toxicity due to the use (years ago) of highly toxic ethylene glycol (car antifreeze) in solar systems. We don't use that anymore and the glycol we use is basically food grade (there are a few inhibitors but since you do not drink the stuff, you would never notice it even if there was a leak).

I would not be concerned about a SDHW system and a HP de-superheater should have an electric element in the back up tank to keep the temp above minimum anyway.

The solar systems I put in almost always heat to a minimum of 50C (120F) and I let the tank get up to 75C if there is enough sun.

It is actually a bacteria. All the reported cases are associated with chillers/cooling towers. I am not aware of a single case associated with hot water tanks.

http://researchnews.osu.edu/archive/legion.htm

But I think it's pretty clear that piping is the primary risk. 160F in the tank and then all the way to a mixing valve just before a shower covers almost all of that piping.

Always enjoy learning new things.  Learned something here.  Thanks!

Best regards,

Bill

Posted By jonr on 09 Jan 2013 10:21 AM
http://researchnews.osu.edu/archive/legion.htm

But I think it's pretty clear that piping is the primary risk. 160F in the tank and then all the way to a mixing valve just before a shower covers almost all of that piping.

Jon, what is that press release supposed to tell us?
"The researchers found Legionella pneumophilia in 6 percent of the homes studied -- a prevalence similar to that shown in previous home water surveys."
Meaning that 94% of the patients suffering from Legionella they could not find the bacteria in their homes or water systems, and that the frequency they found legionella in the homes was the same as in homes where people do not suffer from legionella infections!
The study appears to be skewed towards a rural population in Ohio:
"Although the Legionnaires' patients were more likely to have electric than gas hot-water heaters, the finding was confounded with water supply source -- that is, people with electric water heaters were also more likely to have non-municipal water in their homes. Therefore, we can't conclusively determine whether water heater type itself is associated with Legionnaires' disease risk."
It can all simply mean that people in rural settings are more likely to get legionella disease, and yes, they are more likely to be on electric heaters, and more likely to repair their old farmhouse's plumbing. But because of the plumbing repair, did they find more bacteria in the water? No.
Keep in mind that they also found an association to people working more. Does it mean that farmers have been exposed more to farm dust?
They also found an association for people spending night away from home before becoming ill. Where have they been?
Pretty much all the legionella outbreaks have been associated with nebulized air and A/C systems.

Legionella is a bacteria not a virus.It can be deadly to people with a weakened immune system.It is our job as professionals to protect our customers by using good designs and proper installations of plumbing and hot water systems.Municipalities treat the water but Legionella is dormant below 68F.The bacteria likes to grow in water temperatures from 68F to 122F.158F is the temperature that is used to disinfect water heaters,etc.from legionella.It is a common practice to heat water to 160F and then temper the water at the sink,shower etc. eliminating the risk .

It can be deadly to anyone, not just with a weakened immune system. One of the discussion is wether there is actually a risk and danger which is proven, likely or at least associated, or are we just following "common practice" without any scientific need or evidence for it.

Sorry, yes it s a bacteria, not a virus. 160F is a waste of energy. Millions of water heaters have run at 120F for years with no issues. Either keep the temps above 120F all the time or raise it every day or two to kill whatever possibly could be there. Either way, it is a small worry and the OP should get on with his life, and regardless, it is no reason not to put in a solar system if he wants one.

> Legionella pneumophilia and accounts for at least 1 to 5 percent of all adult cases of pneumonia

So say Legionella kills around 1300/year in the US which is about 1/2 of the deaths due to fire. I'm haven't seen any conclusive evidence that a large number of these are or aren't coming from home plumbing. But it's important in the choice of say a home run pex hot water system (lots of dead legs) or closed vs open loop radiant systems (where allowed, perhaps not in the US). Or the selection of hot water temperature in the tank or the piping. IMO, 145F in the tank and a mixing valve at each shower is probably a good risk/expense (about $15 to $75/year) trade off, more so if any residents are at high risk.

Our (Florida) ground water ranges from 65-85, depending upon latitude and time of year, often well above the legionalla dormancy temperature cited above, so I'm skeptical of the risk.

It is my (limited) understanding that mists (water and air mixed) pose the greatest risk, but even that seems a reach given the large number of decorative outdoor fountains. Based on what I have heard and read, I don't advocate operating domestic water systems at 145 or forgoing the economic benefits of solar thermal or DSH preheat or what have you.

As a side note, when I lowered my hot water tank to 120F, I got hydrogen sulfide problems (caused by bacteria) which, after much effort, I still have some of. So I would never go that low again.

To each his own.

Sure, you have sulfur and sulfur reducing bacteria in a well or in the ground, you get hydrogen sulfide issues. It is more an issue with your water quality than your tank temperature. It is not the growth of bacteria in your water tank but the contaminated water likely already contains hydrogen sulfide. It does not help that you raise the temperature temporarily to 160F. I assume, that is why you still have issues.

The cold water is fine, they are definitely growing in the tanks. I replaced the anode with aluminum, raised the temp to 160, used heavy chlorine. Only thing that worked permanently was aerating the water for a week. But they are still living in a small point of use hw tank (I need to aerate it too).

Are you on well water?