Legionnaires' disease with SHW system

27 Posts
Apr 15, 2012 06:14 pm
Legionnaires' disease with SHW system

I've set my 80 Gallon Richmond SHW tank's backup element to 115 degrees to save money since my system is usually crankin in the CO sun. But I just learned that having hot water tanks below 140 F is not a good idea because of this. I see 160 degrees from collectors coming into the tank so I'm wondering if this alone can flash kill the Legionella. Should I turn up the element on the tank to stop at 130 or 140 instead of the 115 where it's at right now? Only reason I'm worried is the electric backup element is located at the top of the tank vs the bottom and I'm wondering if the bottom temp is ideal for Legionella. Anyways, wondering if anyone has come across this question before. Thanks!....Ben

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47 Posts
Apr 16, 2012 11:04 am
Re: Legionnaires' disease with SHW system

The bacteria that causes Legionnaire's disease grows best between 70°F and 110°F. Generally, you want to keep your storage tank at about 120°F, so I'd suggest turning yours up just a bit more. "Research has shown that at 158°F legionella bacteria is killed in seconds, at 140°F over 90% of the bacteria is killed after two minutes, whilst at 122°F it would take two hours to achieve the same level of elimination."

Home Power Magazine addressed this question a few years back, saying there is no known case related to solar hot water. http://homepower.com/article/?file=HP127_pg12_ATE_4

Amy Beaudet
Solar Thermal Queen
AltE Store
462 Posts
Apr 17, 2012 11:10 am
Re: Legionnaires' disease with SHW system

Hey Ben, as Amy said, most pasteurization of water borne bacteria is a time -temperature relationship. Messing with temperature settings would be more of a safety concern to  me, unless you drink a lot of hot tap water.
  I really don't like having the electric back up in a solar tank since they tend to compete with each other. Normal setting for most any tank is 120-130 deg. so by time it gets to a faucet it is down to 110. Shower valves are to be set at this temperature too, so irregardless of incoming hot water temp. you can't burn yourself in the shower. 
   The thing with electric backup, if you use hot water at night, then the electric kicks in. Then in the morning, when sun is available, and depending where your sensor is located on the tank, the solar will stay off until it gets up to well over the already hot, tank water temperature. This adds lots of energy real quick and can raise the top of the tank temperature well over safety levels, This is great to keep the electricity off but you may notice your relief valve on the tank start to drip. 
    If your sensor is located near the bottom of the tank, and is set for a high limit of 160, then the solar will keep heating and possible bring top of tank temperatures near boiling. 
  If you are getting plenty of sun and use minimal hot water, I would just shut the electric off all together and locate the sensor properly on the tank and use your hot water wisely
. If you know you need hot water and it is cloudy, it is easy enough to turn on the electricity a few minutes before you need it.
  If pathogens are that much of a concern,  a UV light type device may be your answer.
99 Posts
Apr 17, 2012 11:32 am
Re: Legionnaires' disease with SHW system

I highly recommend inserting an on-demand hot water heater in-line with your solar.  Let solar charge your tank as efficiently as possible, then when it goes to your fixtures, the on-demand unit will check to see if it's hot enough, and if not, add more heat.  I have mine set to 167 degrees to kill all pathogens.  Before the fixtures, I have a mixing valve to bring all faucets down to 120.  I leave my dishwasher and pot-filler at full temp though.  By setting the on-demand temp relatively high and putting it on a mixing valve, you only need a very small unit -- just a one-bath rated one -- because you only need a little bit of 167-degree water to produce quite a bit more 120-degree water after the mixing valve.  Just make sure to buy a unit that can achieve high temperatures.  I'm using the Takagi TK-Jr, and it works beautifully.

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