[Greenbuilding] Low powered (wattage) water heaters availablethrough Hydro Quebec

Michael O'Brien obrien at hevanet.com
Fri Dec 30 11:54:38 PST 2011

Hi, guys--

I've read some of the research on Legionella in hot water systems, and here's what I think I learned so far:

--Gas water heaters and boilers don't harbor Legionella at 120 F/49 C, because of the way they heat stored water from the bottom.

--Electric water heaters do sometimes harbor Legionella in the sediment that collects on the bottom of the tank, where the temperature is lower than in the water above.

--Stagnant water in distribution pipes, such as underfloor hydronic loops in summer, can harbor Legionella. Possibly a reason to separate domestic hot water from heating water, and to have a recirculating system for DHW.

--Raising the tank temperature from 120 to 145 F or 49 to 60C does not guarantee Legionella will be killed, especially in the distribution system. 

My own conclusion is that *only* raising tank temperature is not an effective strategy, especially in electric tanks; better to eliminate stagnant cool spots in the tank and distribution pipes. However, I'm certainly open to other views.


Mike O'Brien

On Dec 30, 2011, at 11:03 AM, Corwyn wrote:

> On 12/30/2011 1:35 PM, RT wrote:
>>> - Keep your water heater’s temperature at 49°C (120°F).
>> Probably not the best idea to save a few pennies, recalling a previous
>> discussion on this List where one Maritimer had the heebie-jeebies about
>> the high potential for legionella contamination even when the thermostat
>> is set for 60 degC
> http://wwwnc.cdc.gov/eid/article/10/3/pdfs/02-0707.pdf
> Thank You Kindly,
> Corwyn
> -- 
> Topher Belknap
> Green Fret Consulting
> Kermit didn't know the half of it...
> http://www.greenfret.com/
> topher at greenfret.com
> (207) 882-7652
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