[Greenbuilding] Opinions on electric tankless HW heaters?

[Paul Eldridge]

Hi Bob,

Given that your electric water heater is a fairly new and therefore 
energy efficient model and given that you've also added an insulating 
blanket, your standby losses in continuous duty are likely to be less 
than 20 kWh per month.  Turning off the power to your tank won't 
appreciably reduce its standby losses unless the water it holds is 
allowed to cool considerably (it's going to continue to dissipate heat 
whether the elements are energized or not, just at a slightly slower 
rate as water temperatures are allowed to fall).  My guess is that these 
additional savings are no more than 3 to 5 kWh per month.

However, as a direct consequence of this practice, your water heater is 
almost certainly heavily contaminated with Legionella  bacteria which 
poses a serious health risk to anyone with a compromised immune system, 
i.e., those living with HIV/AIDS, anyone undergoing cancer treatments 
and taking anti-rejection drugs, the elderly and infirm, etc.  You have 
to weigh these risks for yourself, obviously, but for the little extra 
energy it might cost, I would encourage you to keep your tank at its 
highest setting (mine is set at 72C) and to install an anti-scalding 
mixing valve; for an extra 50 or so cents a month, it's well worth the 
peace of mind.

For more information, see: 
http://www.hydroquebec.com/advice_hot_water/index.html.

For a more detailed discussion, see: 
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC183576/pdf/aem00061-0260.pdf

Cheers,
Paul

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Probably we were using that much.  At that time we our household had
increased temporarily by the addition of 2 teenagers, 2 adults, and 2
smaller kids, all of whom seemed to like long showers.  IIRC, one of my
reasons for turning the heater on and leaving it on was to see what the bill
would be like for a more conventional household (the additional folks living
with us had not been habituated to our lower level of energy consumption).
So maybe its not completely apples to apples with our present lifestyle 5
instead of 8, with all 5 (one of whom is age 5 going on 15 today) having
become habituated over time to being frugal with energy.

We already have a blanket around the heater.  I've never been able to test
how long it takes to cool to the ambient temperature by itself without
withdrawing hot water and adding cold water.

Bob Waldrop, Okie City

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