Demand heater for SHW backup [RE-wrenches]

Todd Cory, Mt. Shasta Energy Services toddcory at finestplanet.com
Wed Aug 22 12:37:03 PDT 2007


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Right, an electric water heater on an off grid system is not practical. 
Off grid folks are limited to solar with propane / wood backup. The 
standby loss on a propane tank heater mandate an on demand unit.

If anodes do not work, then why are they put in the tanks and why to the 
"extended life" tanks have two of them? Our tanked heater is now over 25 
years old. I change the anode every other year. I expect it to last 
indefinitely, which is great because I hate plumbing. Perhaps the 
calcium build up issue is related to high mineral content water, which 
will also cause scaling in a tankless heater's heat exchanger.

While I do not do SDHW installations anymore, I get inquiries about this 
all the time from my off grid solar electric customers. Most of them do 
not have SDHW systems, so go with propane demand units as they have 
little other options. I am mostly commenting on the on grid folks that 
erroneously think they are going to "save so much energy" by changing 
out their electric tank heater with a demand use one... or adding a time 
clock (sigh) to their existing tank heater.

There are sure lots of myths out there about water heaters.

Todd




Matt Tritt wrote:

>
> Todd,
>
> If you take into account the rapid buildup of calcium carbonate on 
> electrical water heater anodes, and the resulting loss of efficiency, 
> how do you figure that electric is the way to go? In my experience, 
> people rarely (if ever) service their water heaters, and that 
> especially includes the heater elements. Also, the huge hit from 
> electric on an off-grid system seems counter intuitive for folks with 
> smaller battery banks, no?


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