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Re: Looking for Hot Water Heater Efficiency comparison - Page 6

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Posted by Solar Flaire on April 14, 2007, 2:51 am
...but not the mother-in-law. oooops...

Posted by nicksanspam on April 12, 2007, 5:27 pm

Perhaps we can use a vertical sidearm with lots of insulation and a heater
and a counterflow heat exchanger to pasteurize water in a tank-type heater:
cold water flows out of the tank bottom at 10 C into an inner pipe, a heater
warms it to 60C, it rises and cools to Tco and re-enters the top of the tank.
Meanwhilst, 45C water flows from the tank top into the space between inner
and outer pipes and back down into the tank bottom at Tho.

We could pasteurize 55 gallons per week with Cmin = 8.33x55/168 = 2.7
Btu/h-F. A 6'x1/2" inner pipe would have A = 0.785 ft^2. With U = 30
Btu/h-F-ft^2 and NTU = AU/Cmin = 8.7 and E = 0.897 = (45-Tho)/(45-10),
Tho = 13.6 C. The temp diff is 1.8x3.6 = 6.5 F, so we need a 6.5x2.7
= 17.5 Btu/h (5 watt) heater.

We need a few more details, eg the residence time and thermosyphoning
flow rate, which depends on the pipe diameter and the temp diff and
the water viscosity and expansion coefficient. Daestrom can do those
calcs in his sleep :-)


Posted by Steve Spence on April 11, 2007, 11:38 pm
 140C? What are you smoking?

Steve Spence
Director, Green-Trust

Posted by dold on April 12, 2007, 7:00 am
I searched quite a bit, and asked a couple of plumbers and construction
guys in this area.  No laws on the books, or code requirements concerning
the water temperature in a private home in California.  140F and then
downmix to a non-scalding temperature (110F?, I forget), and the VA health
care facilities were finding Legionella at the flat, wide faucets that are
intended to prevent scalding and splashing, but cause the laminar water
flow that allows Legionella to hide and colonize.

Clarence A Dold - Hidden Valley Lake, CA, USA  GPS: 38.8,-122.5

Posted by Solar Flaire on April 12, 2007, 11:26 am
 Some still recommend a hot water flush periodically to cleanout tap
storage spots, especially water flow restrictors. That kind of
nullifies the usage of the temperature down mixers.

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