SHW with dual-coil tanks [RE-wrenches]

[Karl Schwingel]

I've used single tank options when either the water use is very low or
sporadic. The only time I'd use a single tank system is when space is an
issue.  I have an 80 gallon single tank system at my house.  That's all we
had room for and there are only 2 of us here.    We turned off the element
in late april, and haven't had a shortage of hot water since, even with all
the rain (in WI)  enough storage for 2 days at least.

There is definitely a loss of efficiency, as the sides of the steel tank
conduct heat downwards, for the appropriate use, I'd have no problem with
using a single tank system, either boiler or electric.  But know that some
of the boiler tanks still have the aquastat lower than the top coil, which
is fine, but it's lower than the thermostat in a single tank electric, which
means that it'll keep a larger proportion of the tank hot with the boiler.  

One customer (96 sf panel, facing SSE) and 120 gallon 1 tank system hooked
to a munchkin contender, has a switch hooked to the aquastat in the
bathroom, and only turns it on when there's company, or in the winter.  It
enables an easy way to make sure that the heater only works when they're
running low on hot water.  The boiler's capacity means a short recharge
time. 

I like internal coils because they don't lime up with the extremely hard
water in part of our service area.  As the coils expand and contract, they
shed any lime that's built up. Only one pump, and high efficiency. 

For smaller system, internal coils make all the sense in the world.  Our
standard is an 80 gal SS with internal double wall HX from HT products.
Lifetime warranty.  Everything else will last for 20 + years, why put in a 7
year tank. 
 

Karl Schwingel
Nabcep Certified Solar Thermal Installer
NorthWind Renewable Energy
Karl at northwindrenewable.com
PO box 723 
Stevens Point Wi. 54481
C: 715 209 0446
F:7 15 952 4501

Solar thermal, solar PV and wind system design and installation
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-----Original Message-----
From: Nicholas Ponzio [mailto:nponzio at buildingenergyus.com] 
Sent: Friday, June 13, 2008 10:46 AM
To: RE-wrenches at topica.com
Subject: SHW with dual-coil tanks [RE-wrenches]

Solar Hot Water Wrenches,

I've noticed several manufacturers are offering dual-coil solar
storage tanks. The idea is that you can combine your backup boiler
storage and solar storage all in one tank. Has anyone adopted this
strategy? How does this effect system efficiency? I understand that
stratification in the tank will allow a certain volume of "cooler"
water in the bottom where the solar HEX is but does that mean your
usable storage volume is diminished? And how do these internal coils
compare to tube-in-shell HEX?

Thanks in advance for any insights.

-Nik Ponzio


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