SHW with dual-coil tanks [RE-wrenches]

[Brad Bassett]

Nik,

I installed a single tank system with dual coils a few years ago. I use 
mine slightly differently than it was made for, but with similar effect. 
The bottom coil is of course for the solar, the upper coil is for my 
radiant floor. I use a Rinnai tankless and circulator for backup heat to 
the top half of the tank using potable water from the tank (we have very 
good water around here). There are a couple of advantages to this, one 
is that only one tank is needed saving money and space. Two is that when 
no hot water is being used the solar will keep the backup heater from 
firing from standby heat loss. I think this is a larger energy savings 
than expected, and that most people do not consider enough. 

Having all of this in one tank does reduce the effective size of the 
storage of cold water for the solar to heat. In my case the bottom half 
is 40 gallons and I have a 40sqft collector. I'd consider this to be 
minimal for this collector. However, once the lower half of the tank is 
up to the backup temperature, the solar will then be heating the entire 
tank, this reduces the overheating compared to thinking of only the 
lower half of the tank being available to take all of the solar heat. 
Whether this is as efficient as a two tank system I could not say as I 
have only very basic data monitoring, and having the house heat involved 
would make for too many variables to do it easily anyway.

Having the house heat integrated into this was just for the convenience 
of having one system do it all. I find that the solar does not 
contribute noticeably to the house heat even though it theoretically 
could. By the time the solar is heating the upper half of the tank where 
the floor coils is, the sun coming through the windows has already 
heated the house.

I did not really feel the need to, but just to see if it would work, I 
installed a mixing valve on the solar loop at the exit from the heat 
exchanger which sends solar fluid to a loop in the crawl space to 
dissipate heat when the temperature is over the desired set point. This 
setup works like a charm. Mine is currently set at 150 F and it holds 
the temp extremely steady at that point. I think this could be 
effectively used to prevent overheating in a glycol PV pumped system, or 
other system types as well.

BTW, I do not necessarily recommend the Rinnai for this. It works fine 
but I'd like to see more efficient heaters being used, like condensing 
boilers or heaters. Then the upper coil would probably have to be used 
for the boiler fluid since most boilers are not rated to use potable 
water. 

Another product that looks like it may have great potential is the Heat 
Transfer Products Phoenix Solar, which is a submerged condensing boiler 
in the top half and solar coil in bottom half of a tank. Check out 
www.htproducts.com

I've found in-tank coils to be quite effective at transferring heat to 
the water for SDHW compared to tube in shell. Higher transfer rates used 
in heating systems might be better handled by a counterflow or tube in 
shell HX.

Brad
AEE Solar



Nicholas Ponzio wrote:
> 
> 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



Brad Bassett


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