# Water Mass



## Virgil (Sep 29, 2009)

Does anyone temporarily reduced the amount of water in the primary storage tank to compensate for the seasonal drop in solar energy radiation during the winter months..????


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## biggkidd (Aug 16, 2012)

In relation to what? What is the water stored for?

Larry


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## Virgil (Sep 29, 2009)

Solar heated thermal water storage tanks....such as used in recirculating fresh water drain back collector systems.... where the BTUs gained are used to increase the temperature of water to domestic hot water system via a coiled heat exchanger and/or via piping to concrete floor thermal mass or wall mounted register pipes for the purpose of providing heat to the interior environment of a structure....


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## biggkidd (Aug 16, 2012)

Oh ok.

Sorry I have no idea about that.

Larry


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## Harry Chickpea (Dec 19, 2008)

Don't have that type of tempering system, but it would seem to make sense. (During winter months I try to fill the cistern on the warmest days to take advantage of any heat I can.) Have you tried it?


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## Virgil (Sep 29, 2009)

No problem Biggkidd,,, I should have been more specific with my original question.... 

Currently we are in the planning stages of a larger building project which combines both passive and active solar type systems.... We have test ran a much smaller thermal drain back system for about four years now... Recently I have attempted to extrapolate the data collected on a daily basis from this small system to that of the proposed larger one... Running various ,what if, operating conditions and parameters through an Excel program suggests that reducing the amount of water thermal mass during times of seasonal low angle solar radiation would help boost the useable water temperatures.... These are very ballpark figures.... Was just wondering if any one else on this network had came across this possibility.... or had done that and if so,, what was the results... good, bad, or ugly....????


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## biggkidd (Aug 16, 2012)

Virgil said:


> No problem Biggkidd,,, I should have been more specific with my original question....
> 
> Currently we are in the planning stages of a larger building project which combines both passive and active solar type systems.... We have test ran a much smaller thermal drain back system for about four years now... Recently I have attempted to extrapolate the data collected on a daily basis from this small system to that of the proposed larger one... Running various ,what if, operating conditions and parameters through an Excel program suggestions that reducing the amount of water thermal mass during times of seasonal low angle solar radiation would help boost the useable water temperatures.... These are very ballpark figures.... Was just wondering if any one else on this network had came across this possibility.... or had done that and if so,, what was the results... good, bad, or ugly....????


 Thats cool. Please let us know what you find out.

Larry


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## SolarGary (Sep 8, 2005)

Hi,
I've thought about doing that on my system, but while the tank tends to run at a lower temperature in the winter we do occasionally get a few really sunny days when its not to cold, and the tank temp comes up enough that if I had less water in it it would be short on mass. This happens mostly in the fall.

The collectors will of course be more efficient if the tank does not get so hot, so as long as you are able to make use of the stored heat at its somewhat lower winter temperature, the system will be more efficient and generate more total heat. It depends on how low a temperature your heat distribution system can effectively make use of.

Gary


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## Virgil (Sep 29, 2009)

Yes,,, it is a balancing act Gary... I tried to introduce as many variables as practical into the Excel modeling program...though, I am sure there are addition interaction unknowns... A key assumption I may was that all heat/energy transfers are linear functions... a very very simple analysis... For the cold weather months the two major variables which could be manipulated would be, of course, the amount of water weight in the tank and the programmed maximum storage tank temperature... Above the maximum tank temperature the excess BTUs would be pumped into the concrete radiant floor heating system.... while maintaining the target temperature within the tank lets say at 120f for domestic hot water use... As I make my own stand alone microprocessor control boards such devices are programmable to fit that need..... I currently use the Parallax microprocessors though, there are several other development processors that are available... Not to waste fresh water the down side that I see at doing this would be the need of an additional storage tank at about half the size of the primary tank... This tank would be used as a temporary holding tank to either pump down or add water to the primary tank as the seasons changed.... A simple submersible sump pump could be used on occasions to transfer the water between the two tanks... However, more space would also be needed for the additional tank... We live in a very poor solar gain area....and for the winter months the modeling program showed a noticeable impact on temperature gain by reducing the amount of water mass from that which is needed in the warm months to reduce the likelihood of tank over heating.... However... this is only projected numbers on paper.... So...Any body tried this..??? All comments are welcomed....


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## MichaelK! (Oct 22, 2010)

For my own water storage tanks (not for heating), I keep them as full as possible in the winter (5000 gallons) so they have enough thermal mass that they won't freeze. The previous owner specifically warned me about this when we bought our property. Leaving the tank level low would allow the smaller amount of water to freeze and damage the tank.

The tanks have been there since 1981, so I follow his advice.


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## fordy (Sep 13, 2003)

...............I was always under the assumption that a large mass , partially buried , and insulated form the extreme variations in outside temps would maintain a stable temperature and therefore remain exempt from the large fluctuations in outside weather conditions . , fordy


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## Virgil (Sep 29, 2009)

MichaelK! said:


> For my own water storage tanks (not for heating), I keep them as full as possible in the winter (5000 gallons) so they have enough thermal mass that they won't freeze. The previous owner
> 
> It appears that your tanks are exposed to the weather elements... and would be absolutely right for that situation... Maintaining a large static thermal mass such as thousands of gallons of water retains lots and lots of btus which reduces the likelihood of freezing...
> 
> In the question that I am addressing is in a system which is very active.... and is much smaller in water thermal mass... A volume of water in the 300 to 500 gallons range... At 450 gallons the thermal mass weight would be approximately 3870 pounds... As opposed to 5000 gallons which would weigh in at about.... 430,000 pounds... nearly a 11 times ratio difference... The other big difference is that the smaller active solar system is expending the gained solar btus daily into the structure for use as domestic hot water and heating... Energy which needs to be reclaimed again on the next solar radiation day.... However, for the northern latitudes the amount of available solar radiation is greatly diminished by the lower sun angles and increased cloud cover, (Northeast), during the colder winter months.... So perhaps,,, to compensate for these btu losses decreasing the amount of water volume in the tank would require less btus to help maintain the water temperature at a useable amount.... A comparison would be that circulating 50 degree F water from a 1000 gallon tank, which is heated by solar collectors, through baseboard registers will do little good at warming up a house to 70 degrees F... Whereas, if the solar collectors could provide enough btus to heat up that same tank of water but, at half that volume to 100 degrees F it would seem that positive heat transfer would occur in the living environment... at least for awhile... The total amount of btus would also be reduced... Any comments ??


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## Virgil (Sep 29, 2009)

fordy said:


> ...............I was always under the assumption that a large mass , partially buried , and insulated form the extreme variations in outside temps would maintain a stable temperature and therefore remain exempt from the large fluctuations in outside weather conditions fordy


Yes I agree,,,, The larger the thermal mass the longer it takes for that thermal mass to loose or gain energy/btus....


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