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Old 06-21-11, 01:08 PM   #6
Daox
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I redid my calculations on the tank sizing and decided that going with a larger tank will actually be worth it. The end result is the lower temperature increases collector output enough to be worth the reduced tank temperature.

Here is a quote from another post where I explained the calculation:

Quote:
Thats all quite true AC Hacker. It made me rethink things (as well as S-F's comments). So, I ran my own numbers on using a 400 gallon vs 650 gallon tank. There are a few effects to consider.

First off, this change lowers the tank temperature. The reduced tank temperature reduces the heat loss of the tank due to the water being cooler. However, the tank is also physically larger, so it has more surface area to loose heat from. In the end, the larger tank does loose more heat, but the effect is very minimal. All in all, the 650 gallon tank looses ~300 btu per day more than the 400 gallon tank. This is nothing when you consider even in December (worst month for solar), my average solar heat input is ~79,000 btu.

Second, the tank change also increases the collector efficiency. This is because cycling hot water through a collector makes it picks up less energy. The panel also stays cooler and looses less heat to the environment. This is where most of the benefit comes from.

Lastly, going to a larger tank supplys you with cooler water. In my case, going from 400 to 650 gallons will likely lower the average tank temperature about 10 degrees (from 115 to 105F), and lower the peak temperature from 140F to 120F. 105F is just barely warm enough to shower, so some additional energy may be needed for DHW. However, 105F is fine for hydronic heating. Considering that space heating is by far the larger load and energy hog, I should primarily design to its needs. In addition for 6 months of the year I'll be collecting more heat than will be used, so hot water won't be an issue then.

All in all, going from a 400 to 650 gallon tank will allow me to collect about 6.7% more heat from the panels. I just need to be aware that I'll be dealing wih a lower temperature water and design accordinly for it.
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