solar storage, 2 tanks in parallel or series?

peacefulsoular

Another project that I am going to work on will have a 10 gal drain back tank and an external heat exchanger. A double pumped system.  I am going to have two 120 gallon storage tanks (water heater tanks), and then the water will enter the existing gas condensing 120 gallon water heater. 

I have seen plumbing diagrams on solarhotusa.com website showing storage tanks plumbed in series.  Basically, it maintains one storage tank hotter than the other.

The plumbing is such that the output from the heat exchanger puts the hottest water in the top of the second storage tank, and then exits from the bottom (diptube) and into the top of the first storage tank, and then circulates from the bottom of that tank and into the heat exchanger.  I like the way it maintains the second storage tank hotter than the first.  Cold incoming water enters the first storage tank. and the coldest water goes to the heat exchanger.



My question is: I like this idea, but many companies (such as heliodyne suggest installing 2 or more tanks in a reverse return configuration.  So that both tanks are in parallel,  the water flows equally through both.  The solar thermal book also suggests this plumbing arrangement.

I am looking forward to your experienced comments. Gordan

hot_rod

Two tank

So one tank for DHW and the second for heating? I would use a 3 way zone valve to chose which tank loads. Take the DHW to 120 or so, then switch to the second tank.



The new solar controls can select between the tanks, or load the tanks together.



Loading the tanks together takes tank 1 to 100F or so, then takes tank 2 to the same temperature and goes back to tank 1. This loading allows the collectors to operate at max efficiency, instead or trying to load the dhw to 140F first.



Post a drawing of your piping idea, it's easier sometimes to see instead of a description.



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