How to tie together two non-pressurized tanks

All good plans of mice and men......

While laying out my pipes, I decided to measure up the area I'm going to put my non-pressurized heat storage tank. I'm building out of wood and using a liquid paint-on rubber for waterproofing.

I realized, I don't really have the room for an 8 x 4 tank without it getting in the way. I can, however, put in 2 4 x 4 tanks right around the corner from each other. I'm already planning on using some brass thru-hulls to tie the heat exchanger into my system, and I was thinking I could put a pipe between the tanks to keep the water level equal between the two, and pump from one tank, through the heat exchanger, and into the other tank. Since water likes to "find it's own level" so to speak, the second pipe would act as a return between the tanks.

The problem I'm finding is that I think I'd end up with a HUGE temperature difference between the two tanks. Any ideas on how I could get the tanks to equalize in temperature? The only thing I can think would be to use a "T" before and after the pump/exchanger. Similar to this drawing. Does this seem like it makes sense to people, and do you think it would stay relatively equal in temperature? (sorry for the crummy drawing, only have paint at home)

bmblank said:

I think the problem would be unknowingly creating more resistance in one loop than the other. One tank would get more flow than the other and therefore be warmer than the other. That being said it would probably be one of the closest ways to reach your target of equal temps. Can i ask though, why would it be so bad to have differing temps? You could place any coils in the hotter of the two tanks. Or, have two coils in series, so then the cold goes into the cooler tank and gets pre heated for the warmer tank. I'm just thinking out loud, as many people here target stratification in their tanks.

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I'm going to be using an active (plate) heat exchanger, so I'll be circulating water through it and returning it back to the tank. Stratification is off the table for me, but no worry...I'm using old cast iron radiators so I can get benefit from low temps in the tank.

I'm thinking of equilizing temperatures using valves....its a rather manual solution...but once I find the sweet spot for valve position uneven heating should be OK.

Nofossil said:

Note that for the cost of an extra circulator you could achieve stratification - just connect the two ciculators nose-to-nose. You can pump backwards through an inactive circulator - doesn't hurt a thing. Just make sure you don't have check valves....

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Some stratification would be possible, but with actively moving water mixing in the tanks, I don't think I'm going to be able to do much with stratification. I have big old cast iron radiators, so I'm not too worried about stratification since I can use water down to a lower temperature.