heat transfer between storage tanks

scott markle_2

I have a two tank system and have some concerns about heat bleeding back from the boiler heated indirect to the solar tank.



The arrangement incorporates a stainless circ to mix the two tanks (differential controlled) this pump line runs from the hot outlet of the boiler tank to a mid- level taping on the solar tank, it has a check valve.



The Hot outlet of the solar tank feeds the cold inlet (perforated dip tube) on the 80 gallon

smart indirect heated by the boiler, this line has no check valve.



I was by the other day after it had been cloudy for a while and the top of the solar tank was around 90 degrees, which seemed surprisingly hot, is it possible that heat is being bled through the single pipe connecting the tanks? should I have a check valve here as well?



This got me thinking that should the check valve stick open it could seriously impact the amount of solar energy collected, it's not easy to detect this sort of thing, and I can imagine a situation like this could go unnoticed for years. Guess thats another reason why energy metering is a good idea, it provides some baselines that would identify a problem like this.

Fortunat

seems unlikely

unless there is a difference in elevation, it seems unlikely that heat would move back to the solar tank in this configuration, but I suppose it is possible. What diameter is the water piping? I don't know if it is true, but I've read that single pipe thermal siphoning really only happens in larger diameter pipe.



You could test it by running all the hot water out of the solar tank some day and the checking to see if that line heats back up.



I'd also have a careful look at the mixing valve plumbing as without check valves, they can sometimes allow hot water to siphon back into the cold line.



We have lots of systems set up in the way you describe and I've never witnessed any of them bleeding heat into the solar tank (though as you note, unless I happen to visit after a series of crummy days, I'm not sure I'd notice).



Just another reason to prefer dual coil tanks systems.



~Fortunat

www.revisionenergy.com

hot_rod

single pipe convection

is possible, but I have only seen it in vertical piping. A simple preventive feature would be a deep, 4 feet or more, thermal drop loop.



I have seen tanks in Europe where all the H&C taps enter from the side only. The upper hot outlet is into the tank about 12" from the top and has a bend upward inside. So it pulls the hot water out from the uppermost portion of the tank, but has a built in "thermal stop" or "gravity break" they sometimes call them.



Is the recirc intended to move solar hot water to the top of the boiler fired indirect?



hr

scott markle_2

flow direction

The tank mixing pump, is sucking from the hot outlet of the boiler powered indirect, pushing to a mid-level tapping on the solar tank, and returning to the boiler indirect via the top of the solar tank to the "cold" inlet on the boiler indirect. Not a lot of hours clocked on this pump yet but I have seen the 80 gal boiler tank 15 degrees above it's set-point in march, so I'm glad it's part of the setup.



Home owner called yesterday, the controller was flashing a red alarm, it was an error related to the flow sensor, by the time I got there it had reset and everything was running fine. I did get a bit of air out of the bleeder, I like that the control has these alarm features just hope I don't get more nuisance warnings.