Tank Stratification Issues

[OldStoneHouse]

I haven’t posted in ages but nice to be back!
My unpressurised storage is in an old cistern, lined with insulation and then an EPDM liner. This has worked well since 2013 when we put it in. We didn’t realise until this fall that there had been a lot of evaporation (maybe 75%, it was crazy) and since it was all sealed and difficult to access, I couldn’t tell. It now has a layer of wax on top that seems to have solved that issue nicely.

I had forgotten that my heat exchange coils (3/4 copper) don’t go all the way to the top of the tank. Now I’m really noticing this is an issue since it is full again. I’m seeing stratification of more than 50 degrees from top to bottom. The heat at the top seems to be trapped though as the coils don’t reach that far up. There’s no easy or safe way to move the coils higher which is why they were like that.

I’m wondering about a small pump that could circulate the water in the tank to move the hot water down and mix everything. Does that seem like a bad idea?

 

[eauzonedan]

Stratification is your friend to extend storage. Pull hot off top and return cold down low. Sounds strange but compare the average temp of what you would have if fully mixed vs higher temp that you could pull off the top - unmixed. Mixing IS a good thing to do when charging your storage however! My mixing pump is just sync’d with the blower motor and thus is active durning a burn. FYI - I see 40 degrees top to bottom of my 1500 gallon Garn tank before a burn and nothing after.... dan

 

[OldStoneHouse]

Thanks - that all makes sense but my issue is most of the supply pulls from about a third from the top of the tank. In time it may be possible to get the coils extended higher but right now it seems like I’ve got this layer at the top that has lots of energy in it but I’m not able to extract it.

 

[maple1]

Can you leave the ones that are there in place, and add more at the top?

As is you are kind of wasting 1/3 of your storage. And not sure how effective mixing would be. May help, if you can pull off the top of the tank and direct pump outlet right at the coils. But depending on the route the hotter water would take before hitting the coils, it could lose most of its heat before it hits them.

Sounds like an odd setup, hard to make suggestions without a clear understanding of it.

 

[ABMax24]

Not sure exactly how yours is set up, but would it be possible to add a almost like a dip tube but in reverse to allow the supply port to pull within inches of the top?

 

[OldStoneHouse]

I’m going to try and find a photo of it because that will make more sense I think!

 

[TCaldwell]

As a pita as it sounds, I’d raise the coils. Lowering the top temp sensor to the coil level will offer a accurate supply temp but might overtemp the liner above it, depending on what you charge it to.

 

[tom in maine]

As much of a pain as it is, you are best served by raising the coils. (Or just leaving the level so the heat exchanger is closer to the water level.)
The hottest water at the top will not be useful in a timely fashion otherwise.
The coils induce convection but tend to not disrupt the top layer. Active pumping the tank to mix it is okay until something breaks.

 

[OldStoneHouse]

I was thinking that adding to the coils might be the solution. I‘ll have a look and see how easy and when the best time to actually do that would be.

 

[Bad LP]

Sooner or later the upper temp in the tank comes down into your range.
My best results to date is a packed tank of heat top to bottom.
I’ve slowed my pump speed hoping to get better stratification in the tanks.

 

[tom in maine]

Adding coils that intersect with the uncovered water strata also make it work properly.
.
We did a ton of testing with a local electric utility years ago while developing an off peak electric water tank.
If the heat exchanger was not all the way to the top, there was significant stratification in the top level.
We could see a top temp of 160F and one foot down could be 50-100F cooler when extracting heat.
This is extremely pronounced with DHW systems. The cold water drops right to the bottom. It does not stir upwards.
Removing space heat is similar, with significant stratification, just not as extreme a delta T.

Heating a tank with a coil that spans the height of the tank is better since the hotter fluid rises and mixes the whole tank.
The whole tank will de-stratify to one temperature if left to heat long enough.
There is mixing at the top since the reverse is occurring from heat extraction.

Took a while to wrap my simple mind around this, but the simple physics is heat rises. Cold drops.