Stratification - when it doesn't matter

[jebatty]

Several prior posts of mine have demonstrated the extreme stratification in a 1000 gallon horizontal storage tank (19' x 3'). In my application, it seems that stratification only matters when charging the tank, because that's the only time it is important to keep the delta-T as high as possible for as long as possible to insure the most efficient operation of the boiler by keeping it at full burn and without idle cycling. This seems to be best accomplished by maintaining as low a flow as possible which still keeps the boiler from idling. My system accomplishes this goal.

But stratification during the draw down is another matter. My only hot water draw is heating a 1500 sq ft concrete radiant floor. Others may have a similar situation. I am easily able to heat the storage to 185-190F, top to bottom. The mixing valve is set to deliver hot water to the floor at 100-105F, and heat from storage still functions well with a draw down to as low as 85F, the lowest I've gone so far, although this winter my normal boiler firing occurs when storage drops to about 100F, and firing is occurring every 2-3 days. In this scenario, there is little benefit to maintaining stratification during the draw down.

I've noticed that during draw down considerable mixing occurs in the tank. Between draw downs the tank quite quickly re-stratifies with about a 10F temperature difference between the top and bottom, with only the very bottom 6" or so having the low temperature and everything above being very close to the high temperature. The tank is quite well insulated.

I have been logging boiler/storage performance, as well as floor, inside, and outside temperatures, and wood consumption by weight, nearly continuously since the beginning of November. This will present the best data picture yet which I have of real world performance in a cold climate.

 

[in hot water]

excellent observation. I've noticed the same when I pull back out of my 500 gallon tank into the separator tank. If my 500 falls to lets say 140 during the night, I want that sep tank to be blended 140F top to bottom not stratified with a 140 top 110 or lower bottom. So there are times when a blended not stratified tank works better. The same goes for the 500 storage. I would like it to be entirely 140F in the morning, not cold at the bottom and warm at the top, when the boiler is unfired.

hr

 

[Hunderliggur]

Of course it all depends on your application. If you need high temps (legacy radiators) statafication is good. If you can use low temp radiant it is not as necessary. I have run my tank down to 105F. The tank tends to mix when drawing from my in tank HX but that is fine. Higher return temp to the boiler is fine also.

 

[sweetheat]

my experience with an 806 gallon open storage tank has been stratification temps are about 20 degrees apart at each probe level. It stays that way during the draw down too. When I hit my set point of 185F the mid level and bottom level are around 20 to 25 degrees F lower. My only draw down is the radiant slab also. It is set to use 100 degree water temps but still works fine at lower temps. My wood consumption has been extremely efficient. I have not weighed it but by knowing what was stacked and used per season I've got a pretty good idea how much wood I've been burning each winter. Last winter was about 2 cords, the first winter about 3 cords. This winter we'll see. Sweetheat

 

[sdrobertson]

Another reason to make your system use the lowest possible water temp to heat your house. If you have two tanks, with the same amount of "usable" btu's, the one tank that was mixed would have a lower top temp, which I would think would mean that it would lose less heat to stand-by losses than the tank with extremely high temps at the top and really stratified. Does this make sense?

 

[woodsmaster]

I think that's fine if your only heating a slab, but for making dhw it seems to me you would want the hotter water stratifacation will produce over an extended period. When I desighned my system I thought I
was going to use baseboard in the house so I stood the tanks verticle. Then I found a great deal on some panel rads and went with those. Hoefully it will allow me to go longe inbetween fires and
use it as a dump zone in a power outage.

 

[in hot water]

with radiant in slab I can run my tank down to around 100F. But at the very end of the draw down I wish the entire tank was 110 to store more useable BTUs. If the top layer is 110F and the lower 2/3 below 80- 90F it doesn't have as much to offer. I think that is the point Jim is making.

hr

 

[Singed Eyebrows]

I can maintain good stratification with the Laddomat into the 720 gallon pressure tank. The problem as Jim mentions is my boiler will go into idle as it overruns the Laddomat pump. I either need to speed it up somehow or go to the next size up. If I pump to my heating loop out of the tank I get some mixing, I thermosiphen though & get no mixing at all. As I'm taking hot water off the top of the tank, stratification matters at all times for my setup, Randy

 

[in hot water]

As the tanks heat up, the heat exchange slows down. Hot goes to cold, always. And the rate of heat transfer it tied to the delta T. So with a cold tank you notice the temperature climb rapidly. As the tank hits 160F or so the rate of heat exchange slows way down. I've tried high speed on the pumps even larger GPM circs, the laws of thermodynamics can't be fooled. So as the tank gets up there in temperature the boiler will go in to idle more and more frequently. Loading the boiler with a lighter load will help minimize the idle time.

hr