why stratify during the burn?

[fabguy01]

reading all the post on this forum would lead one to think that stratification is a good thing? Conventional thought would tell me that getting the water mixed as well as possible during the burn to get entire system up to 180-190* and then let it stratify out and draw only off the top of stratified storage? maybe run a anti-stratification circulator during the burn then turn it off after it is complete? this would allow you to store more heat.

 

[kabbott]

I struggle with this sometimes myself. My thoughts: If you are going to heat the entire tank to whatever is the max temp then it really does not matter.
In fact it may be better if it is mixed. However in my case I almost never max the tank and if it's mixed it may be low enough that it won't heat properly.

As far as the water separating after it's mixed I ain't happenin in my tank. Does anybody see this happen. I think once it's mixed your stuck with what you got.
Anyway with pressurized storage and the ability to heat it higher I see this as less of an issue.

 

[twofer]

To me stratification during the burn is necessary because using the lowest possible return water temps leads to a higher BTU extraction from the boiler. As soon as the return temp to my boiler gets up to about 178 degrees it will begin to short cycle if the fire is still raging. If the whole tank were mixed then this would happen even sooner and I would have 500 gallons of water to pump through the boiler because the temperature delta is so small.

This is just a hypothesis so if I'm wrong please let me know.

 

[kabbott]

To me stratification during the burn is necessary because using the lowest possible return water temps leads to a higher BTU extraction from the boiler. As soon as the return temp to my boiler gets up to about 178 degrees it will begin to short cycle if the fire is still raging. If the whole tank were mixed then this would happen even sooner and I would have 500 gallons of water to pump through the boiler because the temperature delta is so small.

This is just a hypothesis so if I'm wrong please let me know.

Good point. I Guess I keep overlooking this point because I have never had a problem being able to move the BTU's. If I had to move 2x the BTU's
then I would need 2x the deltaT....Yes that makes a big difference , Point taken.

 

[hayrack]

I thought like you also fabguy. I just installed an Eko 40 with 1000 gal pressurized storage. I wanted to do it your way, but everyone on here talked me out of it. I am in my second night of heating the tank from 70 degrees, so time will tell. I agree with much of your thought on heating the whole tank much faster if you sent hot supply to the bottom of tank instead of the top.

 

[Nofossil]

As far as the water separating after it's mixed I ain't happenin in my tank. Does anybody see this happen. I think once it's mixed your stuck with what you got.

Nobody ain't ever going to see it - it would be like unscrambling an egg. Thermodynamics: it's not just a good idea - it's the law!

You're right in that you if you're going to heat the whole tank up to max temp then stratification during the heating process is less critical.

However, I think it's still preferable - here's why:

Let's start with a tank that's uniform and cool - say 120 top to bottom. We want to heat it to 190 top to bottom.

In the mixed case we introduce 190 degree water in such a way that it mixes thoroughly, As the tank heat up, the return temp to the boiler increases. Eventually, the tank and return temp get hot enough that the boiler exceeds 190 and starts idling. As you get clser to 190, it idles a higher percentage of the time.

Contrast that with a highly stratified tank where you're introducing 190 degree water at the top and drawing 120 degree water out the bottom, with a sharp thermocline between the new hot water and the old cool water. The thermocline will move downwards as hot water in pumped in. The return temp will stay very close to 120 until the entire tank is heated. Only the last few gallons will show an increased return temp. This gives a much more uniform and useful heat load to the boiler and will absolutely minimize idling.

My $0.02.

 

[kabbott]

This is the point I was trying to make in this thread: https://www.hearth.com/talk/threads/45102/ in post #21

So what are the facts:
1. boiler is most efficient with inlet at 140 degrees and high flow?
2.keep boiler from idling if possible.
3. pressurized storage is best charged with high temp water, keeping strong thermocline between the charging water and the cooler water in the rest of the tank.
I have witnessed this in my DHW tank, with cold well water in the tank and hot water on top you can follow the temp gradient down the tank as it charges, It
hardly mixes at all.
4. as long as storage is as low/lower than 140 you can always mix with warmer outlet water to maintain 140 at the boiler inlet.

Now this is the easy part... Good control logic can accomplish most this with no problem. I think that it would be better to sacrifice some efficiency at the boiler
to only run the water through once to charge the tank. IE maintain 180-185-190 whatever outlet temp and run the tank full through ONE time.

So what are the thoughts on keeping the tank from mixing during the discharge phase? My system returns water ~15 degrees lower than the feed temp so
do i slow down the flow or try some fancy scheme of returning the water to different levels of the tank? I think the answer is slow the flow but then how much
can I get away with and still heat the house. This is what is screwing me up right now, no matter what I do charging it gets mixed during discharge and fouls up
the works.

I think I need to put in an order for that fancy variable speed unit you have Nofo.Hmmmmmmmmmm.

I want to try pressurized storage so bad y'all have no idea. My wife is going to flip. I call it a hobby, She will tell you it's a sickness.

Kris

 

[Nofossil]

The whole issue of return temp and stratification is a real problem. Here's what I'm planning on:

1) Experiment with running the load circ as slow as possible based on outside temp, hopefully getting a much larger than 15 degree drop. There's a limit, though, since baseboards at the end of each zone loop will be getting cooler water than they were designed for.

2) Add a radiant zone that pulls from and returns to the return manifold AFTER the other zones so that it's using water that's been through the baseboards and taking more heat out of it.

This may be an argument for primary/secondary or hybrid systems - you could run the zones at a higher flow rate than the primary loop and essentially control your return temp that way.

 

[chuck172]

How about piping a three way mixing valve into the load piping. Set the temp. to about 150*, so return water above that temp would be re-circulated and mix into the supply.
That would insure cooler return water to the storage tank.

 

[Nofossil]

How about piping a three way mixing valve into the load piping. Set the temp. to about 150*, so return water above that temp would be re-circulated and mix into the supply.
That would insure cooler return water to the storage tank.

Long-term, that's what I'd like to do, except I'd want to use outdoor reset logic to determine the mixing valve setpoint. Next house, maybe.

 

[sgschwend]

From my point of view the simple system is a great solution. I can burn fire for a few hours and achieve a layer of hot water in the tank. In the shoulder season which is quite long for many of us this really makes using a wood fired boiler easy. I can burn a fire for a few hours each day, at any convenient time.

Adding more complexity would seem to be the wrong way to go, at least for the shoulder season solution.

 

[fabguy01]

I struggle with this sometimes myself. My thoughts: If you are going to heat the entire tank to whatever is the max temp then it really does not matter.
In fact it may be better if it is mixed. However in my case I almost never max the tank and if it's mixed it may be low enough that it won't heat properly.

As far as the water separating after it's mixed I ain't happenin in my tank. Does anybody see this happen. I think once it's mixed your stuck with what you got.
Anyway with pressurized storage and the ability to heat it higher I see this as less of an issue.

so, you are saying that the tank will stratify with the into of hot water but will not with cooler water? what if you pumped the cooler water into the bottom with low mix rate and draw off the top wouldent it stratify the opposite of the "conventional" way. in essence you would be driving the thermocline up instead of down