Domestic Tank dynamics

NRT_Rob

Ok!



So, an engineer I have the utmost respect for the other day challenged my assumptions on indirects.



I have always assumed that once you start a domestic demand, the water in the tank mixes pretty quickly. That is, that it gets all turbulent and crazy in there. That means if you, say, had a tank of 110 degree water, and you wanted 110 at the fixture, you didn't have very long to get more heat in the tank because as soon as you get cold water in there it would "mix down" the tank temp very quickly.



However, that engineer seems to indicate that my assumption is wrong, and I should assume that I do in fact have X gallons of hot water available that stays hot even while cold water comes in from the bottom.



Does anyone have any good info on this? papers, pictures, data, measurements, anecdotes, anything?

BobbyG

it depends.

Where, and how, and what it does after it comes into the tank will vary the incoming cold water's influence on the tank temp.

I wouldn't use an open statement about drawdown on all tanks.

I am familiar with the "Lock-Temp" baffle that is supposed to help drawdown a longer stable temp.

bottom of page 1:  [url=http://www.lochinvar.com/_linefiles/LTB-07.pdf]http://www.lochinvar.com/_linefiles/LTB-07.pdf

Gordy

Lots of variables

 In my mind. This all depends on the size of tank, Tank design (tall and skinny short, and fat. Storage temp. rate of draw a biggie. DT of tank water, and incomming water. Mixed temp at the usage end of the line.



Then there is the heat transfer dynamics once hot water production starts while drawing off.





I would tend to believe that stratification in the tank would keep it from diluting quickly. Again this would depend on the storage temp, and incomming water temp. The wider the DT the more it will stay stratified. It would be interesting to see a infra red video of a tank in action this would tell all.



I don't have any studies, but I know by raising my water heater temp. I have extended usage quite a bit. So that tells me the hotter the storage temp the less dilution taking place.





Be interesting to see thoughts, and answers though. I tend to agree with your thoughts on a small tank.



I do think the engineer is wrong in thinking that if you have say 40 gal of hot water at 110* so thats what you get before the temp drops.

Gordy

NRT_Rob

the engineer's

understanding is much more nuanced than that gordy.



and turning up your tank stores more BTUs whether stratification helps or not, so that's not a conclusive observation either. though I suppose measuring the rate of drop of outlet temp on a system without a tempering valve might be more illuminating.



hmm.

hot_rod

once upon a time

I filmed a drawdown on a bare tank with and infrared camera. It was a basic 50 gallon indirect, cold in at the bottom with about a 3 gpm flow. It really does puch that whole layer up and the temperature stays stacked.



I think Dave Yates has some drawdown data.



But so many variables to get a good handle on the question. wasn't that long ago one manufacture had a dip tube with slots from top to bottom? Seemed that would have the opposite effect.



hr

Devan

First hour rating

Perhaps your engineer friend is thinking of first hour ratings on a tank, which is always higher. I always thought this was because of the stack effect.

But on continous use, I would agree that there is a good deal of mixing, dip tube and all.

NRT_Rob

I figured you'd have had some fun with that HR.

Now I need to figure out how to fix my DHW tank load calculations to accommodate the initial stack. sigh.....

Mark Eatherton

Personal experiences...

I have always assumed an 80% draw down factor on ANY storage tank. I've never had occasion to use the Lochinvar tank line.



As it pertains to some tanks (HTP) their aquastat is located about half way up the tank's vertical profile.



If there is a little usage during the evening, it is possible that the coil is almost completely submerged in cold water in the morning when someone jumps into the shower early. The boiler mass has gone cold (summer) and by the time the aqua stat calls for heat, it is playing catchup and the consumer runs out of hot water.



Now, on my Amtrol reverse indirect, the tank is held at 140 degrees F. The aquastst sensor is located on the very bottom of the tank. As I start drawing hot water, the stat kicks immediately and fires the boiler (modcon) which locks into the load and modulates based on draw. I have never seen the boiler shut down on high limit during these conditions.



While in the shower, the water initially comes out hotter and as I continue to shower, the water temperatures droop a bit, then come back up and stay steady. I attribute this to mechanical lag and control hysteresis.



However, upon cessation of the draw, the boiler continues to fire, recovering the tank, shutting down about 5 minutes after the end of the draw, then the temp in the bottom of the tank drops quickly (tank turning over) and re-fires again and runs for about 4 minutes, then shuts down for about 4 minutes, then re-fires for three minutes and shuts down without additional cycling.



I hear it running on occasion to recover standby losses. In fact I hooked a data logger up to it to see exactly how often it would fire, and was shocked at what I found. It cycles for about 5 minutes every 1-1/2 hour.



So, as with all hydronic heating systems, the only correct answer is, (drum roll please.....) IT DEPENDS!



From the stand point of a designer, I think you would be wise to assume an 80% draw down capacity unless the manufacturer (Lochinvar for example) has some special feature that enhances stratification.



Most of the recovery charts I have read take this 80% draw down before dilution into consideration. Their charts also assume the heat source is in a ready steady state of production (180 degrees F) which is rarely the case.



I think that there is also a lot more diversity in loading than what most curves (Hunter et al) take into consideration.



Now you know more about my showering habits than you really wanted to, but it was some good clean fun :-)



ME

Mark Eatherton

BTW....

I hope you are maintaining a tank temperature higher than 110 degrees, unless you WANT to raise a family of Legionella bacteria...



ME

NRT_Rob

Of course

just a number to illustrate a point. we advocate for 140 storage temps whenever possible, though I do think tanks are a fairly low risk given the high dilution factor of the volume of water, and the frequency of movement. but that's just "gut".



I'll probably be a little conservative and figure a 50% draw down. but how that impacts the rest of the recovery I'm going to have to put some thought into.



Thanks for all the feedback, everyone.