Back to the Garn Stratification/Short-Circuiting Question

Rick, I'm a little in the dark as to exactly what you mean by short-cycling. As to no difference in the front and back temp gauges with no fire and no Garn side circulator operating, it likely will depend on exactly where they are measuring water temp.

My only direct Garn experience is with the WHS3200. The Garn side circulator on this beast is set at 75 gpm, and at that flow, whether or not a fire, pretty through mixing occurs. This Garn has a gauge only on the front top. That gauge measures about +5F over sensor measured supply from the Garn.

More to the point might be my experience with my 36" diameter, 1000 gal, horizontal tank. I have sensor fixed to the outside of the tank at top, down 9", down 18", down 27" and bottom. The sensor down 27" is up 9" from the bottom. Then tank is quite well insulated. When the tank is allowed to idle, no circulator running, the tank stratifies quite quickly and the top 3 sensors all will read within about 3F of each other, with the highest reading, of course, at the top sensor. The bottom sensor will stabilize at about 10-12F lower than the top 3. This evidences that the actual location of the bottom sensor is quite critical. If it is "up" from the bottom a sufficient distance, which is not very much, it will read close to a top sensor, at least with a well insulated tank.

Rick, I'm uncertain also what you mean by short-cycle. Seen the term a few times in Garn conversations.

Are you talking about the return water going into the tank and directly back out the supply line, so the rest of the tank volume is not used? More like a short circuit?

I see the 20-30 degree differential all the time Rick. I also turn on the Garn pumps only on demand....and I'm running just a little Taco 00R, likely on the medium speed setting. I wonder if that contributes to this affect, i.e. minimal amount of pump mixing? I want to keep all the hot water inside the well insulated tank!

So after sitting for a good time idle, save a little pump running in the morning to make sure you have no freezing, you see the same temps on the front sensor as on the supply line? So clearly there is no stratification of whatever warm water you have left. What is the water temp at?

So stratification occurs when the higher density water sinks to the bottom of the tank. So when it is burning and churning, it is all uniformly mixed. Fire goes out, returning cooler water sits on the bottom, less dense water is pushed to the top. As this continues, and with nice little pumps like we are using, nice stratification. So now take it to the limit....where we stop taking out heat from the plumbing. The heat loss across the top of the tank is going to be larger than the bottom, becuase of larger delta T to the outside. But this will cool fairly slowly...but the warmer top will cool quicker than the cooler bottom...and they will eventually be equal...and then all cool about the same. I guess the water touching the steel should cool quicker than the rest, and presumably sink, but with a smaller delta T, this rate of cooling is reduced...and perhaps there is no discernable gradient in the water. And as this slightly cooled water falls, it just mixes/is warmed back up by all the water it touches on the way down.

I guess I'm not convinced that it is short circuiting from the return to the supply...I can believe at some point the temp is low enough that the water temp gradients are too small for any stratification to occur....and hence it just sits now appearing uniform.

Put it this way, if we heated the tank up and never took any water out of it, surely it would cool....have a big gradient, but then end up rather uniform...without ever pumping hot water out. Stratification requires a certain amount of delta T to see...

It's like diffusion...once the concentration is uniform, there isn't any noticable migration anymore....although all the water of course is moving around on its own. Must be the current thermal velocity of the water is no longer dominated by the gravitation force acting against the higher density water....pulling it down to the bottom.

Rick, I read your website briefly....very cool! I get a kick out of the Chick's and the 'ol MacDonalds on the farm.

If the outside temp is the same top and bottom (exposed to room air), I would suspect that it would equilibrate in temperature top to bottom.

When you are injecting water at higher and lower temps (normal operation), then you have hotter or colder water to stratify.

Pond water stratifies, but the top and bottom are exposed to different temps. A steel tank- all of the water should fall in a tighter temperature range.

(note: I'm not a boiler owner)

Rick, I've noticed this with my Garn.

I think part of the issue is our little 15-58 circs are pulling out of 1 1/4 pipe, out of a possible 2 inch. I'm very pleased with the stratification in my unit. I have three sensors on the back of the tank. Plan to add three more in the center of the tank. I believe my pump will do 13 gpm. When I've been late to fire the Garn and the pump is running 100%, I see the front gauge drop steadily. The front gauge is extremely accurate after the unit has been run an hour or hour half. I see the mixing of the tank through my sensors. That top layer of trapped btus also exist at the back. Neat to watch that top temp drop and the bot and middle rapidly increase.

I think Jim K said that he hadn't seen this on his. That hoss of a primary pump pulls out those top btus, where our little 15-58s may be allowing short cycling. I expect this issue vanishes when you/I add another pump to our Garn manifolds.

I've seen this happen with chemicals before when injecting them into the tank. The sprayer pump wasn't creating enough suction to draw it down into the pump so it could be sparged back into the tank. I turned my hydraulic regulator up on the pump and solved. Maybe heat is a little different but the concept seems the same to me. Not enough flow from the Garn tank.

I always thought it was best to have the heat exchanger close ish to the Garn, so the Garn water would be pumped around a short distance?

That's what I did Dave. My HX is about 10' away from the Garn, as the water flows out of the supply line. Keep the hot water in the well insulated tank as much as possible. I don't think this is really a problem...it is just misleading to look at the front sensor at 140 and think that is what you are getting at the supply...15 hours after firing.

But it clearly explains why you can have a big fire going and only see a 10 degree rise on the front sensor after an hour....you've heated up the 85% of the water that was so much cooler than at the sensor. I always see the front sensor dive down 2-5 degrees in the first 15 minutes of firing...and then it will usually stay fairly constant for awhile (below the starting temp) and then it begins to rise.

For the performance tests on the WHS3200 a couple of weeks ago, I had sensors installed at the Garn Side A of the HX in and out (also Side B in and out), because that seemed to be the most relevant locations to determine the BTU's actually being delivered. The HX is installed at the back of the Garn with piping just about as short as possible between supply and return. Also during that test the Garn 75 gpm circulator was running constantly and the burn was continuous (demand was sufficient to prevent overheating); stratification was not a concern in this scenario.

If your system uses relatively low temperature water, such as for radiant, and you are mixing down supply, stratification matters little or not all. My radiant floor supply temp is 100F from the 1000 gallon storage tank, and although my tank stratifies, I use the near bottom of tank sensor only for estimating how many BTU's are still in storage so I know about when I need to have the next burn.

RowCropRenegade said:

Rick, I've noticed this with my Garn.

I think part of the issue is our little 15-58 circs are pulling out of 1 1/4 pipe, out of a possible 2 inch. I'm very pleased with the stratification in my unit. I have three sensors on the back of the tank. Plan to add three more in the center of the tank. I believe my pump will do 13 gpm. When I've been late to fire the Garn and the pump is running 100%, I see the front gauge drop steadily. The front gauge is extremely accurate after the unit has been run an hour or hour half. I see the mixing of the tank through my sensors. That top layer of trapped btus also exist at the back. Neat to watch that top temp drop and the bot and middle rapidly increase.

I think Jim K said that he hadn't seen this on his. That hoss of a primary pump pulls out those top btus, where our little 15-58s may be allowing short cycling. I expect this issue vanishes when you/I add another pump to our Garn manifolds.

Click to expand...

Correct - I do not see as much front/back differential as some do, but there are many factors that affect this.

As to stratification, it is going to occur, regardless of the temperature of the room, unless mechanical mixing is induced. Convection within the tank, both the vigorous movement during a burn as well as the gradual movement long after a burn, will continue until stasis is reached (zero delta T betwen ambient and the tank, and no ambient temp change). Even in a heated room, there is a temp differential from floor to ceiling. There are always different rates of heat loss within the room at the floor, walls, ceiling. The water density (temperature) within the tank will always seek equilibrium, so even if you tried to get the warmer water to "stay" at the front of the tank and the cooler water to "stay" at the rear of the tank, you could not. Higher density water (lower temp) will migrate down, low density water (higher temp) will migrate up. Horizontal equilibrium will also be approached in roughly the same manner (via displacement).

As discussed, the pump sizing will affect the velocity of return flow (as well as Btus delivered, of course). Given the design of the lower return pipe within the GARN tank, true short cycling, or short circuiting, is highly unlikely. In fact, even without the extended return piping, short circuiting would a general phenomena, not a true bypass. The return water typically always be cooler than the supply water. Since it is denser than the supply water above the return, it would take a substantial input of energy to make this cooler water rise up to the supply port. The temperature well at the very front of the tank must be regarded as a general indication of tank temperature. To get a true representation of the water densities (temperatures) within the tank volume, a grid of sensors at multiple depths would need to be installed. The single temperture sensor at the front of the GARN can certainly become differentiated from the bulk of the tank volume due to its location and remoteness from the supply and return ports. However, as Jim B mentions, temperature sensors should be installed within the piping to properly monitor S&R temps and delta T, and those data points used to control circulators and determine burn intervals.

I hope I didn't geek out on you guys too much. :red:

I know that stratification does occur in the Garn. I just returned from sunny FL and after 9 days since firing the beast, the temp on the front of the boiler read 132 degrees. I had brought up the temp to 192 degrees prior to leaving. I had turned off my pump which always runs 24/7 that supplies my house heat exchanger. I immediately turned on the pump and started to heat my house prior to making a fire. I noticed that it took a long time to get the temp gauge to move up on the front of the "beast." In an hour I was about 140 degress, which normally I would be much higher then that. I don't have lower sensors, so I don't know how much the tank was cooler at the bottom, but I would say it was quite a bit cooler. I return my water to the front of my tank about mid way up and pull my supply near the top of the tank at the back of the tank. I only used the mid way point as it was existing in the tank, but I purposely installed the supply as high as I could at the back of the tank to take advantage of stratification of hotter water at the top of the tank.