When hooking up the desuperheater to the first of a 2 tank system do you draw water in from the top where the cold water is and send the desuperheater water into the bottom of the tank or the other way around. I have seen several diagrams showing it done either way.....Which way is better?

Posted By redkrypton on 15 Feb 2014 08:25 PM
When hooking up the desuperheater to the first of a 2 tank system do you draw water in from the top where the cold water is and send the desuperheater water into the bottom of the tank or the other way around. I have seen several diagrams showing it done either way.....Which way is better?

Read http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/82138/afv/topic/afpgj/2/Default.aspx#135554

We had an interesting discussion on this point a few weeks ago. I think if you have a DSH that is producing 18 plus hours a day, it won't make much difference. Otherwise go for stratification.

Personally I don't think the bergy diagram works very efficiently because you draw and inject at almost the same spot. Doc has some very interesting variations on the bergy model.

Look for the Bergy diagram here. Know that by drawing from the "top" cold water port the water originates from about 6" from the bottom of the tank via a dip tube.

Returning via the boiler drain ensures that scale or sediment is not drawn in from there.

Drawing from near the bottom and reinjecting at the very bottom does not produce undesirable short-circuiting that FBBP seems to imply. As water near bottom of tank is warmed by successive passes through desuper coax denser cooler falls from above, displacing it. If this process did not work as described, it would be impossible to boil a pot of water on a stove (heat applied solely to base of pot) without stirring it. I've never had to do that.

The movement of warmed water from the desuper outlet to the top of the tank is usefully disrupted during and for a while after hot water draw events. A mass of replacement cold water flows into the bottom of the buffer tank whenever hot water is drawn. and that new cold water sits below the remaining volume of existing warm water until the desuper has a chance to warm it up. This is the desired stratification hard at work.

Stratification is desirable since it directs warmest available water to finishing tank, minimizing its energy consumption. It also directs coolest available water to desuperheater, maximizing its heat transfer.

That a fairly hard thermocline forms in a water heater tank is easily confirmed by anyone having taken the fourth in a series of showers drawn from a tank heater whose capacity is only 3.5 showers. The drop in delivered water temperature is quite abrupt and usually coincides with shampoo still being in hair.

I have thermometers near top and bottom of my buffer tank and know the system performs as written above. It is not uncommon to have 70*F water at bottom of tank and 100+*F water near the top...tickety-boo.

tickety-boo? That sounds like an IPA talking LOL
At the end of the day if you use a buffer you'll get decent results almost despite plumbing method.

IPA? Sometimes one figures in to a post or two, though not often at 0738 hrs.

Joe's prolly right about the plumbing...the one thing I'd avoid is returning desuper water to the hot water line leaving the buffer.

What about taking DSH suction from the storage tank bottom drain valve, but install a Y strainer with screen like in the link below? The Y strainer should catch any scale or sediment prior to the DSH pump. The only downside I can see is that the Y strainer would require some periodic maintenance to remove and clean the Y strainer screen. Then return the DSH hot discharge to the storage tank cold water supply line. This would insure the coldest storage tank water is supplied to the DSH and that hotter DSH discharge is returned to the storage tank above the suction.

Thoughts?

http://www.getzequipment.com/conten...1G0414.jpg

http://www.amazon.com/PneumaticPlus-YBS-50-Y-Strainers-Female-Stainless/dp/B00BMHAHZS/ref=sr_1_1?s=hi&ie=UTF8&qid=1393012256&sr=1-1&keywords=y+strainer+brass+1%2F2%22

engineer - you are correct in everything you say - sort of ;-)

Short circuiting will indeed heat up those few? gallons of water faster and many times this would be a good thing, except that this hotter water must now make its way all the way to the top of the tank giving off heat to the water it passes through until it finds it neutral point. Hence the whole tank would tend to be of a similair temperature and the water at the outlet to the finishing tank will only be somewhat warmer. Yes, it will stratify but very slowly compared to injecting the hottest water in the upper element hole. During a cold snap when the geo is running almost full time, it won't make as much difference as during the shoulder seasons.

As the tank does approach close to its heat capacity, the dsh will shut down on high limit more often and sooner because of the short circuiting. True, not as often as when drawing water off the top without a dip tube but much more then when heat is inject near the top and the colder water is removed from the bottom. So the tank can never store as much heat when push and pull are close together.

When a pot boils, the roiling action of the water displaces the heated water to the top. Unfortunately, we are not boiling water so we must rely on slow convectional rise.

Allowing the buffer tank to operate as it was designed i,e, putting the cold water down the dip tube will always put the coldest water at the bottom. Mixing it up with the incoming water only serves to break the stratification.
Stratification is indeed desirable but doesn't truly happen unless the hottest water is allowed to enter near the top and the coldest water is drawn off the bottom without stirring up the whole tank.

There is no doubt that a fairly hard thermocline exists, in the finishing tank. As we all know electric heaters are designed to stratify. Cold water enters to the bottom and hot out the top with no mixing. When there is no heat in the tank, as in after 3.5 showers, the top element comes on, and only when the top is hot does the bottom heater come on. I believe we should strive to duplicate this in the buffer tank.

The only reason given so far for injecting at the bottom is because we don't want to draw from the bottom. Drawing from the lower element hole will give us a little more clearance from debris then drawing from the dip tube. Drawing from the dip tube while simultaneously stirring up the sediment with the incoming hot water seems to be a certain way to plug up the dsh.

Piping to the element holes is almost free as when you take out the elements, you lay them on top of the finishing tank and you save yourself or your client the cost of their first set of replacement elements. They're not doing anything for us in the buffer tank!

Because we are dealing with relatively lower heat output, Joe is probably right - sort of ;-)

If I need to deliver a whole herd of btus to the finishing tank, I would want to restrict the flow through the dsh to produce the hottest water possible, not because this somehow produces more overall btu's but because I want to keep those btu's corralled such that I can deliver them to the chute for the finishing tank through the upper element hole without them stray all over the pasture.

arkie - I don't know. Seems unnecessarily complicated to me. I like the simplicity of plumbing the domestic water as the tank was designed and the dsh water in and out the element hole. Simple, no complications.

If you wanted to make it a little bit more complicated, maybe add some controls and valves. Maybe draw through a tee at the pt valve till till the top reaches a certain temp and then go back to drawing from the bottom element hole. Don't know if there is sufficient roi for this and would need to think the whole thing through a bit more.

FBBP, In a previous post I mentioned, the DSH delivers a finite amount of BTU's. If you want to argue it is advantageous to get them "corralled" that's one thing, but suggesting you can magically get extra capacity based on piping is folly.

Posted By joe.ami on 22 Feb 2014 09:06 AM
FBBP, In a previous post I mentioned, the DSH delivers a finite amount of BTU's. If you want to argue it is advantageous to get them "corralled" that's one thing, but suggesting you can magically get extra capacity based on piping is folly.

Hey Joe, my apologies if I didn't make that clear but that is exactly what I was trying to say. Restricting the flow will never create more btus overall, but it will deliver a higher concentration of them to the finishing tank if piped this way.

The discussion about DSH piping comes up again and again.

,

2 Things to understand:

1) Stratification does not matter so much since the water is circulated through the tank anyway


2) As long as you pipe it in at one end and and out the other end, so the water circulates through the tank, there is not much which can go wrong.

"Restricting the flow will never create more btus overall, but it will deliver a higher concentration of them to the finishing tank if piped this way."

Yes and I think there is where our design strategies part ways. If X amount of BTU's saves X amount in finish tank operating cost, then it only matters that X amount is delivered not in what concentration.

If you are trying to add capacity to your DHW system, the DSH is not a reliable means.

I think we all get it, we just don't go about it the same.

•••If X amount of BTU's saves X amount in finish tank operating cost, then it only matters that X amount is delivered not in what concentration. •••

If the finishing tank is drawing water constantly and continuously, you would be right.

Posted By FBBP on 24 Feb 2014 10:57 AM
•••If X amount of BTU's saves X amount in finish tank operating cost, then it only matters that X amount is delivered not in what concentration. •••

If the finishing tank is drawing water constantly and continuously, you would be right.

He is right. The amount of total BTUs delivered matters, wether it is with a lot of flow or a lot of delta T, the total usually remains similar.

We are really splitting the hair of a flea here

I was also mildely concerned with stratification of the Buffer and stole some ideas I found online.

I have mine DSH sucking from the bottom drain connection with a Y strainer inline to the GEO. I also inlet my cold street water into this same connection with a T at the tank drain.

I return from the DSH through the original dip tube connection at the top. I removed and modified the plastic dip tube by carefully torch heating it in the center to soften it, then twist it to seal it. Like taffy. Then remove the lower half. I then drilled a bunch of 3/16" dia holes in the lower 10" of the closed off dip tube. This allows the DSH return to enter the buffer tank in the center of the tank with low velocity sidways flow. This allows good stratification. Entering through the side heating element port would like yield similar results.

This set-up is similar to the one FBBP was talking about with some minor changes.

I have city water, so my sediment content may be lower than others. I have taken out the strainer a couple of times. Never found any debris in there.

Observations- I do not have the tanks instrumented, but I do notice the flow out of the buffer is sometimes hotter than the DSH return, so there must be some stratification going on. I do this by just feeling the pipe and sensing the heat. I think the velocity of the DSH pump is a bit too high and causes the buffer to mix a bit too much and kills stratification on long GEO runs.

I think my set-up could be refined by adding a flow control valve to optimize DSH flow. There is diminishing returns on cutting the flow rate, as the most BTU's are transferred with the highest flow and coldest inlet to the DSH.

I am at least sure, this set-up is always drawing the coldest from the tank botton, so that is ideal (except strainer service). I just have not bothered with messing with flow control to optimize stratification. Not sure its worth it, except possibly in the shoulder seasons when GEO run times are brief and far apart and some enhanced stratification could yield some savings.

I need to get a flow valve and some temp data logging so I can fine tune, but that cost would kill any likely savings...

Getting very Rube Goldberg...

Bergy

I believe drawing from the buffer tank cold water inlet (via dip tube) and returning to a tee on the tank drain per Bergy's drawing is the best practice for all the reasons stated by proponents here. One point that has not been discussed is that a DHW tank is either flowing or not. Both states have to be considered in regards to DSH piping and flow. During a heavy DHW draw the highest pressure in the system is at the cold water inlet due to restrictions imposed by the dip tube. I have seen DSH flow nearly stop under extreme conditions if pumped into the cold inlet during heavy DHW demands. Under the same conditions, when piped and flowing into the bottom of the tank per Bergy's drawing, the cold inlet water also flows immediately and efficiently through the DSH loop In the same direction of the DSH pump.

Bergy- Yes, and likely not worth the little extra hassle for most.  I thought I would share for those more on the energy geeky side and less business oriented side.  That seemed to be where this thread may have been targeting.

If it took me an extra 2 hours and a couple fittings, I'm OK with that.  I think it works quite well and may save some finishing tank power, but without data, I'm just speculating.