Pressurized storage plumbing

Heat does rise, but you don't want mixing. Bring the hottest water in the top - that will 'push down' the cold water as you withdraw cold from the bottom.

If you bring hot in the bottom, it will mix a bit as it rises. It will rise, though. When it does, you will be drawing the hottest available water from the top to send to your boiler inlet. Not what you want.

hayrack said:

It seems that I could heat my tank quicker by putting the hot in at the bottom instead of the top. I am not against your way of plumbing. It seems all the gasifier plumbing with storage do it the way you do. I just am not sold on the logic yet.

I do have circulator pumps for each zone instead of zone valves. In what order do you recommend the supply manifold. The way it is now is oil wood and then my storage tank. I am redoing my plumbing to clean it up some. Where is the best place for the supply and retrurn manifold. My return manifold currently has the circulator pumps. Supply has the flowchecks.

I do not want to flow through my oil boiler. I will have it set up to come on with an aquastat to come on at a drop in temp when the tank and wood boiler are not keeping up.

Thanks for any info.

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There are two stickies at the top of this forum that discuss plumbing approaches. In general, you don't want to circulate water through the inactive boiler. Both the primary/secondary and 'simplest storage' approaches described in the stickies accomplish this.

Here is an extreme example of the benefit of hot water into the top of storage and return to the boiler from the bottom of the tank. Yesterday I drained and refilled my 1000 gal storage and boiler after a water treatment procedure. My supply water is from a well, and water temperature of both was 45F. I fired the Tarm, and the Tarm burned full bore until it brought boiler temp up to just over 160F, then the Termovar began to open, bringing in 45F water from storage. Output to storage was also 160F, a 115F temperature rise, and flow was about 2.3 gpm. After about 4 hours of non-stop high burn, boiler output to tank remained at about 160F, storage top of tank was 155F and bottom of tank was 46F. If I had needed to draw hot water for heating or DHW, I would have drawn from the top of tank and had plenty of supply at 150F+, very good for either heating or DHW.

If I had done as your proposed, supply to bottom and draw from top, the top of tank water temp still would have been unusably low, likely well below 100F with all the mixing that would have been occurring.

Although this is an extreme example, it shows the great benefit of having the hottest water available at the top of the tank.

As the others have stated. Utilize the delta T on your boiler to get the greatest temp rise which is also the greatest efficiency.
Cheers. N of 60

I think the physics behind the heat trasnfer is rather hard to argue with. But another "logic" question is how long does it take for the hot water being returned to the bottom of the tank going to take to "heat" the top of the tank when plumbed as you're suggesting? If my tanks are in need of recharging (which occurs daily in the cold season) I want to have usable heat on top of the tanks as soon as humanly possible. I couldn't afford to wait until the heat soaks up from the bottom. Especially since I burn my boiler just long enough each day to get a good 16 hours of heat out of my tanks. When I come back home from work I need hot water quickly to keep the house warm...

All of that being said it would be interesting to have someone plumb their system "backwards" and report on their findings. If you do go this route I'd recommend making it easy to switch at a later date. I have to believe the boiler experts on this forum and elsewhere have been plumbing "hot in top, cold out bottom" for a long time for a good reason...or two....

Here is another scenario just to further confuse matters. Does it make a difference if the storage tank discharge (hot water to load) is sucked from the top or pushed through the bottom of the tank?

north of 60 said:

As the others have stated. Utilize the delta T on your boiler to get the greatest temp rise which is also the greatest efficiency.
Cheers. N of 60

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Good general advice. I try not to be too pedantic, but I just can't help myself - I have to expand on this. Apologies in advance.....

Greatest efficiency, all other things being equal, is obtained with the lowest possible inlet temperature. Given that, it's obtained with the LOWEST possible temperature rise.

What that means is that for peak efficiency, you want a higher flow rate through the boiler HX. With cold inlet water, that results in the whole HX being cooler than it would be with a lower flow rate.

This nuance isn't relevant for most residential applications, but if you have a situation where you don't need really hot outlet temps, you can get a bit more efficiency by running a higher flow rate through the boiler. My baseboards need high temps, but none of the other loads do. When there's no baseboard demand, I run with a lower outlet temp. Don't know if it results in a measurable improvement, but it makes me feel better.

The difference being that... Your heat source is in the DHW tank. In your storage tank the heat source is at your boiler. Pulling from the top of that tank will be bringing back the hottest water possible, therefore getting less temp rise from the boiler. My 2cents.

Be careful comparing electric hot water heaters and wood fired boilers.

I'll agree that you do get the greatest heat exchange with the greatest deltaT and that is why the electric element is at the bottom of your DHW tank so it is surrounded by the coldest water. But that analogy only holds without complication because the heat from the electric element has nowhere else to go.

In the case of a wood fired boiler, the BTUs from the wood fire have a choice of two places to go: into the water or up the flue. By running the circulator slower to get a higher deltaT between the boiler input and output you are lowering the deltaT across the heat exchanger within the boiler. I mean the exchange between the hot gases and the water. More heat will go on up the flue if the hot gases are surrounded by hotter water. So you very well might get measurably more heat out of a given fire by running your circulator faster. I believe (and I trust he will correct me if I am wrong as he has in the past) that was Nofossil's argument and I think he is right in this part of the puzzle.

My own personal hangup is at what cost that extra circulator speed is obtained.

With a Grundfos 1558 at low head conditions the difference between speed 2 and 3 is less than 10% more electricity for over 40% higher flow. That might very well get some extra heat out of the same fire for cheap money, especially in the first 1/2 or 2/3 of the burn where you have the highest BTUs/hour out of your armload of wood. This does mean that the temp in the tank will be lower but more heat would have gone up the flue otherwise. You could capture more heat (BTUs) but at a lower temp. If the tank is completely heated through to that lower temp and the fire is still putting out good heat then the boiler output temp will start to rise again because the inlet temp is higher because you will now be feeding the boiler with water that was previously heated.
So for a full burn of, say, 7 hours those extra 7 watts cost 1/20 of a KWH. That's less than $.01, even here in the land of the highest electric rates in the nation. Do we get more than a penny's worth of extra heat?

How to suss this all out is beyond me. Where is Spreadsheet Man when you need him? And what fiddle factors will be required to put numbers to all this verbal speculation?

hayrack said:

On the flip side why do you never see a domestic hot water tank plumbed this way. The heating element or coil is always on the bottom and you draw your hot water off the top after the cold water supply has been heated . A domestic hot water tank relies on heated water rising and mixing.

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Not exactly true. Many HW heaters have two elements, one on the top and one on the bottom. The top element comes on first to provide immediate hot water, and when the top of the tank is up to the set point, the top elements shuts off and the bottom element comes on to heat the balance of the tank up to the bottom element setpoint. I don't have a single element HW heater, so I don't know where the single element is placed, but my bet is that most single element HW heaters are small (in vertical height), and placement of the single element is not overly critical.

hayrack,
have you decided on how you will plumb your tank?

I had two guys that do it for a living working on mine one day - the guy I hired and another master plumber to help him with the heavy lifting, as he was behind schedule. The second plumber kept trying to tell me why I should leave my Boiler Mate hooked up, in case I got hurt and my wife couldn't run the system. It took me at least 3 trys to get him to understand that my oil furnace was being plumbed in as a complete backup and could take over at any time, and the Boiler Mate was not necessary. He then made a comment about how much oil it would take to heat my storage tank up, which was nonsensical, seeing as all the heat you put into it comes back out into the house or DHW.

Rory said:

I had two guys that do it for a living working on mine one day - the guy I hired and another master plumber to help him with the heavy lifting, as he was behind schedule. The second plumber kept trying to tell me why I should leave my Boiler Mate hooked up, in case I got hurt and my wife couldn't run the system. It took me at least 3 trys to get him to understand that my oil furnace was being plumbed in as a complete backup and could take over at any time, and the Boiler Mate was not necessary. He then made a comment about how much oil it would take to heat my storage tank up, which was nonsensical, seeing as all the heat you put into it comes back out into the house or DHW.

Click to expand...

Flip side, I don't think it's a good idea to try to heat the storage tank w/ fossil fuel if you can avoid it... The most efficient way to store heat is in the original fuel, we only use storage to get around the fact that you can't turn a wood boiler on and off the way that you can with a fossil burner. Every design I've seen that I thought made sense had the storage plumbed in such a way that the fossil burner bypassed it, and only delivered heat to the DHW and house heat loads. If nothing else, you wouldn't want to spend a lot of oil heating the storage just because you missed a single firing period with the wood boiler...

Gooserider

My plumber actually installed a switch on my burner so I can keep it off independently of the setpoint controller. One advantage is if the tank dips below the setpoint, the water path through the oil boiler opens up and flushes it without actually using any oil.

I really only see the role of the oil as backup now, like if we were to take a winter vacation, or if I were unable to make fires for some reason.

Ok, now you've got me thinking.

I'm installing an eko40 with 2 500 gallon tanks and will probably follow nofossils system design but my question is what will actually tell the boiler that the storage is hot enough, which tank will the reading come from and what will the degree range be between the top and bottom tanks.

thanks,
Jim

Using the piping diagram NoFossil provided, the tank will be "hot enough" when the return temp from the bottom of the bottom tank is at the set point temp of the EKO controller. If your set point is 180* and your termovar or laddomat is set to 150*, The boiler will run full-out, pushing 150* water into the top of the tanks, until the bottom of the tank reaches 150*. Then the whole loop begins to rise in temp from 150* to 180*. As the returning water begins to get hotter than 170* the EKO controller will start to ramp down the fan speed to limit combustion and therefore BTU output, finally stopping once the water jacket temp rises above 180*.
However, I think that with an EKO 40 and 1000 gallons of storage you would have to reload the boiler before the tanks were fully charged, given average heat loads during the heating season.

The sensor for water temp that provides feedback for the controller is in the boiler water jacket. In this setup, no external temp sensing is needed.

Charge time depends on starting tank temps, BTU output, and water flow in the tank loop. Degree range will depend on flow rate, flow path, amount of turbulence, tank insulation, etc. in the tank loop.

Those people with two-tank setups could probably tell you what average top-to-bottom temp differences for the two tanks are.