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Flow Rate From Boiler to Storage

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Blue Tornado, Feb 18, 2013.

  1. Blue Tornado

    Blue Tornado Guest

    Hello to All:

    I will be setting up an EKO 25 with storage (300 to 600 gallons) unsure at this point. The water volume of the boiler is 20 gallons. Something that I would like to dial in on is pump sizing for the flow to storage. My plan is to have the boiler to storage and back as primary loop. Although there is a couple of tons of info on this site, I see nothing directed at this.

    The information provided with the boiler does not address this. So I figured I would just compare to other brands and find similar instances. Came up empty.

    Certainly each sized boiler and the size of storage would have different requirements. Has anyone thought this to be worth consideration?

    My thinking says that too fast is not optimum as well as too slow. Are there programmable controllers that would alter the output of a variable speed pump in operating to optimize btu transfer rates?

    My first thought is to use a Grundphos 3-speed and implement a trial and error period.

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  2. maple1

    maple1 Minister of Fire

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    My piping is 1-1/4". My loading unit uses a Grundfoss 15-58 3 speed pump. I run it on low speed on my 40kw boiler, seems to work pretty good.

    Hope that helps.

    (Don't forget to use a return temp protection device).
  3. Blue Tornado

    Blue Tornado Guest

    Yeah, that seems to be a very commonly used pump. Your boiler is approximately 34 gallons and you are charging 660 gallons of storage.Is that about right?

    Have you tried the other two settings on the pump and settled on the low? Do you know your head and resulting gpm's?

    I will also be using 1-1/4" piping and a 140 degree F Danfoss. It seems peculiar that the manufacturers who suggest the useage of a thermal battery do not provide optimum flow rates. Or maybe I am just over thinking this issue.
  4. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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  5. maple1

    maple1 Minister of Fire

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    Heh - weird timing with this one.

    After I did my last reply, I went to the basement for a few hours work on something. I went by the boiler, and noticed the dump circuit had opened up and the boiler was around 200°. Maybe that had happened before, but it was the first time I saw it. Not an overheat, but getting up there. I had a real good fire going (thing really puts out the heat when it gets going) - I think what was going on was my zones had all gotten open at the same time, and since they also have a 15-58 pump on low speed, all the flow was going to the zones & none to storage. So I was getting hotter water returning to the boiler than if it was going through storage - the zones by themselves couldn't get rid of the heat as fast as the boiler was making it. So now I'm thinking I should maybe bump my loading pump up to second gear, so it will pump more than the zone pump & always get some to storage.

    At least with the 3 speeds, there's quite a bit of flexibility. And they're relatively cheap.

    I didn't do any head or gpm calcs, and just went with low speed at the start & it always seemed to do the trick.
  6. Bob Rohr

    Bob Rohr Minister of Fire

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    The pump selection really has nothing to do with the system volume. The pump needs to be sized to move the amount of energy the boiler can exchange. So a 25KW boiler at 75% efficiency would want to flow 25KW x 3.41 x 75% = 63,937 BTU/ hr. At a typical 20°∆T about 6.4 GPM. Next determine the system piping resistance, if it is short piping from the boiler to the tank, probably only a few feet of head. So look at the pump curves and select a pump that can flow 6 gpm or so at 2-3 feet of head, ideally in the mid 1/3 range of the pump curve.

    A typical small 1/12hp circ will easily cover that, if energy conservation is a goal look at the ECM circs that are offered. Grundfos Alphas are one of my favorite.
  7. hobbyheater

    hobbyheater Minister of Fire

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    When storage is approaching its upper temperature limit 190-195 from 180 to the desired limit it takes more flow to keep the boiler from cycling . The 3 speed pump is a good choice and gives you more flexibility in flow rates .
  8. jebatty

    jebatty Minister of Fire

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    Bumping up your loading circ will help and might, or might not, solve the problem.

    I think this may be a design issue with a boiler circ (15-58, for example) or a loading unit on the boiler, circs on zones, and storage where system return water is relatively hot. Simple schematic follows. Keep in mind that water leaving the boiler must also return to the boiler, and water leaving storage must return to storage. Scenarios:

    1) Boiler firing and boiler circ "on", no circ zones active. All boiler output (hot) will flow to top of storage, all return (cold) from bottom of storage: if boiler/storage loop has adequate flow based on delta-T, everything works fine.
    2) Boiler not firing and boiler circ "off", circ zones active. No flow through the boiler. All output will flow from top (hot) of storage, all return will flow to bottom of storage: everything works fine.
    3) Boiler firing and boiler circ "on", circ zones active, and system return water relatively "hot" (the quoted example): all hot boiler output to zones + possible additional hot water flow from top of storage, relatively "hot" system return to boiler + possible additional hot water return to bottom of storage. Boiler "sees" a low delta-T due to "hot" system return, the delta-T is much lower than if return was from bottom of storage, boiler can't get rid of heat as fast as it is making heat, and boiler goes to idle even though plenty of storage availability in the tank.

    Possible solutions:
    4) Increase flow from boiler in high demand situations (bump up the loading circ). This may work, more or less. It will work if the flow rate based on delta-T is sufficient to move boiler heat to prevent idling. Keep in mind that what goes out of the boiler must return to the boiler, so still all system "hot" return will go to the boiler. There may be some flow to storage if by reason of increase in flow the head to system is greater than head to storage, and if so, then there will be some boiler output flow to storage and some "cold" return from storage to the boiler. This may then result in solving the problem due to increase in delta-T. My thought on one way to do this would be to install a bypass circ on the boiler return line to bypass the return water protection valve/loading unit. This circ would activate when return water was greater than 160F (or a higher setpoint), for example.
    5) Change the plumbing so that the boiler only supplies top of storage with return from bottom of storage, and separate plumbing so that circ zones draw from top of storage and return to bottom of storage. The storage would function as hydraulic separator, and boiler would see return solely from bottom, which in most cases should be less than system return. The result is higher delta-T and improved function of the system.
    6) Solutions others propose: ???

    There is another problem, especially with a high output boiler, 160,000 btu for example, and use of a loading unit. Based on the specs of the loading unit maximum flow may be less than boiler output, even at relatively "high" delta-T. Assume output of 160,000 btu, assume maximum loading unit flow rate of 12 gpm (actual spec on one loading unit is a little less than 12 gpm), assume pump head will allow full 12 gpm with the loading unit: If delta-T=30, this will work: 160,000 / 500 / 30 = 10.7 gpm. But if delta-T = 20, then 160,000 / 500 / 20 = 16 gpm required flow rate, which is greater than loading unit maximum flow. The result is that boiler output exceeds capacity of the loading unit to move the output, and boiler will idle. An appropriate design with a bypass circ to increase flow may solve this problem. Of course, even a 160,000 btu boiler does not output at this rate over the full burn, but at high burn output likely will equal or somewhat exceed 160,000 btu. And it is at high burn that it is particularly important to move boiler output to prevent idling.

    In a design where boiler only supplies/return to/from storage, and with somewhat careful loading/firing of the boiler so that high output does not occur as tank delta-T starts to close and boiler output is falling, then even a loading unit with maximum flow of 12 gpm should work fine. But with a design like the schematic, flow always may be insufficient at high output and the problem remains. Boiler Schematic.jpg
  9. Blue Tornado

    Blue Tornado Guest

    Hi to all, Thanks for the good info in your replies.

    While planning this system I am using this formula--Flow rate(gpm) = Boiler output(Btu's per hour) divided by 500 x delta. For the two zone pumps it is clear to me using that formula.

    Figuring for battery charging contains a variable delta (beginning at 50 and ending at zero). I have seen posts about using a second pump set for the narrower delta at the end. Is this condition with the decreasing delta a good place to put an Alpha? Or a second pump with aquastat?

    OK, just saw that Jebatty posted. Covers what i am considering and what occurred with maple1.
  10. Blue Tornado

    Blue Tornado Guest

    My plan is to load storage as suggested by jebatty using one in tap and one out tap. The zones will share one in and one out tap making a total of two taps into storage and two taps out of storage. Boiler pump running 'on demand' determined from aquastat in storage. Zone pumps on demand from thermostats.

    Now concerning the flow rate of the boiler circuit, I am thinking to base the pump on a delta of 20 and include a second pump when delta gets narrow (175 degrees maybe).
  11. Bob Rohr

    Bob Rohr Minister of Fire

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    How much money and technology would you invest to get the last few "degrees"? It's tough to build a heat exchanger that is 100% efficient. Flat plate heat exchangers can be sized with a "close approach" method and get within 3-5° but it takes more surface area and possibly more pumping power. The money invested in the extra exchanger, and the electricity and control logic to drive the exchange may be a case of the "juice not worth the squeeze"
  12. jebatty

    jebatty Minister of Fire

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    This does not need to be expensive. Pressurized storage does not need any heat exchanger. As my posts previously have shown, a weighed wood burn allows charging the entire tank to 190F with no idling. My boiler connects directly to storage, the circ is a 15-58 with 1-1/4" piping, with a Termovar for boiler return water protection. Boiler output falls as the wood load burns down, and the boiler and storage coast to a 190F charge.

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