Injection mixing from storage question

kuribo Posted By kuribo, Jun 23, 2019 at 12:09 PM

  1. kuribo

    kuribo
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    Anyone using injection mixing from storage? Starting to plumb my system and am wondering about something....

    I plan to use a tekmar injection mixing controller to control an injection circulater feeding water from storage to my floor zones, using outdoor reset. The issue I see is that depending on demand and storage tank temps, the injection flow will vary from next to nothing when heat load is small and tank temps are at a peak, to full load flow (12gpm) when heat loads are at the design point (about 120,000btu/hr) and storage is at a minimum usage temp (100F). As a result, if I use a single injection pump and size to max flow, I would be using 1.25" copper. At minimum heat loads and/or storage at peak temps, the pipe would be grossly oversized and flow velocities would be less than the 2 ft/sec recommended.

    Mr/ Siegenthaler has mentioned a multiple injection pumps in parallel scheme to get around this issue, but he doesn't mention what would control this set up....

    So, what sort of experience are those with injection mixing from storage seeing?

    Mr. Siegenthaler's schematic below:

    PME-feb-2017-fig-5.jpg
     
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  2. electrathon

    electrathon
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    Possibly there is something I am not understanding. You are planning on going from one pump the 4 pumps, three of the pumps are to be electronically controlled and modulated. Why not replace the three pumps with a modulating mixing valve in place of the two T's and allowing the main pump to do all the work? During low heat demand the floor system would just loop, during high demand the mixing valve would allow as much heat as called for to enter.
     
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  3. TCaldwell

    TCaldwell
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    My house has mixed emitters, and rooms with different heat requirements. I mix a master odr temp that will cover the poorest emitter on a design day right at the back of the garn with a modulating 3 way valve, to the closely spaced t’s in the injection loop in my basement. Several zones will use this mix temp, from there I mix down again for a radiant loop that has its own outdoor reset curve. This way the closely spaced t’s in the house are always seeing a full odr flow.
     
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  4. kuribo

    kuribo
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    Regardless of whether I use injection mixing via a pump or mix with a valve, the issue is the same: at fully charged storage, the draw from storage, through the pump/valve is very small. When the heat in storage is nearly depleted and the temps are at what the odr is calling for, all the flow will come from storage; at a design temp day, that could be as much as 12gpm. The piping from storage to/from the pump/valve should be sized to handle the design (max) flow. This means that most of the time, the plumbing, sized for 12gpm, may only be transporting a few gpm. That means the flow velocities will be lower than the recommended 2 ft/sec. The reason Siegenthaler proposes this staged pumping scheme is to avoid this, as well as enable the use of a smaller pump to handle the smaller flows, which will be the norm, rather than the exception. It's like using multiple staged boilers, rather than 1 large one....
     
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  5. TCaldwell

    TCaldwell
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    With the valve your always getting 12 gpm, not so with the pump.
     
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  6. kuribo

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    Ah, that's right. They have their own set of control issues though when trying to mix small quantities of hot water...
     
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  7. TCaldwell

    TCaldwell
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    No doubt siggy proposes a interesting concept, I’d like to as you see it in action. I know there are plenty of folks using a circ for injection loop mixing, I’ve wanted and tried multiple times with limited success. The wet rotor cap circs require approx 2v to rotate under load, with a 0-10 v control signal you’ve lost the first 20 percent of the control range to start. The circ on the bench under no load will rotate under a volt. The reason I say multiple times is that I’ve tried multiple controllers, taco, tekmar, nimbus and stratus that’s a mini vfd.
    I monitor the odr injection loop on a trend chart so the affects of losing the first 20 pct are easy to see, in this case espically if your trying to sip from hot storage and maintain a consistent mix temp. That would be my question for siggy, maybe he has that figured out?
    As you can gather, I’m a valve guy, if it’s sized correctly the flow is constant and the temp is more linear to the controller output.
     
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  8. maple1

    maple1
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    What bad things would happen with flows less than 2gpm?

    Wouldn't that low flow, just be around the short boiler/storage loop? Thinking if confined to just there, not much bad? I could see wanting to stay above that for the system. I could be forgetting or overlooking something, of course....
     
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  9. kuribo

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    I think if your loads are high mass (mine are), the slight overshoot might not really matter much. I guess another thing with a valve always pumping 12 gallons is it isn't as efficient if you can only pump 1gpm or much less a great percentage of the time. No doubt each method has it's pluses and minuses....reaching for that last 5% can sometimes be a waste of time and money I suppose.
     
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  10. TCaldwell

    TCaldwell
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    Just curious, do you have a link to the rest of the article?
     
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  11. kuribo

    kuribo
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  12. TCaldwell

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    Very interesting concept with the single variable speed and the two fixed speed circs, just wondering what speed controller could manage 3 circs?
     
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  13. __dan

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    Heat flow in the pipe is not my thing, electrical is, but I am running the Tekmar injection pump controller with the Froling in radiant slabs, no tank but it's planned and tapped for the future. I have the basement slab slaved to the boiler to maintain constant load and flow when firing.

    I don't see your logic. Max load will only happen at cold start.

    If you had HW coils and forced air or typical baseboard convection, those loads are going on - off - on all day, so you may expect max load when it's on.

    With outdoor reset and radiant in the slab, the design premise is very different. Ideally the slabs are almost constant on flow at the lower reset temp, long constant flow.

    Let's say the total load for the day is x kWh. If the load is cycling, on off on for conventional distribution, instantaneous kW is at max but total runtime over the day is low. With radiant slab and OA reset, runtime is on much longer at the reset temp, instantaneous kW never gets to max, but stays at a lower constant demand, total day kWh load is the same.

    It's possible the multiple parallel pumps are for a high instantaneous demand system that cycles on and off, but would be misapplied on a system that runs longer on at a much lower peak instantaneous demand.

    What you need is sufficient or design flow at all times through the heat exchanger, from the boiler to the tank. After the heat is out of the boiler and in the tank, getting the heat out of the tank at a near constant on flow, longer runtime, peak demand is a lot less. Once the slabs come up to temp from off overnight, demand drops way down to a maintenance level.
     
  14. kuribo

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    Supply flows into the injection loop are changing as the supply temp from the tank drops, even with a constant or even at times a decreasing heating demand.
     
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  15. kuribo

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    I received a reply from Mr. Siegenthaler and he tells me that the scheme is not really doable with off the shelf components. He suggested a raspberry pi or the like to program a controller.
     
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  16. __dan

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    If the heat in the tank cannot satisfy demand, you would probably want a low limit control that stops the distribution from pumping and signals time to fire the boiler. An added setpoint control.

    I don't see how more parallel pumps would compensate for the heat not being in the tank. If some heat is still useable, one pump is plenty.

    The article may have been talking about a larger commercial job that has a much bigger load than a standard house IDK.

    The Tekmar I have has a thermistor for low return water temp protection. It will not load the boiler or tank unless water temp is above the setpoint. It may be possible to incorporate this to stop the Tekmar from loading the tank if the sensor placement (supply side) sees supply water temp too low. It most likely will ramp down as it approaches the low limit.

    Not sure what you asked for. One variable injection pump should have plenty flow. It's a short loop of a few feet with no head. Mine only runs at full for an hour or so at cold start in the morning when the boiler is coming up in temp and the slabs have been off for 10 or 12 hours.
     
  17. kuribo

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    I don't think we are on the same page. Multiple pumps has nothing to do with "not enough heat in the tank"...The injection plumbing is usually sized for the max flow rate at a design point. Since the temp of the water in storage will vary from say 190 to perhaps 100 (as the lowest usable temp), the flow from storage into the injection loop will vary from next to nothing at 190 to max flow at 100 at the design point. This spread in flow rates is too great to keep the flow velocities in the 2ft/sec to 4ft/sec regime recommended as good design- a pipe sized for 12 gpm at 4ft/sec will be much too large to maintain 2ft/sec at 1gpm... By using several staged pumps in parallel, you can keep the velocities in the proper range and use smaller pumps. As I said above, it's like using several staged smaller boilers instead of one large one. It's more efficient. Read the explanation in the link above.

    When the tank temps drop too low to satisfy demand, it's time to fire the wood boiler or let the back up propane mod con step in. The mod con can be controlled to do this automatically. The wood boiler, not so much....
     
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  18. maple1

    maple1
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    I am still not sure why flows less than 2fps (I typed 2gpm above by mistake - typo) in that short injection loop that has next to no head, would be so bad? I could still be missing something, and am certainly far from a pro at this stuff.
     
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  19. kuribo

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    I am not sure either and at this point am hoping it won't be- this is why I posted this question- to see if others are experiencing and issues as undoubtedly using injection mixing from storage with one pump will inevitably lead to this situation...
     
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  20. TCaldwell

    TCaldwell
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    What this tells me is that hydronically it will work however, no control logic has been specified, or tested. I’d suspect time and temp like staging another boiler in theory would work , however it would take a lot of tuning to be seamless. I wouldn’t bite until siggy shows working documentation.
    How long is the loop your worried about low velocity in, honestly I’ve only heard of a low flow concern with fouling a heat exchanger.
     
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  21. maple1

    maple1
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    I was thinking the main low-flow concern was with air issues, and not keeping micro bubbles etc. moving through the stream. Which could cause issues in system but maybe not so much in a short 'no-head' loop like that?
     
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  22. TCaldwell

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    I can tell you that I service a deceased friends garn that his wife runs now, he happened to be a hydronic engineer.
    The garn is 200 ft away from the house and main distribution system, he uses a taco outdoor reset circ with his design day at 120degf, he’s got all radiant home run loops, so only one temp to the house. The garn gets fired to 180, I’d wager 80 percent of his load is 115 or less, so the injection flow through 1-1/4 copper is pretty low. Now add the fact this is system is totally unpressurized, it’s been running fine for the last 14 years.
     
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  23. kuribo

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    Good to know. Thanks.
     
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  24. kuribo

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    My injection circuit will be about 16 feet, there and back. 4, possibly 6 elbows before heading in and out of a buffer tank with 2" headers. Siggy recommends using a pressure/ valve in this loops to provide some resistance to the pump since they are usually very low loss.
     
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  25. chew72

    chew72
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    I'm curious as to what benifet injection mixing has over a 3-way valve?

    Sent from my moto g(6) play using Tapatalk
     

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