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Thread: Navien Boilers

  1. #1

    Navien Boilers

    Gentlemen, .... What's our feelings on these new wall hung Navien 210/240 boilers, .... Gas fired.... heat and domestic hot water???...... On the secondary loop, how effective is a 600 sq. ft. radiant loop on the secondary with the use of a 3 way mixing valve?? Are there any recovery problems with domestic hot water usage, (without the use of a D.H.W. storage tank) ?.... Thanks, Frank S.

  2. #2
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    Installed many,, very few problems. Domestic water is priority so you will never run out (within flow rates). They work fantastic in radiant heat applications. The proper installer is a 100% must on these units. If your contractor has not installed and serviced wall hung units look elsewhere!
    ...

  3. #3
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    One caution; and this is true with others:
    The on board circ is only good for producing 5gpm through the primary loop. You cannot change it out for a larger one as it is proprietary.

    This should not be an issue with radiant as the will be a large Delta T between the primary and secondary. However, if you're connecting it to baseboard system that's looking for 170* average water with a 20* Delta T, you're only gonna get 50k btu's regardless of the boiler's input. None of the manufacturers tell you this and I've pinned the the ears of more than one rep to the wall in training classes over this issue. They ought to disclose this, but they don't. To me that's deceptive.

    Other than that, I've had no issues with Navien combi. They work great.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  4. #4
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    Quote Originally Posted by bobboan View Post
    One caution; and this is true with others:
    The on board circ is only good for producing 5gpm through the primary loop. You cannot change it out for a larger one as it is proprietary.

    This should not be an issue with radiant as the will be a large Delta T between the primary and secondary. However, if you're connecting it to baseboard system that's looking for 170* average water with a 20* Delta T, you're only gonna get 50k btu's regardless of the boiler's input. None of the manufacturers tell you this and I've pinned the the ears of more than one rep to the wall in training classes over this issue. They ought to disclose this, but they don't. To me that's deceptive.

    Other than that, I've had no issues with Navien combi. They work great.
    Can you explain this last paragraph again? No wall hung boiler actually puts out more than 50k btus?

  5. #5
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    It's bunk. The theory I believe is that with a 20 degree delta you can 10000 btu per gallon thus 5gpm would be 50000btus.... But the delta is not 20 degrees so the btu output changes.

    Think if if this way,,, if I am putting 140,000btus into the water with a stack temp of 130 degrees where are all te btus going????? Into the water

    Think of it as a tankless water heater application... I put 5gpm through it at 60degree incoming water temp and 120 degree outlet temp (70 degree delta) that is 41 pounds x 70 degrees x 60 minutes = 172,200btus. It is no different in a boiler application. It does not use a 20 degree delta through the boiler. It is a constant 5gpm and the delta falls as it may. As it approaches limit temp it will modulate the burner down.
    Last edited by kangaroogod; 01-06-2013 at 11:37 PM.
    ...

  6. #6
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    No, that's not what I said. The statement was referring to combi boilers that employ a plate heat exchanger to heat domestic.

    The boiler and its components are sized of necessity for domestic, space heating is secondary. Domestic is sized @ 77* Delta T. With an output of about 192k btu's, 5 gpm is necessary.

    When the diverting valve is switched to space heating, you still have the same 5 gpm internal circ. If this is connected to a high temp system such as baseboard which is designed for a 20* Delta T at 170* average water temp, then you only get 50k btu's.

    Universal Hydronics Formula: Btu = Delta T x (gpm x 500).

    The formula proves it. My field measurements and testing with instruments confirms it. And the reps admit it when challenged.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  7. #7
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    Quote Originally Posted by kangaroogod View Post
    It's bunk. The theory I believe is that with a 20 degree delta you can 10000 btu per gallon thus 5gpm would be 50000btus.... But the delta is not 20 degrees so the btu output changes.
    kangaroo,

    Are you saying that the universal hydronics formula is "bunk"? Are you also saying that the normal design for high temp baseboard is not 20*?

    If you are, then I'd like to know where you learned hydronics and which state hospital you reside in. If I'm mis-understanding you, then please clarify what it is you're trying to state.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  8. #8
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    Applying a 20 baseboard delta to the boiler is not correct. The 20 delta for baseboard you speak of holds true at 1 GPM. The secondary loop is not confined to only 1 GPM per 10,000 BTUs. Many run 4 GPM, and use a lower delta.

    The boiler at max firing rate will have more then a 20 delta per GPM.
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  9. #9
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    If you piped the space heating loop directly I the boiler on a single loop then yes the 20 degree theory is correct. This would be used in less than 1 % of applications. Primary/secondary piping which is used in all other 99% if the applications will allow the full btu output of the boiler in space heating application. Using a hydraulic separator will also satisfy the load
    ...

  10. #10
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    What goes into a T, must come out of that T

    The late Gil Carson, who was a pioneer of modern hydronics and inventor of pri/sec piping, would stand before a gathering of engineers that he was lecturing. He would begin his presentation the same way: "What goes into a Tee, must come out of that Tee." Profound. Then he would repeat himself: "What goes into a Tee, must come out of that Tee." Though he was a true genius and excelled as an engineer, he never let theory override common sense. Men are prone to do that as they get caught up in technical things.

    "What goes into a Tee, must come out of that Tee." If that is true, and common sense tells us it is, then this must also be true: nothing more than what goes into a Tee can come out of that Tee.The logic of that is irrefutable.

    While Gil may have been referring specifically to flow, it applies to btu's as well. You can't get anymore btu's out of a Tee than what is put into that Tee. The universal hydronics formula, BTU = Delta T x (500xGPM), proves this also.

    High temp baseboard has almost always been design and installed in North America with an average water temp of 170* (180* supply, 160* return). That's a 20* Delta T. That's what's out there and that's what we have to work with in the field. I'm not talking about designing a new system; I'm talking about what's existing. You cannot widen the Delta T on a system like that without also causing a lower average water temp and starving the rads farthest down the line for btu's.

    Likewise, you cannot expect any more btu's to make it to the emitters just because it's piped p/s. " Nothing more can come out of a Tee than what goes into that Tee" - my words, not Gil's, but proved above.

    The Navien has a maximum return water temp of 160. That means that with a maximum supply temp of 180* and a circ. that's limited to 5GPM, you only have 50k on the btu train. It doesn't matter how many more btu's the burner can produce, that's all that can fit on the train. And that's all that's gonna make it to the station (emitters) no matter how it's piped because that's the design of the system. Piping it p/s doesn't make it produce more Btu's. There's no magic in a Tee that would cause that to happen. "You cannot get any more out of a Tee than what goes into that Tee".

    If we widen the Delta T on the system, then the Delta T on the primary would begin to widen proportionately and then we could get more btu's. But we can't do that on an existing system due to the obvious reason stated above.

    Thank GOD for men like Gil that were great thinkers because their imaginations were tempered with common sense.
    Bob Boan


    ​You can choose to do what you want, but you cannot choose the consequences.

  11. #11
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    Bob is right.

    The formula works on btu input and delta T. The reduced output is due to the reduced delta T on the space heating side of the equation.

    While making domestic hot water, any tank-less water heater is rated for a fairly high temperature rise as the object is to raise ground water to bathing temperature. Here in Minneapolis we would use their NPE-240A chart and following 199,000 input find the approximate output at 77 rise, would find a flow rate of 5.5 gpm at 122F domestic hot water. Like all tankless water heaters the Navien controls burner and flow to maximize the efficiency of a very small low-mass heat exchanger. If you try to transfer the same amount of heat at a higher flow rate the capacity of the heat exchanger is diminished.

    So, the space heating output is reduced to roughly 55mBtuh on space heating with a flow rate of 5.5gpm and a 20 delta T.

    If you grab your B&G systems sizer scale no. Place gpm under 5.5 and read 20 and 77 delta T outputs. It starts to make sense.

    It can be confusing if you install "regular" condensing boilers everyday, as we do and are used to low delta T and high output. The standard condensing boiler has a large heat exchanger and is made for one purpose. Naturally, you can use an external heat exchanger such as Viessmann does with their new Combi-PLUS kit or with an indirect water heater but the secondary heat exchanger and the flow rate will dictate the potential heat transfer.

    I don't think the folks at Navien intends to be deceptive, since improperly sized equipment always makes more trouble than any sale is worth. But it always pays to know what you are doing.

    I had to think of the Navien as a tankless water heater that also heats space rather than a condensing boiler that also heats domestic hot water. With this in mind, it is easier to specify the right piece of equipment for a particular job. We just specified a Navien NPE for a shop/studio with one shower/tub and radiant heat load at roughly 50mbuth. Perfect.

  12. #12
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    Ok,, so 75 degree delta x 500x 5gpm = 187,500 btu? Not the plate but delta across the heat exchanger. Just curious
    ...

  13. #13
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    200,000/77/500=5.19gpm. The CH output is restricted to this number by flow. 5 gpm at 20 delta T is 50mbuh. 5x20x500= 50m.

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