Replacing electric boiler with gas. need piping help

Discussion in 'Boiler Forum' started by Becks, Aug 25, 2015.

  1. Becks

    Becks New Member

    Joined:
    Aug 25, 2015
    Location:
    Minnesota
    Hello, local techs have been no help so i am turning here. I had a electric boiler feeding my shop floor. 5 runs of 250 ft pex. The electric had one of the menards pre fabbed panels so not much thought installing that. I bought a used munchkin 80m rev 2 that was set up primary secondary with a hot water heater on it. So it has 3 pumps available and 5 zone valves. I dont need the hot water heater and will only be setting up one zone. Can anyone show me the way to plumb this? I dont understand the primary secondary advantage over using a mixing valve. Also for a bonus if anyone has any info on the munchkin 80m as far as what i should check before firing it off it would much appreciated. Thanks!
     
  2. Dana

    Dana In the trades

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    01609
    Whether or not you can get away with a direct-pumped or mixing valve approach depends on the flow requirements of your radiation vs. those of the Munchkin. With 5 loops of PEX the odds are your radiation flow requirements will differ than what is ideal for the Munchkin (1 gpm per 10,000 BTU of total output per Figure 5-1 page 23 of the manual, or figure 6 page 22 of the other manual, or Figure 5-2 page 15 of the Rev-2 manual, (so ~8 gpm for the -80m.) If that's a reasonable pumping rate for your radiation and you size the pump to be able to deliver the full 8gpm at the calculated pumping head for the installed plumbing it should be fine. But if your radiation only needs 2 gpm and fairly high head that's going to be a lot of unnecessary pumping power, and may even erode some of the radiation plumbing, depending on what it all is.

    The flow requirements of your radiation requires a level of analysis not necessarily well suited for "design by web-forum", but at the very least we'd need to know the design load, how many square feet of radiant floor, and the radiant floor type & size of the PEX to even take a WAG at it.

    The bonus question find the answers in the documentation here. Read it all to develop sufficient familiarity with what you're dealing with.

    Hopefully the gas plumbing was a dedicated home-run between the boiler and the regulator, and adequately sized for the BTU and equivalent-length of the gas plumbing. With only an 80K burner you have a bit of leeway, but modulating burners are sensitive to pressure fluctuations.

    The primary/secondary question is something of "hydronics 101" question, and there are plenty of ways to screw it up (some subtle, some not-so-subtle), which isn't a confidence builder here. Read up a bit on it prior to hooking it all up, and post a picture of the installation that shows all of the near-boiler plumbing before firing it up. If you're going to plumb it primary/secondary with closely spaced tees as the hydraulic separator, make sure the tees are as close together as you can possibly make them. Be sure to install the expansion tank close to the input side of the pump driving the highest volume (or on the hydraulic separator itself in a primary/secondary, with both pumps close to the hydraulic separator) to limit impeller cavitation.
     
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  4. Becks

    Becks New Member

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    Aug 25, 2015
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    Wow, thanks for you time already. I understand your"design by web" issues (I almost didn't even post this for fear of being flamed) but I'm gonna try to learn this thorough so I appreciate you not just blowing me off. I have a pretty decent understanding of plumbing and am really excited to get this done. The shop is 1200 square ft, r40 in ceiling, r21 in walls, 18x8 overhead door at r8, 96 sq ft of argon windows. Pex is 1/2 oxy barrier in concrete. 3 inches of foam under slab. Gas line is not run yet but I'm switching to high pressure nat gas and running 1/2 cps(?) 120 ft right after meter outside. This info help?
     
  5. Becks

    Becks New Member

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    Aug 25, 2015
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    Minnesota
    image.jpg Here is the boiler I bought as it was set up.
     
  6. Dana

    Dana In the trades

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    We need to know the total area of the walls (less the windows & doors) to estimate the heat load. The basic arithmetic is:

    U-factor x square feet x temperature difference= BTU/hour

    To that we have to insert a WAG or fudge-factor for air infiltration.

    Are the slab edges insulated, as well as below?

    The 99% outside design temp in Red Wing is about -12F (Minneapolis' is -11F, Eau Claire's is -13F, Rochester's is -12F.), so assuming a 68F interior design temp that's a temperature difference of 80F.

    Your U-factors are approximately:

    2x6/R21 wall (assuming 20% framing fraction): 0.065 BTU/hr per square foot per degree-F

    R40 attic (assuming blown insulation in trusses 24" o.c. with 2x6 chords): 0.028 BTU/hr-F-ft^2

    R8 door (assuming 2" thick foam, with wood spline structure): 0.17 BTU/hr-F-ft^2

    Dual-pane argon single low-E windows w/vinyl frame: 0.34 BTU/hr-F-ft^2

    So your door losses are:

    0.17 BTU/hr-F-ft^2 x (18' x 8') x 80F= 806 BTU/hr

    The window losses are:

    0.34 BTU/hr-F-ft^2 x 96' x 80F= 2,611 BTU/hr

    Assuming a 1200' ceiling the ceiling losses are:

    0.028 BTU/hr-F-ft^2 x 1200' x 80F= 2688 BTU/hr

    That's 6105 BTU/hr so far.

    The remaining losses to estimate are primarily wall loss, infiltration loss, and slab loss. The infiltration is going to be WAG, the slab losses are too, but dependent upon the rough estimate of the slab temperature needed to support the load.

    It's a bit difficult to figure out the near-boiler plumbing from that picture, but the expansion tank appears to have a less-than-ideal placement relative to the pumps.

    Assuming the existing radiation pump has been adequate and is pumped direct with the electric boiler (rather than primary/secondary), what has the temperature difference between the return water and boiler output been, and what is the make/model of the pump?
     
  7. Becks

    Becks New Member

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    Aug 25, 2015
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    Minnesota
    I sold the electric boiler and the nibco rhp1 with it. I never checked what the temp difference was. I think I am a bit high with the 80000 btu munchkin but i got a good deal on it and with the overhead door being opened a couple times a day i think loss will be higher. If i was to copy the nibco rhp-1 with my own components, would this be the simplest solution? Would adding a circulator on the return line help with flow? rhp1.jpg
     
  8. Dana

    Dana In the trades

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    It would be better to know the delta-Ts and flow numbers rather than hack at it. An electric boiler can tolerate much bigger deltas than a mod-con boiler (typically 50F max in to out with 20-30F being more optimal.)

    Opening and closing the garage bay doors several times a day may have a modest effect on fuel use, but is completely irrelevant to your design heat load. If you intend to leave the door open on the coldest 1% of hours in in a year, that will have an effect on your design heat load & boiler sizing.

    Did you calculate your wall losses yet? (They are probably the lion's share.) It's true that an 80K boiler is probably a bit oversized for any 1200' building. But as long as the min-fire output is lower than your average winter load it's fine. Taking a WAG at it I'd estimate with infiltration and slab losses you're probably going to come in between 15-25,000 BTU/hr @ -12F. The min-fire input to the 80M is 27,000 BTU/hr, so at 95% efficiency that makes it's output ~25-26,000 BTU/hr. It won't really be modulating with very long or continuous burns, but with high-mass radiation like a slab it shouldn't short-cycle (provided it is set up correctly.)
     
  9. Becks

    Becks New Member

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    Aug 25, 2015
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    Minnesota
    I come up with around 28000 heat loss combined.
     
  10. Dana

    Dana In the trades

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    Tall walls, with lots of wall area? Whatever...

    If that's the conducted heat loss, throw in another 15% of fudging for other factors, call it 32,000 BTU/hr. With 1200' of slab to work with that's ~25 BTU/hr per square foot, which is going to require a slab surface temp of about 80F on design day. I'm not a radiant slab expert, but I'd guesstimate you could get there with 110-115F average water temp, not more, which means it would be in the condensing zone even under design conditions.
     
  11. Becks

    Becks New Member

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    Aug 25, 2015
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    Minnesota
    So with that being said, are you willing to point me in a direction for piping this thing? Primary/secondary or copy the nibco panel with a mixing valve? Just a link to a online sketch would be great! Thanks for your time!
     
  12. Dana

    Dana In the trades

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    01609
    Without doing the math on the exact plumbing, it's safer to run it primary/secondary using the manufacturer's recommended primary pump, which gives you a lot of latitude for playing with the radiation flows. This is the "design-by-web-forum" nut that can't be cracked- nobody is going to run all of the numbers for you for free, even if they had the hydronic design/radiant floor design software (which I don't.) With the napkin-math design and a primary/secondary configuration it's harder to screw up badly. Read the installation manuals carefully.

    In you case it probably CAN be done with a mixing valve, but without actually designing it there is going to be real risk of damaging the boiler, or an under performing system. The down-side to primary/secondary is the extra pump, and perhaps unnecessary pumping power, but that's worth paying to avoid the potential down-sided.

    The alternative would be to pay somebody to actually design it for you using the real numbers. Ping Morgan Audetat (posts here as BadgerBoilerMN) with a private message if you think want to go that route- he's even reasonably local to you, and has the necessary design tools & experience
     
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