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Combi Boiler Design Suggestions

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GRod
GRod Member Posts: 13
in Gas Heating

Greetings,

I'm upgrading my boiler to a new Navien NFC-250/200H combi boiler. Here are my particulars:

3500sqft colonial home, 3 zones, fairly new construction (2013), 3 ecobee thermostats, NY (Long Island)


1st floor - 69 ft of slant fin baseboard

2nd Fl. - 71 ft of slant fin baseboard + bath has a small air coil in the bottom of a cabinet( bath is always the coldest room)

Basement - Hot Air Coil at Air Handler


I'm upgrading from my current system due to my indirect water tank needs replacement and going with this Combi unit.
My biggest question or concern is whether to go with individual pumps (like the 007e or 0015e, or Grundfos Alpha 1558) for each zone (3 circulator pumps)

OR

go with a delta-T or P pump with zone valves (sentry valves) ? Really trying to see which one is best for my case now that Im going with a mod/con boiler and trying to minimize short cycling. Dont know which one will be the best efficiency wise so that I can operate the boiler in condensing mode for the longest time possible even though I do use baseboards for heating my home.

Any feedback is greatly appreciated.

Thanks

Comments

  • hot_rod
    hot_rod Member Posts: 27,667

    69' at 500 btu per ft= 34,500 btu/hr, call that a 3.5 gpm required

    71' is 35,500, call that 3.5 gpm also

    What size is the air handler? call it 30,000

    so a total load of 100,000 @ 20 delta you need to move 10 gpm

    10 gpm is within reach of the small delta P circs like an Alpha 15-58

    So 1 pump and 3 zone valves is my choice. The delta P circ will modulate with ZVs opening and closing.

    Individual zone pumps to move a small 3 gpm load seems overkill to me, but plenty of opinions for pumps vs circs

    That boiler can modulate down to 13,000 btu/hr. You want the boiler to be able to modulate down near the smallest zone requirement, so you are good there.

    I would go into the control and limit heating output to maybe 100- 120,000 btu/hr

    So now you have a boiler with 200,000 output in DHW mode, but you downsized it for heating loads. That helps limit cycling.

    Outdoor reset is a nice feature if you take the time to dial it in.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    GRod
  • GRod
    GRod Member Posts: 13

    Revisiting this installation for my home. I have a Cast Iron boiler operating with an indirect water heater. The water heater needs replacing (12 yrs old and leaking inside) so it got me investigating mod/con combi units. My plan for now is to get a turbomax 30-3 as a water heater with the same boiler I have now and replace with a mod/con when the CI boiler craps out. What intrigued me was that the Turbomax can be used as a buffer tank as well as provide me DHW. That should help with short cycling of the mod/con and less abuse of the unit. Currently looking into the Bosch Greenstart boilers. Whats the consensus of this boiler? I'm also getting a quote for a Greenstart Combi unit from a plumber just to get an overall picture.

  • mattmia2
    mattmia2 Member Posts: 17,120

    are you going with a combi boiler (a boiler that has a heat exchanger built in that makes hot water instantaneously from the boiler(and some models have a very small storage tank to provide hot water while the boiler fires up))

    or

    are you going with a modulating condensing boiler and indirect water heater. an indirect is heated by boiler water through a coil in the tank (or in the case of a turbomax stores a volume of boiler water and heats domestic hot water through a large coil in that boiler water) but hot water is stored in a tank and the boiler only fires to reheat that tank as the stored water is depleted

    setting up the indirect water heater as a buffer tank can get complicated.

    it may be simpler and less expensive to buy an indirect water heater and size it to store enough hot water to cover your needs with a boiler sized to the heating load.

    if you size the boiler to the heating load, a modulating condensing boiler should be able to modulate to the load of the smallest heating zone unless your zones are very small. if you pick the right boiler the controls for this are built right in to the boiler and ot can match the load without a buffer tank.

  • HVACNUT
    HVACNUT Member Posts: 7,505

    Does the combi have the same output as the Turbomax? Do you need the output of a Turbomax? Rain heads? A lot of people? Jacuzzi? Laundromat?

  • GRod
    GRod Member Posts: 13
    https://forum.heatinghelp.com/discussion/comment/1896577#Comment_1896577

    You are correct. The buffer tank idea will be more complicated and dont know if someone will be able to properly install it. Also it will be more expensive. I think I will just rather keep it simple and just get a HTP tank (thats what I have now, although its the basic unit contender …non Stainless Steel) and when my CI boiler goes, I'll replace it with a heat-only mod/con boiler. Seems that a combi unit has limitations eventhough we are family of 4 with full baths.

  • mattmia2
    mattmia2 Member Posts: 17,120

    a combi might keep up with 4 showers at once in some place that is warm where the incoming water is say 60f but where i'm at the municipal water is like 35f at some times in the winter and a single residential size boiler isn't going to be able to heat that instantaneously.

    if you are buying a stainless indirect, look at the chloride content of your water and the specs of the tank, places with a lot of road salt tend to have municipal water with a lot of chlorides.

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