Help with Nat Gas Sizing

Discussion in 'Tankless Water Heater Forum' started by tjk031, Dec 5, 2013.

  1. tjk031

    tjk031 New Member

    Messages:
    7
    Location:
    Roswell, GA
    Hi,

    I'm replacing a tank WH with tankless. Below is my scenario for the gas run. The run to the furnace is existing.

    What do you suggest for Gas pipe sizing? Should I run a 3/4" home run from meter directly to tankless WH? Or size up the main run to 1" and then branch off 3/4 to WH and Furnace? Or do you think I will be OK with 3/4 all around?

    Thanks for the assistance.

    Gas Pipe Size.jpg
  2. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    A 3/4" home-run to the water heater more than 26 feet away doesn't cut it, not even close. Remember count about five equivalent-feet for every 90 degree ell or tee. Even if your piping lengths add up to 25-30 feet, you're probably somewhere well north of 40 equivalent feet. Figure on a 1-1/4" home-run to the water heater, and a 3/4" or 1" run for the furnace, both teeing in as close to the regulator as possible.

    The K-BTU supported for the different pipe lengths and diameters are found in this matrix:

    [​IMG]
  3. Reach4

    Reach4 Well-Known Member

    Messages:
    2,806
    Location:
    IL
    Do you think he should get a meter upgrade or just new piping starting at the meter output?
  4. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    Most meters/regulators on single family homes can handle the ~300K max draw, but he needs to check the the max rated capacity (usually found somewhere on the meter or regulator or both.) If it's a smaller service like those on some apartment buildings he may need to upgrade the gas service, or back off on the tankless.

    FWIW: There are almost no normal sized homes in Roswell GA with a true heat load big enough to call for an 88,000BTU/hr furnace as long as there is glass in the windows, and doors that shut. When it's time to replace the sucker it's a good time to down-size to something closer to the true heat load. (I suspect even a 40K furnace has enough burner to cover the actual load almost 2x over.) Recovery times from overnight setback would increase, but the runs would be longer and noise levels lower, and the average temperatures more even. Often times these oversized burners get installed as part of a package HVAC unit with not quite as badly oversized air-conditioning, which may be the case here.
  5. tjk031

    tjk031 New Member

    Messages:
    7
    Location:
    Roswell, GA
    Thanks again Dana!

    I called the Meter company and they said I should be fine with the exhisting meter and pipe from the main... But I'm kind of confused because on the meter it says Cubic Feet CL 125-250. So would it max out at 250?

    Yeah the furnace is backup heat to a heatpump so it only gets used on cold temp days and nights. The furnace was a package with 3ton Heatpump and 4ton coil (at this point I wish it were matched so it would be more effective taking out the humidity during the summer). Below is the AHRI certificate... and link to Lennox furnace spec page. I'm I correct with reading 88,000 btuh input?


    http://www.lennoxcommercial.com/pdfs/datatables/Lennox_Furnaces_data.pdf

    AHRI Cert.jpg
  6. tjk031

    tjk031 New Member

    Messages:
    7
    Location:
    Roswell, GA
    Sorry text is a little small...

    COIL LENNOX CH33-49C-2F Coil/4Ton/Cased
    FURNACE LENNOX ML180UH090E60C-01
    HEAT PUMP LENNOX XP14-036-230-10
  7. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    Yep- looks like 88K in.

    And yes, the meter nomenclature indicated 250 cubic feet per hour. Most natural gas is good for about 1000-1030 BTU/cf, so unless your local gas grid has exceptionally high BTU content gas you're probably maxed out at less than 260K.

    But I suspect your controls are set up to only kick on the gas burners below some fairly low temperature, and as long as you aren't taking two simultaneous showers or filling the tub at 10gpm while the furnace (rather than the heat pump) is on, it won't be an issue. The heat pump can probably still deliver the full ~32K heating at your ~20-22F outside design at at high efficiency, which is probably more than your actual heat load at that temp if your ducts are tight, and fully inside insulated conditioned space. If the ducts are in an attic above the insulation, maybe not. If the ducts are in the crawlspace under the house, that's another good reason to insulate and seal the crawl.

    If you backed off to a ~150KBTU/hr tankless you'd have some margin and could still take two simulatneous showers, but your tub-filling rates might fall off a bit. That's not a problem for standard sized tubs, but could be slower than ideal for a big soaker tub or spa.
  8. tjk031

    tjk031 New Member

    Messages:
    7
    Location:
    Roswell, GA
    Thanks Dana! For the Gas I will have the plumber either re-work main line coming into the home from the meter or have a larger diameter run as close to the meter as possible.

    I'm now think about going with Tagagi (T-H3-DV) or (T-H3S-DV).

    If anyone has any comments on Takagi or these models it would be greatly appreciated.

    I have 2.5Baths/Kitchen/cloths washer in home
  9. Dana

    Dana In the trades

    Messages:
    3,028
    Location:
    01609
    I have no direct experience with their condensing models, but Takagi is one of the top 5 vendors in the biz with a fairly long and mostly good track record. (I'm using an older Takagi KD20 tankless as a modulating space heating boiler in my house.) Distributor support in my area is all but non-existent, but their national telephone support are willing to talk homeowners through debugging things if/when things go awry.


    Even the smallest of that series has enough output to serve up two simultaneous showers at 2 gpm/per shower at typical GA incoming water temps.
  10. lifespeed

    lifespeed Member

    Messages:
    333
    Location:
    California
    There is some variation among gas sizing charts depending on the pressure drop, eg: 0.5" or 1" of Water Column. That gas sizing chart appears rather pessimistic. You do not need to use 1-1/4" pipe to the water heater, 1" is more than adequate. You can either run 1" directly to the meter, or run 1-1/4" from the meter to the tee where the water heater pipe splits off and then drop down to 1".

    I suggest you look at gas pipe sizing charts from a reliable source and make sure you're getting good information to plan your install. I used about 60' of 1" pipe reduced from a 1-1/4" main with 3 elbows and it works very well. Depending on the chart you use, 60' of 1" gas pipe is plenty for 200K BTU.
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