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Thread: Oil to natural gas conversion - seeking advice - 1950 Cape

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  1. #1
    DIY Junior Member bepoint's Avatar
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    Default Oil to natural gas conversion - seeking advice - 1950 Cape

    This forum has been a real education. Thank you.

    Converting from oil to natural gas heat & hot water from old 1950 American Standard furnace with tankless coil.
    Cape style 1950 house located in northeast (Stamford, CT area) with original insulation and full unfinished basement.

    Here are my measurements:
    Original Double Hung Windows with storm windows: 15 windows
    Total sq ft of windows: 225
    sq ft of Exterior Walls: 1520
    sq ft of Interior Walls: 1570
    Radiators: 13
    Total Length Radiators in ft. - fin type: 44 ft (photo attached)
    Zones: one
    Unfinished Basement (214 x286) sq ft: 608
    House sq ft: 1475

    I've had a number of estimates suggesting Prestige Excellence, Baxi Luna Duo Tec series Model 40GA, and Navien - including a price range of $4,000 with the same Baxi Luna 40G.

    No one has done heat loss analysis, although one, when I inquired, suggested I take above measurements and provide them to him (still waiting for him to get back to me).

    Interested in a Modulating condensing boiler

    Would appreciate some comments on above or alternate suggestions.

    Thanks for taking the time to review this.
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    In the trades Dana's Avatar
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    Getting a reasonable heat load calculation is the first and more critical part to getting it right. Anybody quoting on it WITHOUT running the heat load calc first is pretty much a hack, to be avoided.

    When running a heat load calculation, don't put any thumbs on the scale such as unrealistically low outdoor design temps or high indoor temps. If if you like to keep it to 75F and it's been -10F at least once in Stamford since the last ice age, use 70-72F at most, and the 99% outside design temp (which for Stamford CT is about 14-15F.)

    Let's take a rough-cut first stab at it, which is probably close enough to question any body else's calc that comes in higher:

    Assuming the 1950s construction was 2x4s 16" o.c. with R11 rock-wool or fiberglass the U-factor on the walls is about 0.11 BTU per square foot per degree-F after the thermal bridging of the framing is factored in. Assuming +15F & 70F as the outdoor/indoor design temps, that's a 55F delta, so your wall losses are about:

    0.11 x 55F x 1520'= 9196 BTU/hr

    Assuming you have clear glass (not low-E) storm windows over wood sash double hungs the window U-factor is about 0.50 and the window losses are about:

    0.50 x 55F x 225' = 6188 BTU/hr.

    Lets' assume you have at least R19 in the ~ 608' of attic on 2x6 16"o.c. joists, which is a U-factor of about 0.075 after thermal bridging, so the ceiling losses are about:

    U0.075 x 55F x 608' = 2508 BTU/hr

    Assuming an uninsulated poured concrete foundation you have about a U-1 on the above-grade portion of the foundation. You have a perimeter of about 100', and assuming 4' of above ground exposure (counting down to a foot below grade to account for the ground losses) that would be ~400 square feet. And when it's +15F outside it's probably only 60F in the basement so your looking at a 45F not a 55F delta, making your basement losses:

    U1 x 45F x 400= 4500 BTU/hr.

    Figure another ~45' of U-0.5 exterior door for another:

    U0.5 x 55F x 45'= 1238 BTU/hr.

    Add it all up you get 23,630 BTU/hr.

    If the place is pretty drafty add a 25% fudge-factor for infiltration and you're at (1.25 x 23,630=) 29,537 BTU/hr, call it 30 KBTU/hr

    If the place is pretty tight it's probably more like 25K.

    A modulating condensing boiler that has any more than 15K of output at minimum-fire is a model to be avoided, since even at your AVERAGE mid-winter outdoor temp it would be cycling rather than modulating. (There's simply no-point to a modulating boiler unless it's modulating MOST of the time in winter!)

    The Baxi Luna Duo Tec series Model 40GA has a min-fire of 18K- a bit more than you really want, but maybe.

    The TT Prestige Excellence has an even higher min-fire of 30K which makes it even worse- forget about it. (The Prestige Trimax Solo 60 dials back to 16K-in 15K out, which is about right.)

    The Navien CH-180-ASME-NG (smallest of the series) pulls down to 17K in/16K out, which is probably going to be OK. The bigger versions have more hot water heating output, but also a higher min-fire. This is the one, if you go with that series.

    But if you really have that much uninsulated basement it may be better to go with something cheaper a tiny forced draft cast iron boiler or a combi-tank HW heater and spend the difference in upfront cash on insulating the foundation walls and air sealing the whole house, which would likely bring your load to under 20K. When heat loads are under 20K there's no way you can get much modulating, and depending on the fin size on those convectors it's likely that you can do just fine even at +10F outdoor temps with 140F water, making a condensing tank-type water heater solution pretty reasonable.

    So, what are the depth & height of the fins on those fin-tube convectors?

  3. #3
    DIY Junior Member bepoint's Avatar
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    Thanks for taking the time for such a detailed response.

    The fins are 7.5"D x 2"H.

    The unheated basement is 22" thick exposed field stone with 9' high exposed wood beams (uninsulated) ceiling.
    Basement has 3 old casement windows (34.5 sq ft glass) and 1 exterior wood door.
    Basement walls: 2 sides exposed and 2 sides not exposed (448 sq ft each).

    For what it's worth: there is an attached garage next to first floor and 2nd floor of cape extends over the garage.

    An inside house temperature of 68 degrees would be fine.

    "Assuming you have clear glass (not low-E) storm windows"
    Yes, that's correct.

    Also would prefer to avoid having to install chimney liner.

    I have not heard back from contractor who I initially provided measurements to. Another I spoke to on the phone who I also provided measurements to is still working on it but mentioned that maybe Baxi Luna Duo Tec 1.33 GA may be an option. He wants sq footage of each room first.

    I do really appreciate your help.

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    In the trades Dana's Avatar
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    The Baxi Luna Duo Tec 1.33 GA has the same min-fire ratings as the -40GA, but a lower high-fire output, but need an indirect tank for the domestic hot water.

    Square footage of the room doesn't matter AT ALL! It's the square footage and U-factors of the exterior surfaces. Contractors that use a crummy rule of thumb such as "Lessee, ya got 1475 feet times 25 BTU a foot gives ya 'bout 37K, round up to 50K just to be sure..." are real hacks, always oversizing the equipment, often to the detriment of both comfort & efficiency.

    With your better foundation description your basement losses are probably between 10,000-12,000 BTU/hr, which brings your heat load up to about 28-30K + infiltration. You'll note that the basement losses are by far the biggest single number (between 20% & 35% of the likely total, despite the lower presumed basement temp!). That's due to the lack of insulation. It's worth touching up the mortar, then applying 2" of closed cell polyurethane to both the foundation & band joist, painted with an intumescent paint for fire protection. That would bring the basement losses down by a full order of magnitude, cutting your heating fuel use by at least 20%, and keeping the basement both warmer & drier.

    I don't have a good reference source for the output of finned convectors of 2" x 7.5" dimensions, it's probabably comparable to the 8" deep Modine convectors (see p11.) but my WAG is that they'll run on the order of 2800 BTU/foot or better @ 190F average water temp, 1400 BTU/ft @ 140F AWT, 700 BTU/ft @ 120F AWT. With 44' and a heat load of 30,000 BTU / hr you're at 680 BTU/ft, which means that you'd need maybe 130F boiler output on design-day (130 F out, 110F back), which is PERFECT for using condensing burners. (It's all one zone? Or is it two?)

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    DIY Junior Member bepoint's Avatar
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    It's one zone.
    Maybe down the road a couple years add a zone for small heater/radiator to take chill out of basement for work area only (not to be finished).

    "Square footage of the room doesn't matter AT ALL! "
    Sadly, this contractor seemed the most knowledgable (although, now that I think of it, he mentioned that the Prestige Excellence might be too small).

    I went back and checked fin measurements (only this time on another radiator) and discovered that this one is 5.25 x 2. I will have to go around house and confirm others. I'll get back later with this info.

    Thanks for taking the time on this.

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    DIY Junior Member bepoint's Avatar
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    Here are the updated and more accurate fin measurements after taking all covers off and measuring exact length of fins rather than length of radiator from pipe to pipe:
    7'4" length of fins that are 7.25"x2" (not 7.5"). These wider ones are installed under 2 bay windows.
    33'3" length of fins that are 5.25"x2"
    Total Length: 40'7" (not 44')

    Thanks.
    Last edited by bepoint; 10-08-2013 at 02:46 PM.

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