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Thread: Help With Quotes for NEW Oil Boiler

  1. #1
    DIY Junior Member Greatwhitewing's Avatar
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    Default Help With Quotes for NEW Oil Boiler

    First of thanks for all your help over the past few weeks. I learned a lot...

    I have my oil usage of 775 gallons for winter 09-10 and 1107 for all of 09. 720 gallons for 10-11 winter with 987 for 2010 total. 430 so far 2011 total. Still don't have heat load calculations in hand, might be in mailbox tonight. 1700 sq foot heated space.

    1. Viessmann VR1 with Superstor and controls VERY well respected contractor at $9893, Very detailed quote.

    2. Same system without controls from a decent vendor at $8490

    3. Buderus G115WS-3 no controls with TraingleTube 56 gallon from decent vendor at $9700

    None of the vendors recommended a storage or buffer tank. I like the TT for added mass so asking the first two contractors to price it. The Viessmann seems like the best "sized" unit at 80k BTU. I have the Weil Mclain quote but at 98k it's way over sized.

    leaning towards option 1 because of reputation and TriangleTube depending on additional cost.

    Open to opinions and discussions.

    Thanks

  2. #2
    In the trades Dana's Avatar
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    Using the NORA FSA calculator and putting in an 83% boiler w/tankless coil with 96K of output, 2% idle loss, using Hartford's weather data, 990-1000 gallons/year gives you an estimated whole house heat load of ~38K at a design temp of -1F. If we back off the steady-state efficiency estimate to 80% it's giving us ~36K @-1F. Backing off to 78% gives us ~35K. No matter what your real-actual design day heat load is no more than 40K. For analysis purposes lets use 36K. Heat loss calculators typically hit 15-25% to the high side, so don't through the contractor out on his ear if he's saying 45K, but if they come back with 60K they've made significant errors or very wrong assumptions.

    The average as-used efficiency due to oversizing for the 83% steady state efficiency case is 68%.

    The average as-used efficiency due to oversizing for the 80% steady state efficiency case is 65%.

    Modeling the Viessmann + indirect at 90K output, 0.3% standby idle-loss the as-used AFUE up to ~80% (after adjusting the fuel use numbers down to where it indicates a ~36K load with that boiler which, is at ~800 gallons/year.)

    Modeling the Buderus G115WS-3 w/85K of output, same standby ends up with similar results- ~800 gallons/year, ~80% as-used AFUE.

    Unless you have a big tub to fill there's little to be gained by going with a bigger indirect- the burner output alone is enough to support an endless 2gpm shower flow, the mass is only buffering for the intermittent peaks of multiple draws. If you have a big soaking-tub or spa to fill the bigger indirect lets you fill it somewhat faster, but that's about it. When operated as a separate zone the mass of the tank does nothing for your space-heating efficiency.

    The water temp required to deliver 36K to the house depends on the total running length of baseboard. To be able to run with 140F water at design conditions would take between 110 -120 feet of fin-tube. What ultimately determines the design condition water temp is the zone-by-zone heat-loss/baseboard length ratio though, which could be higher than the whole-house heat-loss/baseboard ratio. But measure it- the optimal outdoor reset curve depends on it, and it may affect how the system is plumbed to the boiler.

    DO ask the contractors about the short-cycling on zone at low water temps using outdoor reset, and make them calculate a minimum burn time at the lowest operating output temp that the boiler tolerates. If it's under 5 minutes (which it almost surely is) ask about the cost to add a buffer. (Print out the system diagram pictures as well.)

    FWIW: As a quick exercise, while propane is more expensive in terms of source-fuel BTUs, a right-sized modulating condensing propane boiler is possible, and would deliver north of 90% efficiency, which could make it cheaper to operate. There are MANY propane fired mod-cons with minimum modulated output under 20K, and the max output of even the smallest are typically over 45K.) Something like a Triangle Tube Solo 60, Peerless Pinnacle T-50, or Burnham Alpine ALP080 would be a perfect fit, and would deliver long modulated burns at high efficiency (fewer and higher efficiency if the system is buffered with some mass), and you'd be able to pull a realistic 92-93% as-used AFUE out of it. In source BTUs/gallon terms you get ~138KBTU out of a gallon of oil, 92KBTU out of a gallon of propane. So at your projected new-boiler 80% AFUE you're averaging 110KBTU/gallon of heat delivered to the house (or hot water), or with a 92% AFUE propane burner, ~86KBTU/gallon. So you'd get 28% more delivered-BTUs/gallon with oil than with propane, but is the price per gallon 28% more than propane? (Usually not, in MA anyway.)

  3. #3
    In the trades Dana's Avatar
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    I should note that for the NORA FSA to be accurate it must be either a buffered or single-zoned system. With it cut up into 3 zones that 65-68% AFUE estimate on your existing boiler is probably more like 55-60% due to short-cycling on zone calls. (This also means your design heat load is somewhat less than 36-38K.)

    A new-school highly insulated boiler will have lower standby losses, but will take a similar hit in efficiency if allowed to short-cycle- the modeled 80% AFUE might really be closer to 70% (which is WAY better than 55-60%.) By eliminating short-cycles you'll be at about the estimated 80%, (maybe slightly better) and save 12-13% on total fuel use compared to short-cycling operation, which means the ~800 gallons/year can drop down to as low as ~700gallons/year, which is the whole point of buffering the system. With fin-tube there's not enough combined mass in the emitter & boiler for it to NOT short-cycle, even set up with a large temperature hysteresis on the boiler's output temp, particularly. (And a big hysteresis to lengthen the burn using the modest mass of the boiler itself would also make the whole outdoor reset point moot.)

    Running it at higher temp would also lengthen the burns, but it takes about 155-160' of 180F baseboard to be perfectly balanced with 90K of boiler output (or 275-300' of 140F baseboard) and I suspect that 150' may be more baseboard than all three zones combined, which means it'll short-cycle on any zone-call, even with 180F+ water. Try timing the burns on your existing system, which is probably 1.5x higher mass than any of your newer prospects, and still bound to have burns shorter than 5 minutes on zone calls, even if you set it up the HI/LO limits to 180F/ 140F (40F of hysteresis.)

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