Question on boiler Sizes

Discussion in 'Boiler Forum' started by BillTheEngineer, Feb 14, 2013.

  1. BillTheEngineer

    BillTheEngineer Member

    Messages:
    34
    Location:
    Hauppauge, NY
    I read a fair number of posts related to heating system sizing. I always read the same info, every unit always seems to be 2 or 3 or more time the size it needs to be.

    That I can accept, but what I don't understand is that there doesn't seems to be many appropriate sized heat units for the typical home and even less of a selection for a small home. The only units that seem to be capable of being properly sized are modulating condensing gas units. Even then the only ones that seem to be sized correctly would generally be the smallest size from just about any mfg.

    Are there any oil fired boilers in the 40k BTU range?

    Seems like a rant to me, and it is, but just trying to see if I am missing something.
  2. BadgerBoilerMN

    BadgerBoilerMN Master Hot Water Mpls,MN

    Messages:
    303
    Location:
    Minneapolis
    With low-efficiency cast iron boilers it is generally accepted that any size up to 150% of design load is OK. The high mass of cast iron absorbs and holds heat somewhat mitigating the short-cycle penalties. Since you have a low efficiency appliance and natural draft the size factor is not a great.

    If you are condensing steady state and shoulder season performance can be dramatically compromised if the low-fire exceeds the design load...not all that uncommon. The answer is for the homeowner to insist on a sample of a proper Manual 'J' heat load and his own once the deposit is made and before the unit is delivered.

    They look like this:

    http://www.badgerboilerservice.com/images/SampleHeatLoadAnalysis.pdf
  3. Dana

    Dana In the trades

    Messages:
    2,793
    Location:
    01609
    There are physical limit to how small you can jet an oil burner and keep it clean, which puts the smallest oil boilers in the ~60KBTU range. SFAIK there has been no commercial release of burner technology that takes a different approach.


    AFUE testing is done at 1.6x oversizing, so if your actual design condition heat load is < 35KBTU/hr even the smallest oil boilers out there won't quite hit their numbers. But how far it slips down the efficiency cliff varies. If it has the dumbest bang/bang controls it can be a fairly severe hit, but much of that can mitigated if smarter heat-purging controls (and buffering thermal mass) is incorporated. As a retrofit solution the Intellicon 3250 HW+ is pretty good, but some of the newer oil boilers have their own smart controls (and better insulation) to better deal with the oversizing problem.

    FWIW: In a L.I. location the better class ductless air source heat pumps (Fujitsu Halcyon, Daikin Quaternity, Mitsubishi H2i, etc.) can cost less than half as much to heat with than oil at current oil & electricity pricing, and in open floor plan homes a 1-2 ductless-head solution can work pretty well, most of the time. If your 3-4x oversized oil burner is still in decent shape, buying a ductless system and using the oil boiler as the Hail Mary backup heat is usually a better investment than the latest-greatest oil burner that's "only" 2x oversized, or a propane-fired mod-con.

    The math sort of works this way: A right-sized ductless in a L.I. climate will run a seasonal average coefficient of performance of about 2.7-3.0, and a 3x oversized oil boiler rated 85% AFUE will run about 75% efficient.

    At $4/gallon and 75% efficiency, you get (0.75 x 138,000=) 103,500 BTU of heat into the system for $4, or ~26000BTU/$

    At 20 cents/kwh and a COP of 2.7, you get (2.7 x 3412= ) 9212 BTU directly into the rooms (no distribution losses, which you likely have with the oil boiler), for 20 cents, which is ~46,000 BTU/$.

    You'd have to have Block Island type electricity pricing for a ductless to be more expensive than oil, and even if you bought the smartest-best oil boiler that actually delivered 87-88% efficiency in your actual system (not likely) the ductless would still beat it on operating costs hands-down.

    And unlike oil boilers, it IS possible to right-size ductless systems for your actual heat load. Of the three vendors/series mentioned, all have a specified output at -15C (+5F), which is below your 99% outside design temp. And when a ductless is oversized by 25-30% it actually improves the efficiency slightly over what a perfectly-sized system would deliver (but not by enough to make intentionally oversizing worthwhile), since as fully modulating systems they get higher efficiency at part load than when running flat-out.

    If oil, propane, and electricity are your only options, you really owe it to yourself to look at ductless heat pumps (mini-split or multi-split.)
  4. BillTheEngineer

    BillTheEngineer Member

    Messages:
    34
    Location:
    Hauppauge, NY
    Dana, thank, you pretty much confirmed what I was thinking. Plus answered a second question that I did not even ask, at what point would a ductless air source heat pumps make sense. I haven't looked to far into it, but I know Long Island electricity rate are one of the highest in the country, but I seen oil isn't to cheap either. For the shoulder season a ductless air source heat pumps probably make sense.

    I am in the midst of buying a house, so most of this does not effect me yet. But as I see different places where natural gas is not available or prohibitively expensive to have installed, and there is a 50's or 60's vintage boiler with a DHW coil, I consider what my options are and what it's going to cost to heat and make DHW.
  5. Dana

    Dana In the trades

    Messages:
    2,793
    Location:
    01609
    In the shoulder seasons the average coefficiency of performance of the mini-split will be more like 4 when the daily average temps is above 45F. At the same time the idle-losses of an oil boiler rpresent an even larger fraction of the fuel use than it was during during mid-winter. Altogether it makes heat solely with the ductless during the shoulder seasons a slam dunk from a financial point of view.

    I thought LIPA residential rates peaked about 6-8 years ago at about 25 cents- delivered(?), and had now fallen to about 20cents, but I don't have an easy means of verifying that since the fuel charge portion of the bill varies from month to month and isn't published on the LIPA website. But if you do the math on 25 cent electricity or even 30 cents rather than 20 cents it doesn't change the general picture- it's still going to be a lot cheaper per BTU-delivered than $4 oil (or $4.35, the current L.I. average price for heating oil.)

    Of course when oil drops to $3 again (in reality, rather than just our dreams) and electricity passes 35 cents on the way up the equation takes a finer-tipped crayon to work out, but I'm not holding my breath on those coming to pass any time soon.
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