Change from fuel oil burner to propane.

Discussion in 'HVAC Heating & Cooling' started by Rughead, Jun 30, 2009.

  1. Rughead

    Rughead New Member

    Messages:
    48
    Location:
    Scarsdale, NY
    Hi there. We're back to our new (old) house in NY in July. It's small, like 1200sq ft, and has a big old fuel oil tank in the cellar that occupies too much space. Am getting Paraco to install a large propane tank out back under the deck for a gas stove/oven, BBQ/grill and a few deck lights. Our current heating system is fuel oil forced hot air combined with central AC ducts. We'd like to convert this to a propane fired heating system. Is this an easy change? It may also affect the size of tank we get. There's no natural gas on our street, hence the LPG. Figure that since we're doing it for the cooking we might as well do it for the heat as well and rid ourselves of the diesel tank in the cellar. Any advice and opinions are most welcome. Cheers and best regards, Rug.
  2. PEW

    PEW DIY Senior Member

    Messages:
    487
    We did it when we moved into a new house. Wanted to get rid of the inside tank, noisy burner, and oil smell.

    Installed an underground tank, moved the heater location to the attic. Installed a high efficiency heater and went from a 125k btu oil to an 80k btu gas. (house about 2800 sf) Kept the same ac compressor, but changed the evap. Ended up converting one fireplace, cooktop to gas, and added the outside grill to the mix.

    Was not big thing, would do it again in a heartbeat.
  3. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    Retrofit burners are relatively easy (if not in the realm of DIY stuff.) What's more, propane/gas burner outputs can be sized lower than the lowest jet that works with oil, so you have a better shot at "right sizing" the burner to improve the seasonal efficiency. Odds are pretty good on a 1200' place that you're running the smallest practical jet on the oil burner and it's still 3-4x oversized for the actual heat load. Being oversized for the load that much drops it's actual as-used efficiency from an ~80-85% AFUE efficiency number to the ~60% range.

    If you haven't already, have a Manual-J type heat loss calculation done on the place, and if you can, don't oversize the propane burner by any more than 10% greater than the calculated number. (Don't settle for a "40 btus per square foot" or similar wild-assed-guess ballparking method or anything similar- they're guaranteed to be wrong, usually significantly on the high side of actual load. Measuring glazed area & window type, doors, wall & attic insulation, wall area, foundation type etc are all necessary to get it even close.)

    Alternatively, you might get quotes on what a right-sized (based on a heat-loss calc) condensing propane furnace costs. The air-handler for the oversized oil-unit is probably overkill for a right-sized burner (unless it's a multi-speed, and you can crank it down), which ends up using more electricity and makes it feel a bit drafty. With a smaller lower-speed air handler on a new properly-sized 90%+ AFUE condensing furnace it'll feel more comfortable, use half the electricity and 15-25% less fuel than with a retrofit propane burner on the old oil-fired beast.

    But if the air handler speed is married to the AC system design, it may turn into a big design headache, and the retrofit burner may be "good 'nuff". Just be sure to not make it any higher output than it needs to be and it'll be as-cheap or cheaper to run than the oil burner. (If only 2x oversized & adjusted for proper combustion efficiency by a competent installation tech, it could still hit ~75% for as-used efficiency. At 3x oversizing & up it's already well down the slope of the efficiency cliff.)
  4. Rughead

    Rughead New Member

    Messages:
    48
    Location:
    Scarsdale, NY
    Many thanks for this very useful information. I will engage a reputable HVAC company in NY to do all the calculations and make offers based on the alternatives. Am so glad we can do this. Might return with a few more questions once we get started with the process. Again thanks and best regards, Rug.
  5. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    This may be obvious, and a good contractor will not fail you. BUT....I would personally ensure that the VERY FIRST step in this project is to remove the oil fill tube. There have been horror stories in the news about where the tank was removed, and a zealous oil truck guy wanting to keep all his customers topped off....fillled the basement!!!
  6. Rughead

    Rughead New Member

    Messages:
    48
    Location:
    Scarsdale, NY
    Thanks Jimbo. Yes it would seem obvious but stuff happens, I know. Good advice and I'll be sure that's the very first item removed once we get started. Cheers, Rug.
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    1 Therm of energy can be had from approx. 0.71 gal of fuel oil or 1.1 gal of propane so depending on your relative equipment efficiencies and fuel prices, your bill will probably change!

    So your sq. footage is 1200 (including basement?). If you know your therms used and Heating Degree Days
    http://www.degreedays.net/
    for a month in winter you can figure your relative heat loss, sort of a poor man's Manual J.

    E.g.
    SW Ontario, Canada
    398.7 cubic meters = 140 therms of NG in 28 days = 5 therms/day
    2900 sq. ft. including basement gives 5/2900 = 170 BTU/day/sq. ft.
    664 HDD/28 = 24 HDD in one day
    170/24 = 7.1 BTU/sq. ft./HDD
    Effic. factor 0.8
    final 5.7

    Here's my data so far in BTU/sq. ft./HDD for single family resi's in North America. The average house uses 6 BTU/sq. ft./HDD

    1.7 least heat loss
    3.8
    4.0
    4.4
    5.7
    8.0
    8.8
    11 most heat loss

    Histogram
    0 to <3|x
    3 to <6|***x
    6 to <9|xx
    9to<12|x
    Last edited: Jul 2, 2009
  8. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    ANY estimate based on square footage of living space is lousy. Square feet of floor area mean NOTHING! It's all about the square feet & U-value of exterior wall & roof area, and the amount of air-leakage into the structure, which vary a LOT for the same amount of floor space- heat loads per ft^2 can & will vary as much as 300%, or more in the same climate, similar sized house.

    Manual-J often misses the mark to the high-side by as much as 50% (which is way better than 300%, but still not great). Knowing the average BTU/ft^2/HDD for homes in your area may be a useful starting point to see if your fuel consumption or efficiency (heating system or building-envelope) is off the charts, but it's clearly the wrong way to design a heating system that's both efficient and delivers the heat even when it's cold out.]

    But doing what you're doing, MEASURING your fuel use per HDD is better than manual-J for calculating your as-used heating load. (Forget the per-square foot part, which isn't irrelevant.) By knowing (or looking up) your burner output, and fuel use per HDD, you can then get to within 10% of the your true heat load. If based on your burner output, system type you're running less than 100% duty cycle on the burner at "design day" temps, you have to derate the efficiency a bit and re-calc based on regression curves for oversized systems.

    Here's how to do it:

    Let's say your 97th percentile lowest binned hourly temperature (the "design day" temperature) is 0F. If you use 65F as your base heating degree-day temp (better insulated houses can use 60F), Then your design heat output need to support the difference, 65F-0F= 65F.

    IIRC, ACCA uses the 97th percentile hourly-temperature bin is usually the recommended design-temp number, and ASHRAE uses 99th, but even 95th percentile would be generally "safe" in most instances. Temps below the 95th percentile usually don't persist for more than a few hours, and they're usually at night when you're all cozy in bed. To estimate what your design-temp should be, look up the coldest temp your area saw in the past few years, add 5F, and you'll be "in the range" (unless it was a 25 or hundred year all-time low or something.)

    If by looking at the published heating degree days for a particular billing period (there are weather-history data sources online-pick a city near you) you can calculate your fuel use per HDD: Let's say you used 100gallons of oil for a period that added up to 500 total degree days, is 100/500= 0.2gallons/degree day.

    To convert that to BTUs for heating oil its 138000BTUs/gallon so that's 138000 x 0.2= 27600 BTUs/HDD fuel use.

    Assuming your furnace/boiler has an efficiency rating of 80%, the actual heat delivered to the heating distribution (ducts or radiators, etc) is 0.8 x 27600=22080 BTUs/HDD output.

    Now, a heating degree-DAY is 24 hours, and heating equipment is rated in output per hour so you have to do more arithmetic. The 65F design temp delta would be 65HDD if it stayed that cold all day, so your hourly output need would be (65 x 22080)/24= 59800BTUs/hour.

    If that's the rated output of your current system, congratulations- you just won the lottery- it's PERFECTLY sized.

    But as is more typical, let's say the burner rating is 225000 BTU/hour in- 180000BTU/hour out (80% efficiency), that means it is at least 3x oversized. That's where the regression curves come in for a closer/better guesstimate. Take a look at figure 1:

    http://simulationresearch.lbl.gov/dirpubs/42175.pdf

    If you have a 180K output and at most a 60K load, you're at the 0.33 load mark for DESIGN DAY load (for a few hours on that day). Most of the heating season you're in the 0.11-0.22 fractional load range (1/3-2/3 of of your design-day load.) So, if you have a hot-air furnace or very low mass boiler, the SDL-C111 curve, which looking at the curve, you need to derate your efficiency estimate to ~0.8x the stated thermal or AFUE efficiency of the equipment: 0.8x 80%= 64% -as-used efficiency. (If you have a cast-iron hydronic boiler other high-mass system or a steam boiler use the other curves. Just take a guess at which one, it'll be close enough.)

    Then using that as your newer-better estimate your actual design-day heat load based on derated estimate is:

    0.8 x 59800 BTU/hour= 47840 BTU/hour.

    THAT's your real as-used heat load to within 10%. (I'ts actually slightly to the high side on average, since at a ~47.8K load a 180K burner is~3.7x oversized, not ~3x, but the regression curves aren't perfectly matched to YOUR equipment.). So the 47800 BTU/hour output is the number to use when sizing the equipment for maximum annual efficiency. If you keep it to no more than 15% over that you'll likely beat AFUE numbers for efficiency. Alternatively, if you go very much below that, count on needing some supplemental heat for some the coldest of cold nights (got a decent small wood stove?)

    And that's WAY more accurate than Manual-J (poor-man's or rich.) It's a snapshot of how well your insulation etc is performing, and how you're actually utilizing the heating system. Manual-J numbers often run 25-35% over real-world measurements like this, unless you're the type who keeps it at 72F day & night. Going more than 10% over Manual-J you're bound to see a measurable difference in fuel use. But oversizing it by 25% from your measurement won't kill it on efficiency- look at the 75% then the 25-50% range on the curves- it's not ultra-derated (but some.) Still, if you're planning on doing another round of envelope-upgrades (new windows, air-sealing, insulating, etc.) you could soon find yourself 2x+ oversized and slipping over the efficiency cliff on the left edge of regression curves again if you oversize it much based on your measurements.

    Oversizing a heating system typically costs you more up front, and for every year thereafter. It's better to undersize it a bit, spend the difference on envelope upgrades, and make your annual-payments on upgrading the building envelope rather than burning the extra fuel.
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,460
    Location:
    MD
    First let me say: Holy Moley!

    Then:

    I think this
    http://en.wikipedia.org/wiki/Law_of_large_numbers
    says that the more samples I get, the better the correlation.

    This method may have a wide tolerance, but this info is obtained for "free". The more days you use, the closer the tolerance.

    Here's my offering
    http://www.focusonenergy.com/files/Document_Management_System/Evaluation/weshbillingstudy_report.pdf
    on the HDD sq. ft. stuff. My 1.7 and 4.4 is from this study, and strictly should be given a lot more weight in my histogram.
    Last edited: Jul 3, 2009
  10. kitehill

    kitehill New Member

    Messages:
    1
    Location:
    Central Maine
    I'm not sure if I'm on the right site...but after this past winter in central Maine where heating my home ran over $725 a month in fuel oil and I'm looking for a less expensive source. I do not have enough to change the heating system, but am wondering if this blog is talking about changing the part of an existing furnace from burning oil to burning propane. If so, what is the average cost for something like this.
  11. DonL

    DonL Jack of all trades

    Messages:
    3,812
    Location:
    Houston, TX

    You are at the correct site for help.

    You may want to start a new Thread because this one is a few years old.

    What make / model of furnace do you want to convert ?

    Pricing would mater on your location, and local law requirements.


    Good Luck.
    Last edited: Apr 28, 2013
  12. Dana

    Dana In the trades

    Messages:
    2,785
    Location:
    01609
    You will NOT save money converting to propane. If you're on the natural-gas grid (unlikely, in central Maine), a conversion burner on your existing system would cut your heating costs by more than half.

    Depending on your floor plan you may be able to heat the place comfortably with 1-3 "ductless heads" of mini-split air source heat pump, at a comparable cost to natural gas. Even if you heat only one large zone with a ductless (the way you might heat a portion of the house with a wood stove) the savings are significant on fuel oil for the thing to pay for itself. If you can get a low-interest 7 year loan it's usually cash-postive on heating cost savings in the first year. A typical 1-1.5 ton mini-split runs ~$4-4.5KUSD, installed, and would likely cover something like half of your total heating for the season. A 2-head 2.5 ton multi-split would typically run $6-7K installed, and may be able to handle the whole-shebang (tbd.)

    If your oil dealer stamps a "K-factor" on the billing a mid-winter bill and your zip code (for weather data) would be able to tell you how much ductless it would take to do it all in terms of tonnage, but not how many interior heads.

    There are many models/makes out there that have a guaranteed output at -20C/-4F, with unspecified output down to about -10F or so before they automatically shut down. One manufacturer (Mitsubishi) has a series with output ratings down to -25C/-13F, that is still chugging along at -18F, which may be the only models appropriate for central ME locations.

    In snowy locations it's important to mount the outdoor compressor units of mini-splits where they won't be swamped by drifting snow or clobbered by roof-avalanche/cornice drops, since they DO need ample access to outdoor air, and would spend an inordinate amount of time in "defrost" mode if snowed in. Bracket- mounted above the historical snowpack depth, protected by the overhanging eaves or porch roofs is the rightto go IMHO:

    [​IMG]

    [​IMG]

    Building an open shed-roof dog house over it is better than nothing, but will still require keeping it dug-out in snow country:

    [​IMG]

    But they work, and work well, provided you size them right for the heating loads, and install them with adequate foresight.


    The most common interior heads aren't exactly a thing of beauty, if arguably better looking that window air-conditioners:

    [​IMG]

    It's the thing on the wall up in the corner.
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