efficiency in this application

Discussion in 'HVAC Heating & Cooling' started by Thatguy, Dec 19, 2009.

  1. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    For a [rated] 83% efficient, 169,000 input BTU/hr, single stage gas furnace that is on only 1/4th of the time in January and the "on time" is ~6 minutes, what efficiency may I expect?
    Thanks, folks. . .
  2. Dana

    Dana In the trades

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    Location:
    01609
    You're likely only about 3x oversized for the peak load on design day, and only ~2x above where was tested for it's AFUE numbers (IIRC the standard size factor for AFUE is 1.7x design load.)

    Let's make some worst-case scenario estimates.

    Look at the curve (SDL-C111) for the furnace in figure 1:

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

    If it's 83% efficient at the 1.7 oversized point (roughly the 0.6 point on the curve) that's about 95% of steady-state, which means the raw combustion efficiency is a near-condensing ~86-87% (which is credible- it may be controlled to be just below the condensing threshold). Have you had it's raw combustion efficiency tested?

    If you're 3x oversized for the max load, move to about the 0.3 point , which is more like 90% of steady state, or 87% x 0.9= 78%, which is on design day.

    Your AVERAGE load for the season is likely to be roughly half your design day load, which is the 0.15 point , or about 85% of steady state. 87% x 0.85=75%. (Which believe it or not, isn't terrible for a non-modulating single-stage system! Most real-world heating systems in the US are pulling less than 70% true efficiency.)

    With the low mass of a hot air furnace, a 6 minute minimum burn isn't nearly as lossy the way it would be in a cast iron boiler. The abandoned BTUs in the heat exchanger are likely less than 10% of what it would be in a high mass system. Bumping the hysteresis a bit probably wouldn't hurt, but won't be miraculous.

    Now, if it's an electronic ignition, induced draft- sealed combustion unit, see the comparisons on figure 6. You could be getting 85% out of an 83% AFUE furnace at your level of oversizing.

    At 169K-in and ~86% raw thermal efficiency, it's steady state output is ~145K, so if 3x oversized means your design-day heat load is ~48K- which might be high for a small house in MD, but reasonable for a 3000'+ pre-1980s stick-built. If it's 4x oversized, it means your design-day peak is ~36K, which might be about right for a pre-1980 2500' house.

    Even a perfectly sized furnace won't run more than 50% duty cycle throughout the day except on the coldest days of the year. The actual design heat load typically occurs for only a few hours at a time, usually between 2-6AM. But by the afternoon of the same day the outdoor temps will have have risen 15-20F from the low, and if there's a bit of sun the heat load for the afternoon will be less than half what it was when it was between 2-6AM when it was running constantly, yet keeping up. So 25% average duty cycle for the entire month of January means it isn't terrible. But unless it's a very large or very drafty house, I'd be surprised if your actual heat load is ever the full 145K.

    Assuming you're actually running in the ~75% range (which is likely) a 2-stage 2x oversized condensing furnace will likely cut your fuel consumption by 20-25% (true as-used AFUE ~90-94%). A multi-stage with a continuously variable ECM motor blower like the ekoComfort would likely cut it by ~30% (true as-used AFUE over 95%).

    Done a manual-J heat loss calc (or similar) on your place yet? Things could be better (or worse) than I've outlined.

    Better than manual-J: Measure your fuel use and correlate it against heating degree days. Then with 25 year weather data for you zip code your 97th percentile binned-hourly heat load can be calculated to a high degree of accuracy.
  3. Runs with bison

    Runs with bison Member

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    897
    Location:
    Midwest
    And I thought mine was massively oversized at 110k input/89k output! I figure mine at somewhere over 2x.
  4. Thatguy

    Thatguy Homeowner

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    Location:
    MD
    You guys are good.

    My Outside Design Temp to the 99% level is +14F, house built in '64, ~3000 sf.
    I'm working on a spreadsheet that tells me the optimum input BTU/hr for a new furnace based on the existing furnace, using HDDs, therms used for that period and ODT.
    It assumed a linear relationship between gas in and duty cycle and heat out, but now I can modify that assumption.
  5. Dana

    Dana In the trades

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    The "norm" in my neighborhood for existing housing older than 40 years seems to be more like 3-4x oversized for boilers, 4-6x oversized for hot air furnaces, where they're all sliding down the steep slope of the efficiency cliff. I'm not sure how the world got there, maybe a combination of oversizing on day-1 based on a WAG by the original designers, then an unwillingness to downsize on replacement units by heating contractors happy to take the extra margin up-front and never worry about getting that 5AM call that the client's freezin' their tuchus off.

    Another factor might be that the thermal envelopes of some of these houses have been improved over the years, an issue ignored at replacement time. There's no excusing the idiot who installed the previous cast-iron boiler at what is now my house, but to be fair, it was only 3x oversized at the time, but rapidly became 5x oversized when I started tightening up the place and installing more/better insulation (and I've hardly gone crazy on it.)

    The even wilder idiot who came along at a later date and hooked it up to a ~300' radiant floor zone unbuffered (at least they mixed the water down to 120F :) ) had the thing short-cycling miserably while serving that zone, (sub 3-minute burns!) and I felt obligated to rectify the situation rather than wait for the equipment to wear out. (And these were both licensed professional contractors who have "Heating & Plumbing" right there on the side of their trucks, eh? :) I smile & shake my head a bit when I pass their trucks in trafffic, but won't be calling on them any time soon.)
  6. Dana

    Dana In the trades

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    Location:
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    It's "mostly linear" for hot air furnaces, when you're mostly above the 0.4 mark though. It only get's interesting when you're sliding down the steepening parts below 0.20 on the left side of the graph, eh? ;-)
  7. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    2800 sq. ft. here, 1994 construction...missing insulation in many areas that should have had it (I'm working on that.) 99% design temp here should be in the sub 5 F range. 97.5% given as 6 F. I'll probably use 0 F to see if it changes the sizing increment. We seem to get a few nights a year in the -5 to 0 F range.
  8. Runs with bison

    Runs with bison Member

    Messages:
    897
    Location:
    Midwest
    When you say "on time" are your referring to the whole furnace cycle or just the burner time? My cycle in 20 F weather at night with sustained 10 mph winds (20 mph gusts) is: 35 seconds ID fan operation, ~4 mins burner operation, 2:15 mins of blower operation after burner shuts down. The period from one start to the next start is 11 minutes. So burner wise that is 4/11 = 36% of furnace rating.

    This corresponds pretty well with daily gas use. After subtracting water heating gas use I used 9 therms yesterday at an average temperature of 16 F. My set point is 69 F and the balance point is about 60 F. With 110k input/89k output this would be about 30,000 Btu/hr output at what I consider to be ~75% of design load. In a typical year we get three days that average below 10 F. I'm waiting for these to fill out the bottom end of my curve (actually, it's a pretty decent linear fit so far.)
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Here's my spreadsheet assuming efficiency is constant over duty cycle. I have to modify the last few steps to incorporate the graphs from the link. On time is blower on time.


    Confirming a right sized single stage or multi-stage furnace


    Find your Outside Design Temperature (ODT) from the link
    http://www.hrt.msu.edu/energy/Notebook/pdf/Sec3/Lower_Your_Energy_Bill_by_AJ_Both.pdf

    calculate the A factor
    A = (65-ODT)/(-100)

    Then calculate your avg. daily temp T
    over a (30 day) month from
    http://www.degreedays.net/
    using the sum of HDD days from that month, and
    T= 65-HDD/30

    Then, using your gas usage from your bill for that month
    you can determine if your furnace is right sized for your house and climate


    Given an Outside Design Temperature from the link of +14F
    A = (65-ODT)/(-100)
    14 >enter ODT
    -0.51 =calc'd A factor

    For your gas usage in BTUs and a 720 hour month
    calculate your furnace duty cycle (DC) at
    your average daily temp. for that month.

    1000 >enter HDD for a 30 day month
    T= 65-HDD/30
    31.7 = calc'd avg daily temp over 30 days

    Then
    DC = (T-65)/A
    65 = avg. DC in percent over a month

    300 >enter therms/month from your gas bill
    1 therm = 100K BTU,
    a 30 day month has 720 hours
    BTUs/hr = therms/mon x 100,000/720
    41667 =calc.d BTU/hr usage = CB
    at
    31.7 degrees
    and
    65 DC

    so the ideal furnace size would be
    (100/DC)xCB

    This is
    63750 input BTU/hr
    assuming the same efficiency
    as your present furnace.
    Last edited: Dec 26, 2009
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