Results 1 to 13 of 13
  1. #1
    Join Date
    Oct 2005
    Posts
    88
    Old Lennox draft G12D2 82,000 in / 62,000 out, used in an apartment with a worst case heat loss around 20,000. Five minutes of heat, hour of shivers, five minutes of heat....

    Given that no landlord will replace anything until a red tag goes on (and this HX isn't dead yet, pity) ... has anyone ever gone to smaller orifices in a case like this? how much of a difference is achievable in practice? any problems likely as long as delta T is kept above 70F (spec is 70 - 100)?

    Thanks,
    -Chap

  2. #2
    Join Date
    Jul 2003
    Posts
    178
    There is the problem. Derate and TR drops.
    You can't derate far and keep TR up.

  3. #3
    Join Date
    Oct 2005
    Posts
    88
    Natch. I was just wondering if there might be someone on the forum with a quantitative sense of how far that might be, or knows of any other adverse effects as long as delta T is in range.

    Thanks,
    -Chap

  4. #4
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
    Posts
    2,492
    Unfortunately derating by changing orifices will cost you considerable increase in fuel cost. Blocking an orifice does work as long as proper temperatures are maintained in the flue to prevent condensation.
    captain CO

  5. #5
    Join Date
    Oct 2005
    Posts
    88
    See, I knew why I asked - the fuel cost increase is something I never would have guessed. What's the explanation?

    I would have worried that just blocking one orifice would lead to undesirable temperature gradients across the HX and increased thermal stresses.

    -Chap

  6. #6
    Join Date
    Jun 2005
    Posts
    171
    Originally posted by chapmanf
    Old Lennox draft G12D2 82,000 in / 62,000 out, used in an apartment with a worst case heat loss around 20,000. Five minutes of heat, hour of shivers, five minutes of heat....

    Given that no landlord will replace anything until a red tag goes on (and this HX isn't dead yet, pity) ... has anyone ever gone to smaller orifices in a case like this? how much of a difference is achievable in practice? any problems likely as long as delta T is kept above 70F (spec is 70 - 100)?

    Thanks,
    -Chap
    Five minutes of heat and an hour of shriver. You mean to say it trips on high temp?

    [Edited by kelvin on 10-31-2005 at 04:50 PM]

  7. #7
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
    Posts
    2,492
    Originally posted by chapmanf
    See, I knew why I asked - the fuel cost increase is something I never would have guessed. What's the explanation?

    I would have worried that just blocking one orifice would lead to undesirable temperature gradients across the HX and increased thermal stresses.

    -Chap
    The flame temperature will be cooler, radiate less to the heat exchanger, less gasses to scrub the heat exchanger and the gasses will travel faster through the heat exchanger and transfer less heat.
    captain CO

  8. #8
    Join Date
    Feb 2005
    Posts
    344

    Derate

    Years ago a rule of thumb was up to a 25% derate. However, remember whenever you make an unautorized modification to equipment, it is your baby.

    Good luck...

  9. #9
    Join Date
    Oct 2005
    Posts
    88
    Five minutes of heat and an hour of shriver. You mean to say it trips on high temp?
    No, just that a 62,000 btu/h furnace in a (max!) 20,000 btu/h space'll drive the temp up pretty quick, and then you have a nice hour while the place gets gradually cooler before the stat will call for heat again. The electronic stat installed last year enforces a five-minute minimum on time ... with the old stat it was shorter than that! 'course the 5-minute minimum makes the temp swings /bigger/, but it made a *big* difference in efficiency compared to the year before: graphing gas therms each month against HDD (to control for weather as a variable) the slope of the graph went down (get this) 36%. The old super-short cycles were hardly longer than the time delay for the circ blower to start. Nice way to send gas money straight up the chimney....

  10. #10
    Join Date
    Oct 2005
    Posts
    88
    unautorized modification to equipment, it is your baby.
    Or with equipment of this vintage, not so much baby as a second-childhood sort of thing.

  11. #11
    Join Date
    Dec 2003
    Location
    Location!, Location!
    Posts
    929
    We are actually repalcing an old G12 that has this done- there are 4 out of 8 orifices removed and the heat exchanger cells for those have blank covers over them-I will post pix when we rip it out next week. The blanks are cut and shaped to fit over the cell nicely. The property is a church rectory, and the monster G12 was installed by volunteers, the rector thinks it was already a used furnace when this happened > 20 yrs ago and it has run (derated) since then. He says it does OK, didn't need to change out until now, when they have $$, I was surprised, with a 50% derating, and the original blower.

  12. #12
    Join Date
    Oct 2005
    Posts
    88
    We are actually repalcing an old G12 that has this done- there are 4 out of 8 orifices removed and the heat exchanger cells for those have blank covers over them ... I was surprised, with a 50% derating, and the original blower.
    Fifty percent! out of curiosity, what IS the temperature rise - is it in range? (how?) the blank covers - do they block the combustion passages, or the circulated air (or both)?

  13. #13
    Join Date
    Dec 2003
    Location
    Location!, Location!
    Posts
    929
    The blanks only cover the cells(section inlets), I'll look when we rip it out to see if they did anything on blower outlet. I don't think so, it all looks original. This monster is sucking through a 14 X 8 return drop(side) and trunk with a 14" high plenum, and only 2- 10 X 8 supply trunks. Total supplies in system = 5 x 6"round. It worked good enough for them for all those years, we're not gonna touch it until we yank it-no way.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event