Can a gas-fired furnace be successfully derated?

Discussion in 'HVAC Heating & Cooling' started by jadnashua, Jan 23, 2014.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    NYS has a program for older residents with certain income levels for some free energy related fixes to their homes...I signed my mother up and she had a guy out this week. They then send their recommendation to the state, and if approved, the work gets done - materials and labor up to $4K at no cost to the homeowner...seemed like a no brainer to me.

    As is true with many older furnaces, hers is probably a bit over 2x what's needed. The guy said he would have his tech look at it, and if possible, derate it a bit.

    While I don't disagree that it's oversized...can a typical furnace be derated? I suppose some of them may be modular, and you could do this. I'm wondering if it is generally feasible, and if so, how it is done.

    The rest of the stuff he recommended (I was listening on the phone, she's 400-miles away) sounded reasonable, air sealing the rim joist and insulating, adding insulation in the attic and a few other places, sealing better and insulating the wall better between the garage and the living room. I'm still wondering about the furnace.
  2. guy48065

    guy48065 Member

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    Jim did you find any answers elsewhere? I have a similar problem--oversized furnace that's been bouncing off the high limit since bought 20 years ago. Fat stack of work orders and no fix. I've asked in other forums about "turning it down" and have been told IF you can find someone cavalier enough to try, you can only safely reduce output about 10%.

    I was a little shocked that although this IS a common problem nobody has invented some solution. I would have thought with such a ready market some eager manufacturer would have invented a modulating gas valve that could be retrofit into older furnaces. Either I don't understand the complexities (they're common in industrial environments) or the perceived liability is too great.

    For myself, this winter has been too frustrating to allow my furnace to continue to run in such an inefficient manner so I replaced the 220 limit with a 240 and it's happily purring along now. I'll start saving my coins til I can afford to replace my perfectly-healthy oversized furnace with a new unit...from an honest company.
  3. DonL

    DonL Jack of all trades Master of one

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    I do think You can.

    I can buy new different size brass orifices here at my local Tru-Value Old school hardware store, just for that purpose. But you do have to ask for them.


    I see no reason why it can not be done, as long as the derating is within reason.
    Last edited: Jan 27, 2014
  4. guy48065

    guy48065 Member

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    It's entirely possible the answer I get from some HVAC pro sitting at his keyboard is very different from what I'd get from one kneeling in front of my furnace. I've heard it can be done but online it's been "good luck finding someone who will".
  5. DonL

    DonL Jack of all trades Master of one

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    Many will not do it, because they can make more money selling and charging you to install a new unit.

    If you find one that will, they will normally put a smaller orifice, and adjust fan speed. At a nice labor price.

    Upping the BTU rating or changing the type of gas, can be a hazard and many pros will not touch it, for good reason.

    If you do it, it may or may not require a adjustment in blower speed, vent temp will normally be lower.


    People should be very careful playing with gas, and any mod. It is not a average DIY project.
    Last edited: Jan 27, 2014
  6. Dana

    Dana In the trades

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    Unlike high-mass boilers, with low mass hot air furnaces there's no efficiency to be gained by derating the burner- it may reap 1-2% on fuel savings, but the longer burn cycles mean a uptick in air-handler power use, and something of a wash (or worse) on total operating cost.

    Even if it's possible to do, the only reason to go there would be for the comfort issues, and it would have to be a really SIGNIFICANT derating to feel the difference if you're starting with a 2-3x oversized furnace. Leaving the burner/heat exchanger balance well enough alone is probably the best approach.

    A better expenditure of funds on the furnace system would be to re-commission the ducts to current tightness and design standards. That includes air sealing all the seams and joints of the ducts with mastic and taping the seams of the air handler will make the thing run quieter, providing a bigger net uptick in system efficiency. If there are any doored off rooms with only supply ducts inadequate return paths, jump ducts /door cuts/ transom grille etc to keep the room-to-room pressure imbalances well bounded would work too. (These suckers actually work pretty well, and while not exactly beautiful, might be more appealing than some other approaches.) Air handler driven infiltration can easily run into a double-digit percentage of the instantaneous load when you have leaky unbalanced ducts, a load that only appears when the system is running. Fixing the duct leakage and lowering the return path impedances can reap high-single digit percentage fuel savings in many cases.

    At the same time it increases comfort in colder climates by not drying out the air with unintentional ventilation that only increases during the coldest/driest weather!
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    After years of suggesting it, she did install central air this past summer and in the process, essentially they redid all of the ductwork to include expanding the return ducts quantity and locations...so, I don't think that area is likely to help much.

    It would be primarily for improved comfort and the decreased cycling and ultimate longevity. At her age, since the existing furnace still works well, it does not make sense to replace it at this time.

    I know you could re-jet at least some burners, but didn't know how well that works. The potentially lower heat exchanger temp could decrease the efficiency, but on the other hand, you'd have the larger area than may be normal for that sized unit, and recover some of it. Just don't know enough about it, which is why I asked.

    The biggest things they're going to do with the sealing and added insulation should also go a long ways towards improving the comfort levels and ultimate economy. Built in the early 50's with maybe R-9 fiberglass in 2x4 construction with no house wrap, the plaster walls can get pretty cool. Sealed up all of the boxes in the walls earlier, so that helped. The (relatively) new vinyl siding added a little insulation on the outside, but they did not really seal much. She does things without telling me, so I can't guide her...parents, can't live without them, but can't guide them as much as may be useful.
  8. DonL

    DonL Jack of all trades Master of one

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    Good post Dana.

    And I agree.

    The plan sounds like a Gov plot to take more of Your Tax dollars.

    Just like the inflated price of Propane, because of a so called Propane Shortage.

    If we could just capture all of the smoke and gas , that the gov blows up our ass, We would have plenty of Hot, Heated Air.

    Get a Fat Lady, She can keep you warm.


    Bad me, but I think that is the truth.


    If I am wrong, I would like to hear, from anyone, that thinks so.
    Last edited: Jan 27, 2014
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    FWIW, the de-rating of the furnace, that was an o, by the way, if we can easily, we'll do it. The primary focus of the program is to air seal and add insulation. For someone living primarily on SS, being able to stay in their own home and stay warm is a lot cheaper on society than housing them in a group home. That is where the program pays its best dividends.
  10. DonL

    DonL Jack of all trades Master of one

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    I understand that Jim.

    I use 750 Watt space heaters, If my girl is not here to keep me warm. Most of the bigger electric space heaters are not safe, when running on a 120V outlet.

    It saves on NG instead of heating the whole house.

    When it gets real cold, I will crank the central gas heat, just to keep the pipes from freezing. They put them in the unheated attic, because houses here are on slabs.


    If a heater is way overrated, some times just Increasing the Fan speed can help.
  11. Dana

    Dana In the trades

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    Air sealing the house AND the ducts (just 'cuz they'er new doesn't mean they're tight) are the cheapest and most cost effective upgrade you're ever likely to make. Taping the seams of the air handler still counts too. The only time I've heard of folks pressure-testing and remediating duct leakage in this region is in high-R energy-nerd demonstration houses, but most of those were already built with much tighter ducts than the industry average.

    They can blow cellulose into the wall cavities from the exterior, and when they do it's going to tighten up the house by quite a bit too, and may exceed the amount of improvement she gets even from the band joist/foundation sill (which is usually one of the big 'uns.) Popping a row of vinyl siding every 4-5 vertical feet and drilling it's pretty easy to get reasonable densities compressing out the R9-R11 batts, and since the new fiber flows the path of the air leaving the cavity during installation, it clogs & mostly-seals the leak points. Cellulose even tightens up plank siding to a very high degree. With the inherent back-ventilated character of vinyl siding it doesn't much matter if there's housewrap or not- even interior wintertime moisture drive loads the sheathing will still dry pretty well toward the exterior, and bulk wetting blow by the siding also dries quickly.

    The other ways these weatherization programs have a big dividend is in delaying the need and size of grid improvements required for new development, often for decades, and the avoided capitalization costs keeps the ratepayer per-therm or per kwh cost bounded. For most large utilities in the northeast their bottom line is decoupled from the per-unit-energy delivered, and has been for years. This differs quite a bit from the go-go 1940s/50s/60s where regulators guaranteed the utilities fat fixed profits on the capitalization of new generation & transmission line capacity, etc. Under the newer regulatory environment/business they get rewarded at a higher levels for meeting their output and reliability targets, which has for more than a decade been easier/cheaper to do by decreasing the load than by upgrading their transmission line or generation capacity. Given the age/condition of the regional housing stock, that's likely to continue to be the case for at least another decade, probably two. Fixing up three 1950s houses frees up enough capacity to fully supply a brand new code-min house, and it's a heluva lot cheaper than building up transmission lines or upgrading the gas-grid capacity by the same amount. Who pays & how can be argued about (and most certainly is :) ), but the basic approach is sound. The already high utility rates don't really need to go any higher, but using tax revenue from the general coffers to achieve that end has a cost-shifting aspect that many find onerous. I'm not sure how it's currently done in NY, but it's a moving target in most of the northeastern states, sometimes it's the rate payer base who pays, reaping the returns in lower utility rates, other times it's a combination of general revenue and utility money.

    At typical NY utility rates running electric space heaters would be prohibitively expensive (but mini-splits would be comparable in operating cost to 80% efficiency gas, on a per-BTU basis.) Many places in NY are north of 20 cents/kwh for residential-retail electricity, and very few are as cheap as 15 cents. ( By Houston standards upstate NY is "real cold" about 80-90% of the year! :) ) NY doesn't have quite as open a marketplace for retail electricity as TX does, but that's not necessarily why they have historically higher electric rates for decades.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    It will be interesting to compare before and after energy use once they finish their improvements. I'm not sure where or how they are adding insulation, and I won't be there while it happens, but anything they do to help the utility costs and comfort will improve her life and I'm glad I found the program and got her signed up. I'll try to get her to take note and let me know, and I'll post the results if I get them.
  13. Dana

    Dana In the trades

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    A fuel-use against heating degree-days analysis on recent billing for a base line, compared the fuel/HDD to a mid-winter bill either next year or later this season WOULD be interesting. If the house was really air-leaky the delta could be well into double-digit percentages from air-sealing alone (as was the case on the 1920s vintage house I'm currently living in.)

    Do know whether they use blower doors and infra-red imaging to direct & verify/quantify the job, or just a "looks pretty good" visual?

    At my place the foam crew missed a 3" x 150" gap under a beam between a kneewall attic space and the vented porch roof (which I later remediated myself) that would have roared like a speeding train if blower door tested. That's effectively a three square foot hole!
  14. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    They were there on a nearly zero degree day and I asked about a blower door, they said it wasn't a great idea since she's 87-years old...they used IR to check things out as I understand it.
  15. guy48065

    guy48065 Member

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    WTH is that--she's too old to receive good service?
    We've heard about the "death panels" of socialized medical care, are the elderly not worthy of ANY gov't service now?
  16. DonL

    DonL Jack of all trades Master of one

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    She needs to leave for the test is my guess.
  17. Dana

    Dana In the trades

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    At ~0F outdoor temps IR cameras can easily spot major infiltration points without benefit of blower doors, but the exfiltration points can be much trickier.

    It's better to depressurize the house and find 'em all- even tiny leaks become apparent at 0F outdoor temps when depressurized to 50 pascals. But you're sucking in a lot of cold air as well as some dust/grid etc. during a depressurization test.

    Houses with known histories of asbestos aren't allowed to blower-door test rather than risking blowing otherwise hidden-away fibers to get spread all over the domicile. An 87 y.o. would need to worry about the asbestos, but the folks running the blower door test would incur a measurable occupational hazard if doing 100 of those a year. You can still air-seal an asbestos-risk house, but you usually can't use blown insulation until/unless it's fully remediated. (It's usually on the check-list on these weatherizing inspections.)
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