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  1. #1
    Join Date
    Mar 2005
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    Milwaukee, WI
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    In the process of replacing furnace & AC, and have had a recommendation to install dual fuel heat pump system. Does this make sense for SE Wisconsin? I realize there may not be a single "right" answer to this question, but any insight would be appreciated!

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    33,916
    How are your electric rates? If low I would expect to see some savings with a dual fuel setup with today's gas prices. In most cases, a heat pump alone can handle the house needs from about 35 on up and in fall and spring, and some sunny winter days, you could have some cheap heat by running a heat pump.

  3. #3
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
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    27
    BaldLoonie -

    Thanks for the response. Electricity rates are $0.08/kWh - gas rates are $0.95/therm. Average monthly heating degree days during winter months is 1100 - 1200. Long-term average winter temp is 25 F, although average spiked this winter at about 30 F.

    A few more details about the house - 1800 sf brick ranch, with a newly remodelled basement (previously unfinished). Although our existing equipment is rather aged (18 yr furnace, 16 yr AC), the new basement space is what pushed us to re-evaluate our equipment at this time. On the topic of this new space, there is another debate regarding capacity of existing trunk and its ability to support 5-8 additional 8" supplies, but that's for a separate thread...

    So, assuming a dual fuel HP system can be justified, is a standard efficiency furnace sufficient, or is a high efficiency still worthwhile?

  4. #4
    Join Date
    Apr 2004
    Posts
    7,680
    A duel fuel system is the right choice for where you live.

    The heat pump will heat the home most of the time in the 3 seasons and the gas will pick up in the dead of winter when it is below the balance point, primarily at night. The kicker here is, that heat pums are sized for the cooling load.

    IF, and I say IF... If you do not get much use out of your a/c in the summer except for the hottest of days 10-30 days a year, I would personally oversize the heat pump by 1/2 or a ton provided the ductwork can be improved along with it. This would allow you a lower balance point and delay switching to fossil fuel in the winter.

    IF, you require good dehumidification in the summer, this advice is to be ignored.

    As far as 80% versus 90%, that is up to you. I use a duel feul system and use a 90+ 2 stage. This gives me heat staging that more closely matches the current conditions.

  5. #5
    Join Date
    Mar 2004
    Posts
    1,042
    Yep, dual fuel looks good at these rates. You'll get lots of use out of it in the spring and fall, and a good bit in the winter, too. If you use a programmable thermostat, be conservative about using large setbacks. Using small (or no) setbacks, especially daytime setbacks, will help a dual fuel system do more using the heat pump, further cutting gas consumption. The only exception will be in the worst weather of winter, in which case most recovery, especially in the morning, will be done with fossil fuel.

    For best results, use a dual fuel aware thermostat (not a "fossil fuel kit"), and preferably a heat pump with a demand (versus timed) defrost control. Best comfort will come with a variable speed unit indoors, as always, but it's possible to get away without that.

    While any new equipment will be more efficient, and dual fuel will save money in heating season, you're also trying to heat more space than before, so a 90+% efficient furnace should by no means be out of the question. With either furnace type, don't let them oversize it.

    -What are your winter gas bills like right now?
    -Do you intend to fully heat the basement, or just take the edge off the chill? Will you heat it all the time, or just some of the time?

    To achieve decent temperature balance, you should consider zoning the main floor and the basement separately. Then you can heat the basement more when you're using it, and less when you aren't. If you go this way, it will probably work out to keep the existing trunk as-is, and adding a second trunk for the basement zone.

  6. #6
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    Wow - thanks to all for the input. I've done a bit more homework, so perhaps additional info will add clarity?

    The existing system features a Heil NUGK100AH02 furnace (4 ton??) and a Heil 2.5 ton AC (don't have model # handy). This system was fit to a 1750 square foot brick ranch (main floor square footage w/unfinished basement), 1967 construction, installed late 1980s. Main (only) trunk off plenum measures 17.5" x 8.5". It reduces after about 8 ft to 16" x 7". 12 ft later, it reduces further through a 90 deg radius to 14" x 7" for the final 32 ft. First section has 2 8" supplies, second section has 2 8" supplies, and third section has 7 8" supplies. I checked the motor wiring, and it's wired for 1885 cfm cooling/1660 cfm heating (0.1 ESP), 1750 cfm cooling/1545 heating (0.3 ESP), other ESPs tabulated. Return is 12" x 6" for 24 ft, parallel to last (32') leg of trunk. At 90 degree trunk turn, sheet metal return duct merges with sheet metal-capped 14.5" x 9.5" joist cavity for 24 ft back to furnace, through Aprilaire 2200 air filter.

    Regarding equipment upgrade, keep in mind that support for remodelled basement is added to load. Basement is 80% fully below grade, with 20% gradually exposed to fully exposed south walk-out through 72"x80" sliding door. Stud walls are raised against 1" rigid insulation (R-5) with R-11 unfaced cavity batts.

    Contractor A did not do a detailed load analysis, recommending an 80K BTUh Trane XV90 matched with 3 ton Trane XL14i. His conclusion regarding ductwork was to add secondary (smaller) trunk to supply basement HVAC requirements, leaving existing ductwork undisturbed. Heat pump was dismissed as viable alternative. Zoning discussed, but not recommended.

    Contractor B did a Manual J analysis (53K htg, 28K clg), recommending 60K Trane XV90 matched with a 2.5 ton Trane XL16i heat pump. He also suggested 80K XV80 as supplemental heat alternative. Regarding ductwork, his conclusion was that new supplies could be added to existing ductwork to meet demand of new basement space. Only alteration to existing ductwork was to replace "box" plenum with "radius" plenum. Zoning discussed, but not recommended.

    After reading this forum all weekend, I purchased homeowner version of HVAC-Calc to do an independent load analysis. Compared to numbers from Contractor B, we matched closely on heat load, but are way off on cooling load (18K vs 26K). As only the second owner of this home, I'm fortunate to have original building plans, so I have advantage of knowing construction materials. Contractor B did not inquire about ceiling or wall insulation, or measure windows, so perhaps this accounts for the difference?

    The loading question is not what concerns me most, nor does the issue of whether to heat pump or not. Most concerning is the difference of opinion on ductwork. I've pressed each on their position, and they're both holding firm. It seems there is only one correct answer, so I'm trying to figure out which it is! I'm slowly trying to digest volumetric flow rates (set by furnace blower, right?), air velocity (flow rate/cross sectional duct size, right? 700-900 fpm, max?), static pressure (no idea), etc. Cooling load sets size of AC (350-400 cfm/12K BTUh, right?) Blower capacity >= AC size?

    I've read enough this weekend to know that long, detailed messages like this one are discouraged, so again, I apologize. This is a big $$$ we're making, and I just want to get it right...

    Thanks again...

  7. #7
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    Wyounger -

    Sorry, you asked a couple of specific questions which I did not answer:

    1. Gas bills are high ~$200/month in winter, heating only, no other gas appliances.

    2. We'd like to heat the basement to a "comfortable" level - ~68 F. Primary feature of new basement is master bedroom suite, so it'll be a full-time load.

  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    If money is no object, stay with the super SEER pumps they are proposing. But in your area, I highly doubt you see any kind of economic payback going beyond 12 SEER. You can get 13 SEER and a very healthy HSPF out of the XR12 and XV90 combo with the right indoor coil. And that would be mucho less bucks up front than for the XL14i or XL16i. But if you like the funky cabinet of the "i" models, spend the extra but don't expect the minimal energy savings in the summer to pay much.

    Sounds like contractor A wants to oversize because he didn't do a calc and is dismissing your idea of dual fuel. Makes B look good!

  9. #9
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
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    27
    Money is always an object! Are there significant advantages of XL14i's 2-speed fan over XR12's single speed? What are other major differences (besides funky cabinet!) I know discussion of $$$ is taboo, but is it within forum rules to discuss relative % cost difference between these 2 models?

    Regarding the "right" indoor coil, is that separately determined, or dictated by selection out outdoor unit?

    Regarding A vs. B, I agree that B did more homework on the equipment sizing and suggested units more appropriate for load. As well, he first suggested heat pump as an option (I wasn't even considering this). My concern with B, however, is plan to add new supplies (6-9?) to existing trunk. Is the capacity there?

  10. #10
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,916
    The 14i puts the comp under a cover, 2 speed fan motor & the top. The XR12 does not but with scroll, isn't that much noisier than the 14i. Normally those XLi units are salty but only way to know how much less the XR12 would be is to ask the dealer. To get 13.00 SEER out of it, requires a TXC-S coil.

  11. #11
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27

    Lightbulb

    Correction - return air is drawn through Space-Gard 2200 air cleaner, not Aprilaire (that would be the humidifier, but I'm sure you guys already figured out my mistake!)

  12. #12
    Join Date
    Mar 2005
    Posts
    12
    I am from northwest Indiana. If you want heat pump, dual fuel is my recomendation. Also you might want to take it one step further and have a detailed room by room load calculation done. They can answer a lot of unknowns.

  13. #13
    Join Date
    Mar 2005
    Location
    Milwaukee, WI
    Posts
    27
    rabadger - you from the Badger state? Thanks for the input...

    BaldLoonie - on your mention of XR12, I asked dealer and was told it was discontinued and could no longer be ordered. Is this a phase out in preparation for 2006 13 SEER minimum?

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