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  1. #1
    Join Date
    Apr 2005
    Location
    Atlanta, GA
    Posts
    3
    I've got to replace an old (17+ years) condensing unit and gas furnace in my home in Atlanta, GA. I'm leaning towards a Rheem 90+ modulating furnace with a dual fuel link to a heat pump.

    The advantages as I see them are: 1) the HP will be at least as economical during the cooling season as a straight cool condenser; 2) the HP will be more economical heating in the 60-40 degree range than a gas furnace, given gas & electricity prices for the foreseeable future; 3) a modulating furnace will give me better efficiency during the below 40 degrees time of year, and 4) the variable speed blower will give me better comfort during the cooling season and the heating season than a single speed blower.

    The disadvantages as I see them boil down to greater upfront costs than a straight cool c/u + standard 90% furnace, and a greater likelihood of service calls because the system is so complex: dual fuel boards, modulating gas valve, variable speed blower, all of which would (presumably) be expensive to service/replace after the warranty runs out.

    Is this kind of system overkill in a practical sense? I love the idea of the variable speed blower for both seasons, but do I really need a complicated modulating gas valve when the furnace will only be running at temps below 40 (and then at a fairly high rate, I assume)?

    Should I default to a straight cool c/u (eliminating the dual fuel complexity and upfront costs) and hope that a 90+ modulating furnace is so efficient that I don't spend too much more in the 60-40 degree season than I would with a HP?

    Any advice would be greatly appreciated, and feel free to point out any misconceptions I have about any of this stuff. I'm a classic example of "a little knowledge is a dangerous thing."

  2. #2
    Join Date
    Mar 2004
    Posts
    1,042
    Dual fuel has real operating cost savings in Atlanta. Natural gas is just... messy here... and the winter electric rates are pretty good.

    I would say that this is a bit of overkill, though. Unless you're replacing a 90% furnace and don't have a regular flue for an 80%, consider getting an 80% furnace. Given the price difference and that the heat pump is going to be your primary heat source, you're never going to save enough gas to pay the price difference for getting a 90.

    The modulating furnace is great, don't get me wrong. For replacements of gas/electric systems in this market, though, the sweet spot in upfront cost versus operating cost is probably dual fuel with an 80% furnace. If it will be easy and cheap to do it, going all-electric is actually the lowest on operating cost around here. When I say that, though, I mean cancelling your account with the gas company completely so you can stop paying all the monthly fees. Variable speed is still great, though, so consider an 80% variable speed furnace. It helps with humidity control and making for warmer output when you're using the heat pump for heat.

    If your AC/heat pump location is in a noise-sensitive area, think about noise ratings carefully. Switching from straight cool to a heat pump is going to probably double the amount time the condensing unit runs (on an annual basis).

    Ask for a dual fuel thermostat instead of a fossil fuel kit for a dual fuel system. There are a number available now, but I do have a link handy to this one for an example (it just came up in another thread).
    http://www.xpedio.carrier.com/idc/gr...tstat-18si.pdf

    I think there is a dual fuel versions of the Carrier Thermidistat and Honeywell VisionPro available now, too, if you'd like to take an extra step towards dehumidification during cooling season.

    [Edited by wyounger on 04-28-2005 at 03:03 PM]

  3. #3
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,371
    To get the comfort of the Mod, you'd have to do some creative engineering of the low voltage to use the Mod stat and have dual fuel. It's not set up to be a heat pump stat. Sure you could use another type of control but lose the modulating feature, would have to use the timer which pretty much defeats much of the comfort.

  4. #4
    Join Date
    Mar 2004
    Posts
    1,042
    Yeah, the Bald One is (almost?) always right. I don't see how you could set up a modulating furnace with a modulating thermostat AND dual fuel.

    I hadn't noticed that you said that the old furnace was a condensing unit... which means you aren't going to have a B-vent flue available for use with a new 80% unit. That makes this tougher. Have you picked an installer already, and if so, do they sell any other brands? If you really want dual fuel it may work best to go with another brand. Nothing against Rheem or the mod, but I don't see how you could set up dual fuel with a modulating unit and preserve the modulation.

    Two stage furnaces are no problem with dual fuel, though... so a Rheem 80% or another brand's two stage condensing furnace can still do fine here. With any of those setups, you're still going to end up with three heating stages, so the system will still have nice long, slow, and gentle heating cycles under pretty much any condition. It will work a lot like the mod, though not exactly.

    Or you could go for ultimate comfort and just get the modulating furnace without dual fuel. You're not likely to see a drop in operating costs, though, relative to your old condensing furnace.

    [Edited by wyounger on 04-28-2005 at 03:38 PM]

  5. #5
    Join Date
    Apr 2005
    Location
    Atlanta, GA
    Posts
    3
    Sorry wyounger, I may have misspoke -- I currently have a Janitrol c/u outside the house with a probably 60%AFUE Janitrol gas furnace with whatever vent flue was needed for that 17 years ago.

    I have to confess that you guys have lost me with some of these tstat issues -- my knowledge about this stuff is broad but shallow -- so I'll have to run them by my installer. He can sell me anything, of course, but can give me a better deal on Rheem and Ruud than anything else, so that's why I started there. The posts on the Mod seem pretty positive, but the complications you and Baldloonie raise about the tstat controls and your other comments incline me to take your advice and look into an 80% furnace with variable speed blower and a HP, set up as a dual fuel system. Cheaper furnace, less complicated tstat issues, and the benefits of HP economy most of the year, right?

    Good point about the HP noise levels -- I hadn't thought about that. Unfortunately, it's hard to get that info off of some of the spec sheets, so I'll have to do some digging. Any central location for comparison ratings, accessible to mere mortals?

    I appreciate your insights on the gas vs. electric issue -- sadly I've got a gas water heater and clothes dryer, so I'm stuck with the monthly fees for the foreseeable future.

    Thanks, and give me any other advice you can (appropriately dumbed down, if possible)!

  6. #6
    Join Date
    Apr 2004
    Posts
    7,680
    It is sort of overkill for your area. It can be done but you would use a 2 or 3 stage heat pump thermostat and not get the full mod functions as the mod thermostat cannot be used without some real creative control strategies. You would however get the 40% firing rate versus 60 or 70 in other makes. Its a great set-up but I just dont know if in your area, it would be worth the cost in comfort. You would be the envy of the neighborhood no doubt.

    I think the 80%, 2 stage, variable speed furnace with dual fuel would be the wiser choice financially without giving up much in comfort. If I were your contractor I would let you decide. On the otherhand if electric rates shot through the roof you could just replace the thermostat with a mod thermostat and have be in pretty good shape.

  7. #7
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,339
    Heeldad, to answer your question of the duel fuel unit, we need to the cost of fuel in your area. In my part of the country, natural gas is $11.44 per MCF and electricity is $0.070 per KWH. The differance in savings with heat pump over gas is $0.78 per million BTU's.

  8. #8
    Join Date
    Apr 2004
    Posts
    7,680
    What COP did you use to determine the cost savings to be so low? The guy is in Atlanta.

    Weather data has an average LOW temperature of 31 in January, 35 in February and December and an average HIGH temperarature of 50 in January, 55 in February and 54 in December. I wouldnt expect him to see going below the blance point more than a few times each year.

    His cooling load will be high, so his HP will be sized large enough that (and this is a guess) his balance point may be 25 degrees or less.

  9. #9
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,339
    Originally posted by docholiday
    What COP did you use to determine the cost savings to be so low? The guy is in Atlanta.

    Weather data has an average LOW temperature of 31 in January, 35 in February and December and an average HIGH temperarature of 50 in January, 55 in February and 54 in December. I wouldnt expect him to see going below the blance point more than a few times each year.

    His cooling load will be high, so his HP will be sized large enough that (and this is a guess) his balance point may be 25 degrees or less.
    Doc, the example given was for my part of the country. You're right that his COP will be greater.
    Do you have Excel? I could email you what was given to me.

  10. #10
    Join Date
    Apr 2004
    Posts
    7,680
    yeah send it.... docholiday rgw-hq . com


    BTW, if there are any Georgia contractors here, can you figure this for us? Lets use for figuring sake a 35,000 btuh total cooling and a 55,000 total heating home. Lets use a 13 SEER HP with good HSPF and an 80% furnace.

    [Edited by lusker on 05-03-2005 at 05:51 PM]

  11. #11
    Join Date
    Mar 2004
    Posts
    1,042
    Yep... we don't spend much time below the balance point around here. Mine is about 30 degrees.

    Most residential customers in Georgia are paying about $1.05-$1.10 per therm (essentially the same as a ccf, thus $10.50 per mcf) and 6-7 cents per kwh in the winter.

    Heeldad, If you're in Georgia and you haven't got a good fixed rate for gas, Walton EMC is offering 99 cents per therm for signups this month. This is probably the best rate that will be available this year, so I recommend signing up with them. They can service anyone in the former Atlanta Gas Light service areas, which means anyone in metro Atlanta. (http://www.waltonemcgas.com) I've been with them for two years myself.

  12. #12
    Join Date
    Apr 2005
    Location
    Atlanta, GA
    Posts
    3
    Thanks wyounger! At your suggestion I checked into Walton EMC gas rates, and compared them with the rates for Scana Energy that I have now. The difference in per therm rate is 5-8 cents, and I've noticed that in the past. What I never thought to do until your post was to check my annual consumption -- I used about 2500 therms last year (old inefficient gas furnace, gas water heater, gas clothes dryer, gas stove). I had no idea how much difference a few cents per therm could make!
    On your advice, I'm getting an 80% two-stage gas furnace and a 13 seer heat pump.
    Thanks for your generosity (and the generosity of everyone else who responded) in taking the time to respond to my post with helpful, simple advice above and beyond the call of duty. As in any industry, there are plenty of folks out there practicing this trade without the knowledge and experience (and sometimes without the integrity) to provide good advice at a reasonable price. Unbiased, independent sources of information like this board are an irreplaceable aid to homeowners who know enough to "trust, but verify."

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