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  1. #1
    Join Date
    Oct 2005
    Hey all,

    Currently have a 13 yr old gas pack/ AC unit package. 1500 sq. ft. house with 2.5ton unit. THe gas pack just gave up the fight and i need to replace the unti. The automatic control valve started screwing up last year, limped along until now.

    Anyhow, i'm trying to weigh replacement with another 10-12 seer gas/ac unit, or go with DF HP with gas aux.

    Rates (TN)
    .815 cts/therm....probably going up 50-70 % next month
    .0652 cts/KWH above 800KWH .0608cts/KWH

    I'm looking over last couple years bills and HP is not looking like a good option even with the gas price increase.

    For instance....1/16/05 - 2/15/05

    gas bill 149 CCF for 126.82 + some PGA charge......this is gas pack, insulated gas water heater, and sometimes gas light outside(not going to be lit this year!) I need to look up lamp flow rate, but can't really judge how much the water heater contributed. During june/july my NG usage is around 12ccf, so i figured double for winter months for water heater and around 12ccf for the lamp (it was on continuously)...

    so guessing approx 125 CCF
    113CCF * (102,000BTU/CCF)= 11526000BTU's

    80% efficient so 100% electric would put it at 9220800 BTU's

    1KWH = 3412 BTU

    so this equals 2702.5 KWH (.0608) = 164.31

    If NG goes up 50%, then i'm at 190.23 using that same month, but no appreciable saving unless NG continues to skyrocket in long term. Electric rates are to go up 7% locally in same time. This will take nearly a decade to see the savings with the $1500 offset from 10Seer NG to a "approx" DF with same Seer rating. 12 Seer gas pack to 12 Seer NG is a little under $1000 difference.

    Granted, the DF i have been quoted are 12 Seer and up, which will affect that price as well, just not sure about picking DF in my market.

    Alot of fudging, but this is just a one month approx.

    Any advice i really appreciated.

  2. #2
    Your math is wrong.

    You're not including the HP efficiency.

  3. #3
    Join Date
    Oct 2005
    I changed the Btu value to reflect a 100% efficiency compared to the 80% gas pack...

  4. #4
    Join Date
    Feb 2004
    Kingsport, Tennessee

    Tn Dual Fuel

    The heat pump is more than 300% efficient at temperatures of 40 & above. The efficiency drops as the temperature outside declines. The weather data in Tennessee is very good for heat pumps since we do not see extremely low temperatures for very long over an entire heating season. If you are more comfortable with the warmer supply air temperatures of natural gas heat, & the current instability in the natural gas market is only short termed, then perhaps a new gas pack is your best choice. If this natural gas instability is long term, you could have a quicker payback with the dual fuel unit. Your natural gas supplier will not like you switching to a dual fuel unit since you will only use gas during their peak load times. Some gas utilities in the past have charged people more for gas when they switch to dual fuel, but not sure now. Your electric rates appear to be TVA rates which are relatively low compared to many parts of the country. I quess it depends on how well you like the feel of gas heat, your money, & whether this natural gas "shortage" is short termed. Good luck with your decision.

  5. #5
    Join Date
    Oct 2005
    Yeah, i realize the COP values change drastically relative to temperature, but wasn't sure how to factor that in.

    I would like to go straight electric, but since i'm already setup for gas packs, it makes more sense to go with NG again or as aux with Dual Fuel. The cost to add the additional line and such is not economically practical.

    I don't see NG supply overcoming demand anytime in the future. Only way to lessen the hit is legislation to slow the price fluctuations every winter, and that opens a whole other bag of worms. However, when everybody ditches gas packs for HP or other heating methods this winter it might make a dent in the NG price.

    Charging for duel fuel is completely bogus. They will charge me more for not using it? I'm already charged $9 bucks a month just for the connection. This does not make me feel all warm n fuzzy inside.

  6. #6
    Join Date
    Mar 2004
    Like it or not, dual fuel is your best bet if it's not practical to run a big circuit up there to run electric backup. Once you figure in the COP of the heat pump, all the heat you can get from the heat pump is dirt cheap compared to anything gas fired.

    It's tough to figure exact numbers, but in a dual fuel setup, I'd say it's fair to assume an average COP between 2.8 and 3. In your climate, it's probably fair to assume an average COP of 2.5 or so for an all-electric system. Either way your heating cost will be way lower than on an gas only system.

    My experience recently in Atlanta (fairly similar climate and winter electric rates) in switching from natural gas only to dual fuel is that my winter electric bills went up about $12 a month to feed the heat pump. In exchange, winter gas consumption was cut about *65%* (in quantity of gas used, not dollars, so you can take that as you will based on your gas rate structure and increases). The part I forgot to anticipate is the shoulders of heating season. I used to use about 50-60 therms (ccf) in months like November, March, April. Since it typically never gets super-cold in those months, the heat pump now does all of the heating, and my natural gas consumption just goes to the water heater. So now I only use gas for space heating during December, January, February. Spring and fall gas bills look just like summer gas bills- just enough for the water heater.

    Bottom line, you aren't going to have any trouble recovering your $1000 upcharge for getting dual fuel. Ballpark, you will probably save $90 a month in the winter and $50 a month in November and April. At that point you're looking at payback in two or three years, which is outstanding.

    2006 is shaping up to be the year of the heat pump.

  7. #7
    Join Date
    Oct 2005
    Hey, thanks for the post! I was hoping to find some people in the general area for their opinions.

    I've been doing more number crunching using those COP numbers and the number of heating days we see here, and i'm figuring a cost savings of around 300-400 bucks a year (50% gas increase to 5% electric).

    I'm getting a heat load analysis done and i might have to replace my ducting....It is flex duct that was installed very badly. The duct tape is falling to pieces and the insulation is coming apart. They even put ducting out the end cap to my bed room, which explains why the bedroom near the unit stays -2 deg in winter and +2 in summer! I think it is blow-by.

    So, now i've got the added cost of replacing the duct. I think i want to run metal in the straight runs and flex to the registers, just not sure.

    Also, around here an all electric system is going to cost about the same as dual fuel, within couple hundred bucks....get rid of my old gas water heater, kickout erpud gas ($9 bucks a month) and i make that up pretty quick.

  8. #8
    Join Date
    Mar 2004
    Sealing up ductwork makes good sense regardless of what sort of system you get. Metal trunks are great stuff if you can afford them.

    I'm surprised at your earlier comment about not being familiar with heat pumps. When I lived in TN I thought the place was fairly crawling with them. Maybe I got that idea because I am originally from California, where heat pumps are just mythical creatures that no one has ever seen or worked on (it sounds like a joke, but they really are about that rare there).

    The legacy of TVA and coal fired power plants is that your electricity is some of the cheapest in the world, and it's going to stay that way.

    The cost of heating water with electricity will probably be higher with electricity than with gas, but the gap has been narrowing fast lately. Your best space heating cost will be all-electric, though, even compared to dual fuel. That's because you can keep using the heat pump in subfreezing weather with an all-electric system; the electric heating elements just kick in to cover the shortfall between what the heat pump can put out and the total amount of heat that you need. You can't run gas heat and a heat pump at the same time, though, so with dual fuel, the heat pump shuts down at around freezing.

    An all-electric package unit will cost somewhat less than a gas pack or a dual fuel. You'll need another big circuit run to it, though, for the backup heating elements, and if you're to get rid of gas, you'll also need a new water heater and a circuit for that. If you don't have 200 amp electrical service, unfortunately, that whole equation may not be practical. You won't be using those big heating elements all that much, but if you consider the power needs of running the heat pump, the auxilary heat, the water heater, the usual household loads, and an electric dryer, you can start to see where at some point, 150 amps might not be enough to meet a peak of demand. If you need to upgrade your electrical service just to switch to all-electric, that will probably wash out any potential cost savings of going all-electric. At that point, I'd point you back at dual fuel.

    Check with your electric utility for incentives and other rate structures you could potentially qualify for. Sometimes you get a totally different (better) winter rate if you have a heat pump. Lots of utilities will give out rebates for switching from gas to electric, too, so that may be something to partially offset your switching cost.

    What part of TN, by the way?

  9. #9
    Join Date
    Jul 2004
    West TN

    If you have a contractor you know and trust, ask them about this program. (They get a little incentive if you let them get you started through the program)
    If you don't have a contractor, contact your power and light distributor about the heat pump program. They have a list of qualified contractors.

    Its a financing program, but you don't have to finance it.
    You can have it put in and inspected then pay the power and light company when its all done.
    Or you can have them add it to the utility bill.
    The dual fuel has a higher interest rate than the electric backup for what its worth.

    Its a very very nice program.

    Quality Contractor
    Proper load calculation
    No Air Leaks
    Air Balanced
    Air speeds in proper range (Heat pump blowing hard will 'feel' cold in the room even if its warm in the room.
    Air speed is important with heat pumps. When its right, its real nice)
    Capacity of equipment checked and compared to manufacturers data
    Straps holding up duct (not tie wire)
    No flex
    2" duct wrap
    Seams on duct wrap stapled and taped
    Properly sized supplies and returns
    Properly inspected (The power and light company will send an inspector out to check the job over and run tests on the system when its finished)
    This is a quick list off the top of my head....

    Their are also guidelines for weatherization.
    Proper insulation in attic (if not, they can finance it as part of the job)
    Proper attic ventilation
    Dryer vent run outside (not under house)
    Leaks around doors.

    Be sure to get a 10 year parts and labor warranty.
    Some power and light distributors require it while others don't. So be sure to ask about it and GET IT.

    Don't know how to link to my post directly,
    so go here....
    and scroll down to my post.

    Be sure to get a unit with a variable speed blower.
    The humidity here in TN is pretty rough in the summer.
    The variable speed blower will make the unit remove more humidity than a standard blower.
    The lower the humidity in the house, the more comfortable you are at higher temp settings.
    If 78 feels like 74.... then setting the stat to the higher temp will save you money.

  10. #10
    Join Date
    Oct 2005
    Thanks for the replies...

    Wyounger, i'm hailing from Coffee county, Tullahoma to be specific.

    I'm familiar with HP systems, but i'm originally from Southern IL, and they are not as plentiful. Mostly split systems up there....Heck, my grandmother has a boiler with baseboard heat. I can hear the pump kickin in right now...

    Considering ducting, i think i want to go all metal (round), especially considering the flow field of round compared to rectangular type duct. The drag induced by rectangular trunks to round metal/flex can be substantial thinking back into my pupil days. I'm going to have to bust out my thermo and fluid dynamics books.

    Good point about the electrical load, i will need to look into that. I'll definitely check out any potential savings in switching. Brown nosing never hurts...winter rate would be most excellent.

    I agree electric water heaters haven't caught up yet, but with NG prices soaring, it won't be long till the efficiency will not matter.

    Thanks wormy for the advice, i have a contractor working on a load calc and bids on the duct. Out of four contractors, he was the only one to even mention it. Since i'm an engineer with two years of thermo, i didn't ask anyone to do such calculation. If they asked, they were the guy for me, if not, too bad. I mentioned to him that he was only one and i guess most laypeople do not care of such things.
    However, I will mention this energyright program to him for sure.

  11. #11
    Join Date
    Oct 2005
    Well, here goes..

    Out of five HVAC companies, only one guy even mentioned a heat load (j manual) calculation and offered to crawl around with me under the house to check the ducting. One other company at least poked his head under there. I didn't mention it to any of the companies, but the fact that "the one" brought this up won the bid in my estimation. He was a little higher than the rest, but he has put in the effort to win my confidence. I then told him i was an engineer and my thermo/fluids background, and yet he was the only one to offer this information. His opinion on it was that most companies don't want to put forth the effort to do Manual J or Manual D calculations and not win the bid.

    I guess most laypeople don't care or understand, but after seeing my house HVAC system (probably lowest contractor bid) when the house was built (well built house though), how could one assume this is the correct setup? Crazy!

    Anyway, after much discussion and bids, i've thrown out Dual Fuel and i'm going to go straight electric.

    American Standard WCX030G100 Premium

    However, this unit doesn't have the variable speed motor. Since i'm keeping the current flex ducting (doh!) and just having them fix the end cap issue, would it still benefit me to go with variable speed? Flex duct has so much drag compared to round metal. I just can't afford the initial cost of metal duct install right now.

    Thanks for the input

  12. #12
    Join Date
    Jul 2004
    West TN
    Get the variable speed.
    It will control itself to the correct airflow.
    Its alot easier for it to do than a regular blower if the duct is a little out of shape.

    It removes the humidity in the house better, and that
    contributes to overall savings. You will feel cooler at a higher temp. Humidity is a big concern in TN.
    Compare 95 degree in Arizona compared to 95 here.
    The humidity drives the heat index up. So it can feel
    like 105 here and still feel like 95 in Arizona.

    Replace the flex duct as soon as possible
    and be sure to get the airflow adjusted properly
    (air speed of supplies that is).

    The variable speed will also allow the heat pump to start blowing warmer air sooner.
    It takes a little while for a heat pump to come up to temp
    with a standard blower.
    The variable speed will run the blower at 50% for a minute, then 80% for 8 minutes, then 100%
    When its at 50%, the heat pump has a chance to warm up real good. This cuts down on a drafty feeling when it comes on.

    The ramping up is especially important for cooling.
    It allows the cooling coil to drop to a colder temp much faster than a standard blower. This allows it to remove more humidity.

    IN the end, you end up with a more effecient unit than what they rate them at. So you get more bang for your buck.

    Did you ask about the Energyright program?

  13. #13
    Join Date
    Oct 2005
    Thanks for the info, i'll make sure to see if i can bump that up to a variable speed motor unit.

    I definitely want to replace the flex, but not sure i want to deal with that so close to colder temps...

    I've asked the contractor if he could do the duct calculations since he already has a j-manual load calc. He's using wrightsoft software, and if i'm not mistaken, it can systematically change the duct calcs based on the house design and load calc. Shouldn't take much effort to compare the existing flex and then find out the proper metal round ducting...

    The energyright program is mentioned on my utilities board website, but all it gives is contact info. I'll call and see.

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