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  1. #1
    Join Date
    Sep 2005
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    Texas
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    2,793
    Ive been getting a few inquiries from customers about heatpumps being a better choice for heating over a gas furnace due to fuel cost.

    Any opinions?

  2. #2
    Join Date
    Nov 2001
    Posts
    1,874
    To have both is better (cheaper)then just gas.

    Heat pumps are cheaper to operatethen gas , down to a given outdoor temp. If it's a newer unit, I'll add.
    That given number depends on location, up north it may be about 40*, down south it could be 25*
    If you try to fail, and succeed.
    Which have you done ?



  3. #3
    Join Date
    Sep 2004
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    Memphis
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    2,502
    I know my opinion isnt the same as the majority on here. But I prefer going with a 90% furnace as my primary heat. I also am not seeing these huge fuel costs that were supposed to hit us this winter. I have a 90,000 btu 90% furnace downstairs & a 50,000 btu 80% furnace upstairs & my total utility bill is still only around $200 a month which is about the same as the summer. We never keep our stat below 70. I cant imagine me saving hardly any money at all by having a heatpump.
    Life is like a jar of jalapenos. What you do today might burn your ass tomorrow.

  4. #4
    Join Date
    Sep 2005
    Location
    Texas
    Posts
    2,793

    Im in the Dallas area

    and my heating bill went from $80 per month to $180 per month with stat at constant 69 degrees (80% furnace)

  5. #5
    Join Date
    Jul 2004
    Posts
    253
    This can be calculated. For example, lets assume a therm of gas is $1.20 ( 100000 BTUs ) and electricity is $0.08 per kwh. Also assume a 90% efficiency furnace and a heatpump with a COP of 2.5. The cost for 100000 BTUs of real heat would be as follows:

    Gas Furnace : $1.20 / 0.9 = $1.33
    Heat Pump : $0.08/kwh * 100000 BTUs / ( 3413 BTUs/kwh x 2.5 )
    = $0.94

    OK, so the heatpump would cost less to run. The COP of the heatpump will vary with temperature and you should use the appropriate numbers for the cost of gas and electricity.

    Originally posted by air2spare
    Ive been getting a few inquiries from customers about heatpumps being a better choice for heating over a gas furnace due to fuel cost.

    Any opinions?

  6. #6
    Join Date
    Sep 2005
    Location
    Texas
    Posts
    2,793

    our KWH rate

    is now at 14.5 up from 9.5 less than a year ago

  7. #7
    Join Date
    Mar 2004
    Posts
    1,042
    From what I've seen in metro Atlanta most people who have switched to dual fuel or all-electric have had their heating cost cut in half or better.

    Tuccillo is on the right track, but to get the numbers exactly right you have to also have some idea of the balance point of the structure. For example, if you assume a COP of 2.5, that happens at about 17 F outdoor ambient, but there aren't many applications where a heat pump is above the balance point at 17. So for dual fuel applications, assuming a typical balance point of 30-35, your COP will be closer to 3 whenever the heat pump does run. Below the balance point, you'll burn gas. For electric backup, at 17 your COP is 2.5, but you also have to account for the shortfall between your structure's heat loss at 17 and the heat pump's output at 17. Once you know how much resistance you're using, you have to account for that in your operating cost at that temp.

    In my own case (the numbers work out extra-well here because heat pumps are rated at 17 and my design temp is about 17):
    Heat loss 34,000 btu @ 17
    Heat pump output 15,200 btu @ 17, COP 2.36
    Shortfall at 17: 18,800 btu (=5.5 kw)

    The question can't really be the cost per 100,000 BTU when you're below the balance point, because it doesn't account for auxiliary. So let's instead consider operating cost per hour at design temp (17 degrees here). I'll keep tuccillo's assumptions of $1.20/therm and 8 cents/kwh. What I'm calculating here, then, is the cost per hour to heat this specific structure when at outdoor design temp (which is below balance point). Note that under this condition it doesn't matter if you're dual fuel or gas-only heat, because the dual fuel setups wouldn't be running the heat pump anyway.

    90% Gas Furnace : $1.20 / 0.9 = $1.33 (per 100kBTU output)
    $1.33*0.34 = $0.452
    80% Gas Furnace : $1.20 / 0.8 = $1.50 (per 100kBTU output)
    $1.50*0.34 = $0.51
    HP with electric: [HP cost ]$0.08/ kwh * (15200 BTUs / (3413 BTU/kwh X 2.36 COP)) = $0.149
    [Electric aux cost] $0.08/ kwh * 5.5 kw = $0.44
    So at 17 F ambient the HP with electric-backup cost per hour is $0.589. In this case, the all-electric HP is the most expensive to run. But you spend very little time at design temperature, and in this area, not even much time below the balance point. At 25 degrees (I'll spare you repeating the numbers) the HP cost is about the same but the aux is down to 2 kw, so $0.16, and your cost per hour is already down to under $0.30, which beats the pants off the other choices. So with these assumptions (utility costs, efficiencies, heat loss, balance point) the heat pump is still running cheaper than a 90% furnace down to somewhere in the 20-22 degree range. The farther you get above that mark, the more operating cost advantage there is.

    Anyway, in my book, heat pumps are a great heat source anywhere that electric rates are moderate or low during heating season. In the sun belt, they can get you through the winter with very little use of backup because you typically don't get very far below the balance point. Farther north, you still get a lot out of a heat pump, but it's more during the spring and fall. At that point you need to make sure that you make a good choice on a backup heat source because you'll not only spend a lot of time way below the balance point (that's important for dual fuel) and will be seeing temperatures way below the balance point (that's important for electric backup, 'cause when you're fairly close to the balance point you don't use that much electric backup to cover the shortfall).

    So there are a lot of factors to consider... utility rates, sure, but also you have to consider local weather, the structure's heat loss and balance point, etc. There are very few places where electricity is so expensive that the heat pump isn't cheaper to run than fossil fuel heat when ambient temperatures are above 30-35, though, which suggests that dual fuel can achieve some savings over fossil fuel almost everywhere. The only question is how much, and if the savings is enough to justify the extra upfront cost of dual fuel over other equipment choices.

  8. #8
    Join Date
    Mar 2004
    Posts
    1,042

    Re: our KWH rate

    Originally posted by air2spare
    is now at 14.5 up from 9.5 less than a year ago
    Ouch. But how much did gas go up over the same time period?

    HP = $0.145 * 100000 BTU / (3413 BTU/watt * 2.5 COP) = $1.70
    Not so hot at COP of 2.5, call that 20 degrees. But how about at 47?

    HP = $0.145 * 100000 BTU / (3413 BTU/watt * 3 COP) = $1.41

    OK, so at 14.5 c/kwh and $1.20/therm gas, it has to get pretty darn warm before the heat pump has a cost advantage. But I'd bet that gas has gone up well over $1.20, too. It's really just a battle of numbers that depends on your weather and your utility rates.

    My winter electric rates are about half that, so heat pumps are cheap fun in Atlanta. Most people are paying about $1.50-$1.70 per therm for gas around here now. I locked in a fixed rate last May, before Katrina, so I'm still at 99 cents per therm. Sounds nice now, but I also remember paying 45 cents a therm in 2000. One way or another, energy prices have nowhere to go but up for the forseeable future, so the winner will be those who are ready to deal with high-tech high-efficiency solutions that weren't cost-effective five years ago. With this big adjustment in energy prices, they're looking more and more attractive.

  9. #9
    Join Date
    Jul 2004
    Posts
    253

    Re: Re: our KWH rate

    wyounger, very nice analysis! I did a similar thing when deciding on a water heater for my new house. The choice was electric or propane - for 100000 BTUs, assuming 95% efficiency for electic water heater vs. 80% for propane water heater:

    Electric: $0.08/kWh * 100000 BTUs / (3413 BTUs/kWh*0.95) = $2.47

    Propane: $2.50/gal * 100000 BTUs / (91600 BTUs/gallon*0.80) = $3.41

    If you assume a tankless propane water heater, the efficiency is about 95% and you probably save another 5% because of no standby losses ( compared to an electric tank water heater - didnt consider electric tankless, heard too many bad things ), the numbers are as follows:

    Electric: $2.47
    Propane Tankless: $2.72

    In the summer, my electric rates go to $0.10 kw so the tankless propane my have a slight advantage assuming propane prices remain fixed. I have more faith in the stability of electric prices than propane.



    Originally posted by wyounger
    Originally posted by air2spare
    is now at 14.5 up from 9.5 less than a year ago
    Ouch. But how much did gas go up over the same time period?

    HP = $0.145 * 100000 BTU / (3413 BTU/watt * 2.5 COP) = $1.70
    Not so hot at COP of 2.5, call that 20 degrees. But how about at 47?

    HP = $0.145 * 100000 BTU / (3413 BTU/watt * 3 COP) = $1.41

    OK, so at 14.5 c/kwh and $1.20/therm gas, it has to get pretty darn warm before the heat pump has a cost advantage. But I'd bet that gas has gone up well over $1.20, too. It's really just a battle of numbers that depends on your weather and your utility rates.

    My winter electric rates are about half that, so heat pumps are cheap fun in Atlanta. Most people are paying about $1.50-$1.70 per therm for gas around here now. I locked in a fixed rate last May, before Katrina, so I'm still at 99 cents per therm. Sounds nice now, but I also remember paying 45 cents a therm in 2000. One way or another, energy prices have nowhere to go but up for the forseeable future, so the winner will be those who are ready to deal with high-tech high-efficiency solutions that weren't cost-effective five years ago. With this big adjustment in energy prices, they're looking more and more attractive.

  10. #10
    Join Date
    Mar 2004
    Posts
    1,042
    Interesting post on the water heater question. I think 94-95% is spot-on for an electric storage water heater; the heating itself is basically 100% efficient, but it does lose some heat through the insulation. But from what I've seen with gas/propane storage water heaters, the energy factors are 0.58-0.62- meaning at best, 62%. That makes sense because you can't insulate the flue, and when you aren't burning fuel you leak heat from the water back into the flue like mad.

    The tankless gas water heaters I looked at had energy factors around 0.78 (natural draft) or 0.85 (induced draft). That's from having looked around at Rinnai, Noritz, Takagi, Bosch. I'm sure they can both do better at steady state, but energy factor is like AFUE- it's supposed to account for big-picture total efficiency.

    I suppose somebody makes a condensing tankless, but that's got to be even more expensive than the noncondensing versions, and even with gas/propane prices doing what they have, I still don't see the economics of even the more run of the mill tankless units working out very well.

    Anyway, given my thoughts on water heater efficiencies, I think you did even better by choosing electric than you think you did


  11. #11
    Join Date
    Jul 2004
    Posts
    253
    Hi wyounger,

    Thank you for the efficiency numbers for the propane water heaters - I guess I was being a bit optimistic on their efficiencies so the case for electric (at least compared to a tank propane water heater) is better than I thought. I wound up going with a "Marathon" electric water heater. Nice product - well insulated with a couple of inches of foam and the tank is some sort of composite that wont corrode - the whole thing sort of looks like a torpedo - the outer shell is plastic. I get a lot of "what the heck is that thing" comments from friends when they first see it.

    Originally posted by wyounger
    Interesting post on the water heater question. I think 94-95% is spot-on for an electric storage water heater; the heating itself is basically 100% efficient, but it does lose some heat through the insulation. But from what I've seen with gas/propane storage water heaters, the energy factors are 0.58-0.62- meaning at best, 62%. That makes sense because you can't insulate the flue, and when you aren't burning fuel you leak heat from the water back into the flue like mad.

    The tankless gas water heaters I looked at had energy factors around 0.78 (natural draft) or 0.85 (induced draft). That's from having looked around at Rinnai, Noritz, Takagi, Bosch. I'm sure they can both do better at steady state, but energy factor is like AFUE- it's supposed to account for big-picture total efficiency.

    I suppose somebody makes a condensing tankless, but that's got to be even more expensive than the noncondensing versions, and even with gas/propane prices doing what they have, I still don't see the economics of even the more run of the mill tankless units working out very well.

    Anyway, given my thoughts on water heater efficiencies, I think you did even better by choosing electric than you think you did


  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,248

    Thumbs up Heat Pumps $ave

    Originally posted by Toolpusher
    Heat pumps are cheaper to operatethen gas , down to a given outdoor temp. If it's a newer unit, I'll add.
    That given number depends on location, up north it may be about 40*, down south it could be 25*
    In a Northern climate, the heat pump balance point may be as low as 22ͺF, whereas in a southern climate it may be close to 31ͺF due to amount of insulation and total window area and U-value.

    Modern heat pump will be MUCH[*] cheaper in nearly all of Eastern U.S.

    * NY or ... > $0.16 /kW .. a little cheaper)



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  13. #13
    Join Date
    Jan 2006
    Posts
    3

    Heat Pump in Southern California

    Would a duel fuel system be ideal for southern california?

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