electric heat pump in cold climate?

Discussion in 'HVAC Heating & Cooling' started by flatlander dave, Jun 9, 2009.

  1. flatlander dave

    flatlander dave New Member

    Messages:
    2
    just registered to forum. we are building a new home in northern idaho. electric rates are fairly cheap. no natural gas available. propane is available. my builders hvac man wants to install american standard heat pump with back up propane furnance. he says that heat pumps are efficient down to 0 with gas defroster on coils. is this correct. my concern is using propane all winter long, don't want to pay high fuel cost and rely on someone having to delivery. we wanted propane back up because power can go out frequently where we are building and will have a standby generator. with electric backup it was going to take huge generator. is propane backup going to be expensive to operate or are heat pumps really that good in cold weather. thanks for any responses.

    dave
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    I'm not current on the latest, but I'd be leary about a heat pump in really cold weather unless you go ground source. There just isn't that much energy easily available from an air-air heat exchanger...much more available from the ground, since if you go down far enough, it probably stays above 40.
  3. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    1 Therm of energy can be had from approx. 1.1 gal of propane, 29 kwh of elec heat or 8.4 kwh of heat pump heat.

    "Air conditioners and heat pumps usually operate most effectively at temperatures around 10 to 13 degrees Celsius (°C) (50 to 55 degrees Fahrenheit (°F)). A balance point is reached when the heat source temperature falls below about 4 °C (40 °F), and the system is not able to pull any more heat from the heat source (this point varies from heat pump to heat pump)."

    http://books.google.com/books?id=A7...mguIAK&sa=X&oi=book_result&ct=result&resnum=3

    You should probably get a Manual J heat loss/gain calc. made so this can all be predicted within a few percent before you decide on an HVAC system.

    Here's some HDD data for your area
    Description: Farenheit-based heating degree days for a base temperature of 65F
    Source: www.degreedays.net (using temperature data from www.wunderground.com)
    Accuracy: No problems detected
    Station: Airport: Coeur d'Alene, ID, US (116.82W,47.77N)
    Station ID: KCOE
    Month starting HDD
    6/1/08 271
    7/1/08 99
    8/1/08 132
    9/1/08 272
    10/1/08 585
    11/1/08 795
    12/1/08 1322
    1/1/09 1214
    2/1/09 990
    3/1/09 1007
    4/1/09 626
    5/1/09 376

    Here's some recommended insulation values
    http://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_insulation_table

    While I have your attention, what is your elec. rate? Here's some data I collected, in cents per kwh.
    42
    20
    17
    17
    16
    14
    11
    9
    Range 9 to 42
    Median 16.5
    Average 18.3

    75% pay between 11 and 20
    50% pay between 17 and 14

    Thanks!
    Last edited: Jun 9, 2009
  4. flatlander dave

    flatlander dave New Member

    Messages:
    2
    thatguy, thanks for the technical info. i am going to sit down with hvac guy. i'm not convinced that air heat pump will be my best bet. i don't want to heat with propane all winter. my kwh is only .057 cents. i'm sure it goes up with more consumption. as mentioned, the reason for propane back up was so i didn't have to install such a big generator. i'll do some more research on my own. we are having the house insulated with foam which should help with heat loss but we do have a lot of glass and one large room with cathederal ceilings. thanks again
  5. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Fwiw

    SW Ontario, Canada
    398.7 cubic meters = 140 therms of NG in 28 days = 5 therms/day
    2900 sq. ft. including basement gives 5/2900 = 170 BTU/day/sq. ft.
    664 HDD/28 = 24 HDD in one day
    170/24 = 7.1 BTU/sq. ft./HDD
    Effic. factor 0.8
    final 5.7

    D.C.
    838 therms of NG in 90 days = 9.3 therms/day
    3100 sq. ft. including basement gives 9.3/3100 = 300 BTU/sq.ft.
    2658 HDD/90 = 30 HDD in one day
    300/30 = 10 BTU/day/sq.ft./HDD
    Effic. factor 0.8
    final 8.0

    SF, CA
    2.3 therm/day
    2700 sq. ft. gives 85 BTU/day/sq.ft.
    540/30 = 18 HDD in one day
    85/18 = 4.7 BTU/sq. ft./HDD
    Effic. factor 0.8
    final 3.8

    San Diego, CA
    29,000 BTU/hr = 21 therms/30 days = 0.7 therm/day
    1560 sq ft including basement gives 0.7/1560 = 45 BTU/day/sq.ft.
    270 HDD/30 = 9 HDD in one day
    45/9 = 5.0 BTU/sq.ft./HDD
    Effic. factor 0.8
    final 4.0

    victoria, BC
    So 750 gals of oil giving 1100 therms for 4900 HDD for 180 days for 2000 sq. ft.
    6.1 therms/day
    2000 sq. ft gives 6.1/2000 = 305 BTU/day/sq.ft.
    4900/180 = 27 HDD in one day
    305/27 = 11 BTU/sq. ft./HDD
    Effic. factor 0.8
    final 8.8

    Cincinnati, OH
    600 gals/yr, 680 therms for 5096 HDD for 365 days for 1250 sq. ft.
    1.86 therms/day
    1250 sq. ft. gives 149 BTU/day/sq. ft.
    5096/365 = 14.0 HDD in one day
    149/14.0 = 10.6 BTU/sq. ft./HDD
    Effic. factor already in there
    final 11

    Wisconsin Energy Study gives
    4.4
    1.7

    Ranked
    1.7
    3.8
    4.0
    4.4
    5.7
    8.0
    8.8
    11

    Range 1.7 to 11
    Median 5.0
    Average 5.9
  6. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    A Mr. Slim heat pump can be had with a 24 SEER rating...how would this factor in...
    Last edited: Jun 11, 2009
  7. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    "1 Therm of energy can be had from approx. 0.71 gal of fuel oil, 0.77 gal of gasoline, 100 cubic feet of natural gas, 1.1 gal of propane, 29 kwh of elec heat, 8.4 kwh of heat pump heat, 14 pounds of wood, sunlight falling (insolation) on 5 sq. meters of absorbent surface for one day in CA or 14,000 gal/min of water falling 10' for one hour.
    You could also use 12 billionths of a pound of radioactive material decaying."

    The SEER rating is the Btu of cooling output during a typical cooling-season divided by the total electric energy input in watt-hours during the same period.

    100k BTU/10 = 10kwh at a SEER of 10.

    EER (Btu/(W*hr)) is converted to COP (Btu/Btu (Note: some may write W/W)) by dividing by 3.413 Btu/(Hr*W).

    Typical EER for residential central cooling units = 0.875 X SEER

    So let's say you need 1 therm (100,000 BTU) of energy to heat your place.
    It's 29 kwh of elec. heat.(COP = 1), it's 8.3 kwh at a COP of 3.5 with a heat pump, it's 10kwh at a SEER of 10, it's 10kwh at a EER of 8.8.

    There's also energy expressed in therms or kwh and there is power expressed in therms/hr or kw.

    A handshake, a fatal dose of radioactivity, and 45 cal. bullet all have the same energy, about 45 foot-lbs. But their power is much different, because a handshake takes seconds, a bullet energy probably dissipates in 1/10th of second, and radiation is a whole 'nother thang.
    Last edited: Jun 11, 2009
  8. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    It depends a lot on the heat pump. the SEER rating is for one temperature range. So a 3 SEER -40F air source heat pump might be better than a 24 SEER air source heat pump that is speced for Florida.

    As I see it there is a progression of possible systems.

    1. The most efficient is a closed loop ground source heat pump running off of a "Time of use", "off-peak" or "dual fuel" electric rates. This of course requires a second heat source.

    2. The next is a air source heat pump running off of "off-peak" or "dual fuel" electric rates. Or a ground soruce heat pump running on regular rates.

    Given the push to finally make "Time of use", "off-peak" or "dual fuel" electric rates available to everyone... I would say we are likely to see a lot of combination based heat systems.
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
  10. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
    As the outdoor temperature gets colder it gets harder to pull heat from it. At some point based on the units design vs the cost of the other heat source you are better off running a different heat source.

    This "balance point" shifts based on the price of electricity and the cost of other fuels.

    The "spot" price of electricity varies widely from about $0.01 per KWH to as much as $40 per Kwh. The power utility averages the price into what is called a "pool". This is why systems that let the power utility turn off the power when electric is costly help everyone and also help your pocket book due to the lower rates you pay.

    The "bivalent" system in your link looks interesting. There are some issues since it can't take advantage of all of the heat from the flame and also there is some extra energy involved with running the heat pump compressor even without any pressure loading on it. (The temperstatic valves would fully open)

    Edit: Summary
    1. When it's really cold outside... have the system use other the heat source.
    2. when electric prices are high... have the system use other the heat source.
    Last edited: Jun 11, 2009
  11. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Here's some elec. rate samples I got in cents per kwh; the 42 outlier is from Hawaii.

    42
    20
    17
    17
    16
    14
    11
    9
    6

    Range 6 to 42
    Median 16
    Average 17

    75% pay between 11 and 20
    50% pay between 17 and 14
    Is your $40 a misprint? I pay 16 cents/kwh.
  12. Bill Arden

    Bill Arden Computer Programmer

    Messages:
    584
    Location:
    MN, USA
  13. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
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