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  1. #1
    Join Date
    Nov 2006
    Posts
    20

    Heat pump in cold climate ?

    We recently purchased a lake cabin in MN that has a Carrier heat pump (25HPA 536A0031010) plus two electric assist (I believe 18kva each) and an Infinity control. It is a 4-season cabin that we use 2 or 3 weekends per month during the winter. My initial plan was to keep the temp at 40f or 45f when we're not there and then heat up to 65f when we are. I'd like to limit the use of the electric assist to save money. Looking for advice on all of this though.

    Is this a reasonable plan?

    How low of outside temp will the heat pump function? EG, as long as the temps won't be below +5f or 0 or -5f or ?? can I switch to heat pump only on the thermostat and have it maintain temp (40f during week, 65f weekends)?

    I assume I need the electric assist to heat it up from 45f to 65f? At what outside temp could the heat pump likely handle this alone in a reasonable time like maybe 4 - 8 hrs?

    I've scoured the web and haven't come up with much.

    Thanks,

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,401
    You can have a tstat installed that has an outdoor sensor to lockout the electric heat above a certain temp (usually ~30F). Heat pumps have a rated btu capacity for corresponding temps they are rated at 47F and 17F but a graph would show the curve. You would then need to know what the heat loss of the house is to determine how many btu/hr it needs to maintain temp at X outdoor temp. The heat pump shouldn't have a problem keeping it at ~40-45F without electric heat coming on but just in case something happened to the heat pump I wouldn't disconnect the electric heaters below freezing or you may get some busted pipes.

  3. #3
    Join Date
    Nov 2006
    Posts
    20
    Thanks. The thermostat is a Carrier Infinity (haven't been able to determine model #, but installed in 2008 and not zoned). It does display outdoor temp. Not sure if lockout is set, but will check tomorrow. Am I correct then that anytime the outdoor temp is below 30f it needs the electric? In Brussels I was told that they were good down to -18c (about 0f). If locked out above 30f, does the HP run in combo with the electric or does electric do everything? We are below 30f about 90% of Jan & Feb.

    Thanks,

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,242
    You'll be using nearly all (ONLY ) heat strip in Jan - Febr
    with temperatures < 30'F

    What Size cabin?

    EXAMPLE:
    with many simplifications -

    1500 Square feet with 10 windows / U 0.3 and semi-tight for infiltration
    Heat load might be ~15,000 BTU/hr for 40'F temperature rise above outside air.
    ... which means > $300 / month at $0.10/kw

    5 kW =~ 17,000 BTU/hr

    5 kW * 60 days * 24 hours/day * $0.10 / kw = $720 for 2 months
    if temperature was a constant 0'F and 40'F inside.

    A constant 0'F means Heating Degree Days (HDD) would be 2100 which may be significantly more than real temperatures.
    Typical HDD would be close to 1,600 per month.
    So, based on decreased HDD's, electric cost might be 80% of $720 or $560 (5,600 kw) for 2 months.

    __60 days
    __24 hours
    1,440 hours

    5 kw
    7,200 kw-hr

    $0.10 $/ kw
    $720.00 total
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,902
    It is usually recommended not to run the heat pump when the indoor temp is below 60.

  6. #6
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    1,986
    BaldLoonie is right, you will have defrost problems when the inside temp goes below 60. The outside unit will ice over. So when your gone and for warm up you will want to run Emerg. heat, then switch it to heat. The elements won't use that much energy if you have the stat set at 40-45*.

  7. #7
    Join Date
    Nov 2006
    Posts
    20
    Thanks all. I'm guessing by your comments that the thermostat will not automatically avoid the HP when inside is below 60? Or will it? If not then I assume I need to set it to use only emergency/auxiliary when we're not there?

    Is there a difference in emergency heat and auxiliary heat? I think both coils are setup as auxiliary?

    Why would someone install a HP in MN?

    BTW, the 'cabin' was built in 1972, 2200sf (1.5 levels above + half w/o basement). It was constructed as a full-time residence so typical insulation for 1972.

  8. #8
    Join Date
    May 2008
    Location
    Missouri
    Posts
    3,422
    As Bald mentioned, it's not wise to attempt to heat a home below 60 with a HP alone as there is very little "available" heat with which to defrost the heat pump if aux. heat is left off, or disabled. The heat pump uses the heat inside the home (along with heat from the aux. heater) to heat up the outside condenser coils and remove frost. I'd leave the Aux./emergency heat working and simply turn the stat. down. The heat pump will do what it can, and electric strips will make up the difference.

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,242
    Quote Originally Posted by jsfox View Post
    Is there a difference in emergency heat and auxiliary heat? I think both coils are setup as auxiliary?

    Why would someone install a HP in MN?
    WHY install a HP?

    Apparently, They Must reduce _overall_ electrical power consumption.
    Otherwise, WHY would B.C. Hydro ( even NORTH of MN)
    offer upto $1,500 for installing an Air Source Heat Pump.?

    http://www.bchydro.com/rebates_savin...y_rebates.html

    http://www.livesmartbc.ca/attachment...Incentives.pdf
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,156
    For part time use calculate how long the payback time is for heat pump vs electric heat strips.

  11. #11
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    6,007
    No the stat doesn't automatically stop HP when indoor temp is low.

    Set to emergency heat it should only use strips

  12. #12
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,700
    How much do you expect to save keeping it at 40? Just leave the place at 60 and let the unit figure itself out. Likely when temps get around 0 the strips may stage on and off to help carry the load.

    If you leave it at 40 you may create all kinds of problems not just with your system, but with your structure. And if the power goes out there will be very little temperature in your mass for margin before pipes freeze.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  13. #13
    Join Date
    Jan 2008
    Location
    Texas
    Posts
    1,662
    If indoor temp is set at 40, inside the exterior walls containing the plumbing may be boarderline freezing temps also.

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