HVAC question about heat pumps, how cold will they work without using the backup electric heater?

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  • edporch

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    We already have a heat pump that the previous owners had installed from some years back that works fine til it gets below 25F or so.

    How cold outside can TODAY'S best air-source heat pump heat a home WITHOUT having to use the internal backup electric heater?

    I'm having people on another forum claim that today's air-source heat pumps will heat down to -18F with JUST the heat pump and no backup electric heat.
    I'm very skeptical of this.
    PLEASE educate me. :)
     
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    Gingerbeardman

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    Just cuz I don't know anything, how does a heat pump work? Is it a heated electric element with a system to pull air through the heated wires? Or maybe heats by pulling air through a screw type compressor?
     

    edporch

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    Just cuz I don't know anything, how does a heat pump work? Is it a heated electric element with a system to pull air through the heated wires? Or maybe heats by pulling air through a screw type compressor?
    I'm no expert, but this is my understanding.

    In heating mode, a heat pump works like an air conditioner in reverse.
    It draws the heat from outside air and releases it inside the home.

    But as the outside air gets colder, the amount of heat it can draw from the outside air gets less, while at the same time the home needs more heat.

    As it gets colder outside, there comes a point when the heat pump has to rely on an internal backup electric resistance heater to produce the needed heat.

    The heat pump we have, works OK til it gets down to about 25F, then the electric resistance heater in it kicks on, and uses massive amounts of electricity.

    I'm just skeptical that today's heat pumps can heat down to -18F without having to resort to using the internal backup electric resistance heater.
     

    edporch

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    A heat pump works like a window air conditioner installed backwards. I had a heat pump and hated it. They need to run constantly in the winter.
    We have a heat pump, and it's fin til it gets down to about 25F.
    Then the backup internal electric resistance heat kicks on and we get insane electric bill.

    SO I have a fireplace insert and burn wood when it gets down below 25F or so.
     
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    mattpepper97

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    We already have a heat pump that the previous owners had installed from some years back that works fine til it gets below 25F or so.

    How cold outside can TODAY'S best air-source heat pump heat a home WITHOUT having to use the internal backup electric heater?

    I'm having people on another forum claim that today's air-source heat pumps will heat down to -18F with JUST the heat pump and no backup electric heat.
    I'm very skeptical of this.
    PLEASE educate me. :)
    -15 to -18 is where the best heatpumps become ineffective at drawing heat out of the air. The best units out their can probably maintain temp in a house down to around 15 without electric backup. That said it depends heavily on set temperature, insulation, and how much air leakage you have. Also the type of system I mentioned typically is running proprietary software in the thermostat to run the heat pump and air handler as efficiently as possible while maintaining the set temperature within tenths of a degree.
     

    jamin

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    It all depends on the brand and its efficiency ratings. I haven’t seen any heat pump that will still operate at 100% capacity down that low. For example, Mitsubishi’s HyperHeat line is rated at 100% heating capacity down to -5.
     

    mattpepper97

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    It all depends on the brand and its efficiency ratings. I haven’t seen any heat pump that will still operate at 100% capacity down that low. For example, Mitsubishi’s HyperHeat line is rated at 100% heating capacity down to -5.
    That is probably about the limit for 100% but there are some units like Bryant/Carrier variable speed units that can operate down -15 or so before you have to rely solely on electric backup.
     

    Leo

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    It has been 30 years since I had a heat pump. It was new, state of the art and ton higher cost than a conventional system. It ran on electric element back up pretty much anytime it was cold enough to need a winter coat. Had the techs out several times. The techs said "off record" that heat pumps do not work well in Chicago winters. Right after the warranty was up, I had to pay for a sticky reversing valve to be replaced. Serious money. A couple years later I had it taken out and went to conventional natural gas with a central air unit. Even without the electric heat coils running, you are still feeding electricity to a large compressor unit, double the size of the central air compressor.

    They say the new systems are really improved, but it will take a lot to convince me to try one again.
     

    Gingerbeardman

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    Given all this information, what's the argument FOR a heat pump? If you don't have natural gas you could have propane, there's electric baseboard, wood, or even one of those outdoor burners that will eat anything, tires oil trash etc right?
     
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    Leo

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    Given all this information, what's the argument FOR a heat pump? If you don't have natural gas you could have propane, there's electric baseboard, wood, or even one of those outdoor burners that will eat anything, tires oil trash etc right?
    I am far from an expert on current technology, But if you live somewhere like Georgia, Florida, etc. where you need a substantial sized A/C most of the time and only sometimes need heat for a few days, you may do well with a heat pump. 35 outside and you want 70 inside appears to be the best use.
     

    Brandon

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    Greer claims their stuff is the cats meow.
    I ran my bryant down to 0⁰ prob should have stopped it at 15 or 20⁰

    How well your insulation is, windows and door seals make a difference.
     

    tmschuller

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    Greer claims their stuff is the cats meow.
    I ran my bryant down to 0⁰ prob should have stopped it at 15 or 20⁰

    How well your insulation is, windows and door seals make a difference.
    This.. new technology and better electricity consumption.. DC inverted all help but if your home isn’t well insulated, windows doors/ air infiltration and poor ductwork design/insulation all will impact the performance of a heat pump. Perfect world, heat pump with gas backup.
    I’m installing a heat pump at home and have a wood boiler and wood stove.
    We have have several months that they would work well. Mild winters ok but transitioning from a gas or electric heat to a heat pump is rough the first year. Above stated issue’s will affect the output temperature and the misses happiness.!
     

    rhamersley

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    I'm no expert, but this is my understanding.

    In heating mode, a heat pump works like an air conditioner in reverse.
    It draws the heat from outside air and releases it inside the home.

    But as the outside air gets colder, the amount of heat it can draw from the outside air gets less, while at the same time the home needs more heat.

    As it gets colder outside, there comes a point when the heat pump has to rely on an internal backup electric resistance heater to produce the needed heat.

    The heat pump we have, works OK til it gets down to about 25F, then the electric resistance heater in it kicks on, and uses massive amounts of electricity.

    I'm just skeptical that today's heat pumps can heat down to -18F without having to resort to using the internal backup electric resistance heater.
    You are exactly right. Essentially in the winter your condensing unit ( heat pump od unit) becomes the evaporator and your indoor coil becomes the condenser. That is why there is an orifice or expansion valve at the outdoor unit service valve as well as at the indoor coil. The reversing valve kicks over on a call for heat and the refrigerant flows in reverse everywhere except through the compressor. This pulls whatever heat is in the outdoor air into the refrigerant and releases it into the home.

    When I was designing them, I believe the manufacturing literature stated that the switchover between full heat pump mode and using the electric heat was between 25-30 degrees, going full electric heat somewhere around 17. Also, the outdoor unit has to go back into cooling mode to defrost the outdoor coil periodically, which then also kicks the emergency heat on to maintain temperature.

    I personally don’t care for heat pumps because the heat feels “drafty” to me, so I have a traditional air conditioner/propane furnace system in my home.
     

    indyblue

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    I have a dual fuel system (gas heat/heat pump). Since last summer the heat pump is going, may need a new reversing valve or both TXV's. Just the cost to evacuate/recover the refrigerant is not worth the effort to replace them. I'll likely replace the heat pump with straight A/C since I've been running on the gas heat only since fall and my energy bills are less than with the heat pump+gas was.
     

    Brandon

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    I have a dual fuel system (gas heat/heat pump). Since last summer the heat pump is going, may need a new reversing valve or both TXV's. Just the cost to evacuate/recover the refrigerant is not worth the effort to replace them. I'll likely replace the heat pump with straight A/C since I've been running on the gas heat only since fall and my energy bills are less than with the heat pump+gas was.
    Unless on propane or all elec. I do not see the value in having a traditional heat pump with ng.

    If we are talking evolution or infinity... well... i wouldn't pay for it.
     

    indyblue

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    Unless on propane or all elec. I do not see the value in having a traditional heat pump with ng.

    If we are talking evolution or infinity... well... i wouldn't pay for it.
    It was existing when we bought the house, I wouldn’t have gone heat pump myself.

    I read a slashdot thread and they were claiming >100% efficiency. Sorry, I don’t believe it. Power to run pump plus the fan is more than gas needs.
     

    wcd

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    Would not recommend them for North of the Mid South. Once that emergency heat kicks on you are going to need the Emergency room for when you see the Bill. We have one and use it for AC only, for the most part it’s good at cooling but our home is well insulated so there is that.
     
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