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  1. #27
    Join Date
    Jul 2004
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    Southern Alabama
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    448
    Posted By Pokerface:

    Since the balance point is the temperature outside at which the unit cannot maintain the set temperature without using the auxiliary heat, lowering the outside temp will raise the balance point. Right?

    No. The balance point is a single point. We know that capacity of the heat pump to provide heating DECREASES as the outside temp goes down (less heat, or enthalpy, to pull from the air. So, to determine balance point you plot the structure heat loss over temperature. This line will slope down and to the right indicating that the higher the outside temperature the less heating you will need to maintain space temp.

    On the same graph, plot the heat capacity of the HP. This will be a line going from left to right and sloping up. This show that as outside temp increases, HP capacity increase. There will be only one temp where these lines intersect; that is the Thermal Balance Point. Temps above this point and the heat pump has enough capacity to satify the load. Below this point, aux heating is required. To do this correctly a Man J needs to be done and you have to know the HP ratings.

    BTW - Once you have this data, you can size the aux heating properly. When using electrical heat, you don't want the strips to come on until, theoretically, the balance point is reached. Remember a HP COP is always greater than 1 (heat strip is 1) and WILL always be more efficient than strip heating. In reality you would set the heat strips to come on arounf 2-3 degrees BEFORE the thermal balance is reached.

    Posted By CT2:

    have to vote for "D" because I havnt learned about calculating the ratio yet and I dont understand how lowering the oat would raise the compression ratio

    Just think about your freezer. It is pulling heat out of the freezer air to maintain a temp below 32 degrees and has to reject the heat into a house at 70 degrees.

    comp ratio = head pressure in PSIA/low side pressure in PSIA.

    It takes more compression ratio to get a lower pressure. A compressor has to be designed for the temp rating that you want to use it for. Be it low, medium, or high temp rating (comfort cooling). The lower the temperature you need to achieve, the higher the compression ratio needs to be.

    This is easy to prove. Run the compression math for what a comfort cooling compressor has and a freezer has. HP is like the freezer when in low temp heating.

    Shophound explained it exactly while I was typing this. So I am being redundant.

    [Edited by on call on 08-09-2005 at 07:48 AM]

  2. #28
    Join Date
    Jul 2005
    Posts
    19
    Thanks for the excellent post.

  3. #29
    Join Date
    Dec 2002
    Location
    davenport, iowa
    Posts
    778
    even though I don't work on heat pumps, I'd say 'D'.
    Da Threadkilla

  4. #30
    Join Date
    Mar 2002
    Location
    Richmond, Virginia
    Posts
    4,264
    C
    There is nothing more frightful than ignorance in action....Mark Twain

  5. #31
    Join Date
    Nov 2004
    Posts
    2,927

    Re: Re: Blance Point

    Originally posted by shophound
    balance point is at a fixed outdoor temp for the structure's heat loss rate to heat output of heat pump.
    So,if you lower that "fixed outdoor temp",you will lower balance point.
    Or is the ARI standard design temp of 47 degrees used here? 17 degrees?


    (I gotta get a passin grade here ,or my momma whups me..)

  6. #32
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,357

    Re: Re: Re: Balance Point

    Originally posted by jacob perkins
    Originally posted by shophound
    balance point is at a fixed outdoor temp for the structure's heat loss rate to heat output of heat pump.
    So,if you lower that "fixed outdoor temp",you will lower balance point.
    Or is the ARI standard design temp of 47 degrees used here? 17 degrees?


    (I gotta get a passin grade here ,or my momma whups me..)


    Here's how I understand it. At a given OAT, a building is going to lose a set amount of BTU's to the colder air surrounding the structure. I'll just toss out some numbers without a care as to accuracy for the sake of the discussion. Let's say at 28 degrees F, a building is losing 25,000 BTU per hour to old man winter. The heat pump is running and has reached a point where it can't produce any more heat than 25,000 BTU's at this OAT (without heat strips). At a higher OAT, it can make more heat, and the structure loses less heat, because the temperature difference between the building and outdoors is less.
    At lower OAT, the heat pump cannot extract as much heat from the winter air, so when capacity drops below balance point, the HP can't meet the heat being lost by the building to the outdoors. The system either pulls in the heat strip contacts or, failing that, the indoor temp setpoint won't be satisfied with just the heat pump.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #33
    Join Date
    Jul 2004
    Location
    Southern Alabama
    Posts
    448
    Posted by Shophound:

    " Here's how I understand it. At a given OAT, a building is going to lose a set amount of BTU's to the colder air surrounding the structure. I'll just toss out some numbers without a care as to accuracy for the sake of the discussion. Let's say at 28 degrees F, a building is losing 25,000 BTU per hour to old man winter. The heat pump is running and has reached a point where it can't produce any more heat than 25,000 BTU's at this OAT (without heat strips). At a higher OAT, it can make more heat, and the structure loses less heat, because the temperature difference between the building and outdoors is less.
    At lower OAT, the heat pump cannot extract as much heat from the winter air, so when capacity drops below balance point, the HP can't meet the heat being lost by the building to the outdoors. The system either pulls in the heat strip contacts or, failing that, the indoor temp setpoint won't be satisfied with just the heat pump "

    EXACTLY!!!

    The only way to change the balance point is to either change the thermal dynamics of the house (more/less insulation, more/less SQFT, new windows, etc.) OR change the capacity of the heat pump. The intersection of the temperature graphs determines the balance point with the current situation. The ARI rating is used to graph the capacity of the heat pump.

    Suppose you graph the structure heat loss vs. hp capacity and find that the balance point is 10 degrees but you want more hp heating capacity. HPs should always be sized for the cooling load, but there is a slight deviation allowed for more heat capacity. HP standards say that you can oversize the HP by 25% in cold climates and 15% in warmer climtes. Oversizing the unit would give you more heating capacity and lower the balance point even more.

  8. #34
    Join Date
    Sep 2004
    Posts
    3,157
    shophound
    Professional Member




    Compression ratio = suction pressure in pounds per square inch absolute (psig + 14.7 @ sea level = psia) divided by discharge pressure in psia = compression ratio.

    As OAT drops, compression ratio will rise due to lower temperatures and pressures in outdoor coil (serving as an evaporator when the system is in heat mode), compared to fairly consistent discharge pressures as one would find when the system is in cooling mode. The wider the spread between suction and discharge pressures, the higher the compression ratio.
    __________________


    thank you shophound

  9. #35
    Join Date
    Jul 2005
    Posts
    171
    Norm, you shouldn't post these questions here in an open forum. With the answers about evenly split, it kinda gives us "outsiders" the idea that half the hvac pros are wrong on any given issue...

  10. #36
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,357
    Originally posted by dx
    Norm, you shouldn't post these questions here in an open forum. With the answers about evenly split, it kinda gives us "outsiders" the idea that half the hvac pros are wrong on any given issue...
    I disagree. It's an excellent opportunity to revisit old information in one's head to see if it is right or not. An open forum widens the opportunity for knowledgable persons to step up and present their case as to what may be correct info and what is not. I have both corrected and have had myself corrected during an open forum exchange that has proven beneficial in many ways. I will say, although I enjoy passing on info I understand to be right and useful, I also benefit greatly when I learn "I was always taught so and so..." may be hogwash.

    Oh, and CT2, you're welcome. Still hotter than blue blazes out there in Sactown?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #37
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,308
    Shophound.

    I think he meant Norm shouldn't post the questions in the resi section, where ho's can see them.
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  12. #38
    Join Date
    Aug 2001
    Location
    forney texas
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    17,890
    I still say A. the cooler it gets, and the question just says cooler, the lower the amp draw, the more efficient it becomes.

  13. #39
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    Jan 2004
    Location
    Lancaster PA
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    68,308
    Originally posted by acmanko
    I still say A. the cooler it gets, and the question just says cooler, the lower the amp draw, the more efficient it becomes.
    Think of it this way.
    The amps drop, but the amount of heat your getting from it drop more, so your getting less heat per watt, so it is less efficient.
    Remmember the C.O.P. gets lower the colder it is.
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