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  1. #1
    Join Date
    Apr 2008
    Posts
    21

    Question Does Heat loss calculation change with time?

    Does heat loss calculation changes with time?
    I have a copy of the original heat loss of the townhouse when it was built in 1983. I need to install new HP/AC system due to the age of the outside unit (1983), and many of the contractors will use the existing calculation. I'm in central CT.
    Is this OK?

    Here’s the data:

    Design condition:
    Outside temp: summer 95F, winter 0F
    Inside temp: summer 75F, winter 70F
    Daily temp range: summer 15-25F, winter n/a
    Summer swing temp: 4.5F, winter n/a

    SqFt: 1438 (1st floor, 2nd floor)
    Type: Multistory
    Exterior walls: frame, 3.5 inch, R-11
    Windows: double pane, weather-stripped double hung
    Doors: sliding glass door weather-stripped

    First floor: over unconditioned space
    Insulation: no insulation

    Ceiling: under vented roof or unconditioned space
    Insulation: (not legible). I personally checked the space, and seems to be about 4-6 inch of lose fiberglass.

    Heat loss: 27,980

    Heat gain: 16,261


    The current system is a 1983 carrier 2.5 ton (mod.#38RQ0033400), air handler is a 3.0 ton carrier (replaced in 1994 by previous owner) mod# FB4ANF036000ACAA, with 10kw electric strip.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,601
    There can be some slight variations with different versions of the load calc programs but probably not major. Biggest change is people tightening up their homes with more insulation and better windows. Sounds like you are oversized on the cooling if you have a 2.5 ton and need under a 1.5 ton. Heat sizing is good.

  3. #3
    Join Date
    Apr 2008
    Posts
    21

    Question

    Thank you.
    When sizing the heat pump for heating (as the system is designed @ 0F) do the 27,980BTU’s have to be provided by the heat pump alone, or by the heat pump plus the electric strip?
    I will use Goodman as example, just because I found extensive literature on their website.
    2.5 ton HP alone @ 0F provides ~12,100BTU’s.
    If I add staging 10 kw electric strip, I will have ~29,160BTU’s with 5kw @ 0F, and ~44,420 BTU’s with the full 10kw @ 0F. How does it work? What number should I use?

  4. #4
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,909
    The heatstrips are used to supplement the heatpump in cold weather.

    If the HP was sized for heating at design conditions, it would be grossly oversized for cooling.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  5. #5
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,777
    Your strip heat is sized to handle the full heating load in case the HP fails to work.
    That way if the part in on back order, you don't have to wait 2 weeks with a house temp of 45*.
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  6. #6
    Join Date
    Apr 2008
    Posts
    21

    Question

    Thank you guys...
    Please let me understand…
    1. HP+electric strip have to provide heat loss requirements of 27,980BTU’s at designed temp (0F). In this case I can stage electric strip, as HP+5 kw will provide enough BTU’s.
    2. The electric strip alone (10 kw total) has to provide the entire BTU’s load in case of compressor’s failure. Is this correct?

    BaldLoonie:
    I also noticed the discrepancy heating/cooling requirements. How can this be solved with proper sizing? I’m thinking I will either have a properly sized heating system, or a grossly oversized a/c. I’m more concerned about heating, as I live in central CT (HDD=5,614 and CDD=879).
    If I undersize HP (i.e. going to 2.0 or 1.5 ton), I’m afraid I will have the electric strip coming in more often as the HP can’t keep up, resulting in higher power bills (COP HP>COP electric strip even @ -10F).
    On the other extreme, what if I upgrade to a 3.0 ton? Will it be more efficient (cost less to operate) as the electric strip will not be used as often? I know that this may require duct evaluation for proper functioning.

  7. #7
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,777
    Was your current 2.5 able to keep the humidity reasonable in teh summer time.
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  8. #8
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    have you been comfortable?
    has shading changed?
    has roof color changed?
    has insulation in attic settled?

    the ducts may well need sealing with mastic, especially at the branch takeoffs= probably hand made junction there --

    tightening the house envelope, sealing penetrations, around doors & windows, will save $$$$$ in first costs, & thereafter energy costs.

    did old unit run for hours on 7/20?
    if not, it was too big!
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  9. #9
    Join Date
    Apr 2008
    Posts
    21
    Yes, I think so...but it is my personal opinion.
    In summer time, we usually leave the t-stat set at 75F during the day, and 77F at night. We set the fan to run continuosly, to insure similar temperatures downstairs/upstairs. The air handler is a 3-speed, and it is currently set at medium speed. I assume ~700-800 nominal CFM for a 3.0 ton system. Usually, we have high umidity days, but they are very limited in summer time.
    The townhouse is sandwiched between other two. It is in the middle of the building, leaving only two two-story high walls exposed.
    In winter we set the programmable t-stat low in the morning (when nobody's home), then 68F in the afternoon/evening, and at 60F at night.
    As average, I use ~500Kwh per month in the June-July-August (summer peak) period, and ~2,200kwh per month in the Dec-Jan-Feb (winter peak) period.
    My concern is winter time, and costs to run the HP+electric strip backup. Thus my question: what if I upgrade to 3.0 ton?
    I'm looking to replace with V/S air handler, or multispeed w/energy saving motor to decrease cost.

  10. #10
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,777
    Oversizing will tend not to remove enough moisture from the air from cooling the house too quick. So you end up not as comfortavle. Then you turn the stat down lower, and end up using more electric in the summer then you did with the old unit.

    In the winter, if the ducts weren't increased in size enough, it can lock out on safety and switch over to strip heat needlessly, raising your electric bill again.

    Many newer heat pumps put out more heat then the old ones did. Good chance that a new heat pump the same size as your current one will be able to heat your house at lower temps without the aux heat.
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  11. #11
    Join Date
    Apr 2008
    Posts
    21
    Thank you.

    cem-bsee:
    have you been comfortable? yes
    has shading changed? no
    has roof color changed? no
    has insulation in attic settled? not sure...

    the ducts may well need sealing with mastic, especially at the branch takeoffs= probably hand made junction there. re-sealed with aluminun foil tape, both in basement and attic vented space. existing one was very loose in some spots.

    So, how the unit has to be sized? I'll need ~27,980BTU's at 0F. Do I need to include the HP alone, HP+5kw, or HP+10kw strip?
    Would it be beneficial to downsize to 2.0 ton?

    beenthere:
    I share your same thoughts...

    Thanks.
    Last edited by Polibio; 07-14-2008 at 05:18 PM. Reason: adding comments

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