Results 1 to 6 of 6
  1. #1
    Join Date
    Sep 2006
    Posts
    1,253

    Interpret Man J results

    My home is about 3,100 sq ft on two floors with a single open staircase at the front of the house. I done a Manual J using HVAC-Calc inputting my current configuration and then re-running it after making changes to the home like more insulation and reducing infiltration. Therefore, I have an as is and an as will be analysis.

    The current as-is results show that the home has heat loss/gain as follows:

    Heat loss: 49,877 First floor
    41,967 Second floor

    91,920 Total Heat Loss

    Heat Gain: 26,565 (20,506 Sen) First floor
    21,099 (18,310 Sen) Second floor

    47,965 (38,830 Sen) Total Heat Gain

    The as will be results are:

    Heat loss: 43,680 First floor
    33,586 Second floor

    77,324 Total Heat Loss

    Heat Gain: 24,217 (18,440 Sen) First floor
    17,568 (15,475 Sen) Second floor

    41,801 (34,923 Sen) Total Heat Gain

    My current equipment is as follows

    First Floor:
    Oil fired hot air furnace rated output 86,000 BTU
    Cooling with 2.5T York A/C (single stage)

    Second Floor:
    York 3T heat pump (single stage) strips used only for defrost

    Here is what I am trying to figure out from the Man J calculation.

    My current as is configuration calls for nearly equal heat loss for both the first and second floors at the design temp of 0*. But, the real world configuration shows that the first floor oil furnace is providing the majority of both the first and second floor requirements. In making this statement I am assuming that the heat pump has output of around 10K BTU when the outside temp is around 5*, the low that we have had this winter. I should note that the house has even temps from room to room and maintains the set point when the OD temp is 5*.

    Similarly, while the Man J calls for more cooling on the first floor than the second, the reality is the second floor heat pump is larger than the first floor A/C and, in fact, provides the majority of the cooling for the entire house. When at or near the summer design temp of 96* the heat pump is running continuously but the first floor A/C is cycling albeit not a lot. As during the winter months the home is generally comfortable during the summer with consistant room temps and the set point being maintained.

    I understand the physics of heat rising in the winter and cool air falling during the summer, but when it comes time to replace equipment how should it be sized? If the equipment were sized for cooling based on the Man J, it appears as though from real world experince the second floor is going to be warm while the first floor unit will constantly short cycle. Also, it looks like the heat requirements would not be met if the Man J is followed as the heat second floor unit would short cycle while the first floor unit could never meet demand.

    It seems like when there are upper and lower zones, each with separate equipment and controls, then the burden of the heating and cooling must shift accordingly. Im interested in real world experience and comments.

    Thanks

  2. #2
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    go with the facts -- from the real world, your existing systems --
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  3. #3
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,241

    Thumbs up

    Quote Originally Posted by mchild View Post

    My home is about 3,100 sq ft on two floors with a single open staircase at the front of the house. I have performed a Manual J using HVAC-Calc

    The current as-is results show that the home has heat loss/gain as follows:
    Heat loss: 49,877 First floor
    41,967 Second floor
    91,920 Total Heat Loss

    Heat Gain: 26,565 (20,506 Sen) First floor
    21,099 (18,310 Sen) Second floor
    47,965 (38,830 Sen) Total Heat Gain

    Here is what I am trying to figure out from the Man J calculation.

    My current as is configuration calls for nearly equal heat loss for both the first and second floors at the design temp of 0*.

    ... the first floor oil furnace is providing the majority of both the first and second floor requirements.

    Similarly, while the Man J calls for more cooling on the first floor than the second, the reality is ...
    ... consistent room temps and the set point being maintained.

    It seems like when there are upper and lower zones, each with separate equipment and controls, then the burden of the heating and cooling must shift accordingly. Im interested in real world experience and comments.
    It looks quite obvious that the loads on the 1st and 2nd floors
    are identical for Practical purposes.

    Manual J is a nice guideline
    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

  4. #4
    Join Date
    Sep 2006
    Posts
    1,253
    So I should expect for any heating equipment that is located on the first floor to continue to provide the majority of the heat requirement and similarily the cooling requirement will be primarily carried by the second floor unit.

    Therefore, the reports are more helpful in the total heat gain/loss and equipment sizing should be weighted to the where it is located.

  5. #5
    Join Date
    Feb 2007
    Posts
    6
    Did you use the free download? I'm not real sure, but I don't think you can change your climate variables w/ that version.

  6. #6
    Join Date
    Sep 2006
    Posts
    1,253
    No I used the HO version. Set the summer temp at 96 winter at 0 and grains 107.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event