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Manual J calc inputs and outputs and sizing

user-7501857 | Posted in GBA Pro Help on

Good Morning, 

I recently found your website and absolutely love the sound advice that is given here. 

I am in the process of switching my AC and heating system at home, from oil and window units to a mini split heat pump multi zone system. 

My house was constructed in 1970 by the previous owner, so I don’t have a lot of the specifics.  It is a single story rancher, with a full unfinished basement that is used for exercise equipment and work bench and tool area.  It is located in Southern NJ. 

Howerever, I have put alot of time into trying to do the Manual J calculations which I have just somewhat completed.  

I drilled a few small holes in my walls to check for insulation.   It appears to be full depth, so I am assuming it is R11.  When starting the Manual J, it asked about the level of bei g airtight, I choose average but not sure if I should change that to Leaky or in between based on the standards of construction in 1970.  The only other real concern I have regarding the inputs is how to ac out for window awnings.  It list a box for external shading with a default 1.0 in there but no explication or choices.  I am not really sure if that is where it would go.  I am thinking to add more insulation to my attic  and will run change that in the software and see how that changes the load. 

Anyway, my house has 2,782 between the 1st floor and the basement.  The basement does not require too much conditioning, mainly too add a little heat in the winter and a little dehumidification in the summer. 

The results of the draft load calculations thus far is stating total heating 46,278 btu/hr, total sensible 22,507 btu/hr and total latent 3,745 btu/hr.  Does that sound even close to being right based on the year of my house, etc.?  Also, how is the equipment sized based on the far apart heating and cooling loads? I can post a copy of one of the reports if  needed. 

Any opinions and advice would be greatly appreciated. 

Thank you! 


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  1. Expert Member
    Dana Dorsett | | #1

    >" It appears to be full depth, so I am assuming it is R11."

    R13 was more common in 1970 than R11. Unless it's known to be R11, assume R13.

    >"When starting the Manual J, it asked about the level of bei g airtight, I choose average but not sure if I should change that to Leaky or in between based on the standards of construction in 1970."

    Since it's usually fairly easy and cost-effective to tighten up the house, select "Tight" , not "Average", and unless it's sheathed with planks without any tar-paper on the exterior definitely not "Leaky". Most houses in 1970 were sheathed in 4 x 8 sheets of plywood, not ship-lap or plank. Even if it's not super-tight now, it's often pretty straigthtforward to make it tight.

    A heat load of 46K+ @ ~+15F for a 2x4 framed 2800' rancher on a full 2800' basement is on the high side, but not ridiculously high. How much of that 46K is air infiltration? How much of it is the basement?

    If the basement isn't finished, it's worth taking on the project of insulting the foundation walls to the current R10 c.i. code minimum. Right now I'm guessing the standby and distribution losses from the oil burner are keeping basement temps from plummeting during cold weather, but that won't be the case after switching to a mini-split solution. With R10 or higher foundation walls (and air sealing) it will stay pretty reasonable without active conditioning all year round.

    If you have a winter's worth of oil use behind you it's possible to use exact fill up dates and amounts and heating degree-day data to crudely measure the heat load using the fossil-burner as the measuring instrument. That makes a good sanity check against IBR or Manual-J load calculations. Fo best accuracy limit it to wintertime fuel use only, otherwise solar gains and other energy use inside the house inject a large error. See:

  2. user-7501857 | | #2

    Hi Dana,

    The actual area of the main floor is 1375 sqft and the basement is 1407 sqft.; sorry, I should have made that clearer.

    Also, the exterior siding is painted cedar bevel siding. Not sure if they used plywood sheathing under that or tar paper or perhaps nothing; not sure how to determine that without damaging something.

    In addition, I believe the 46k was based on 11 degrees. I attached copy of the Manual J summary and full report which will hopefully describe the scenario better then I am able to do.

    Thanks much!


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