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  1. #1
    Join Date
    Mar 2013
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    10

    Manual J question

    I recently moved into a new build house in Northern Virginia from a production builder that is two stories plus basement. They installed a 100k btu 92% efficient furnace for the basement and first floor along with a 3 ton ac unit. The area for the two floors is 2,648 sf. For the top floor they installed a 3 ton 13 SEER heat pump for an area of 1,676 sf.

    The gas furnace seems to be running for short periods of time which concerns me. I asked for the Manual J report for the house and it calls for a 37,000 btu gas furnace and a 2.5 ton AC for the basement and first floor. The second floor calls for a 1.5 ton heat pump. Do these numbers sound right for northern virginia and the square footages? The heating design temp is 18 degrees and the cooling design temp is 91 degrees.

    Why are my systems so much larger than the manual J report? Should I be concerned? I have attached the manual j for reference.

    Thank you for any help.
    Attached Images Attached Images

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    If the basement is finished, it will need very little to heat and virtually nothing to cool it. If the home is fairly tight and well insulated, and has osme shade, it takes very littl eot heat or cool it. Those numbers sounds very plausible. You summers are more humid than hot and winters are not too cold. Looks like they used the correct design temperatures for your location.

    I have no reason to doubt hte manual J. Looks like downstairs was right between 2.5 and 2 tons. Since it's humid there and you have 2 systems, I would have gone 2 tons personally downstairs. Better just a little small than a 1/2 ton too big. Manual J has some safety margin already in there.

    Upstairs... well, well 1.5 tons would be plenty if the ductwork is well sealed, insulated and installed correctly. The latent gain however seems really ,really low. It doesn;t factor in revserse stack effect, showering, occupancy and other normal factors. I think you're probably closer to 2 tons upstairs.

    So my summary...

    2.5 tons downstairs, not great but OK.
    3 tons upstairs... Way oversized unless it leaks a lot of air and you have a lot of unsealed can lights, air leaks, etc. Otherwise 1.5 or 2 tons, but I'd have a new manaual J done factoring in air leakage and occupancy. Keeping mind that most air leaks in summer are upstairs.


    The furnace is an absolute monster. That's rediculous! More than 2X what the load calculations call for. I could see 60k BTU jsut because it's a 2.5 ton AC (2.5 tons is pushing the limits of the blower in a 40k BTU unit).


    AM I suprised at what was installed. Aboslutely not at all...sadly. That's what you get these days in new construction... where low cost wins over quality and proper design and installation... especially tract homes. SOrry you have ot learn thsi the hard way. I think for more Americans, ignorance is bliss. If you don't know how a properly designed and isntalled HVAC system is supposed to feel, then you wouldn't know better.

    I've drastically downsized all the equipment in my home and made my wife a believer. She didn't know a furnace and AC system could be quiet and every room could be the same temperature. She figured that's just what you lived with in most homes.


    From a building science perspective, there's no excuse, for any home over 2500sqft to need more than 20 BTU/sqft heating and 15 BTU/sqft cooling. If it needs more than that, its' not designed and built right. We have mpg limits for cars. Why not energy performance for homes of different size categories? We have insulation standards, but that doesn't nessesarily deliver. It would be like having aerodynamic drag limits for cars, weight limits, then having seperate fuel consumption testing for crate engine on a dyno. It's not until you put it all together that you really know what the package is.

  3. #3
    Join Date
    Mar 2013
    Posts
    10
    Thanks for the reply motoguy. The house in an energy star certified house and has r-15 in the walls, r-38 in the ceiling and the ducts were checked for air leakage. I have been in the attic and the duct work seems to be sealed really well but when the heat pump kicks on and it is 40 degrees outside, the temperature rise from the heat pump is only 8 or 9 degrees. The heat pump shuts off once it reaches the 8 or 9 degree peak differential. It seems like it should run longer and less often to be running at the optimum temperature difference. Does that sound right?

  4. #4
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,058
    Quote Originally Posted by drt125 View Post
    I recently moved into a new build house in Northern Virginia from a production builder that is two stories plus basement. They installed a 100k btu 92% efficient furnace for the basement and first floor along with a 3 ton ac unit. The area for the two floors is 2,648 sf. For the top floor they installed a 3 ton 13 SEER heat pump for an area of 1,676 sf.

    The gas furnace seems to be running for short periods of time which concerns me. I asked for the Manual J report for the house and it calls for a 37,000 btu gas furnace and a 2.5 ton AC for the basement and first floor. The second floor calls for a 1.5 ton heat pump. Do these numbers sound right for northern virginia and the square footages? The heating design temp is 18 degrees and the cooling design temp is 91 degrees.

    Why are my systems so much larger than the manual J report? Should I be concerned? I have attached the manual j for reference.

    Thank you for any help.
    The didn't even get your design temperatures correct.

  5. #5
    Join Date
    Mar 2013
    Posts
    10
    What should the design temperatures be??

  6. #6
    Join Date
    May 2008
    Location
    Central Fla.
    Posts
    311
    Is your lot #191 ? They maybe gave you the wrong #'s? Upstairs is way off in my opinion.

  7. #7
    Join Date
    Mar 2013
    Posts
    10
    Yes, my lot is 191. Do you think I should get different HVAC company to come in and check things out?

  8. #8
    Join Date
    Mar 2013
    Location
    Central IL
    Posts
    103
    I would definently look into a new inspection. you could also look into using a few ceiling fans upstairs and heat and cool with the down stairs unit.

  9. #9
    Join Date
    Mar 2013
    Posts
    10
    I'm not sure what you mean by the ceiling fans. How would they get the air to move between the floors? We have a 2 story open foyer which allows some intermixing of the air already, but the upstairs heat pump has twice as many cycles as the downstairs gas furnace which I find odd. I though the downstairs unit would run more cycles in the winter.

  10. #10
    Join Date
    Sep 2008
    Location
    New York
    Posts
    131
    drt125

    would you mind contacting me. I have an opinion that I prefer not to voice on the open forum.
    My email is in my profile.

    Thanks
    "The only way in which a human being can make some approach to knowing the whole of a subject is by hearing what can be said about it by persons of every variety of opinion and studying all modes in which it it can be looked at by every character of mind.
    No wise man ever acquired his wisdom in any mode but this."
    John Stuart Mill

  11. #11
    Join Date
    Jan 2009
    Location
    Keokuk, IA
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    5,520
    Quote Originally Posted by second opinion View Post
    The didn't even get your design temperatures correct.
    Dpes seem too far off to me. But I could have missed something. I'm not sure what city to use, but Richmond for example is 18F winter, 92 F summer. The whole state only ranges from 10F-25F(along coast) and 90-93F.

  12. #12
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    Quote Originally Posted by drt125 View Post
    Thanks for the reply motoguy. The house in an energy star certified house and has r-15 in the walls, r-38 in the ceiling and the ducts were checked for air leakage. I have been in the attic and the duct work seems to be sealed really well but when the heat pump kicks on and it is 40 degrees outside, the temperature rise from the heat pump is only 8 or 9 degrees. The heat pump shuts off once it reaches the 8 or 9 degree peak differential. It seems like it should run longer and less often to be running at the optimum temperature difference. Does that sound right?
    Something either not set-up or installed correctly. The temp rise at 40F outdoor should be close to 25F. Are you sure you have a 3 ton outdoor unit? I wonder if it's a 3 ton indoor air handler unit set to high fan speed nd outdoors thre's only a 1.5 or 2 ton unit. IF your heat pump was performing that poorly, you'd have some really high electric bills. considering the insulation level of your home. Somthing doesn't sound right.

  13. #13
    Join Date
    Mar 2013
    Posts
    10
    Both indoor and outdoor units are definitely 3 ton units, I checked them myself. The HVAC company changed the indoor unit from medium to high fan speed because one side of the house was cold. Maybe I should turn it back down to medium and play with the dampers instead.

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