Results 1 to 12 of 12
  1. #1

    What Heat Load Calc looks like?

    Ignorance is bliss... or a curse. I am not an HVAC pro, hence have no idea what Manual J calcs look like. I had several contractors come out to give estimates for system replacement. Some based new size on existing - I learned to pass on those. Three others (so far), when I asked them what the size is based on, told me it's based on heat load calculations. Great!

    But...
    1st contractor: only walked around the outside of the house, but said that he has access to public records with sq footage. He did not measure anything inside. His conclusion is 2.5 and 3 tons (I have two systems to replace).

    2nd contractor: also walked around the outside and quoted two 4 ton units (w/ 2-stage compressors). Told me his calcs yielded 36,000BTU for one floor and 48,000BTU for another.

    3rd contractor: came yesterday. He's the first one to measure all rooms and windows inside. Took notice of skylights too. Told us the house is not standard colonial, but contemporary with lots of irregular and open spaces. He would need some time to do a heat load calc, which is okay by us. I don't have his numbers yet, but now I started wondering whether the other two did the calc correctly, by just walking outside.

    If I ask the contractors to show me the calculations, what should I look for to tell if they were done right? The difference in suggested size between 1st and 2nd contractor is big. I don't want to over- or under-size the system. How could they both be right? I'd like to be able to determine who's done it right.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,184
    Unless they are good guessers, probably neither 1 nor 2 is right. #3 will be if he goes back and punches his data into a program or does a Manual J by hand!

    http://www.hvaccomputer.com/hvac/newfeatures.asp

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,108
    Sounds like 1 and 2 are guessing.

    3 sounds like the real deal.
    Many of us don't show the load calcs until the contract is signed.
    The reason is so other contractors can't use our leg work for free, and under bid us.
    So if #3 is reluctant to show it to you, that may be the reason.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    This is an excellent question. You see everyone tell you you need a load calc but how do you know if you are looking at the real deal. They should be able to show you the forms and method without showing you the final numbers. They should also look in the attic to know how much insulation you have. Here is a sample I found on the web.
    http://www.southface.org/web/resourc...alculation.pdf
    I use Wrightsoft computer software which has a number of reports I can print and I am always happy to show how thorough I am without giving the exact size until the job is accepted. Here is another good article.
    http://www.homeenergy.org/archive/he...95/950509.html

  5. #5
    I will be sure to study those exhibits and an article. Thank you.

    While I can understand not willingness to show the results until the contract is signed, but I personally would not just take the result and give it to another contractor. It's not just because it's unethical, but also it's because if that other contractor did not care to perform Manual J calculation properly, what other corners will he cut while installing the system. To me it's not just the result that's valuable, but also the fact that it was done right, which serves to me as a test of quality and care.

  6. #6
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    I wouldn't go with either Contractor 1 or 2 unless they had a really good way of indicating how they came up with their load calc. Sounds like contractor 2 just guessed based on the outside perimeter. If you believe Contractor 1's statement that he had access to public records (but more importantly that he would actually use them), you still have to wonder how he knew the insulation value, window sizes/types, etc without walking through all the rooms inside.

  7. #7
    Join Date
    Dec 2007
    Location
    Cedar Grove, Wi-Sheboygan
    Posts
    1,582
    To Mbarson, I found that article you linked in your post to be very informitive to myself as I am sure it will be to other HO's, I surely did learn alot from that reading, many thank's !!! I plan on keeping that for furture reference when it becomes time for me to have my ac unit replaced which will be within the next several years if not sooner depending on engery prices of wheather they continue to rise or hoover as they are now, but the one question I have in regard to that article is how does this relate to HP or does sizing a HP require a different load calc to be used ?

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,108
    Your heat pump is your air conditioner.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #9
    So, one contractor stopped in for a second visit. He showed me how he came up with two 4-ton units. He measured the perimeter outside, counted the windows and even had some measurements for them (I think those are guesses, but anyway). His calcs indicated that I need 71K BTUs, or 3 and 3.5 ton units (not sure why, since it looks like two 3-ton units to me: 36x2=72). Since 2-stage units are only available in whole numbers, he suggests two 4-tons.

    Also, he said there is an acceptible range for sizes. The boundaries depend on what temp I want to keep my house at, what is outside temp, how much I entertain and cook. So, his two 4-ton units would fit higher in that range, but other contractors' 3 and 2.5 ton units would be somewhere towards the low end, but still acceptible.

    Also, since units are 2-stage, he feels it's better to get 4-tons each since they will run longer in lower stage and it will help to dehumidify. If we were to get a 3-ton, it would need to run in higher stage, but shorter.

    And finally, he said that b/c my house is big (3650 sq ft) going with two 4-tons is not really oversizing it, since it's only a marginal increase over calculated 71K BTUs. If it were a small condo, going up a ton would make a big difference in short-cycling.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,108
    It sounds like he's using the 450 sq ft to the ton rule of thumb.

    A 2 stage 4 ton unit, in first stage can have a capacity as high as 3.2 tons in first stage.
    So the area the other contractor says takes 2.5 tons, would be oversized in first stage, and short cycle.

    I can see going up a half ton to 3 tons to get 2 stage, but 4 ton could be asking for more trouble then you want.

    Done a lot of houses that were between needing x ton or a half ton more.
    But never seen a house that could use either a 2.5 ton, or 4 ton. Someone has guess work some where.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  11. #11
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    DanW13, heat pumps are still sized to the cooling load and do not over size by more than 25%. Size back up heat for entire heat loss. Use the balance point to determine when to switch over from HP to back up heat. The heat load calcs do not change.

  12. #12
    Join Date
    Mar 2008
    Location
    Baltimore MD and Ridgebury PA
    Posts
    542
    Quote Originally Posted by dafreerider View Post
    And finally, he said that b/c my house is big (3650 sq ft) going with two 4-tons is not really oversizing it, since it's only a marginal increase over calculated 71K BTUs. If it were a small condo, going up a ton would make a big difference in short-cycling.
    Marginal increase? That's 35%! (96-71)/71=0.35
    Why not a 3 ton and a 4 ton instead? (84-71)/71=0.18 Thats better at least if you must use whole numbers for tonnage.

    Regardless, I'm still concerned that he did a proper Manual J. Considering the other contractor walked through every room and took various measurements and came up with a considerably different load calc, my gut feeling is his was more thorough and therefore more likely to be correct.

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