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  1. #1
    Join Date
    Nov 2007
    Location
    Rio Rancho, NM
    Posts
    14

    It's hard to find a good contractor

    House is 25 years old and since we're getting ready to retire, decided to replace the furnace and switch evap to refrigerated air. The existing furnace is 130,000 BTUH. The house is brick with 6 inch walls. Outside (realtor dimensions) measures 1850 sq ft, inside rooms total only 1511. R19 in walls and ceiling, and new low E double pane windows. 4 contractors looked at the old furnace and based estimate solely on that, adjusting down a little for 80% AFUE vs old 60%, and all quote 4 ton AC. Last guy though does a sort of Manual J (I guess) and tells me I only require 38,000 BTUH and 3 ton AC. So then I am concerned that the other guys are way oversized and then what good is a 2 stage variable speed system? But again, what if the last guy is wrong and the new system can't handle the job? So I try to get a mechanical systems engineer to come out and actually spec the requirement. NO LUCK. Nobody seems to want a small few hundred dollar job. So I buy HVAC Calc (Homeowners) and it says 2 tons AC and around 50,000 BTUH. The floor edge insulation jacks it up - seems to just be 1/2 inch fiber board expansion joint. So I asked the HVAC contractors to estimate 75,000 BTU furnace with 3 ton AC and they all - ALL - tell me that they know their business and that software crap on the internet is worthless and yadda yadda and one even says I will have to sign a waiver if I want that size system. None of them looked at the insulation or windows or the floor edge or my heating bills. I am discouraged.

  2. #2
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,903
    Last guy though does a sort of Manual J (I guess) and tells me I only require 38,000 BTUH and 3 ton AC.
    This is the only contractor you should consider.

    If you're not in a very warm climate, 3-tons sounds like a lot.

  3. #3
    Join Date
    Nov 2007
    Location
    Rio Rancho, NM
    Posts
    14
    It is a hot, dry climate in the summer - Albuquerque NM. HVAC Calc uses 94 deg/16 deg, which I changed to 99/11.

  4. #4
    Join Date
    Nov 2007
    Location
    Livermore, Ca
    Posts
    163
    Quote Originally Posted by cokoliso View Post
    So I try to get a mechanical systems engineer to come out and actually spec the requirement. NO LUCK. Nobody seems to want a small few hundred dollar job. So I buy HVAC Calc (Homeowners) and it says 2 tons AC and around 50,000 BTUH. The floor edge insulation jacks it up - seems to just be 1/2 inch fiber board expansion joint. So I asked the HVAC contractors to estimate 75,000 BTU furnace with 3 ton AC and they all - ALL - tell me that they know their business and that software crap on the internet is worthless and yadda yadda and one even says I will have to sign a waiver if I want that size system. None of them looked at the insulation or windows or the floor edge or my heating bills. I am discouraged.
    Amen, brother. I'm starting to think that a person could make a nice living doing such a thing. An HVAC consulting firm. I'm an electrician by trade and work closely with the tin knockers. Over the years I've learned enough to know that there are way to many variables to use the rule of thumb but at least it puts you in the ball park.

    Then when you look at AC/Forced hot air systems, the AC tonnage and the gas furnace coincide. ie, 2ton with 50k 3ton with 75k and so on. So your quote of 38k with 3ton doesn;t copy the numbers.

    try acdirect.com and one of my fav's is hvac-webdirect.com

    On the tool bar is a "system sizing" window. Click on it and three red circles pop up. The right one is a quick reference chart. This helped dial me in.

  5. #5
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,380
    Quote Originally Posted by cokoliso View Post
    It is a hot, dry climate in the summer - Albuquerque NM. HVAC Calc uses 94 deg/16 deg, which I changed to 99/11.
    Just for comparison change the conditions in HVAC -Calc back to default for your region. See how close that is to the contractors calculations. If they are close his experience might say the default numbers work. Very important is to make certain the house orientation is correct (N,S,E,W etc).

    Or keep calling contractors until you find another that will run the numbers.

    The science is hard to believe, but if the input is correct, the output is also.
    Climate Control Solutions for your Home or Office

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  6. #6
    Join Date
    Nov 2007
    Location
    Rio Rancho, NM
    Posts
    14
    To be fair, the guy's smallest furnace was rated 70,000 BTU input, and at 80% that comes out to 56,000, then he derated it for altitude like 20% (we're at 5000 feet) so it was then 45,000 BTU output. So he was quoting a 70,000 BTU furnace with a 3 ton condenser. Do all furnace brands get derated for altitude? I guess orifices get changed and some use pressure switches or other adapters...?

  7. #7
    Join Date
    Oct 2006
    Location
    So. NH
    Posts
    746
    Quote Originally Posted by comfortdoc View Post
    Just for comparison change the conditions in HVAC -Calc back to default for your region. See how close that is to the contractors calculations. If they are close his experience might say the default numbers work. Very important is to make certain the house orientation is correct (N,S,E,W etc).

    Or keep calling contractors until you find another that will run the numbers.

    The science is hard to believe, but if the input is correct, the output is also.
    I agree.
    I did work for a utility co using other programs but the result is the same, only as good as info put in. I had to MATCH computer model with actual bills and most often but not always found errors in the model. When corrected you could assess improvements much more accurately.

  8. #8
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by cokoliso View Post
    It is a hot, dry climate in the summer - Albuquerque NM. HVAC Calc uses 94 deg/16 deg, which I changed to 99/11.
    You need to use the design temperature values for your area. Do not chage the temperature values if you want to have an accurate load calculation. I review a lot of Manual J calculations that are done wrong, so just because someone says that did a Man. J does not mean they know what they are doing. Many have started doing it because in most cases it is required by the code/law. I have found some that when said they did a "calculation" and what they meant was they took the square footage of the house and divided by 500, which technically is a calculation, just not the right one.

    Anyway, 3 tons sound very high for your area. You should have a low latent load so most could be used for sensible cooling.

    Make sure you check and then check again ALL of the inputs to the program. I do a lot of load calcs and I find that you can miss changing a default value or enter another value wrong, etc. I like to print out the results and check the building inputs to make sure.

    I have done load calcs for people from the east coast to the midwest. If you do it right it works. However, the load calculation is only the first part of the process. You have to select the correct system, you need the duct system designed and installed correctly, you need the equipment installed and charged correctly, etc. Too many people jump on the load calc wagon and then jump right back off before the trip is finished.

    Any one that tells you that "they know their business and that software crap on the internet is worthless" is someone that I would run away from. I would also tell them, assuming that it is correct in your state, that it is required by law. But I would not use them because with that attitude they will most likely not do it right.

    There are several people on this board, including myself, that do load calcs based on the plans and specifications. If you are interested I'm sure you could get help if you need it.

  9. #9
    Join Date
    Nov 2007
    Location
    Rio Rancho, NM
    Posts
    14
    Using the HVAC Calc default values for my area, the load comes out a little smaller of course. About 47,821 BTUH heat and 19,535 AC. The only thing I think the guy who did the calculation missed is the slab loss, which he never tried to check. In fact HVAC Calc only has choices for "none", then 1 inch and up edge insulation. So I had to select "none" and I can see that the slab edges and the infiltration are the biggest loads. If I change it to 1 inch, the heating load comes out at 39,756 - pretty close to his calculation. I can see the joint between the wall footer and the slab in the garage, and it is not 1 inch. But perhaps it is different inside the house?

  10. #10
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,541
    Without looking I really don't know what you need. I will say that 4 tons is just a tad big if you have 1500 sq. ft. My house is 1450 sq. ft. I have a 3 ton in Texas that cools my house just fine, its an older house with single pane windows and I need more insulation. My furnace is 50K BTU and works just fine as well. We don't get quite as cold as you, but our heat is similiar. We have more humidity than you. You can take that into consideration with your decision. What the manual J guy said sounds pretty close from my experience and from what I have.

  11. #11
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by bmathews View Post
    Without looking I really don't know what you need. I will say that 4 tons is just a tad big if you have 1500 sq. ft. My house is 1450 sq. ft. I have a 3 ton in Texas that cools my house just fine, its an older house with single pane windows and I need more insulation. My furnace is 50K BTU and works just fine as well. We don't get quite as cold as you, but our heat is similiar. We have more humidity than you. You can take that into consideration with your decision. What the manual J guy said sounds pretty close from my experience and from what I have.
    You can't make that comparison and conclude his is ok. What can be done, since oversizing is common, is to question a sized that appears to be based on some rule of thumb and not based on sizing. You would have to know everything about both houses. His appears to be built better, with good windows and insulation (except for the ceiling, which is not enough if it really is R-19). There are too many variables. Your house probably only delivers 1.5 to 2 tons to the registers because of duct leakage, incorrect charging, inadequate return air, etc.

  12. #12
    Join Date
    Dec 2006
    Location
    Arkansas
    Posts
    292
    Quote Originally Posted by cokoliso View Post
    Using the HVAC Calc default values for my area, the load comes out a little smaller of course. About 47,821 BTUH heat and 19,535 AC. The only thing I think the guy who did the calculation missed is the slab loss, which he never tried to check. In fact HVAC Calc only has choices for "none", then 1 inch and up edge insulation. So I had to select "none" and I can see that the slab edges and the infiltration are the biggest loads. If I change it to 1 inch, the heating load comes out at 39,756 - pretty close to his calculation. I can see the joint between the wall footer and the slab in the garage, and it is not 1 inch. But perhaps it is different inside the house?
    Do you have slab insulation? If so, is it on the perimeter, underneath, or both? Slab insulation makes a big difference, especially in a cold climate.

  13. #13
    Join Date
    Nov 2007
    Location
    Rio Rancho, NM
    Posts
    14
    I do not see any insulation on the foundation on the outside of my house. In fact, there are sidewalks against the foundation all the way around the house.

    Inside (in the garage), I see about a half inch "joint" between the floor slab and the footer for the wall. It appears that the footers for the walls were poured and then the slab was poured with this expansion joint material against the inside of the footer.

    My ducts are under the slab, by the way. I don't know whether that's good or bad, but there is no way to do much about it. Can ducts under the slab leak air?

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