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Thread: Adding Central AC to 1920's home

  1. #1
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    Adding Central AC to 1920's home

    We just bought a house in Wisconsin. The house was built in 1920 and is just a hair under 2000 square feet. It has a furnace that was installed in 2016, the vents throughout the house are old, they look like original, very large and there seems to be a mix of old and new duct work. Humidity is high, you could smell it in the basement when we first moved in. We have been running a dehumidifier in the basement since we moved in and the musty smell is gone now.

    We want to add central air. So far I have had two contractors come out and give me quotes. Their prices were very different. I think the main difference is that one quoted a 3.5 ton until while the other quoted a 2.5 ton unit. The one who quoted the 3.5 ton until was the same company that originally installed the furnace. Part of his quote was also upgrading the filter to a 4" filter while the less expensive quote did not and did not think it was necessary when I asked about it. How do I know which one did a more accurate heat load calculation? Do I need to be more concerned with under sizing or over sizing?

    The town we live in is right on Lake Michigan and is well known for being cooler than the rest of WI in the Summer. This is why there wasn't already AC in the house, however there are short periods of hot and humid weather, and we moved in during a truly miserable week. Possibly the smaller size unit takes this climate more into consideration and will provide better de-humidification?

    The windows are old as well, I think they're original windows, definitely not modern double pan windows, but they are mostly in good shape with outer storm windows. I only mention that because I would assume it would have an impact on the heat load.

    I appreciate any advice on how to choose between two very different quotes.

  2. #2
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  3. #3
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    “How do I know which one did a more accurate heat load calculation? “

    Do your own, here’s a free one https://www.google.com/url?sa=t&rct=...wxqi0c65CNPb1r

    There are more detailed ones online for a small one time fee.

  4. #4
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    You say there is a mix of old and new duct work, did the bid include updating all of the duct? I have seen many houses that the old supply registers were still in use on inside walls and the returns were still in the floor under windows. This needs corrected or equipment size will not matter, you still won't be comfortable. On some houses I have done I used the old supply registers for returns and put new supply registers at the outside walls. This keeps some of the appearance of an old house yet gives better performance out of the equipment. The thing to remember is any equipment will perform better on properly sized and installed duct work. That does not mean that the 2.5 ton and 3.5 ton systems quoted can go on the same duct system. You need a good Man J first though so everything gets sized correctly.

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  6. #5
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    Quote Originally Posted by Pex View Post

    We just bought a house in Wisconsin.
    The house was built in 1920 and is just a hair under 2000 square feet.

    It has a furnace that was installed in 2016, the vents throughout the house are old, they look like original,
    very large and there seems to be a mix of old and new duct work.

    Humidity is high, you could smell it in the basement when we first moved in.
    We have been running a dehumidifier in the basement since we moved in and the musty smell is gone now.

    We want to add central air.
    So far I have had two contractors come out and give me quotes.
    Their prices were very different.
    I think the main difference is that one quoted a 3.5 ton until while the other quoted a 2.5 ton unit.

    The one who quoted the 3.5 ton until was the same company that originally installed the furnace.

    Part of his quote was also upgrading the filter to a 4" filter while the less expensive quote did not
    and did not think it was necessary when I asked about it.

    How do I know which one did a more accurate heat load calculation?
    Do I need to be more concerned with under sizing or over sizing?

    The town we live in is right on Lake Michigan and is well known for being cooler than the rest of WI in the Summer.
    This is why there wasn't already AC in the house, however there are short periods of hot and humid weather, and we moved in during a truly miserable week. Possibly the smaller size unit takes this climate more into consideration and will provide better de-humidification?

    The windows are old as well, I think they're original windows,
    definitely not modern double pan windows, but they are mostly in good shape with outer storm windows.
    I only mention that because I would assume it would have an impact on the heat load.

    I appreciate any advice on how to choose between two very different quotes.
    How do I know which one did a more accurate heat load calculation?
    The contractor who measured the windows and determined the air infiltration.
    The contractor who let you review the ACCA Manual J calculation.

    No mention of the furnace nameplate data.
    No mention of specific city. ... I can guess within ~ 130 miles.
    No mention of number of windows and typical size ( 3' x 6' or __ x __ ' )
    ___________ 18 or 28

    No mention of the house perimeter, Width x Depth
    Wall Height: ___ '
    No mention of size or type of basement.
    No mention of current or the historical monthly energy use.

    ***** ***** ***** ***** *****************
    Blower Door Test indicates the amount of infiltration.
    0.5 or 1.3 ACH Natural ?
    *** *** *** **********

    29,000 BTU/HR +/- 29% Summer
    78,000 BTU/HR +/- 24% Winter


    2.5 or 3.0 Ton +/- 29% depending on total glass area and orientation ( N, S, E or West) + Leakage
    Attached Images Attached Images    
    Last edited by dan sw fl; 09-16-2020 at 06:27 AM.
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  7. #6
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    Quote Originally Posted by Pex View Post
    We just bought a house in Wisconsin. The house was built in 1920 and is just a hair under 2000 square feet. It has a furnace that was installed in 2016, the vents throughout the house are old, they look like original, very large and there seems to be a mix of old and new duct work. Humidity is high, you could smell it in the basement when we first moved in. We have been running a dehumidifier in the basement since we moved in and the musty smell is gone now.

    We want to add central air. So far I have had two contractors come out and give me quotes. Their prices were very different. I think the main difference is that one quoted a 3.5 ton until while the other quoted a 2.5 ton unit. The one who quoted the 3.5 ton until was the same company that originally installed the furnace.
    Part of his quote was also upgrading the filter to a 4" filter
    while the less expensive quote did not and did not think it was necessary when I asked about it.


    How do I know which one did a more accurate heat load calculation? Do I need to be more concerned with under sizing or over sizing?

    I appreciate any advice on how to choose between two very different quotes.
    ALWAYS to$$ out the Lowest Quote.

    https://www.energystar.gov/ia/home_i...ors/qispec.pdf
    Designer Dan __ It's Not Rocket Science, But It is SCIENCE with Some Art. _ _ KEEP IT SIMPLE & SINCERE ___ __ www.mysimplifiedhvac.com ___ __ Define the Building Envelope & Perform a Detailed Load Calc: It's ALL About Windows & Make-up Air Requirements. Know Your Equipment Capabilities

  8. #7
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    Has either contractor addressed the existing ducts, uninsulated, probably designed for a Low Velocity heating system, and may have to be removed and completely redone?

  9. #8
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    Culver, Oregon (Central OR)
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    Quote Originally Posted by dan sw fl View Post
    All good advice except for this, Dan. Not always is the lowest quote the worst. Sometimes the others are just extremely high. Sure, we would all love to fall in the middle, but that doesn't happen in every type of install and/or service.

    Case in point. I typically come in cheapest on manufactured home heat pump installs. It isn't because I am going to cut corners or not deliver a quality job. I just happen to get very good pricing on my mobile home products (Intertherm).

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