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  1. #1
    Join Date
    Sep 2012
    Posts
    18

    Sizing Furnace Help

    I have an existing home that I have completely remodel, demo and replaced. It is approx 1600sqft on the 1st floor with 750sqft basement. I have had the ductwork reworked when I tore off and replaced and added on approx 1000sqft of the home.
    I had a load calc done to see for myself what size of furnace/ac I should be expecting from quotes I am waiting on.

    My load calc came out with the following
    Heat Gain-Latent and Sensible-14000
    Heat Loss-44000

    I have recieved one quote which is for a 100000 BTU 2 Stage 96% Furnace with a 4 Ton Heat Pump. I was told that part of the reasoning for the size was due to the size of blower I needed.

    I had another contractor stated he was thinking 70,000BTU with a 3 ton Heat Pump, I am still waiting on his quote to see if it sticks.

    Any suggestions welcomed

  2. #2
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,506
    45,000 BTU furnace has a 3 ton blower which is good for 36,000BTU of A/C.
    Looks like a 45K furnace with a 2 ton A/C would be enough.
    I've NEVER seen an undersized gas furnace in the real world...

  3. #3
    Join Date
    Sep 2012
    Posts
    18
    So do I show the HVAC contractor the load calc I had done, or keep searching till I find one that sizes it accoerding to the information

  4. #4
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    7,775
    The contractor should do a load calc of his own, otherwise he is going by rule of thumb and you need to find a contractor that will perform the calculations necessary for a correct install of your system. If they cut corners on the load calc where else are they going to cut corners?

  5. #5
    Join Date
    Sep 2012
    Posts
    18
    My bad I forgot to mention that I would be using a heat pump, would that change how it is sized to be done so by heat loss

  6. #6
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    Quote Originally Posted by remodel80 View Post
    My bad I forgot to mention that I would be using a heat pump, would that change how it is sized to be done so by heat loss
    Some will size the H/P to the heat loss of the home. The problem is that you'll have too large of a cooling system in the summer, even if you went with a 2-stage H/P.

    If ur heat loss and heat gain formula is correct, then all you need is a 2 ton H/P. On the heating size, it sounds like a 60,000 btu input, 2-stage would do.

    Make sure to get a variable-speed blower and a 4" filter rack assembly.

  7. #7
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    You can get a 2 ton HP with 6 kw electric resistance heat. Your indoor blower will need can be 800 CFM. If your ductwork is complex, maybe a 1200 CFM blower.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  8. #8
    Join Date
    Aug 2009
    Location
    Beatrice, NE
    Posts
    2,509
    I have run into many houses that have been remodeled and sealed up so the load goes down 25 - 50%. So that makes it easy to size the equipmetn to match the needs of the house BUT, you need to have a blower big enough to fill the existing duct if it is not going to be changed out. If the duct and registers are sized to move twice the amount needed by the new equipment you won't either get the air spread through out the house as needed or it won't have enough velocity to properly stir the air. Either way your comfort will go down. Long story short if you don't have properly sized equipment, ducts, and registers you have a problem. Make sure your contractor checks all 3.

  9. #9
    Join Date
    Sep 2012
    Posts
    18
    I had the ductwork replaced when I had completed my remodel/addition. At the time the existing furnace was more than big enough is what i was told. I had my ductwork designed and built by a commercial shop that I work with daily insulating there pipe and duct.

    At the time I was told the ductwork was sized to the home not the system, since I wasnt replacing the system at the time.

    I personally took the time to show both contractors the entire duct system (duct,registers) which neither one stated there was problem. I told them that if a component of the system needs to be replaced even though it was recently done that is what I was wanting done to make sure I have a hvac system that can do the job right and effieciently.

    I guess I will just have to start asking on the front end how they size there equipment there quoting.

  10. #10
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    Quote Originally Posted by remodel80 View Post
    I have an existing home that I have completely remodel, demo and replaced. It is approx 1600sqft on the 1st floor with 750sqft basement. I have had the ductwork reworked when I tore off and replaced and added on approx 1000sqft of the home.
    I had a load calc done to see for myself what size of furnace/ac I should be expecting from quotes I am waiting on.

    My load calc came out with the following
    Heat Gain-Latent and Sensible-14000
    Heat Loss-44000

    I have recieved one quote which is for a 100000 BTU 2 Stage 96% Furnace with a 4 Ton Heat Pump. I was told that part of the reasoning for the size was due to the size of blower I needed.

    I had another contractor stated he was thinking 70,000BTU with a 3 ton Heat Pump, I am still waiting on his quote to see if it sticks.

    Any suggestions welcomed
    So, as I read this, ur home is approx. 2,600 S.F. (ranch?) with a 750 S.F. basement? Is that right?

    Regarding the contractors looking at ur ductwork.........did they test the return air side? I've been to homes where the homeowner told me the ductwork was fine, according to the contractors before me, and, after a quick test, I found the return side was 1/2 of what was needed.

  11. #11
    Join Date
    Sep 2012
    Posts
    18
    No my home is approx 1,600sqft with 750 sqft basment. My home is a two story with the second floor being heated and cooled by minisplits, there is a cellar basement as well but is moreless my mechanical room.

    On my return side i have the following
    2-32x8 return grills (2-16" stud cavities per grill)
    1-16x12 ducted return with a 16"x20" grill

    So i should roughly have 1250 cfm of return.

    I personally showed them all the ductwork, I told them what i knew but did not comment on whether it was fine or not. Thats there job to make that call.

  12. #12
    Join Date
    Dec 2010
    Location
    The Quad-Cities area (midwest).
    Posts
    2,744
    Quote Originally Posted by remodel80 View Post
    No my home is approx 1,600sqft with 750 sqft basment. My home is a two story with the second floor being heated and cooled by minisplits, there is a cellar basement as well but is moreless my mechanical room.

    On my return side i have the following
    2-32x8 return grills (2-16" stud cavities per grill)
    1-16x12 ducted return with a 16"x20" grill

    So i should roughly have 1250 cfm of return.

    I personally showed them all the ductwork, I told them what i knew but did not comment on whether it was fine or not. Thats there job to make that call.
    We are getting closer...........are you planning on heating and A/C (or H/P) the whole home or are you going to let the mini-splits continue to take care of the 1st floor?

    How large is each level of the home (not the basement)?.......800/800?

  13. #13
    Join Date
    Sep 2012
    Posts
    18
    Heating and H/P the entire first floor and basement
    The First Floor is the 1600sqft, the second floor is not in this number since it is heated and cooled by the mini splits.
    Basement-750sqft
    1st Floor-1600sqft
    2nd Floor-600sqft (Not to be included since it is served by the mini splits)

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