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  1. #1

    need advise on Duct system

    Advice please to upgrade my heating system. My 12 years old house in BC Canada has 2 levels: Main level 1400 sf and ground level 1000 sf. Currently using 12 years old Luxaire Furnace model# PACE-L12N080A 80% efficiency 80,000 BTU input. It's quite noisy when running.

    I talked to several Contractors who gave me quotes mainly based on the square feet of the house and the size of the old furnace. One recommended Trane 2 stage 95% 80,000 BTU while other quoted Carrier 96.6% 2 stages 60,000 BTU

    I found out lately from this site that my duct system might not be good even for the current furnace's capacity. Here is the details:
    It is the extended plenum configuration where, there are two 4" round ducts, one 5" round duct coming off the plenum. The 20 feet extended plenum with the size of 8"x14" also comes off the plenum. There are about 10 5" ducts coming off from the top of the extended plenum along its 20 feet length.

    Would you please recommend what is the max CFM that my duct system can handle? Based on that max CFM, what is the best size of the furnace for this duct system? Upgrading the duct system is not a good solution at the moment.

    Thanks

    Tony

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,081
    Both of those furnaces will want to move more air then your current furnace does, when they are in second stage.

    The Carrier will move less then the Trane, because it is smaller.

    Your duct system is better suited for a 40,000 BTU furnaces air needs. But your house may require more.

    The question is, how much noise can you tolorate.
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  3. #3
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    What do you have for return duct and filter?

    Post pics of furnace and connecting duct,if you can.

  4. #4
    Thank you all for your replies.
    Dash, here is additional info
    The return duct is 8"x25 at the furnace (pict 1) with the disposable filter 1"x16"x25". However, it is 8"x20" on the ceiling (pict 3).

    Beenthere, would the Carrier have any chance to run in the first stage? if so, it would cause less noise then my current one, isn't it. My main objective is comfort while saving on gas is second as it's quite an investment.

    If the load calc was 55,000 BTU (hopefully I could find a contractor willing to do that later), Would 60,000 BTU furnace is better for my house than the 40,000 BTU under my current duct system?

    Thanks

    Tony
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  5. #5
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    769
    Duct work looks pretty decent, actually.

    Trane TUH2B060A9V3VA (XV95, 2 stage 60mbh) gets my vote.

    You won't regret buying Carrier, either. Go with the better contractor - namely, the one who will do a heat loss calc.

    How cold does it get where you are?

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    You have to get the size furnace that will heat your house.
    Weather that means you put up with noisey ducts, or have alterations done to them.
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  7. #7
    Join Date
    Jan 2006
    Location
    South Western Ontario
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    1,299
    Find a contractor with industry credentials and perform a load calculation. Go to www.hrai.ca to find someone certified. If your ductwork system can't handle a larger furnace there are other heating solutions to consider.

  8. #8
    Join Date
    Nov 2008
    Location
    MI
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    What ever furnace you put in. Make sure you get one with a variable speed blower motor. It will compensate for inadiquit duct sizes and will be alot quieter.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,081
    Quote Originally Posted by jimzcoolz View Post
    What ever furnace you put in. Make sure you get one with a variable speed blower motor. It will compensate for inadiquit duct sizes and will be alot quieter.
    Quiet often, that is not true.

    Many times, it will become a loud system, with undersized duct work.

    Too often, VS blowers are used as a cure all for undersized duct work. And more problems are created.
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  10. #10
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
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    IF the load calc. came out to 55k BTU heat loss then a furnace with less BTU's than this would cause the inside temp. to drop below the indoor design temp when the outdoor temp is also at or even above design temp. This is the problem with undersizing. You would be forced to use portable heaters to keep the temp. up or deal with a colder than desired interior temp. Regarding your question of would it run in low stage: Yes when the outdoor temp. is well above design temp. It would work like a dream. Then as the temp. drops it would ramp up to a high velocity system that could cause noise and blower motor failure and higher than designed heat exchanger temps and more duct leakage etc... Not good.

  11. #11
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    Actually - after re-reading the first post, I realize that the 8 x 14 duct is the only main trunck - I thought for some reason there was two of them (one on each side of the dog house). Huge difference.

    Yeah, you will have some high velocities with a new VS furnace for sure.

    Sorry for the misleading comments before.

  12. #12
    Daltex, portable oil-type electric heaters might be a compromised solution for me if need be. I have the same concern as you mentioned that when the oversized furnace ramps up, it would cause noise, and even worse cause the furnace (especially the VS blower) to fail prematurely.

    As Beenthere recommended earlier, 40K furnace is better suited with my current duct system. If I were to have a 95% 60K furnace, would the furnace still be able to deliver 95% AFUE performance as its spec said: delivering 1000 CFM in high fire? My current 80K furnace has been running for 12 years although quite noisy. Can the new VS blower's motor survive this long?

    Larobj63, The average Temp in Nov is between 38F to 48F.

    Thanks Homersodyssey for the Website, I will have a look at that next week when I have more time.

    Tony

  13. #13
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    769
    Using basic sizing principles, your duct system is "suitable" for about 600 cfm.

    A 60 mbh furnace delivering 1,000cfm would be problematic.

    It looks like the majority of your ductwork is in the basement, exposed. If you get a new VS furnace, I would budget some money for re-doing some or all of the ducting. I know you said it's not an option, but hey, it is what it is.

    I'd be surprised if a 40 mbh furnace (a closer fit to the installed duct, as mentioned) will cut the mustard on design day, but a heat loss calc will determine that for you.

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