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  1. #1

    CFMs vs Square Footage

    I have recently hired a heating company to design and install a new gas furnace and ducting. I'm not sure they really know what they're doing. I have an 1800sf house in a cold climate (Vermont). They are installing a new furnace with 70,000 BTU output and max CFM of 1300 with the blower on high.

    I have two questions.

    1.) Does this furnace have enough CFMs? I've read that a general rule is 1 CFM per sq. ft. Is that correct?

    2.) Shouldn't the total CFM of the supply ducts add up to the max CFM of the furnace?


    Any advice would be greatly appreciated. Thanks in advance!

  2. #2
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    1a. Yes and No. It depends on the Brand/model. Some do and some don't. 1b. No!

    2. Not really. You size the duct to what's needed (CFM) not the other way around. Anyway, what is "total CFM of the supply duct"?

    Advise:
    1. Need model of furnace.
    2. The rule to determined CFM is a HeatLoad of the House!!
    3. Everything is built from (upon) the HeatLoad! Otherwise it's all a guess (rule of thumbs).

  3. #3
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    Quote Originally Posted by M.Dubbs View Post
    I have recently hired a heating company to design and install a new gas furnace and ducting. I'm not sure they really know what they're doing. I have an 1800sf house in a cold climate (Vermont). They are installing a new furnace with 70,000 BTU output and max CFM of 1300 with the blower on high.

    I have two questions.

    1.) Does this furnace have enough CFMs? I've read that a general rule is 1 CFM per sq. ft. Is that correct?

    2.) Shouldn't the total CFM of the supply ducts add up to the max CFM of the furnace?


    Any advice would be greatly appreciated. Thanks in advance!
    We size the blower (normally) to what the A/C or H/P demands. If your homes A/C or H/P is a 3.5 ton, then that furnace does not have enough airflow. Note: The furnace blower will slow down when in the heating mode, so it is not an issue for the heating.

    The 1 cfm per sq. ft. rule applies to the cfm a room needs. Say the room measures
    10' x 12' = 120 sq. ft. so a 6" supply (120 cfm) might do the trick. Again, this is only a rule of thumb. There are exceptions, of course.

  4. #4
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    Quote Originally Posted by George2 View Post
    We size the blower (normally) to what the A/C or H/P demands. If your homes A/C or H/P is a 3.5 ton, then that furnace does not have enough airflow. Note: The furnace blower will slow down when in the heating mode, so it is not an issue for the heating.

    The 1 cfm per sq. ft. rule applies to the cfm a room needs. Say the room measures
    10' x 12' = 120 sq. ft. so a 6" supply (120 cfm) might do the trick. Again, this is only a rule of thumb. There are exceptions, of course.
    While I agree with George 2. While 10x12 usually needs a 120 cfm (not all the time but a good rule).

    Are we talking about metal or flex ductwork? If flex then I would install a 7 inch with a damper. Metal duct work moves more cfm then flex so if metal is chosen then a 6" might work?

    Do you know the static pressure of the system. This is important as with either metal or flex it would let you know what size duct to install. Based on .5 static pressure a 7" flex duct
    will move/provide roughly 120-130 cfm if installed propely. A 6"metal duct drop will provide 85-100 cfm. Either way I install a 7" with damper.

    If the room gets to much air flow/cfm you can also damper it back. Without a load cal. Ran for each room then the company is guessing.

  5. #5
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    Quote Originally Posted by duckman06 View Post
    While I agree with George 2. While 10x12 usually needs a 120 cfm (not all the time but a good rule).

    Are we talking about metal or flex ductwork? If flex then I would install a 7 inch with a damper. Metal duct work moves more cfm then flex so if metal is chosen then a 6" might work?

    Do you know the static pressure of the system. This is important as with either metal or flex it would let you know what size duct to install. Based on .5 static pressure a 7" flex duct
    will move/provide roughly 120-130 cfm if installed propely. A 6"metal duct drop will provide 85-100 cfm. Either way I install a 7" with damper.

    If the room gets to much air flow/cfm you can also damper it back. Without a load cal. Ran for each room then the company is guessing.
    X2

  6. #6
    Quote Originally Posted by arc8 View Post
    1a. Yes and No. It depends on the Brand/model. Some do and some don't. 1b. No!

    2. Not really. You size the duct to what's needed (CFM) not the other way around. Anyway, what is "total CFM of the supply duct"?

    Advise:
    1. Need model of furnace.
    2. The rule to determined CFM is a HeatLoad of the House!!
    3. Everything is built from (upon) the HeatLoad! Otherwise it's all a guess (rule of thumbs).

    The furnace is a HEAT CONTOLLER/CENTURY GLUB 75-E3A

    The trunk is 28" x 8"

    So if I am understanding this correctly, it is not necessarily wrong if the total CFM of the ducts added together exceeds the output CFM of the furnace. Is this correct?

  7. #7
    Quote Originally Posted by George2 View Post
    We size the blower (normally) to what the A/C or H/P demands. If your homes A/C or H/P is a 3.5 ton, then that furnace does not have enough airflow. Note: The furnace blower will slow down when in the heating mode, so it is not an issue for the heating.

    The 1 cfm per sq. ft. rule applies to the cfm a room needs. Say the room measures
    10' x 12' = 120 sq. ft. so a 6" supply (120 cfm) might do the trick. Again, this is only a rule of thumb. There are exceptions, of course.
    Let me make sure I am understanding this correctly.

    If I have three rooms that are each 120 sq. ft. then I should have a furnace that can produce 360 CFM. (enough for each room to recieve 120 CFM)

    Also, what do you mean by "The furnace blower will slow down when in the heating mode, so it is not an issue for the heating." ?

  8. #8
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    Quote Originally Posted by M.Dubbs View Post
    Let me make sure I am understanding this correctly.

    If I have three rooms that are each 120 sq. ft. then I should have a furnace that can produce 360 CFM. (enough for each room to recieve 120 CFM)

    Also, what do you mean by "The furnace blower will slow down when in the heating mode, so it is not an issue for the heating." ?
    With the typcial maximum of a 60F temp rise, a 70k BTU furnace will only need 1050 CFM on high stage.

    Vermont is a pretty cold climate, but if very well insulated, 70k might still be a little big for only 1800sqft. Especially is 2 story.

    CFM as is unrelated to size of the home or a room. Its' realted to the size of the equipment installed and the amount of heat loss... ie capacity needed for each room... unless you have specific fresh air ventilaiton requirements... but that's a who new can of worms and wont; be an issue unless you have a very, very tight almost net zero home (near zero heat loss form high insulaiton values).

    So lets say you have a 15x20 master bedroom with 2 exterior walls and a roof above it ands 8' ceilings and assume is has a conditioned basement. Attic insulation is R40, walls are R10 (we'll average it to factor in windows). Design is -6F out, 70F in, so 280/10*76 = 2128BTU/hr and 300/40*76= 570BTU/hr. Lets assume 0.20ACH for air leakage for that room... that's 8CFM * 1.08 * 76 = 657 BTU/hr. So that's 3355 BTU/hr you need to supply it at design conditions. If the furnace temperature rise is set to lets say a middle road of 50F, you'd need 62 CFM to maintain temeprature. Yup, that's all. OF course, if all the rooms were proportionally the same, you'd only need 20k BTU's to heat the whole house (possible in a well built home that size) if you ignore the basement, so in reality, the furance would be larger, so you'd have to supply it more like 90CFM for a 45k BTU furnace on low stage and 130 CFM on high stage.

    You can only precisely match CFM to heat loss if you have a commerical modulating vairable air volume system (VAV) which has a modulating heating and cooling source ot maintain constant supply temperatures across a varying range of CFM. This is overkill of coruse for a small residential space.

  9. #9
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    Quote Originally Posted by M.Dubbs View Post
    The furnace is a HEAT CONTOLLER/CENTURY GLUB 75-E3A

    The trunk is 28" x 8"

    So if I am understanding this correctly, it is not necessarily wrong if the total CFM of the ducts added together exceeds the output CFM of the furnace. Is this correct?
    1. It's a 3 Ton unit. I haven't seen the air flow chart of the this unit.

    2. The trunk is big enough for 1300 CFM!

    3. It shouldn't, otherwise the velocity would be low (not enough umph!).

  10. #10
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    The AC will need 1200 CFM nominally or 3 tons.

    3 tons sounds pretty big for a cool climate like Vermont where you barely even need AC except for a few weeks mid summer. I bet a 1.5 or 2 tons would be plenty.


    Not enough "oomph" ? I have velocities around 300fpm in most of my branches when I'm on low stage cooling at my home. I still have good throw and air mixing. Low velocities make it much easier ot balance a system

  11. #11
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    total cfm capabilities of ductwork is difficult to determine without complicated charts and formulas.
    cfm and flow of ductwork need to take into account the fittings, the total length of both supply and return, the bends, and the size of the plenum and runs... also the registers and grilles need to be factored into the calculation... it's all based on the pressure available from the blower, and the restrictions of the ducts.
    The TRUE highest cost system is the system not installed properly...
    The three big summer hearththrobs...
    Mel Gibson
    Dwane Johnson
    The A/C repairman

  12. #12
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    Quote Originally Posted by motoguy128 View Post
    The AC will need 1200 CFM nominally or 3 tons.

    3 tons sounds pretty big for a cool climate like Vermont where you barely even need AC except for a few weeks mid summer. I bet a 1.5 or 2 tons would be plenty.


    Not enough "oomph" ? I have velocities around 300fpm in most of my branches when I'm on low stage cooling at my home. I still have good throw and air mixing. Low velocities make it much easier ot balance a system

    I'm sure that furnace requires a 3 ton blower for its heating output. You just have "to" design the ductwork for less than 1200 CFM if that's the case with AC.

    Anyone: How low velocity would any professional venture to design?

    Remember; I mean @ full capacity, not intermediate stages. Lower stages have lower velocities.

  13. #13
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    Quote Originally Posted by motoguy128 View Post
    With the typcial maximum of a 60F temp rise, a 70k BTU furnace will only need 1050 CFM on high stage.
    I think he said 70K is the output. When figuring 60F TR I came up with 1166 CFM!

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