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duct sizing - heat or A/C?

GMcD Member Posts: 477
The ducts are normally sized for the larger of the airflow requirements, if you have a dual speed or variable speed fan. The air temperature being delivered determines the amount of Btuh of heating or cooling into the room. The basic formula is CFM x Delta Tair x 1.08 = Btuh

The "Delta Tair" is the difference in temperature between the supply air being delivered and the desired room air. If you have a fixed CFM then the amount of Btuh being delivered in either heating mode or cooling mode can be varied by changing the delivery air temperature.

So, the reverse calculation is: determine the heat loss of the room, then starting with an assumed warm air delivery temperature (gas furnace usually puts out 110F air), and if the room heat loss is 5,000 Btuh, and you want the room to be maintained at 72F, the CFM can be calculated as: CFM=5000/[(110-72)x1.08] = 121.8 CFM

If the same room has a cooling load (heat gain) of 3,000 Btuh and you are using a DX cooling system that usually provides a supply air temperature of 55F, then the CFM requirement for cooling mode is:
CFM= 3000/[(72-55)x1.08] = 163.4 CFM.

So, your choice is- be economical and size the duct for the smaller airflow (heating) and then lower the cooling supply air temperature to use the same CFM, or if you can't lower the cooling supply temperature, then size the duct for the cooling CFM and lower the warm air supply temperature to use the higher CFM.

It's a bit of an iterative solution, and I've glossed over the whole latent vs sensible cooling load issues and other details, but that's the basic process.


  • KW_2
    KW_2 Member Posts: 27
    duct sizing - heat or A/C?

    A question regarding ductwork sizing: Should the ductwork be sized according to A/C requirements or heating requirements?

    Posed another way - if a duct system is only for delivering cooling, will the ductwork be smaller or larger than if the system delivers both cooling and heating?

    Thanks for all replies!
  • Constantin
    Constantin Member Posts: 3,796
    From the limited reading I've done...

    ... I'd say, it depends on what you're trying to do.

    Normally, the airflow requirements for heating and cooling differ, as the heat gains and heat losses for rooms usually do not match. From my reading of Manual-D, that means sizing the duct to the larger air-flow need, whether it be for heating or for cooling.

    Thus, if the installers follow manual-D, one would expect to find a larger duct system in a home that enables heating and cooling vs. ductwork that only carries air to heat or cool. However, I suppose you could have a home where the heating and cooling air flow requirements are so similar that the ductwork would stay the same, whether its used for heating, cooling, or heating and cooling.

    Perhaps it would be best to repost this question in the AC/HP area of the Wall, i.e. "Ask Prof. Silberstein", since he's an AC expert.
  • Robert O'Connor_12
    Robert O'Connor_12 Member Posts: 728


    Robert O'Connor/NJ
  • John Mills_5
    John Mills_5 Member Posts: 950
    I'm with Geoff

    Needs to be sized to the higher airflow which isn't always cooling!
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    And don't forget that warm air occupies more space than cool air.

    Returns sized for heating only can be somewhat smaller than the supplies since the cold air is going through the returns.

    Returns sized for cooling only have returns larger than the supplies since the warm air is going through the returns.

    Yet another balancing act with combined heat/cool systems...
  • Boilerpro_3
    Boilerpro_3 Member Posts: 1,231
    I think that would be \"unbalancing act\" NM

  • joel_19
    joel_19 Member Posts: 931

    neither , thats the problem it will always be wrong one way or the other which is why you need seperate heat cool systems , and I AM A DUCT GUY !
  • CDM2
    CDM2 Member Posts: 18
    Duct Sizing

    Always size for the higher cfm requirement.

  • will smith_4
    will smith_4 Member Posts: 259
    pipe is pipe-pipefitters wake up!

    You know, I've been reading Dan's books for years-been to his seminars, and loved them all. My eyes light up every time I walk into a boiler room-steam or hot water-in large part because of what his books (and recommended books)and this site have opened my eyes to. Those of us that have poured through The Lost Art know to pay close attention to near boiler piping, A and B dimensions, headers, traps, false water lines-and love it! But for all that, there are 100 times as many forced air jobs dying for someone to take just as much interest in understanding the physics behind moving AIR. Home Dumpo doesn't give quick tips in steam heat, but they sure do for forced air-half-***ed at best. I've seen new 120,000 btu furnaces installed in duct systems designed for 80,000 btus, with failed heat exchangers and a pile of "faulty" limits laying around. Pipe is pipe-it carries steam, water, or air in our chosen field; We need to realise that tin-knocking, though a "sister" to pipefitting, is still a type of pipefitting-and deserving of the same intense attention to detail that we bring to hydronics. Hey-they've been dropping the ball for decades; think about it-we pipe systems in with religious ferver, they kill it with bad duct sizing and miles of flex. If they won't care-we should! We need a Dan Holohan of Duct!
  • Tom_35
    Tom_35 Member Posts: 265
    National Comfort Institute


    Dominic Guarino and Rob Falke head up the National Comfort Institute and they are doing their best to train contractors how to install and/or repair duct systems. They have proven methods showing how to make systems work as they are supposed to.

    Go to their website at www.nationalcomfortinstitute.com and you can view portions of what they have to offer. They don't have as much interaction through the internet that The Wall has, but they have tons to offer.

    Tom Atchley
  • KW_2
    KW_2 Member Posts: 27
    duct sizing

    Thanks to all for your information and assistance. Great to have a resource of dedicated, experienced professionals to call upon like a favorite uncle in the 'biz!!

  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Duct Sizing

    I must have had my head "you know where" while this thread was going on... and I apologize, but I have been dealing with some heavy issues at this end.

    You are 100% correct, Constantin about sizing for the larger CFM, be it heating or cooling. The tricky thing is this... If you look at my post at the bottom of the Wall about using Manual J, the response was less than overwhelming as far as the number of us who actually use it.

    I am quite certain that the number of those who use Manual D would be even smaller. I personally use both Manual J and Manual D and teach them both in the HVAC/R program at the college.

    Assuming that the heat loss and heat gain calculations are correct and acurate, you could very well have a house with, let's say, 6 rooms and have some of the individujal room duct runs sized for the heating air volume while some are sized for the cooling air volume.

    The kicker is this... How many people actually do detailed, accurate calculations? It has become an industry accepted standard that the air distribution system is sized for the cooling load. The rationale behind this was relatively simple. Rooms that had a large solar gain had a heat gain that far exceeded the heat loss, so the cooling load CFM was greater. In rooms that did not have a large solar load, the heat gain and loss were close enough that either selection would work well (not perfect, but well enough). Since the major issue was solar gain, using the cooling CFM became commonplace.

    Installers and system designers of old were very happy to use that rule of thumb and, since systems operate well under their design capacity most of the time, never had major problems with the outcome.

    Have you ever considered that air conditioning systems are designed for a 95 degree outside ambient temperature and have over twice the required cooling capacity when the outside temperature drops to 85 degrees? Check this out...
    A system is designed to transfer 48,000 btuh (4 tons), for example, at a 95 degree outside temperature. As the outside temperature drops, the cooling capacity of the equipment rises and the heat gain of the structure drops. At an outside temperature of about 85 degrees, the system is operating oversized by approximately 100%. Cool, huh?
  • Constantin
    Constantin Member Posts: 3,796
    So much to consider...

    ... when the time came to select the components for our AC system, I was pondering many of the same questions that you have raised. I had to educate myself regarding the ASHRAE design-day requirements for cooling just as I had had to do so on the heating side.

    Our AC system is used just for cooling and I left it to the installer to properly size ducts. Given that he has Wrightsoft to work with (which allegedly incorporates Manual-D in its ducting module), I'd like to think that the ducting is OK. We did have a disagreement on the size of the equipment, however.

    Though our respective heat losses were calculated with two different programs, they came very close to each other. However, the heat gains were way off, by 2 tons, IIRC. I asked for a print-out and went to work. The principal difference was due to design-day conditions. My contractor assumed that we'd want 68°F indoor conditions when it was 95°F outdoors. Meanwhile, our ASHRAE design-day requirements call for 91°F outdoor conditions, and my wife and I are comfortable at 74°F in the summer...

    Next up, latent vs. sensible cooling in conjunction with air handlers that had been sized to meet the larger loads (the condensers weren't installed yet). I went through the equipment pairing charts and eventually had my contractor agree to install both condensers with a ton less capacity than the air handlers they're attached to. That stretches the sensible heat removal capacity of the system to be more in line with the expected load on the system, yet remains in the universe of equipment pairings published by the OEM.

    I wonder how many contractors do the same (i.e. match sensible and latent load as best as possible with creative equipment pairings).
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    Unfortunately... A lot

    How many of us as contractors would like for a customer to tell us that, after we installed a brand new air conditioning system, that the new system is not cooling as well as the old one? None of us, right?

    The installation process is expensive, time consuming and stressful for the contractor and the homeowner. Imagine having to tell the customer that the system that was installed was too small. To cover our butts, we typically oversize equipment so we can "just be sure".

    The problems with that are numerous. For one, oversized air conditioning systems will result in ineffective dehumidification as well as increased power consumption, increased equipment costs and increased installation costs.

    The initial problem and reason for oversizing in insecurity and insecurity comes with a lack of understanding and education. This is not meant to be an insult in any way shape or form, but it is difficult to trust something that we do not have a complete and full understanding of.

    Consider that guy who is dating your daughter. You know the one... the guy with the 37 earrings and the eyebrow piercing? The guy with 4,318 tattoos, 24 of which bear the names of his previous "one and onlys"? You know... the guy who dropped out of school at age 15 to travel cross country with his soon-to-be-famous rock band.

    You know.. It wasn't until after your daughter and this guy got married and had a few kids, that you got to spend some real quality time with him and realized that he was really a quality guy.

    You know... we are quick to put up our guard in the face of uncertainty, but, if used properly, reliability and confidence will come when it comes to heat gain and heat loss calculations go. Trust the software... that's what it's there for...

    Then, go get that tattoo you've always been dreaming about!
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