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cmf vs. btu

sounds like the hvac contractor doesn't realize the diff between heat load and cfm either. Just because you need more cfm to push the cooling around, doesn't necessarily translate into the need for more cooling tonnage. Lot of guys just oversize and tell the cust to run a duhumidifier in the basement and problem solved. Course it is easier to just size it right the first time. Cheaper too.


  • Keith W.
    Keith W. Member Posts: 29
    leaky sheds and tight auditoriums

    First, a caveat: I mostly do solar heating and hot water, radiant floors, and other energy consulting. This all integrates with AC, but I don't consider myself an AC expert. It's very, very useful for me to be able to brainstorm and problem solve AC, since it ties into systems I'm working on, directly and indirectly, all the time.

    In trying to resolve a problem today, I realized I'm confused about something I hadn't even noticed. Let's say I do a heat loss/gain calculation on a building, and it turns out to need 3 tons of cooling. Typically, 400 cfm/ton is recommended, so about 1200 cfm would be recommended. Now, that 3 ton building could be a leaky little shed, or a very efficient (and quite large) house or auditorium... that is, the cooling demand doesn't say much at all about the building volume. (imagine for a moment we have heat recovery ventilation and filtration as necessary, so this question is not much about air quality, but just about cooling).

    Surely the shed and house (or auditorium) need very different quantities of air movement to distribute their AC, no? I'll go hit the books, I'm sure this is basic stuff, I'm just realizing I missed this implication somehow.

  • Empire_2
    Empire_2 Member Posts: 2,343

    Manual J is calculated by the volume not just sq ft. Structor/windows and other ventelation equipment has a value that needs to be added to the total heat gain on the structor. Air change per hour is a reflection of how the building is or is not sealed..
  • Keith W.
    Keith W. Member Posts: 29
    but of course

    I was talking with someone who was installing a unit in a project I'm working on, who kept insisting that, although the Manual J called for a 2 ton unit and we already had a 3 ton unit that was installed and being extended into this addition, we needed to add ANOTHER 1 1/2 ton unit to resolve the air-movement problem caused by the difficulty of ducting the job (the plans for the project didn't include any ductwork, so there wasn't much space to work with).

    Some years ago they had oversized the original to accomodate an expected addition, but then reinsulated and added new windows everywhere, so despite the addition total load went down! And I ended up having a long painful conversation with the HVAC guy yesterday who was convinced the only thing to be done was add yet another unit, and even more excess capacity, even though I'd done a really careful Manual J with HVAC-calc. Which is exactly why I need to understand the ins and outs of AC/forced air better. Thanks!

    In the end we figured out we could move the unit and make it work, though he's convinced it's going to be undersized. He just doesn't believe the Manual J! (He is a pro, and does really nice duct work, so I'm sure his approach oten works, but this is a super-insulated house...)

  • Empire_2
    Empire_2 Member Posts: 2,343
    Bottom line...

    If the man don't like or believe in the program then I guess there is no changing his mind. I respect that. TRUST Your Figures, is all I can say as to the addition and whole house load.... I know sometimes it's a pain to extend the duct, but a properly sized piece of equipment will cool just fine in the long run. I dont have a problem with oversizing by 1/2 ton, but again trust what the program is telling you. Alot of guy's think bigger is better, but may cause more headaches than you want. Can the existing system be increased by a ton or 1/2 ton? Will the blower drive be enough? Zonning is also a way to get out of a tight spot with a slightly undersized system, but it should actually be designed for the whole load...I hope this helped...Mike T
  • Eugene Silberstein
    Eugene Silberstein Member Posts: 1,380
    One (of many) Main Problems

    One problem that WILL be encountered with an oversized system is excessive humidity. The evaporator coil of the air conditioning sytem performs two functions: it cools the air (sensible heat transfer) and also dehumidifies the air (latent heat transfer).

    If the system is oversized, the run-time of the cooling equipment will be greatly reduced. So, although the temperature (sensible heat) of the space will be at the desired setpoint, the humidity will be higher than desired. This is because of the reduced system run time.

    Manual J even recommends that, on homes with AED (adequate exposure divesity) where the rooms in the house experience their peak loads at different times during the day, that the total equipment size can actually be reduced by 10% and the system will work fine. It is relatively safe to assume that a freestanding, single family dwelling with average fenestration on all sides will have AED, so using a slightly undersized system is acceptable. The HVAC contractor you are dealing with wants to OVERSIZE the system by 50%. So, even if the contracot seems to have the "knack" for high quality work, who would want to take the chance (or responsibility) of having a high humdity problem?

    Remember that a relative humidity over 60% greatly increases the rate of mold growth in the structure and nobody, and I mean nobody, wants to be responsible for the cleanup bill for that.
  • Keith W.
    Keith W. Member Posts: 29
    Humidity and variable speed fans

    Thanks for your responses.

    I did understand the oversize/humidity problem, which is mostly why I sat through 2 hours of meetings trying to convince everyone that we shouldn't add another unit to an already oversized system.

    But since the client already had the 3 ton, that's getting moved to a more central location, and will be used. So it will still be oversized (but at least only 3 tons, not 4.5 or 5 total!).

    Here's my question: the unit is a Carrier Weathermaker 9200, I think it is, with a variable speed fan. Will the variable speed fan make it possible to get the humidity reduction we desire, even though the unit is oversized? We do expect to be including an energy recovery ventilator also, so make-up air should be somewhat drier than DC average (which is bad). Thanks again for your help!

  • Tim_24
    Tim_24 Member Posts: 53
    400 cfm/ton

    is only a nominal air flow and nor necessarily "recommended" by anyone. At lease not anyone that has designed HVAC systems for high latent loads. In high occupancy spaces like an auditorium, gynasium or church sanctuary, air flows are usually much lower than 400/ton. One way to reduce the sensible capacity of a system, while at the same time increasing the latent fraction of the cooling (lowering s/t) is to turn down the air flow rate across the condenser. Check the manufacturers published performance tables for the components involved and see how low they have the flows tabulated. Typically if you are operating anywhere within the bounds of the published data, you can operate there with no problems.
  • Keith W.
    Keith W. Member Posts: 29
    Hmmm... further confusion

    Well, I thought I had it, but now I'm confused again...

    When I run HVAC-calc, it determines the tonnage based on the cubic? or square? footage (I assume square, but I'd have to go peruse Manual J to be confident) as well as the insulation and infiltration, etc.

    But it only gives me a tons of cooling number, not total cfm or anything. It then assumes/defaults to 400 cfm/ton if I have it do CFM calculations. Now I know it calculates cooling based on volume/space, but it's still hard for me to believe that this takes into account all possible space/distribution considerations.

    I suppose that cfm could really require an entirely different analysis. For example, let's say you were using passive chilled beams, you might get your cooling and dehumidification without any forced air flow, and the only airflow you'd need to arrange would be for filtration and make-up air.

    Or am I STILL confused? Thanks for further insights, this is definitely clarifying things for me.

  • Keith W.
    Keith W. Member Posts: 29

    I hadn't seen this response when I last responded. It clarifies things for me a bit. Definitely time to hit the books. Thanks again.

    Here's a question, though: it seems to me there are a few ways to slow down airflow: one is to use a lower fan speed. Another is to provide higher "friction" in the system (longer or smaller ducts). I'm used to the ductilator assumptions about set air speed in ducts. When you choose lower airflow in a high-occupancy space, do you always still choose duct sizing for some minimum airflow, or can you choose slower duct flow/greater resistance (which would happen in any case if you have to use balancing dampers much) to slow down the air speed across the coil?

    I realize I might not be asking a completely sensible question, since I might be missing something basic...

  • jeff_51
    jeff_51 Member Posts: 545
    heat loss and gain are based on cubic footage

    and fo course all the other variables like orientation, windows infiltration etc. Cfm is basicly based on how much heating or cooling btu or tonnage you have to move around in a system and is usually based on charts, starting from the farthest run away from the fan unit and increasing in size as you move back towards the unit and add more capacity. cfm also determines your duct size. Too small and it gets noisy, drafty and can cut the amount of capacity. To large and you don't get enough back pressure to force the air out of the main trunk and into the runs. You don't need much, but you have to size right or you will have a hard time balancing the system. Your duct size is based on cfm, not btu's, also fan speed makes a differance. It all has to be set up correctly
  • Tim_24
    Tim_24 Member Posts: 53
    Three issues involved...

    here. Determining loads; determining air flow rates to meet those loads; and how to set up as system to perform as designed.

    For residential cooling there are four significant sources of heat gain to address, no matter what the shape, volume or air flow rate.

    These are: Transmission (conduction/convection due to temperature difference), incident solar radiation, infiltration, which is primarily latent in nature and internal loads (in non-residential applications significant heat gains will be realized from internal sources such as lighting, electronics, motors and people, etc.) which are minimal in most residential load deteminations.

    Second, contrary to one posters "advice", air flow is determined based on the design conditions of the space and the equipment. Nominal values would be for 400 cfm/ton and 55 degF LAT (leaving air temperature). This is good enough for "rule of thumb" and "square foot" designers, various hacks and amatuers, but is inadequate if you were to design an HVAC system for any type of critical application or if you wanted to truly match a system to the loads. Air flow is determined based on the sensible load/(delta T x 1.085) = cfm; where delta T is the difference between the desired space conditions and the LAT. It's that simple.

    Where it gets more complicated is then how do you address the latent load and how do you match the equipment to do so. Manually, this is done graphically with a psychrometric chart and tabular data of the equipment you are basing the design upon.

    Sizing the duct to accomodate the flows you determined is also in and of itself a simple exercise, done manually by means of a ductilator. My design criteria excludes frition above 0.1"/100', and velocities above 1000 cfm (exclusive of exhaust systems), otherwise I try to use stock sizes and keep the fittings to a reasonable amount.

    Adjustment of air flow in a system ,asuuming the design was guesswork, is dependent upon the flexibility of the equipment. Belt driven blowers can be adjusted or re-sheaved, volume dampers at branchs are there for balancing, and some if not most multispeed blower motor have "taps" that can be reset for lower speeds.
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