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7 Ways to Improve Ducts in an Unconditioned Attic

Flex Ducts In An Unconditioned Attic Installed As Low As Possible

A lot of people have gotten the message that ducts for your HVAC system—and the system itself—should be in conditioned space. Researchers at the National Renewable Energy Lab (NREL) studied the effect of ducts in an unconditioned attic and found they add 25% to the cooling load in hot climates.  In their study, the NREL team assumed the ducts were sealed well.  New homes that have to pass duct tests are often sealed well, but the reality for many duct systems is that they’re leaky.  In addition to the losses associated with the leakage, unbalanced duct leakage creates other problems.  So, get those ducts out of an unconditioned attic if at all possible.

But what can you do if have to put ducts in an unconditioned attic?  Maybe you’re buying a tract home, and they won’t encapsulate the attic or move the ducts.  Maybe you’re replacing a system that’s in an unconditioned attic now and can’t afford to encapsulate the attic, too.  Maybe you’re worried about moving the building enclosure to the roofline.  Here are some ways you can make the most of a not-so-ideal situation.

1. Never, ever put ducts against the roof deck

The absolute hottest part of an attic on a summer day is the underside of the roof deck.  It can get so hot that you can’t even keep your hand against it.  Why some contractors think it’s OK strap a duct right up against the deck is beyond me.  In addition to the heat, the roofing nails coming through the decking can puncture the duct insulation jacket or the duct itself.

Duct against the roof deck in an unconditioned attic
Duct against the roof deck in an unconditioned attic

2. Keep the ducts low in the attic

Attics get very hot.  On a sunny summer day, the air temperature in an unconditioned attic can get up to 120° F or even a bit higher.  But what you may not know is that attics aren’t uniformly hot.  The hottest air in an attic is up high.  The coolest is down near the attic floor.

Ducts in an unconditioned attic installed too high
Ducts in an unconditioned attic installed too high

In the photo above, the installer didn’t put the ducts in the worst place, but they certainly didn’t try to minimize heat gain either.  The temperature is lowest at the attic floor, and that’s where those ducts should be, as they are in the lead photo above.  (Note that the lead photo shows the ducts before he attic was insulated.  After the attic was insulated, those ducts were all buried in this dry climate house in California.)

3. Use a horizontal air handler

For the same reason you want the ducts to be low in an unconditioned attic, the air handler should also be low in the attic.  The furnace and air conditioner shown below are vertical, with the supply ducts at the top.  In summer, you’re putting cold air into those supply ducts higher in the attic because of the choice of vertical air handler.

Air handler in an unconditioned attic installed vertically
Air handler (furnace & AC) in an unconditioned attic installed vertically

A horizontal air handler in an unconditioned attic should be installed as low in the attic as possible, for the same reason the ducts should be low in the attic.  The photo in the insulation section below shows a horizontal air handler.

4. Seal the heck out of the ducts

Duct leakage is really bad when ducts are in an unconditioned attic.  Unbalanced duct leakage is even worse.  Seal them with abundant quantities of mastic.

Seal the ducts with copious amounts of mastic
Seal the ducts with copious amounts of mastic

5. Insulate the heck out of the ducts and air handler

Attics get really hot.  Insulation helps reduce heat flow.  Use a lot of it.  Unfortunately, standard duct insulation is available only up to R-8.  In a dry climate, you can bury the ducts in blown attic insulation.  In a humid climate, you might be lucky and get away with that if you put only R-8 on the ducts.  But buried ducts in a humid climate are at risk for condensation.  The building code now allows it, but only with R-13 on the ducts before you bury them in IECC climate zones 1A, 2A, and 3A.

Air handler and plenums well insulated in an unconditioned attic [Photo by Mike MacFarland, used with permission]
Air handler and plenums well insulated in an unconditioned attic [Photo by Mike MacFarland, used with permission]
An alternative is to put closed-cell spray foam insulation on the ducts before burying them.  The photo below is from a Building America report presentation by Robb Aldrich on buried, encapsulated ducts.  Inside that mound of closed-cell spray foam insulation is a duct.  The whole thing was buried deeply in blown insulation after this photo was taken.

Spray foam encapsulated duct that will be buried deeply in insulation in an unconditioned attic
Spray foam encapsulated duct that will be buried deeply in insulation in an unconditioned attic [Photo from Building America report by Robb Aldrich]

6. Increase the air velocity in the ducts

This recommendation helps maintain comfort in the home.  With ducts in conditioned space, slower moving air reduces the resistance to air flow, which is a good thing.  When ducts are in an unconditioned attic, the air inside picks up heat in summer.  The more slowly the air moves through the duct, the more the temperature of the conditioned air will rise.  Moving the air faster spreads the heat gain across more air with less temperature rise in each cubic foot.  So design the duct system for faster air in this case.

7. Keep your attic cooler with a reflective roof or radiant barrier

Another way to reduce heat flow into your ducts from a hot, unconditioned attic is to keep the attic cooler.  The best way to cool your attic is by stopping the heat before it has a chance to enter by using a reflective roof.  A good second choice would be to install a radiant barrier beneath the roof deck.  A really bad choice would be to use powered attic ventilators.

An infrared image of a hot, dark roof
This infrared image of a dark roof shows lots of heat on the roof. Much of that heat makes it into the attic, but reflective roofing can reduce it tremendously. [Photo by Cameron Taylor, used with permission.]
Yeah, ducts in an unconditioned attic are often a big liability.  They require you to get a bigger air conditioner and spend more on your heating and cooling bills.  But follow the guidance above, and you can make that liability much smaller.

 

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.

 

Related Articles

Case Closed: Get Those Air Conditioning Ducts out of the Attic

The Best Velocity for Moving Air Through Ducts

The Best Way to Cool Your Attic

 

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This Post Has 46 Comments

  1. Allison, I got a slight issue with keep ducts low in the attic. I would clarify that unless they are buried under insulation, they should be kept off the insulation by at least a few inches & in some cases can argue that being higher up can be better.
    The first reason is this allows the radiant barrier to work properly & eliminates conductive transfer. The second is the second hottest point is generally the top of the insulation during the summer with the sun beating down / holds the heat longer than the air. The third is this also allows them to pull them tight and/or seal around them easier. A minor fourth as most attics don’t have proper ventilation and location matters is that by being higher up they will be exposed to more outside temperature currents as compared to just stagnant / downward driven heat.

    1. Hi Sean, a few rebuttals to your post:
      (a) A radiant barrier (in this case, an insulated duct’s outer liner) only needs an inch or so to work, and even if ducts are touching ceiling insulation, that only represents a fraction of the circumference. Also, a radiant barrier only reduces conductive heat transfer, not nearly eliminate it.
      (b) I’d like to see research that demonstrates that ducts supported just above ceiling insulation incur more heat gain than ducts located higher in the attic. Let’s just say I’m skeptical that the ceiling insulation is hotter than the air above it.
      (c) In my experience, your third point is moot since mechanical rough is typically completed before insulation. In new construction, I would never allow a duct installation crew to trample through pristine ceiling insulation to do their work!
      (d) My own measurements of attic temperature gradients (from top of insulation to ridge) don’t support your premise that attic air is warmer at the bottom! In particular, I don’t know what ‘stagnant / downward driven heat’ means.

      1. David – sorry I wasn’t clear enough & nice reply. You can have three or more temps in a space, roof sheathing, the air temp, & then the actual temp of the insulation. Air temp is the most variable & I will easily concede that in general air temps are higher towards the top. The catch is the top of the insulation (my issue) which if you take an infrared camera to might surprise you on how hot it really is & how long it holds the temps.

        Missed A – a radiant barrier does not block conductive heat loss or gain, no space = no radiant blocking & conductive is allowed to pass right through. As for any other “R” properties that will help with conductive

        C – yes you are correct, but what happens when they fail the duct test & said builder…. The other problem by only leaving an inch or two is it is harder on subs to install nice & level or what happens if they want to add more. Hmmm just where are those balancing dampers at…

        D – Center of attic = very little air movement if any as most air movement happens in the space between the trusses from the soffit to ridge (yes this can be thrown off by duct leakage / over pressurizing the house forcing air up / or using a dreaded PAV thus pulling from house but we are talking properly built). Downward driven is summertime reverse stack effect & solar drive.

        Hope that clears up my thoughts more though of course my first thought is – yeah don’t do it & if you have to use a properly done Hot Roof system

    2. Sean & David, thanks for your comments. My recommendation for a radiant barrier was to install it on the underside of the roof, not just as the duct insulation jacket or lying on top of the insulated attic floor. They’re often not cost effective to install in efficient new homes, as Michael Blasnik showed in his Houston study, but if you have ducts in the attic, they may help a little bit. I almost left this recommendation out, but there are some cases where it could help.

    3. Sean,
      You say “insulation” generically. Blown fiberglass is different from blown cellulose. Fiberglass lacks the mass and thermal qualities to form an air seal and be an effective barrier between attic-installed ducts and the hot (or cold) ambient environment of unconditioned attic space. Cellulose is required to be installed to a specific depth to account for that warmer several few inches. That means installing the cellulose at a greater depth above the ducts. If the rest of the attic is R-49, above the ducts should be R-49. I do it when I have to but my best advice is never to install HVAC equipment in an unconditioned attic. Doing so is irresponsible…and just plain dumb.

  2. @Allison, regarding radiant barrier… I would argue that it’s never cost effective to install a radiant barrier on the roof in an existing home. Labor costs get rather crazy (especially for products associated with a free steak dinner ;-). Better to spend that money on duct tightening and/or additional insulation. OTOH, the additional cost foil-coated OSB is reasonable in new construction or when roof deck is being replaced.

    Regarding velocity… what Proctor has argued is that downsizing a replacement system (which typically means lower system airflow) can increase conducted losses since duct surface area is larger relative to the volume of air being moved. I’m not sure I buy that the converse is true: that increasing airflow in an existing system (unless airflow is already too low, which we know is a common problem with other implications). I know that in hydronics, increasing the fluid velocity through a heat exchanger will actually INCREASE total heat transfer somewhat. You see also this in expanded performance tables for AC’s and heat pumps — increasing the airflow increases total capacity! The trade-off is in blower energy, not in heat transfer through the heat exchanger. Just the opposite.

    In a new construction project where there’s no other option than to locate ducts in the attic, my target CFM and branch velocities are not be influenced by that. Instead, I reduce duct surface area by focusing on shortening the branches, NOT by using smaller ducts.

    1. David, those are all good points, as usual. On the radiant barrier issue, I’m with you on existing homes. It could be a DIY project, though. My article is aimed at both new construction and existing homes, and I was thinking more of new homes for radiant barriers.

      On the velocity issue, again, I didn’t make it clear but that recommendation was for new duct systems. Lower surface area is only part of the reason for increasing the velocity. The other result of smaller ducts and higher velocity is that the there’s less temperature increase in the conditioned air when it’s moving faster. Even if the heat gain in the duct were the same, faster moving air means the BTUs are spread over more air.

      1. I think we need to clean up some of the physics here. The overall thermal resistance between the air in a duct and the air outside of it depends partly on the convective heat transfer resistance between the moving air inside the duct and the interior duct wall. If you increase the airflow rate, and keep all else constant, you will reduce that internal wall resistance, thus reducing the overall resistance which will then increase the heat transfer through the duct. It is true for this case that the change in air temperature through the duct will decrease, but that is because of the higher airflow rate, not because of the higher velocity. It is the same situation as mentioned for water flowing through a coil. If you increase the water flow rate, you will increase the heat transfer rate even though the temperature change of the water will decrease. Remember that heat transfer rate equals fluid mass flow rate times fluid specific heat times fluid temperature change. Thus, you can have a higher heat transfer rate with lower temperature change of the fluid when the fluid mass flow rate increases.

        If you lower the air velocity in the duct by keeping the airflow rate constant but reducing the cross-sectional area of the duct, that is a different story since you are changing the surface area of the duct. But keep in mind that pressure drop will also increase as well as fan power and duct leakage.

      2. Roy wrote: “…heat transfer rate equals fluid mass flow rate times fluid specific heat times fluid temperature change. Thus, you can have a higher heat transfer rate with lower temperature change of the fluid when the fluid mass flow rate increases.”

        Thanks for explaining this better than I did. My point was that it’s not a zero sum trade-off. Ignoring the pump (or blower) energy trade-off, if you increase flow rate, the reduction in delta-T is not exactly proportional to the increase in mass flow, thus total heat transfer slightly increases with flow rate. The incremental increase in heat transfer diminishes as flow rate increases.

        BTW, in your last paragraph, I think you meant “If you *increase* the air velocity…”

        1. David, thanks for catching that. The last paragraph should have been:

          If you increase the air velocity in the duct by reducing the cross-sectional area of the duct while keeping the airflow rate constant, that is a different story since you are also changing the surface area of the duct. But keep in mind that pressure drop will also increase as well as fan power and duct leakage.

          1. RoyC,
            What is “fan power?” The load on a fan motor is measured in amperes. A heavier load draws more current.

          2. Roy, you’re right that I didn’t explain carefully what I was thinking when I advised to increase the velocity in the ducts. My intent was that this is for new duct systems being designed properly. Increase the velocity while keeping the air flow rate constant. Yes, that increases the pressure drop, but as long as the friction rate doesn’t go too low, that’s not a problem. I don’t know this stuff as well as you and David do, but I think that would help in an unconditioned attic, right? I may have to do some calculations and see real numbers for this because it’s pretty abstract right now.

  3. Hi Allison,

    Another great post.

    A question as it relates to “seal the heck out of the ducts” and the general principle of “do no harm.”

    Should you always measure total external static pressure as a “test-out” procedure when sealing ducts? I think the answer is yes. But I’m curious if there are reasons or situations why measuring the impact of duct sealing on a system’s static pressure may not be necessary.

    1. @Casey, great question! As you surmise, duct sealing naturally and necessarily increases external static pressure. The question is to what extent (the leakier the ducts, the bigger the increase), and whether there’s a meaningful reduction to system airflow. I would say the only exception where you don’t always need to check external static pressure is if it’s a variable speed blower with constant CFM. But even then, if pre-sealing static is on the margin, sealing the ducts may push it past the blower’s limit. With other types of blowers, a reduction in airflow may impair AC performance. Worst case, the evap coil turns into a block of ice long after the duct sealing crew is gone. (Yes, that has happened!)

      The problem we run into is that many duct sealing crews don’t have the technical training & experience to assess system airflow. Also, if test-out static reveals airflow has dropped too low, the technician needs to know how to adjust the blower or in some cases, modify the duct system in order to reduce external static (e.g., add another return, increase filter surface area, etc).

      1. David, of what benefit is it for a laborer who seals sheet metal ducts to have a working knowledge of how to assess system airflow? The assessment was done by the designer.

        And, exactly how does duct sealing increase external static pressure? Upon what is the external air pressure increased?

        Why do you say “pre-sealing static may push the blower past its limit?”

        And how does a variable speed blower maintain a constant volume?

        1. Hi Larry,

          Lots of good questions. As a poor substitute for David, I’ll take a crack at answering a few.

          As a contractor who is selling the benefits of duct sealing, you (hopefully) want to deliver those benefits to the customer. You don’t want to sell the benefits of duct sealing (comfort, IAQ, reduce energy use) and then have your customer experience reduced airflow, higher energy bills, and/or (worst case) a cracked a heat exchanger. Whether the person sealing the ducts is a laborer or not, I hope the builder (in new construction) or owner of the HVAC or home performance company (in existing homes) wants to deliver the results that they promise to their customers.

          As far as your other questions, they might be too detailed for a comment section. But static pressure increases when there are obstacles to airflow (kinks, bends, filters, etc.) and generally has an inverse relationship to velocity pressure and airflow. Higher static pressure means either reduced airflow or more energy is needed to move the same amount of air. PSC motors faced with higher static pressure will move less air. ECM motors will ramp up energy use in order to try maintain airflow.

          For more information you can probably use search terms like “total external static pressure” and “duct sealing.” There are lots of good analogies using garden hoses to make these ideas slightly more intuitive.

        2. @Larry, I should clarify that my comment referred to duct sealing work done AFTER the system has been initially commissioned (i.e., in new construction, the installer is responsible for verifying proper airflow as per the design after the ducts are initially sealed during construction).

          The problem is with existing homes with leaky ducts. If the homeowner decided to have the ducts sealed, this will change system performance. In that case, the duct sealing crew is responsible for ensuring proper airflow after the sealing work is done. Of course, the duct sealing crew may not be aware of the initial design CFM, but there are well established guidelines for airflow based on AC tonnage.

          > exactly how does duct sealing increase external static pressure?
          This is sorta hard to explain without a whiteboard, but leakage paths in a duct system reduce static pressure in the same way that adding an additional return or supply reduces static pressure. Conversely, sealing those leakage paths will have the same affect as a closed vent, clogged filter or crushed duct.

          Manufacturers publish blower tables that correlate external static to airflow. This is typically how a designer or field tech assesses airflow.

          > Why do you say “pre-sealing static may push the blower past its limit?
          What I said was that duct sealing may push a constant CFM blower past its limit if it was already on the margin to begin with. For example, if a constant CFM blower is rated for a maximum external static of 0.7 IWC (inches of water column), and it is already close to that limit before sealing, then sealing the ducts may push external static past its limit. This can cause the blower to buffet or stall. Best case, airflow will drop off past 0.7 IWC.

          Variable speed blowers that are constant CFM (note that not all ECM or variable ECM blowers have this feature!) use electronics to monitor the motor’s torque and RPM’s and adjust as necessary.

          I’m happy to provide this explanation, but as Casey said, your questions are a bit too detailed for a blog comment string, not to mention way off topic. In any case, these concepts should already be familiar to anyone who oversees duct sealing work or otherwise modifies duct systems.

    2. Casey, I’ll come in and add one more thing that I haven’t seen mentioned yet regarding testing the total external static pressure after duct sealing. If the ducts are attached to a furnace, sealing the heck out of them could reduce the air flow enough to cause the heat exchanger to crack. That could put exhaust gases, possibly including carbon monoxide, into the conditioned air stream. So this is a combustion safety issue, too. Even if there’s no furnace, low air flow can freeze evaporator coils in summer and potentially damage the compressor.

  4. Roy C,
    You are assuming a delta T great enough to provide heat to transfer. Again, a well-insulated, but unwisely placed duct will survive in an unconditioned attic. It will never achieve the energy conservation possible if installed in conditioned space, but the loss won’t break the bank. The major “crime” is locating mechanical HVAC equipment in an attic. The industry seems to do everything it can to insure equipment needs replaced sooner rather than later. Shame on them!

  5. Allison, Several years ago I knew some folks who had bought a home that was well maintained and over the years had some improvements including a newer HVAC system with insulated ducts in the attic. During the summer (think South Louisiana) the one story house simply would not cool. After having two HVAC companies evaluate it and tell them the system was too small, they planned to have the 2nd company increase the size of the equipment. I asked them to let me go in the attic and take a look. I bought about $250 worth of hard fittings and sealant at Lowe’s and a friend helped me install them at every turn and stretch out the ducts so they were tight. I removed more than 25% of the duct work. The house cooled fine after that.

    1. Bob,

      It’s great the HVAC system functioned sufficiently after sealing all the duct leaks. But what is the insulation level (R-value) above the ducts. “Cooling fine” does not mean “cooling efficiently.”

    2. Bob, I believe a lot of cooling problems could be solved that way. In addition to the extra resistance to air flow from flex not pulled tight and lack of fittings, I can’t tell you how many disconnected ducts—both return and supply—I’ve seen in unconditioned attics and crawl spaces. Great job getting in there and fixing the real problem!

  6. Larry,
    In loose fill applications in spite of all their benefits, neither fiberglass nor cellulose insulation provides an adequate air seal. While some research suggests that cellulose resists and/or blocks air movement more effectively than fiberglass, these materials are and should only be installed to reduce the passage of heat – not the passage of air. Only foam insulation and certain caulks can block air movement in a way that truly improves comfort and boosts energy efficiency.
    In dense pack wall applications, the Pink L77 product is the equivalent of cellulose in infiltration performance. source:NAHB research study 10/09

    1. There are thousands of “foam” insulation formulas. Some breath, therefore, does not seal. Others shrink. One heavily promoted enclosed cavity “foam” shrinks as much as 36 percent before the installer leaves the job site. I’ve seen spray foam detach from walls after a few short years.

      Building Performance Institute REQUIRES cellulose be dense-packed into walls when retrofitting for the purpose of air sealing. My buildings always outperform using this technique. Anyone who claims otherwise reads more than does. Once you experience the results, any claims otherwise are not credible. I can retrofit insulate a one-and-a-half-story cape cod with cellulose, Attic, walls, dormers, knee walls, sloped ceilings, rim joists, crawl, basement, etc., and the results will ALWAYS outperform the highly promoted wall foam & and blown fiberglass attic every single time.

    2. Chris pfoohl,
      I’m not having any luck finding your “NAHB research study 10/09.”
      Where might I read it?
      Thanks

    3. chris pfohl,

      I should add, Building Performance Institute DOES NOT recognize blown fiberglass as having any sealing value. (Does anyone?) In fact, it’s not valued as thermal insulation. Maybe the manufactures of the blown fiberglass product you are citing should get the research data to BPI? What are they waiting for?

  7. Larry: Fan power is the electrical power supplied to the fan motor, typically measured in watts. It is what your electric meter measures and determines how much you pay the electric company.

    1. I believe I know OHM’s law. I wasn’t understanding “fan power.”

      But thanks.

    2. RoyC,

      All AC electrical power is expressed in watts or amperes, depending on the topic and intent of the user. What threw me is all this talk about “static pressure.” I did a quick search to discover “static pressure” is a generic term synonymous with “pressure” used by the HVAC industry. If we are talking about the force needed to overcome the resistance of the ducts, then why not simplify. No need to talk in code.

      1. Larry, I am not talking in code. Electrical power is usually expressed in watts. Electrical current is usually expressed in amps. It is not the same as power. For simple electrical loads (like resistance heaters), electrical power (watts) is equal to the current (amps) times the voltage (volts). For AC inductive loads like fan motors, the relationship is a bit more complex in that it includes a power factor that is dependent on the inductance and capacitance of the device. Ohm’s Law has little to do with motors, but it is applicable to electric resistance heaters where voltage (volts) = current (amps) times resistance (ohms).

        The static pressure rise of an air handler is the air pressure rise from the return air to the supply air side. The term “static” for this pressure rise is used because it does not include the effect of the air velocity. If you measure the “total” air pressure of an air stream with a device like a pitot tube, you are including the velocity pressure. For the relatively low velocities that occurs in residential air ducts, the velocity pressure is generally negligible so we only use simple static pressure measurements. The fan power required by the air handler is proportional to the static pressure rise multiplied by the volumetric airflow rate. I don’t know if this helps break the code or not.

        1. RoyC,
          Of course, Ohm’s law applies to motors. You were calling out “fan power.” I asked you to explain. Now, I take it you meant “electrical power.” When I spec motors or troubleshoot motors, they are measured by amps, from fractional HP, one and three-phase induction motors, up to 2,000 HP motors operating at over 5,000 volts. Their measure of power is amperes. No one cares about watts. Motors run on electrical power and rated in voltage and amps. The wattage is mathematically derived if someone is curious. Wiring and switchgear, again, are rated (sized) based on amps and volts, not wattage. There’s a much bigger industrial world out there than anemic capacitor-run blower fans. Again, I understand, motors, electricity, air flow, pressure, etc. I did not understand the ambiguous term “fan power.” For all I knew you might have been talking about how many candles a fan can extinguish 🙂 You folks seem deep into minutia. Again, for clarity, is “static pressure” and “pressure” interchangeable in your usage. That is, are you describing the pressure inside an HVAC duct system when the fan is “ON” and moving air?

          1. Larry, you say you “understand, motors, electricity, air flow, pressure, etc.” The questions you’re asking indicate otherwise, though. If you really understand electricity, you would know immediately what “fan power” means. If you understand air flow and pressure, you wouldn’t have to ask about static pressure. I appreciate that you seem to be asking sincerely, but I think you would do well to learn more about these topics before coming here and accusing us of “jabbering” about physics and being “deep into minutia. If an electrician came to my house asking the kind of questions you ask, I’d be afraid to let them touch any part of my electrical system.

          2. Larry: I will try again. Power (energy transfer per unit time) comes in a lot of forms such as electrical power (W), mechanical (shaft) power (hp), heating or cooling power (Btu/hr), etc. Electrical current is not “power”. Motors convert electrical power to shaft power. Fans and blowers convert shaft power to “air power” which can be calculated as air pressure rise multiplied by the air flow rate.

            Motors are designed to operate at specific voltages and are rated in terms of current to ensure that they are wired properly. This is where Ohm’s Law comes into play. If a motor requires higher current to produce the necessary power output, you must use larger wire gauges with lower resistances (Ohm’s Law) to connect it to the breaker panel, otherwise the wiring can overheat and fail, especially if the circuit breaker is improperly sized. This is why larger motors and other equipment are designed to operate at higher voltages. Motors that operate at higher voltages can provide the same power with lower current. Thus, you can get by with smaller connecting wires.

            The indoor blower doesn’t care how much current is required by the motor for a given shaft power requirement. It will operate the same with a 120 VAC motor at 5 amps as it would with a 240 VAC motor at 2.5 amps as long as the shaft speed and torque are the same (note that shaft power = speed x torque).

            As an industry, we do tend to get sloppy when we talk about air pressure. When talking about airflow in ducts and air handlers, should always be talking about pressure rises and pressure drops. We can use static pressures for lower air velocity systems like most residential systems. These pressure changes multiplied by the volumetric airflow rates are related to the required blower and thus motor powers. Motor electrical input power is what we eventually have to pay for.

            Yes, this may seem like minutia to you, but it is all necessary to properly design this type of equipment and should be helpful for installing and maintaining this equipment too.

  8. Larry–There was no insulation above the ducts, before or after. The duct seals were not so bad either. What I did primarily was improve air flow by eliminating all of the turns in the flex duct and stretching it out properly. In my opinion it proved that proper installation of flex duct is almost as good as hard duct. But finding an HVAC company that will properly install flex duct is another matter.

  9. Allison: There is a lot of physics going on in both good and bad ways when you change duct velocity, thus it takes a detailed study to see what is optimal for a given application. AHRI funded a study about 10 years ago in an attempt to do this, but it was poorly executed in my opinion and did not provide useful results. Do you have the right tools to do this type of optimization?

    1. Roy, I have a lot of Excel spreadsheets! Actually, though, you’re right. This is a hard problem, but I have a friend at NREL with a PhD in heat transfer who’s done work on conductive duct losses. I’ll ask him what he knows about this.

      1. Allison Bailes,

        “Heat transfer” through what material? What importance is it?

      2. Current is the quantity of electron flow. Voltage is pressure. Wattage is power consumed. These three factors make Ohm’s Law. Any two will solve for the third. This is basic electricity as is your last diatribe. Instead of trying to impress with factoids about Ohm’s Law, please answer my question. For the record, I did not say, or imply, that the HVAC industry “tends to get sloppy when we talk about air pressure.” That is your confession. Now, and for the third time, considering you have switched and are now saying “air pressure” and not “static pressure,” please state simply whether “static pressure,” as you use it, and “pressure” are interchangeable. You have implied it. Will you confirm? I often use the terminology of an industry or trade or even regional colloquialism so don’t mind calling pressure “static pressure.” I simply want to be on the same page and follow along as you iron out the important details of your examples. I am curious about one thing. That being, once you mathematically derive the ideal static pressure, what next? I mean, HVAC packages are engineered to cover a range, not a finite volume. And once you are satisfied with your numbers, the unit is installed, presumably absolutely in compliance with your all-important “static pressure” numbers, then the next day I insulate the home to the hilt maximizing energy-savings as only I do, what, if anything does that do to all your numbers, both static pressure and those calculated heat transfer values for the duct walls? The new thermal parameters keep the ducted air warmer and cooler, respectively. Focus, please:
        1) Does “static pressure mean “pressure?”
        2) Does thermal mass (duct insulation R-value) change your calculations if they change?
        3) As a point of reference, the motor’s wattage consumed is not of concern to me. The motor and switchgear were sized according to Ohm’s law. If I install a motor according to electrical code, the power consumed is within the specs of the motor. And the performance of the blower fan is limited by its design parameters and only installed with matching HVAC systems at the factory. The true measure of wattage is known only after commissioning the system and calculating actual consumption using Ohm’s Law, which you finally concur has to do with fan motors.

        You are wrong: Current draw is most critical in evaluating a motor, its load compliance, and phase balance. Is it possible you are speaking out of your expertise? Maybe cutting and pasting basic electrical knowledge?

        1. 1. Static pressure is not the same as just ‘pressure’… as Roy and I have already said, total pressure = velocity pressure + static pressure. Although velocity pressure is relatively small in low velocity duct systems as Roy pointed out, we must measure static pressure if we want to use the manufacturer’s blower table to evaluate or diagnose airflow issues. Moreover, a static pressure probe is different than a pitot tube that measures velocity pressure. (Some pitot tubes can measure both.)

          2. Thermal mass is not the same as R-value as your question suggests. In any case, duct insulation R-value has no bearing on Casey’s question regarding the impact of air sealing on external static pressure and whether air sealing can ‘do harm’ so I’m not sure how that became part of this discussion. Changing duct insulation R-value of course changes the heating or cooling load, and thus must be considered when doing load calculations.

          3. Here’s why we’re concerned with blower motor watts: As HVAC equipment has become more efficient, the blower represents a larger and larger share of HVAC energy, which of course is the focus of this blog. Blower watts is a function of external static pressure, among other things.

        2. Larry: Perhaps I am a bit dense and speaking “out of my expertise”, but you seem to want to be precise with some concepts (pressure) and loose on other ones (electricity). But I will keep trying.

          Ohm’s Law is often expressed as V = IR. Ohm just said that the current through a material or object is proportional to the voltage across it. The constant of proportionality for this process is resistance (R) which was given the units of “ohm” in his honor. Some materials obey this relation, thus they follow Ohm’s “Law”, but many don’t, so this is really just an empirical relationship not a law of nature.

          Another important equation for electrical analysis is P = IV. This is based on basic physics and is not an empirical equation. It has nothing to do with Ohm’s Law. You can combine Ohm’s Law with this electrical power equation and derive another equation for the electrical power dissipated as heat in a resistive material as P = IxIxR (I-squared R). This equation shows why appliance current requirements are important for wire sizing in order to limit the power dissipated as heat.

          So now I will address your questions on pressure. Pressure is used many different ways depending on the issue being addressed. Let’s start with atmospheric or barometric pressure which is the actual pressure of the air around us, typically measured with a barometer. There have been many units developed for expressing it. At sea level under “standard conditions” it is 1 atm =14.7 psi = 29.92 in Hg = 101 kPa = . . . . This value is important for determining air density in an HVAC system when doing some energy calculations. It varies with weather conditions, local elevation, temperature, humidity, etc. So perhaps this is what you mean when you use the single term “pressure”?

          The discussion on this thread has been mainly about pressure differences which are typically measured by a manometer with units of inches of water. These manometers (whether fluid-filled u-tubes or electronic) always measure a pressure difference between two points, typically between two points in an air handler or duct system, or between a duct and a space, or between the indoor and outdoor air (building pressurization). Since these pressure differences were typically measured with water-filled manometers, the typical unit of measurement is inches of water column (inwc). Note that normal atmospheric pressure as previously defined is equal to about 408 inwc. This is important since we typically talk about pressure differences in ducts or buildings on the order of 1 inwc or less which is insignificant compared to the actual pressure in a building for the purposes of determining air density. However, these pressure differences in terms of duct pressure drops and required air handler pressure rises are very significant in terms of their impact on blower airflow rate and power consumption. These pressure differences between the indoor and outdoor air can also be significant in terms of infiltration. These pressure differences between the ducts and adjacent spaces can also be significant in terms of duct leakage.

          I won’t address the differences between static, total, and velocity pressures again. But I will repeat that many people in our industry (myself included) throw these terms around loosely because they are typically communicating with others who understand how these terms are commonly used.

          Hopefully David addressed your confusion about the difference between thermal resistance and thermal mass. I can’t add much to what he said other than I prefer the term “thermal capacitance” instead of “thermal mass” since that is a better analogy to electrical processes.

          I hope this answers your questions and addresses your misconceptions.

  10. Bob,
    I agree with you. It seems too much emphasis is placed on “static pressure.” A bend in any duct, hard or flex, creates a lot of resistance. I can’t imagine the homeowner not thoroughly insulating above the duct. If I were “energy czar,” I’d outlaw all insulation attached to the duct at the factory. All should be bare of insulation and then correctly and thoroughly insulated when installed. As is, that step is skipped so often because the impression (to some) is that the insulation attached with the ducting is adequate. It seldom is. My experience is that little amount of insulation causes installers to not realize they don’t have a seal…until the duct blaster tests expose the leaks. Air seal and insulate – why jabber about physics and static pressure and VS motors and constant flow. None of that is meaningful without complete air sealing and complete insulation.

  11. Hello. Non HVAC expert homeowner. We built single level 2100 sqft home in floodplain a few years ago in Central Washington State, though the last flood in our immediate area was Noah’s probably. We have attic ductwork that seems relatively well insulated. When we get into 90s and 100s, we hit mid to upper 70’s inside and thats if we get to 70 in am, keep house buttoned up best we can, and avoid cooking and running dryer. Our unit was probably undersized given attic ducting and was a new code in our area so not much experience. My question, can HVAC unit simply be increased in size without increasing air handler, and given what I read here about attic ductwork bad idea, will it likely make that much difference if we invest? Thank you!

    1. @JohnR, yes, manufacturers often rate air handlers with multiple condensers (or heat pumps) and it’s usually possible to do additional unofficial match-ups with condensers that aren’t officially paired. It depends on the specific AC and AHU model numbers, as well as the (duct system) external static.

      If you post the AC and air handler model numbers, I can tell you what your options are, but you’d need to have someone check your air handler’s available speed settings and external static to see if your air handler and duct system can support higher airflow. If you’re willing to invest $50 in a Magnahelic gauge, I can show you how to do this test yourself (contact me privately for consultation).

      you wrote: “will it make that much difference if we invest”…
      Assuming it’s possible to up-size your condenser, of course it would make a difference! But before you do anything else, you should make sure there’s not something else that’s causing your system to under-perform (leaky ducts, dirty or undersized filter, unbalanced air distribution, improper airflow setting, inadequate duct system or restrictions, improper charge level, etc).

      In any case, I’m not sure it makes sense to replace a relatively new AC because it couldn’t keep up with a once-in-a-century heat wave. You should at least eliminate other possible problems before considering a new AC.

  12. Thank you David. Excellent advisement.
    I’ll get some model info. I can get a gauge too. Appreciate the help.

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