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Community and Q&A

Sizing HVAC Equipment

Ed Welch | Posted in Energy Efficiency and Durability on

If we use ACCA Manual J as a reference, HVAC contractors consistently oversize equipment. For instance, on my house, ACCA Manual J calculated the heating load to be 46,000 BTUs….the Title 24 Environmental Engineers used a Micropas program to calculate a 52,000 BTU heating load….not a huge difference….then, they added a 30% safety factor, for extreme weather fluctuations (I assume, those days when storms hit and drop temperatures by 50 degrees over night)….bringing the load to 68,000 BTUs…..the contractor covered that estimate with a 90,000 BTU boiler, possibly because of size restrictions, etc…..twice the Manual J calculations! And Performance Contractors and others claim that Manual J is already oversized by 15-30%.


(1) Is Manual J an accurate calculation tool? Is it already oversized?
(2) Why does it appear that HVAC contractors pay no attentions to Manual J?
(3) Does the typical HVAC contractor even use accurate heating load or cooling load calculations?
(4) And, as someone recently told me, these guys (HVAC contractors) are not idiots….they install equipment that reduces the chance of getting callbacks.

I understand the inefficiencies related to oversizing (short cycling, etc) but I also understand HVAC contractors not wanting to get callbacks from super sensitive clients who are not willing to wear a sweater or add a blanket when storms hit. Where is the middle ground?

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  1. Doug McEvers | | #1

    The 2 stage forced air furnaces and modulating gas boilers have taken some of the oversizing concerns out of the picture. With a 2 stage furnace sized for twice the design temperature heat load, what is the downside? I know that if you do a deep nightly setback like I do, the recovery time in the morning will be faster, the remainder of the time the furnace runs on low fire. If you are using forced air, insist on a ECM fan motor.

    If I recall, in Canada they call for a design temp load x 1.4, in otherwords a 40% oversize.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    There is increasing evidence that much of the alarm over equipment oversizing is misplaced. As Doug points out, two-stage equipment really doesn't carry an energy penalty. Even the long-standing belief that oversized air conditioners do a worse job at dehumidification isn't always true.

    That said, large furnaces often cost more than small furnaces, so there is a capital cost penalty when oversized equipment is purchased.

    1. Manual J is pretty accurate and doesn't require any fudge factors on top of the Manual J calculation. According to some experts, Manual J already includes a safety factor; I know that some builders of tight, well insulated homes deliberately design heating and cooling equipment that is smaller than Manual J requirements.

    2. You have already answered the question -- why do contractors oversize equipment? The answer is, "To reduce the chance of callbacks." The fact is, many contractors don't do a proper Manual J. They don't know the window specs, they don't want to measure the windows. So they guess, and they throw in a fudge factor, because oversizing doesn't (usually) result in callbacks.

  3. Allison A. Bailes III | | #3

    Ed, to add to what Doug & Martin said, let me say that, no, HVAC contractors, at least here in the Southeast, don't often do Manual J load calculations at all. They mostly use rules of thumb based on square footage (e.g., 500 square feet per ton of AC capacity). When they take the time to enter data and run a Manual J, they usually do it incorrectly. In addition to the window spec problem Martin mentioned, other popular tricks are not using the correct design temperatures, orienting the house to worst case, and throwing in extra people. (I once checked a load calc for a 3 bedroom house. It should have had 4 people, but the contractor threw 11 people in to increase the load and justify a bigger AC.)

    My experience has been that HVAC contractors will do a Manual J load calc when asked, but if they know someone's going to check up on them, they often sub it out to someone who knows what they're doing. Most of the load calcs I've done were for HVAC contractors.

    Martin, what's your justification for the oversized AC and dehumidification claim? Of course, many things aren't 'always true,' but working in mixed-humid and hot-humid climates, I always emphasize the importance of proper sizing for cooling equipment. Yes, for heating equipment, there's little or no penalty, especially with the advanced equipment, but most people are still installing single-stage, single-speed cooling equipment, which needs long run times for dehumidification. Please let me know what news I might have missed about this topic.

    Allison A. Bailes III, PhD
    Energy Vanguard

  4. User avatar GBA Editor
    Martin Holladay | | #4

    In the May 2007 issue of Energy Design Update, I reported on a study performed by Florida Solar Energy Center researchers Jeffrey Sonne, Danny Parker, and Dan Shirey.

    The surprising result of their study: when existing oversized air conditioners in Florida were replaced with right-sized equipment, the new equipment was less effective at dehumidification than the older oversized equipment. The main reason: existing homes have leaky attic ductwork. That's why the findings only apply to houses with bad duct systems.

    Here's what I wrote:

    The three FSEC researchers looked closely at the data to try to explain why the right-sized equipment performed poorly. Clearly, the right-sized equipment had longer run times than the oversized equipment; longer run times allow attic ducts to pick up more heat. Moreover, longer run times also provide more opportunities for humid outdoor air to enter leaky attic return ducts. As Danny Parker explained, “The smoking gun was the humidity situation. Moisture was being pumped into the space, and the condensate quantity was higher when the machine was running.”

    As the paper notes, “The data from the Lakeland site revealed that the new [right-sized] air conditioner, with its higher evaporator air flow per unit cooling capacity, did a worse job at controlling interior moisture levels. … While it would be convenient to attribute this slightly higher interior humidity level to the higher coil air flow, the higher measured condensate removal in the post period, and pre and post dew points, suggest that somehow the moisture load was much greater with the new machine. Note that dew points were lower in the post-monitoring period by over three degrees. If a higher evaporator temperature and lower outdoor dew points were reducing the moisture being removed by the air conditioner, then we would expect to see lower moisture removal rates. In fact, we find just the opposite. … We see that the new air conditioner actually removed an average of 1.4 additional gallons of water each date after the unit was changed out. Given the lower outdoor dew point, this means that somehow an increased moisture load was being placed on the air conditioner. A likely explanation is that with the greater run time of the new air-conditioning system, return-side duct leakage and leakage from the air handler is placing additional load on the AC system. … Since previous FSEC research shows that the average air handler in Florida homes leaks 70 - 80 cfm during operation, … the air handler is drawing in additional outdoor air during the extended run times with the right-sized system. … With the properly sized system, the longer run times relate to greater volumes of attic air being drawn into the air handler, which results in the greater observed condensate removal. What is not as obvious is that the longer run times also are necessarily associated with greater heat gains from the duct system.”

    Parker told EDU, “It turns out that the longer the machine runs, the more duct losses you have. And those losses are large -- they tend to be in the range of 20 percent. The duct losses are bigger than cycling losses. So even though right-sizing saves energy by reducing cycling losses, it goes the other way on the duct losses.”

    Large Ducts Have Too Much Surface Area
    Oversized ducts may also have contributed to the poor performance of right-sized equipment. The researchers wrote, “The longer run times of the smaller air-conditioning systems compared to the oversized systems mean increased duct air leakage penalties, and since cold air is flowing through the ducts for longer periods, heat conduction through the ductwork is also increased. Also, since the ductwork size was not reduced when the properly sized systems were installed, the same duct surface area that was present for the oversized system now has cold air flowing through it for longer periods. The relatively large duct work may also explain why the properly sized AC systems all had higher airflow rates per ton of cooling than the original systems, which may in turn partially explain higher post-change-out RH levels.”

    Bad Ducts Trump Efficient Air Conditioners
    The FSEC report provides a strong argument in favor of installing duct systems inside a home’s conditioned space. “Most attic air handlers suck in air,” says Parker. “That’s why sealing ducts is hugely important.” Most attic duct systems are so leaky and poorly insulated that duct performance issues overshadow any improvement in air conditioner performance. The researchers wrote, “Downsized machines with the duct systems located in attics may see that increases in duct losses substantially exceed the savings in increased air-conditioning system part load performance.”

    The FSEC research raises an important question: if “right-sized” equipment uses more energy and performs worse than oversized equipment, is it really right-sized? “These findings basically indicate that there is a theoretical optimal size for air conditioners with duct systems that are not in the conditioned space, and that the optimal size is bigger than you would conclude from using Manual J,” Parker told EDU. “The problem with Manual J is that it doesn’t assume that the duct losses vary with the size of the air conditioner. To perform the sizing calculation, you must find a solution that keeps revising itself -- it’s a problem with an iterative solution.”

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Here's an important fact for HVAC contractors to remember: Manual J calculations are code-required; they are not optional. If an HVAC contractor isn't performing a Manual J calculation (or another load calculation using a method that is approved by code authorities), then the contractor is probably breaking the law.

    In the 2006 IRC, section M1401.3 requires that "Heating and cooling equipment shall be sized based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies."

  6. Allison A. Bailes III | | #6

    Thanks for the explanation, Martin. I keep learning cool, new things from you. Danny Parker and FSEC do great work, and they've shed some light on an important topic with this research. The real take-away from this study, in my opinion, is that you have to remember that ducts are an important part of the HVAC system.

    Of course a right-sized cooling system with ducts outside the envelope is going suck in more unconditioned air on the return side and lose more conditioned air from the supply side. In a leaky system with, say, 300 cubic feet per minute (cfm) of return leakage, if the AC runs 40 minutes per hour instead of 10, that’s 12,000 cubic feet of outside air pulled in versus 3000 cubic feet for the oversized system. Those extra 9000 cubic feet can bring in a lot of water vapor, too, so it’s easy to see that interior humidity could be higher with a right-sized AC. If the duct leakage is unbalanced (more supply than return leakage, for example), that causes more air leakage in the house as well and can lead to even higher latent loads.

    That’s why it’s critical to deal with the whole system and not just slap a new, right-sized AC on a crappy duct system. The first choice, if possible, should be to get the ducts inside the envelope, so duct leakage outside the envelope isn’t an issue. Barring that, the ducts should be designed and sealed properly. You mentioned that Manual J is in the code, and so is Manual D. Right-sizing and duct design go hand-in-hand.

    I was involved in a sizing snafu a few years ago where I specified the equipment sizes, and left the ductwork to the HVAC contractor. The homeowner, in his brand new house with spray foam in the walls and roofline, couldn’t get the house to the thermostat setpoint, and the three systems were running all day long. When I checked the air flow with a flow hood, it turned out that his ducts were undersized and couldn’t move enough air. The 7 tons of total AC capacity (it was a big house) were moving only 5 tons of air (~2000 cfm).

    Most contractors undersize their ducts, but they get away with it because of greatly oversized equipment. When the equipment is right-sized, the ducts have to be sized and installed properly, or you lose all the benefits of the right-sizing. That’s the real lesson of the FSEC study (and my experience with the house described above)—not that right-sizing isn’t important, but that it can’t be done in isolation.

    Allison A. Bailes III, PhD
    Energy Vanguard

  7. Ed Welch | | #7

    Great information, everyone. Thanks.

    Let me see if I understand. With newer equipment, 2 stage furnaces, etc....the energy penalty for oversized equipment on the heating side appears to be minimal. Sounds like the furnace manufacturers bailed out the contractors who were unwilling to perform an accurate Manual J.....and the new equipment accomodates severe weather fluctuations by using the necessary BTUs for the job. Manual J can be difficult to compute correctly....I've tried....and it can be tweaked to support larger or smaller units. I suppose this rational supports equipment being slightly oversized for those crazy storms. The equipment costs more, as you said Martin, but otherwise it does not appear to be a huge problem. Are they still selling/installing single stage furnaces? I'll have to check on that. Maybe that is the strongest point to emphasize in educating the public.

    On the cooling side, right sizing only seems to be more beneficial if the duct system was sized and sealed correctly. That makes sense. Often it seems like contractors sell high SEER equipment (more expensive!), then fail in sizing supply and return runs correctly, fail to seal ducts effectively, etc.....leaving the customer with an inferior system, but with an impressive sounding air handler.

    Ducts within conditioned space......this appears to be extremely important...although I have heard from Performance Contractors that if the ducts are sealed correctly, it becomes less important. Cathedralized attics without vents, using spay foam, I know, has been getting a lot of press in the green building world....does anyone know of any related problems with such assemblies? Someone expressed a concern about solar driven moisture causing problems in such assemblies, but I have yet to hear of specific problems.

  8. RichK | | #8

    (1)Is Manual J an accurate calculation tool? Is it already oversized?
    Yes I believe that it is fairly accurate if used correctly. Listening to experts like John Proctor speak on the subject they believe the Manual J will in fact oversize the equipment or give you some margin of error . There is no need to add a fudge factor or oversize the equipment on top of the Manual J calculations.

    (2) Why does it appear that HVAC contractors pay no attentions to Manual J?
    HVAC contractors don't have a comfort factor with the other trades and or poor training / lack of knowledge. For example if the insulation contractor doesn't install the insulation meticulously and all the thermal by passes haven't been sealed you'll need larger equipment to satisfy the load. The HVAC contractor doesn't know this in advance or have the tools to performance test so they error on the side of caution. The last thing a contractor wants is a call from their customer saying they can't keep the house warm or cold. Contractors also have the perspective that the difference in cost between a 60k btu/h boiler and a 80k is trivial to the total job. HVAC contractors don't have to pay your energy bills and from my experience they don't really care to much so they error on the side of caution. Your house is a good example of these unfortunate circumstances.

    (3) Does the typical HVAC contractor even use accurate heating load or cooling load calculations?
    Some good ones will but most from my experience don't use the sizing information correctly. Manual J calculations are typically done today using software. If you put garbage into the program you will get garbage back out. A thorough Manual J requires lots of inputs or details about the house and it's easy to make mistake. For example you could change the size of an air conditioner by 30% by imputing the wrong orientation. Sadly many contractors either don't do them at all or manipulate the data to match the size of equipment they want to use.

    (4) And, as someone recently told me, these guys (HVAC contractors) are not idiots....they install equipment that reduces the chance of getting callbacks.
    Some are idiots, most are not but none of them want callbacks.
    I understand the inefficiencies related to oversizing (short cycling, etc) but I also understand HVAC contractors not wanting to get callbacks from super sensitive clients who are not willing to wear a sweater or add a blanket when storms hit. Where is the middle ground?
    Over-sizing a modern modulating condensing boiler will not cost you much in efficiency due to its modulating design. The bigger issue is that the homeowner however will have payed more for that boiler. The market for typical HVAC contractors is a mess. Most only care about selling boxes and not getting a call back. Contractors being trained in the Home Performance markets across the country will be better versed at fixing the existing homes. New home builders and contractors could gain valuable knowledge by obtaining this knowledge also.

    Looking at why house don't perform correctly offers valuable insight into to how to better buildings in the future. Holding contractors to higher standards and performance testing their work is the only way to better buildings.

  9. Ed Welch | | #9

    Thanks Rick for your input. It does sound like manufacturers have bailed out the industry with 2 stage furnaces and modulating boilers....eliminating the need for extremely accurate Manual J computations, which can be difficult. Unfortunately, my boiler does not appear to be modulating...fires everything at once.....I will probably replace it as a result.

  10. Jonathan Smith | | #10

    I really can't help weighing in on this. I am a mid sized HVAC contractor in the north suburban Chicago area. I have devoted my career to energy savings, safety and comfort. Every day I see Grossly oversized poorly installed equiptment. With grossly undersized ductwork.
    When sized correctly using Manuel J and Manual D the systems will run with comfort and economy. Manual J is oversized enough that you need no more fudge factor.
    My competition is just lazy and STUPID (sorry you should see what these hacks are installing especialy huge residential contractors and big box retailers)
    About 5-7% of contactors actualy run a a load 2-3% know anything about duct sizing.

    It is all about education. Most trade schools will touch on duct sizing but I'm not sure the student even knows the basics when graduating. The manufacturers do a poor job of educating the contractors. they are happy to sell larger boxes. I have chosen to educate myself and staff with National comfort institutes procedures. In many cases we have cut the unit sizing in 1/2 and made the customer more comfortable and cut the utilitys by 60-70%. That is being green
    Can I rant a little longer? how about installing a 95% efficient furnace with undersized ductwork. We have seen some brand new oversized systems running at 30% This is what is going on in our industry. My advise for any one make sure the contractor does a manuel J and that your ductwork is properly sized, sealed and insulated. The box doesn't matter It is the design and the instalation that make the difference.
    #4 Yes they are idiots.

  11. AC KIDS | | #11

    Over sizing AC units makes contractors more money, second depending on the tightness of the home is where u get your ac size from.

  12. 5C8rvfuWev | | #12

    @ Jonathan and all

    "My advise for any one make sure the contractor does a manuel J and that your ductwork is properly sized, sealed and insulated."

    And how would I go about that -- as a consumer?

    Thanks in advance.


  13. User avatar
    Michael Chandler | | #13

    Joe Wilson

    How to go about getting accurate manual J calculations is simple. Hire an independent engineer to prepare one prior to asking the HVAC installer for a quote and provide it with the plans when asking for his price. I think I Pay about $350 to have a manual J done on the complex homes with varying 336 and 179 glazing and external shade structures and better than code infiltration and R-values.
    It does dilute the HVAC installers guarantee somewhat as he can duck liability by saying he built it in accordance to the specs but generally when there is an issue it's because the system wasn't ducted and commissioned in a way that assures air delivery in compliance with the MAN J or there is unacceptable duct leakage leading to pressure imbalances. Typically lower flow at the return ducts than the accumulative supply ducts indicating return duct air leakage combined wit excessive flow at bathrooms with exhaust fans with leaky dampers and inadequate flow to bedrooms and remote locations as well as imbalanced flow at multiple linked return air grilles.

    Provide a scope of work that includes an accurate definition of what you want done, what level of commissioning you expect and check the work with a third party flow hood test after commissioning.

    I don't specify duct sizing, only flow per room and return air grille and filter locations. The rest we work out on site after the framing (though we do design the framing around the duct runs, easy for me to do since we design in-house.)

  14. 5C8rvfuWev | | #14

    "I don't specify duct sizing, only flow per room and return air grille and filter locations."

    This makes perfect sense, Michael, and is probably simple enough to at least give a reasonable target. Thanks.

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