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Oversized AC, Screwed-up Manual J, ENERGY STAR HVAC Tirade!

Why Is An Oversized Air Conditioner Bad? Read On.

hvac oversized air conditioning system massive condensing unitI was checking some REM/Rate files for our HERS raters yesterday, mostly submitted for ENERGY STAR homes. I’ve come to expect Manual J heating and cooling load calculations submitted along with the files to be less than perfect. Mostly, I accept them because they’re close enough.

I was checking some REM/Rate files for our HERS raters yesterday, mostly submitted for ENERGY STAR homes. I’ve come to expect Manual J heating and cooling load calculations submitted along with the files to be less than perfect. Mostly, I accept them because they’re close enough.

Yesterday, though, I received a Manual J to go along with a file for an ENERGY STAR home that was beyond the pale. It was egregiously horrific. It was spectacularly sordid. It did come close to meeting the ENERGY STAR Version 2 requirements for Manual J (tight or semi-tight infiltration and correct design temperatures), but whoever put this one together was singularly devious in his efforts to justify the oversized air conditioning systems he wanted to install.

Yeah, he did the usual things to fabricate extra cooling load, but when that wasn’t enough, he resorted to one trick that’s not used nearly as often as it might be. Keep reading, my friend, and I’ll let you in on his secret.

One of the first things I do when checking to see if a cooling system might be oversized is to look at the ratio of conditioned floor area (in square feet) to the cooling capacity (in tons). ENERGY STAR and other high performance homes usually come in at about 1000 square feet per ton or more. The house I built was about 2000 square feet per ton.

A lot of HVAC contractors, though, don’t do Manual J sizing calculations but instead rely on rules of thumb. Mostly they use 500 to 600 square feet per ton. This house came in at 368 square feet per ton! That’s ridiculous, especially for a house in Charlotte, NC.

When I went into the reports, here are the problems I found that are typical of bad Manual J’s:

  • They put 6 people in the calculation when this house should have had 4. (It should be the number of bedrooms plus one.)
  • The HERS rater calculated that the house had 184 square feet of window area; the Manual J had 383 sf.
  • The HERS rater used a window U-value of 0.32; the Manual J had 0.53. (Lower is better.)

Those three items alone inflated the cooling load sigificantly. Not enough for this contractor, though. Evidently he really wanted to install a 2.5 ton air conditioner for the upstairs zone, yet after all those shenanigans, the Manual J result was only 1.5 tons. So, what did he do to get that extra ton to show up in the Manual J? He could have gone in changed wall insulation or duct leakage or any number of other parameters, but there was an easier way.

Manual J calculates the sensible and latent loads separately and adds them together for the total load in Btu/hour. The sensible load is how much cooling you need to do to bring the temperature down, and the latent load is how much cooling you have to do to bring the humidity down. If you take the sensible load and divide it by the total load (stick with me here – we’re almost there), you get what’s called the Sensible Heat Ratio, or SHR.

The Manual J report often submitted shows the total load (sensible plus latent), but it also shows what they call the required total capacity of the equipment at a particular SHR. Whoever does the Manual J can override the default SHR of 0.75, and that changes the required capacity. Most air conditioning equipment comes with an SHR in the 0.7 to 0.75 range.

The crafty calculator who completed this Manual J figured out that by adusting the SHR, he could get the required capacity to equal what he wanted to install. In this case, he needed 0.53 SHR to get his 2.5 tons. Can you even get an air conditioner with 0.53 SHR?

Come on, HVAC guys! Do it right! If you can’t do this for ENERGY STAR Version 2, you don’t have a chance with ENERGY STAR Version 3, which is much harder.

This Post Has 19 Comments

  1. Allison.
    Allison. You’re way to patient with your HVAC contractors! I got sick & tired of it. Now, I create a Manual J Block load just to do my HERS model. The Manual J load I get from my contractor better be close.  
    I no longer accept paper reports from them. I want the file. I will go through their report, fix the file, then decide if what they want to install is within 15% or the next size up based on Manual S procedures. 
    I get a lot of excuses for why it’s not right. Thankfully, I’ve never had a contractor outright try to oversize the system like this person did with you. Most of the time, they don’t want to put the effort into doing it right the first time and then make appropriate changes as the project changes. I had one HVAC contractor tell me yesterday that he’ll draw the building and install the windows and basically ignore everything else! 
    For my builders and HVAC contractors I offer welcome hope. When I model their buildings in parallel, a better discussion happens between the builder and HVAC contractor! Everyone wins!

  2. I am really glad to see some
    I am really glad to see some details on how an AC installer deceives using the Manual J. I hate to say it but many of these installers are just ignorant. Some may perceive it as insurance against design and craftsmanship deficiencies — the customer has difficulty claiming a problem when it’s not “failure to cool enough”. 
    But maybe it’s time to change the definition of the problem. Can we not argue that oversizing is not as harmful as it used to be, and there exist ways to mitigate the problems? There will *always* be customers who want oversizing even if they are not aware of it, there will always be some who insist 68F is the comfortable sleeping temperature, there will always be a few who really do want that July 4 party with the doors open and it’s still cool inside. Maybe it’s time to focus on not butting heads so much with ignorant installers, and idiosyncratic (but legitimate) customers. 
    A professional study by utility company *tried* to find the fabled energy waste from oversizing. Their proposition was they could make “negawatts” by downsizing (and get regulated rate of return on doing so). The actual savings was NEGATIVE. There were complicating factors to be sure, but it’s hard to claim that energy waste is a good enough reason not to oversize. 
    Many smart building science people are pointing to INFILTRATION as the more realistic source of bad humidity in homes. The right-sized AC cannot solve that problem, to point to oversizing as a cause of poor humidity control, is only part of the problem. 
    Duct losses to unconditioned space are always problematic, and in some humid regions I think 95% of houses put ducts there. If you downsize airflow and do not replace the ductwork, the laws of fluid flow guarantee the percent duct leakage will become worse. That thing about flow thru an orifice vs. pressure. I think that may be the leading suspect of unexpected results in that utility study. 
    I’m being devil’s advocate of course. Ducts within conditioned space are a big help. Two stage AC (despite the above paragraph) is a big help. Minisplits. Even window units. 
    We’d all love to have a finely designed house, built with all the advantages from the ground up. However only a few of us can live in one like that. I am grateful for all the attention you can offer, for solutions that mitigate the problems on the ground.

  3. Sam: This
    Sam: This is about a Manual J I received from one of our independent raters, so it’s not an HVAC contractor we work with. Yes, it’s definitely better to have someone who knows what they’re doing complete the MJ, and third party is usually the best way to go. Sounds like you’re doing the right thing.

  4. M. Johnson
    M. Johnson: You’ve packed a lot into your comment, but let me see what I can do to address the issues briefly.  
    First, it’s been a while since the building science community argues that right-sizing saved energy. The actual energy savings are around 2% from what I’ve read. Haven’t heard about that utility study, but they might have an agenda, too. The main reasons to size ACs properly is for comfort, moisture control, and equipment life.  
    Also, in this case it’s required if they want to qualify the home for the ENERGY STAR label. Those customers who insist on oversized systems for all the reasons you state will have to buy houses that don’t have the ENERGY STAR label. 
    Yes, infiltration is the biggest source of latent load in a typical home. ENERGY STAR homes have to be tighter, though, and their latent loads are usually small. I don’t know of anyone who says that right-sized air conditioners alone will solve the problems. The house is a system, as we like to say. 
    And so is the HVAC a system that includes the ducts. You can’t look at any individual component in isolation.

  5. “egregiously horrific
    “egregiously horrific”, “spectacularly sordid”, “crafty calculator”…Love it! I think these terms need to be incorporated into the Energy Star official glossary 🙂

  6. John: And
    John: And let’s not overlook ‘singularly devious.’ Actually, I think I was channeling Ed Voytovich, another PhD building science guy, but one who was an English major and has much more skill in word crafting than I.

  7. Allison – great article, but
    Allison – great article, but you probably missed the reason he picked that SHR – It was the terrible, damp, crawlspace. 😎 
    Sadly many people have not truly enjoyed the benefits of undersized air conditioning. I am in Rochester, NY and we now have 3000 sf of very well insulated home. We use two window air conditioners – total 19,000 Btu’s. For those doing the math it’s 3000 sf divided by 1.58 tons – just under 1900 sf per ton. 
    People are always surprised at how comfortable our house is, even at it’s warmest – 73 to 75 degrees in July. I am quick to point out that the RH is usually 35% to 40%. 
    As I type this I’m in an office complex with the clammy AC running – sad. 
    All I can say to oversize AC is I have been pushing two stage AC for a long time. I wonder how many of these things actually hit the second stage due to demand, and not just the timer on the thermostat. 
    Sounds like a question for John Proctor! 

  8. Sorry for the triple spacing
    Sorry for the triple spacing – don’t know where that came from. 

  9. Rob: Yes,
    Rob: Yes, it’s too bad we don’t get more ‘undersized’ systems. It’s crazy that people want to size for those extreme conditions that occur 1% of the time. I like mini-splits and 2-stage equipment for managing loads better, too.

  10. Fabulous post Allison, it’s a
    Fabulous post Allison, it’s a bit like revealing your favorite camping spot though. Now everyone will need to watch out for the SHR trick! However, I thought these were called “Emmanuel J” calculations…

  11. Taking a step back, while I
    Taking a step back, while I applaud ACCA Manual J sizing there is one additional point I wanted to say. Manual J is intended to size for the hottest summer day, or at least the 97-99th percentile. It is intended to run the AC 100% of the time on that day, and the room temperature will either maintain, or creep up very slowly. 
    All milder days of the year, you definitely have a larger AC than required. So the problems associated with oversizing, deserve to be understood and mitigated even when ACCA methods are followed perfectly.

  12. Steve:
    Steve: That’s Emmanuel Jay! 
    M. Johnson: Yes, that’s absolutely true. Even if a system is perfectly sized, it’s supposed to based on the 1% design temperature, which means that 99% of the time, the AC will be oversized. The truth about Manual J, however, is that even if you do everything perfectly according to the protocol, you still get an oversized unit, so the amount of time you’re oversized is then more than 99%. I wrote about this last year regarding the oversized new AC system I installed in my condo 2 years ago. I did a by-the-book Manual J, and the AC still doesn’t run enough.

  13. M.Johnson said: “Many
    M.Johnson said: “Many smart building science people are pointing to INFILTRATION as the more realistic source of bad humidity in homes.” 
    Building scientists have long understood the sources of moisture, infiltration being just one. There’s been no shift in thinking on this, as your comment suggests. Internal moisture loads can have a significant impact on the latent load. A tight envelope doesn’t eliminate potential for moisture problems. To wit, I’m aware of very tight foam-insulated homes with severe moisture problems, due to oversized cooling systems. 
    Whether or not an oversized AC will create comfort or structural issues depends on many factors, but it comes down to moisture load vs. latent capacity of the system. Latent capacity depends on moisture level of return air and coil temperature. Coil temp depends on volume and temperature of the air crossing the coil, as well as the coil’s bypass factor and ADP (apparatus dew point). For example, I can increase the latent capacity of an oversized system by lowering the fan speed. However, this will kill efficiency. This happens by default in many homes due to undersized or restricted ducts. 
    And by the way, if a system (and airflow) is downsized in an existing home, this will NOT increase duct leakage percentage. Not sure what law of fluid flow you are referring to. There is little downside to oversized ducts. It’s true that a larger duct surface area (relative to airflow) increases the percentage of conducted loss through duct walls. But the absolute value of conducted losses will go down. Hardly an argument for not downsizing. 
    Aside from moisture and efficiency issues, a grossly oversized AC (or furnace) will cause larger temperature swings, especially as envelope becomes more efficient. 
    Right-sizing alone will not solve many of the performance problems related to HVAC. The more serious issue is that many HVAC contractors have no clue how to balance all of the above factors when selecting an appropriate system for a particular home (Manual S). Your suggestion that we redefine the problem and accept the status quo demonstrates a lack of understanding of the Manual S procedure, and the impact this has on every aspect of a system’s performance. 
    BTW, I wouldn’t put much weight on the results of a single utility HVAC study. One of my clients designs and implements HVAC mitigation programs for a number of large utilities. Having reviewed many such studies, I would like to point out that the sort of data you mentioned in your comment is difficult if not impossible to measure with reasonable accuracy, at least not using the methods utilities are willing to pay for.

  14. I know I’ve read parts of
    I know I’ve read parts of that HVAC study, but something bothers me about it. 
    Comparing it to programs for increasing A/C capacity by fixing airflow problems and charge errors that have been done in various states where the average increase in delivered BTU/h capacity was 38%, there seems to be something wrong with the study. 
    It seems to me that either every system the encountered had proper airflow and proper charge 
    the new correctly sized systems were installed just as poorly as the old oversized units. 
    Does any of that make sense? Am I missing something?

  15. Helder, these types of
    Helder, these types of studies raise many issues. For example, is the energy penalty cited by the study based on kW or kWh? Many utility HVAC programs are only concerned with kW demand. However, few homeowners pay a demand charge. In any case, the accuracy of kW estimates depends on the method used. In some programs, the kW impact is ‘deemed’, and in others, some analysis is used to calculate the impact. It is expensive to directly measure kW on individual homes and to my knowledge, this has never been done in a large study. 
    Also, utilities are usually only interested in aggregate impacts, not impacts to individual customers. The impact on aggregate kW resulting from right-sizing is mitigated by increased cycle time (downsized systems will run longer), thus reducing the natural load diversity that occurs with oversized systems. (The concept of load diversity is critical to understanding these types of programs.) All else being equal, downsizing en masse should lead to a decrease in aggregate kW *and* kWh, but rarely is “all else equal”. In particular, occupant behavior and weather are difficult if not impossible to normalize. 
    If you provide a link to the study (or send copy via email), I can be more specific.

  16. In the mountains, where we
    In the mountains, where we have 5X HDD more than CDD, one expects to purchase a system that is both comfortable and economical. However, if I read Version3 right the entire system size is based on cooling alone in my climate zone.  
    I posed this question at a version3 rater training and have never gotten a reply.

  17. George, not sure why your
    George, not sure why your instructor would not know this. Section 5.5 (and footnote 19) of the new HVAC Contractor Checklist (rev. date 2/7/11) says:  
    “Listed output heat capacity (for furnace) is 100 to 140% of Design Total Heat Loss or next nominal size” 
    Section 1.2.9 on the Rater Checklist says heat pumps must be sized from 95% to 115% of the design cooling load in climate zones 1 thru 3 and up to 125% in CZ 4 thru 8. In this case, the balance of the heat load would be picked up by supplemental heat.

  18. I’ve tried finding
    I’ve tried finding information about package units versus split systems. While the seer of my package unit is known it seems horribly inefficient to have the air handler outside in the Florida heat, not to mention the extra duct work required to get to it( in my case it sits on the opposite side of the garage from the AC space). I’ve considered switching to a new split system but without real info it’s tough to make the decision. Is there any information in energy star or manual j or???? That might help me?

  19. @Thomas, your question is
    @Thomas, your question is valid but Manual J, nor any other residential tool I’m aware of accurately models external heat gains or losses for rooftop unit. I can’t speak to Manual N (commercial) modeling tools.  
    I recently inspected a home in Tuscon that had 4 rooftop package heat pumps with exposed trunks. With 110F outside temp that day, a significant amount of the capacity was spent overcoming above-roof heat gains. Unfortunately, none of these systems could be converted to splits.  
    If you have a way to convert yours economically, I can help you estimate the gains based on some simple diagnostic tests. Contact me privately if you’re interested.

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