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Musings of an Energy Nerd

When Do I Need to Perform a Load Calculation?

Part 4 in a series of articles on sizing heating and cooling equipment

Image 1 of 3
Some houses need a big furnace, while other houses only need a tiny space heater. If you're not sure how big a heating system your home needs, you should definitely perform a load calculation.
Image Credit: Fine Homebuilding
Some houses need a big furnace, while other houses only need a tiny space heater. If you're not sure how big a heating system your home needs, you should definitely perform a load calculation.
Image Credit: Fine Homebuilding
Once you've completed your room-by-room heat loss calculations, you can size your ducts. Back in the 1970s, I used this cardboard duct calculator (a type of slide rule) from Lima. According to the calculator, a 6-inch round duct can supply 110 cfm (assuming 0.1 inch water column friction per 100 feet of duct).
Image Credit: Martin Holladay
In the 1970s, the Institute of Boiler and Radiator Manufacturers (I=B=R) distributed this paper-and-pencil form used to calculate residential heat loss.
Image Credit: Martin Holladay

In my last three blogs, I discussed the basics of heat-loss and cooling load calculations. The unfortunate truth about these calculations is that fast methods aren’t particularly accurate, and accurate methods require making measurements, checking specifications, and entering data into a computer program — in other words, a significant investment of time.

So how should builders go about making these calculations? There are several ways:

  • You can use a rule of thumb (along with experience) to estimate your equipment needs.
  • You can ask your HVAC contractor to make the calculations. (I don’t recommend this method.)
  • You can make the calculations yourself using a simple spreadsheet or an online calculator (like this one from the Build It Solar website).
  • You can buy Manual J software and learn to use it.
  • You can hire a consultant (usually an energy rater or an engineer) to perform the calculations for you.

Why do we need to perform these calculations?

There are at least two reasons why we need to perform load calculations: to size heating and cooling equipment (ideally, using ACCA Manual S), and to design heating and cooling distribution systems (using ACCA Manual D). Neither Manual S nor Manual D can be used unless Manual J calculations are performed first.

These are valid reasons, so a room-by-room Manual J load calculation makes a lot of sense. If you perform such a calculation, you may save money on your heating and cooling equipment (because it is less likely to be oversized), and there will be a lower chance that the homeowners will have comfort complaints arising from a poorly designed heat-distribution system.

In most areas of the country, a room-by-room Manual J load calculation is required by code. If you don’t have the software yourself, you’ll have to hire an energy rater or engineer to perform the calculations. Very few HVAC contractors are capable of performing an accurate load assessment, so I’d be wary of leaving this task to your furnace guy.

You don’t always have to perform a Manual J

As long as there is no code requirement in your jurisdiction for a Manual J calculation for the type of work you are contemplating, you may not need a Manual J calculation. To understand why, we need to examine two myths that have long been promulgated by energy experts. The first myth is that rules of thumb are inappropriate; the second is that oversizing of equipment is disastrous.

In fact, if you are an experienced builder who understands Manual J calculations, and if you have already built a few new homes, you probably already have a good rule-of-thumb understanding of how big a heating or a cooling system is needed in your climate. This rule-of-thumb method may be perfectly adequate for sizing heating and cooling systems for new homes.

There are a few caveats, however:

  • A rule of thumb is only useful if the homes fall into the same general category, with similar insulation levels, glazing specifications, and air leakage rates.
  • A rule of thumb won’t work for a house with unusual features (especially unusual glazing features).
  • My advice only applies to people who have actually performed an accurate Manual J calculation or who have hired a knowledgeable professional to prepare one for them. If you’re just guessing, you will almost certainly oversize your equipment.

Why am I comfortable with these seemingly risky recommendations?

  • Even skilled users of Manual J software usually end up with slightly oversized equipment.
  • Oversized equipment doesn’t really carry an energy penalty or a comfort penalty anymore. (Newer modulating or two-speed furnaces operate efficiently under part-load conditions, solving any possible problems from furnace oversizing; and oversized air conditioners aren’t really as terrible at dehumidification as many energy experts claim.)
  • As houses have become better insulated and tighter, heating and cooling distribution systems become less important, because room-to-room variations in temperature are much smaller than in older homes.

Most comfort problems in existing homes are due to a poor building envelope (not enough insulation, low-performance windows, and a high rate of air leakage), stupid design details (like big unshaded west windows), and leaky ductwork. Once you control these factors, it’s much easier to avoid comfort problems.

In an old, leaky house, it’s not unusual for occupants to complain that one room is chronically cold or hot. A variety of factors are usually responsible, but it’s a good bet that the walls and ceiling are leaking air, the windows have terrible glazing specifications, and the leaky ductwork is located in an unconditioned attic. The problem is almost never due to undersized heating and cooling equipment.

Passivhaus designers are now building homes with a single heat source (for example, a gas space heater with through-the-wall venting) in a central location. As these homes make clear, as building shells become tighter and better insulated, distribution system design is becoming less relevant.

Why is Manual J so complicated?

The better the envelope, the easier it is to perform load calculations. Moreover, if your envelope is good enough, errors in load calculations don’t matter as much as they do when you have a leaky envelope.

It could be argued that current code requirements for Manual J calculations are only necessary because so many new homes have lousy thermal envelopes. If code enforcement officials were willing to insist on high-performance envelopes, full-scale Manual J calculations wouldn’t be necessary.

It’s easy to imagine the development of a simplified version of Manual J for homes that exceed certain minimum airtightness, insulation, and window-performance specifications. After all, if we know that our envelope is well built, we should need fewer inputs when performing our load calculations.

The wild card is occupant behavior

Energy nerds can be fetishistic about their load calculations. The everyday variety of this species is the Manual J Fetishist — usually an engineer who warns homeowners that they will be uncomfortable and will face high energy bills unless they invest in more engineering.

The more exotic variety of this species is the PHPP Fetishist — usually a young architect who did a year of post-graduate study in Germany. This fetishist spends days at his or her computer, trying to reduce the U-factor of a troublesome thermal bridge in hopes of achieving the magical goal of 15 kWh per square meter per year.

Both types of fetishists are easily defeated by the Common American Homeowner, a casual oaf who buys several big TVs at Best Buy, installs an extra refrigerator, leaves the bedroom window open, and never turns out the lights.

“Be aggressive”

Whoever performs your Manual J calculations, remember this motto: “Be aggressive.” This advice comes directly from ACCA Manual J, version 8: “Manual J calculations should be aggressive, which means that the designer should take full advantage of legitimate opportunities to minimize the size of estimated loads. In this regard, the practice of manipulating the outdoor design temperature, not taking full credit for efficient construction features, ignoring internal and external window shading devices and then applying an arbitrary ‘safety factor’ is indefensible.”

Last week’s blog: “Calculating Cooling Loads.”


  1. User avater
    Carl Seville | | #1

    On fetishists and homeowners
    Nice series of articles, Martin. Sounds like you need to create a simple app to size HVAC systems in high performance homes. Why not do it on our smart phones, they're more powerful than the computers on the space shuttle anyway.
    I agree on your take on the fetishists who insist on taking everything to extremes instead of just making do with +/-99% accuracy.
    Finally, on your comment about homeowners messing things up, that is certainly the truth and reminds me of something I heard John Straube say: "You can't idiot proof anything, the idiots are too smart."

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

    Response to Carl Seville
    According to Michael Blasnik, Bruce Manclark (out in Oregon) is developing a simplified version of Manual J. As far as I know, however, it hasn't been released yet.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    I think we all are


    I think we all are GBA Fetishists

    (Had to teach my spell checker a new word today Martin)

  4. User avater
    Armando Cobo | | #4

    I wish, I wish...
    Great series...however, I wish you don't stop here with the HVAC systems... Design loads are only one leg of a three legged stool. We know that System, Duct and Terminal design is a second leg, and Commission and Balance should be the third. The last too are hardly ever done in residential construction. Anyone of those legs missing or done wrong, and you’ll have an inefficient and underperforming HVAC system, not to mention waste of money.

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

    Response to Armando Cobo
    I agree -- my blogs on load calculations do not represent everything there is to say on HVAC design. However, I have already covered some of the topics you mention in previous articles. See, for example:

    Keeping Ducts Indoors

    Sealing Ducts

    Duct Leakage Testing

    Return-Air Problems

    The importance of commissioning has been discussed several times -- for example, in Simplicity Versus Complexity, where I wrote:

    "Builders often underestimate the importance of commissioning all HVAC equipment after installation. (“Commissioning” simply means making final adjustments and tuning up equipment to verify that it functions properly.)

    "Unfortunately, most new homes are imperfectly commissioned, leading to one or more of the following errors:
    ● The air conditioner has the wrong refrigerant charge.
    ● The airflow over the cooling coils has not been verified.
    ● Duct systems haven’t been checked for leaks.
    ● The airflow through forced-air registers hasn’t been adjusted to meet specifications.
    ● Pressure imbalances between bedrooms with closed doors and adjacent hallways have not been remedied.
    ● Exhaust fan airflow hasn’t been tested.
    ● Heat-recovery ventilators have not been balanced.
    ● Atmospherically vented appliances haven’t been checked for backdrafting during exhaust fan operation.
    ● The temperature set-points on the solar hot water system controller are improperly set.

    "HVAC commissioning errors almost always result in needless increases in energy costs. Many HVAC specialists can share horror stories about commissioning errors, running the gamut from irritating to outrageous (for example, air-source heat pumps with electric resistance elements that operate for most of the winter)."

    Also here:

    "A mechanical system also requires careful commissioning to ensure optimal operation. During commissioning, mechanical equipment is checked for proper operation, distribution systems are balanced, and controls are all checked. All too often, careless mistakes in installation and setup result in poor system performance. Without a commissioning process, these mistakes might not be caught for years. Commissioning is important in both commercial buildings and homes. Clear, thorough documentation is critical to ensure proper ongoing maintenance and operation of systems of any size."

    And here:

    "Every HVAC system needs to be commissioned. Commissioning includes measuring the airflow at the coil, and the airflow at each register in every room, to verify that the delivered cfm matches the design cfm."

  6. Robert Bean | | #6

    Rules of Thumbs and Software
    Just some comments from the northern peanut gallery (BTW...forecast for snow tonight)

    I get the use of rules of thumbs and for general estimating purposes it’s a nice way to put a preliminary box around project scope and magnitude; I even get it for cookie cutter projects where the “rule” worked…but a caveat: having been involved in arbitration's where residential design reviews were required - the last thing the designer wants me to hear coming out of her/his mouth in a room with all the representative parties, is that his/her design was based on rules of thumb - especially in Alberta and parts of British Columbia where mandatory professional engineers or certified designers are required for some residential HVAC systems.

    Same goes for the use of software…designers frequently shoot themselves in the foot if they say they relied on the output of a program – that doesn’t fly any further than rules of thumbs. Mark Bomberg, Editor for Journal of Building Physics and past NRC/IRC researcher did an engineering lecture called, “what you don’t know and can make your work useless” would beat it into his students that …“you don’t touch the software unless you know by hand calculations the answers to the problem.”

    I believe that to be sage advice for those who dislike arbitration hearings and lawyers.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Robert Bean
    Let me get this straight:

    1. You advise designers of HVAC systems not to use rules of thumb.

    2. You advise designers of HVAC systems not to use computer software programs like HVAC-Calc, Elite RHVAC, Wrightsoft Right-J8, Adtek AccuLoads, or EnergyGauge.

    3. Only calculations performed "by hand" can be trusted.

    I have no idea why the arbitrators and lawyers of Alberta insist on pencil-and-paper calculations. But if your observation is correct, that's nuts.

    We've come a long way since 1975. The software now beats the pencil, Robert.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Robert Bean
    The high bar you set -- only engineers who thoroughly understand and are able to defend the mathematical calculations made by their Manual J software should be allowed to design residential heating and cooling systems -- will certainly guarantee work for a limited number of mechanical engineers. The group of engineers who can leap above your high bar is surprisingly small, as many energy efficiency experts will attest.

    Assuring work for a small circle of engineers will not get us any closer to achieving a basket of desirable goals, including better HVAC systems and lower energy bills.

    We need to train designers in basic principles, and we need to develop useful, easy-to-use software tools to help them.

    Engineers enjoy mystification and complicated algorithms. Your premise -- that only mathematicians and software geeks should be allowed to specify furnaces, boilers, and air conditioners -- serves no one but a limited number of expensive professionals.

  9. Robert Bean | | #9

    Response to Martin Holladay
    1.Yes (unless it can be defended)
    re: insist,(They don’t)
    re: software beats pencil,( I agree).

    Rules of thumb and software are tools and there is never a problem with their use - until there’s a problem. If designers are going to use them be prepared to defend the results mathematically. Conversely if one is not prepared to defend the results mathematically don’t use the tools and hire someone who can.

  10. Robert Bean | | #10

    Response to Martin Holladay
    Martin, it’s not my high bar - the Government of Alberta put it in place.

    In the systems I get involved in you have to be a licensed professional or a certified designer through the Canadian Hydronics Council (CHC), or the Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI).

    I can’t speak for CHC curriculum but the HRAI curriculum does not permit the use of rules of thumb and students are required to complete all exams without software.

  11. oldbelt | | #11

    Why run a heat loss for a temp below equipments operating range?
    I am having a conceptual challenge with sizing mini-splits for homes here in Portland Maine. If a mini-split is specified to work at temps of 5 deg or above and we have a back-up source for colder days (nights more likely) shouldn't we run a heat loss for the equipments lowest operating temp?

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Sam Zuckerman
    If you only expect your ductless minisplit to work when the outdoor temperature is 5°F or higher, then your would perform your heat loss calculation with a delta-T of 65 F° (70°F minus 5°F).

  13. User avater
    Morgan Audetat | | #13

    "Why am I comfortable with
    "Why am I comfortable with these seemingly risky recommendations?"

    I still haven't a clue.

    The whole point of a Manual 'J' load is to properly size equipment for design conditions. Naturally one would have to oversize the equipment, since being a little short is not generally acceptable. How much you are over-size HVAC equipment is what separates the educated from the experienced. If you happen to have both, you are my kind of designer.

    It most certainly matters more than before, since modulating burners are for the most part, not intended to replace proper sizing, rather make a properly sized gas-fired appliance more efficient in less-than-design conditions. Every modulating furnace or boiler has a minimum fire rate, so that loads presented below this minimum will cause the appliance to short-cycle (death to efficiency, no matter what the AFUE). There is at this time, only one boiler, with a range and fire rate, that would serve most residential applications, given radiation was sized properly using, you guessed it, a room-by-room Manual 'J' heat load.

    Finally, my experience designing HVAC systems (using Manual 'J' based software) all over N. America, runs contrary to your opinion that tighter and better insulated homes somehow make " heating and cooling distribution systems become less important". I have never had a client relay this sentiment to me. Our experience does reflect your cavalier (frankly insulting) statements in that the average HVAC installer thinks that close enough is good enough. As the loads get lower with better building practice, the equipment should get smaller. Unfortunately, we regularly see equipment (boilers in particular) at twice the output of the load and a few with "minimum output" exceeding design conditions of the home!

    As we are often called in on HVAC systems that do not or have never worked properly we take Manual 'J' very seriously and find that we can do a better job correcting other designers mistakes but that they were in fact mistakes in sizing equipment.

    With the lack of HVAC design - of any kind - in the residential building market, I think it is irresponsible to suggest that it shouldn't or needn't be done. I attach a sample of a proper heat load. If you can do this in your head, well...

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Morgan Audetat
    I stand by the statements made in this article.

    If you have read all five of my articles on load calculations, and you consider them carefully, you will find that I am a strong proponent of load calculations. I have been performing such calculations since 1976.

    I have also criticized the majority of residential HVAC contractors who use outdated rules of thumb and who fail to properly size equipment.

    You wrote, "Our experience does reflect your cavalier (frankly insulting) statements in that the average HVAC installer thinks that close enough is good enough." Your statement is confusing. Perhaps you meant the opposite of what your wrote -- namely that your experience does NOT reflect my statements?

    Whatever you intended to write, my conclusions about residential HVAC contractors have been confirmed by every academic and technical research study into the issue. The vast majority of residential HVAC contractors never perform a Manual J calculation, and a customer who asks an HVAC contractor to perform one is (in many areas of the country) often met by a blank stare.

    This was not intended to be "frankly insulating" -- merely a fact.

    Concerning the question of whether point-source heating works in a compact house with a tight, well-insulated envelope: believe it or not, it does. I have been living in such a house since 1981. If you read about Carter Scott's homes in Massachusetts, you may become a believer: Just Two Minisplits Heat and Cool the Whole House.

  15. User avater
    Morgan Audetat | | #15

    "The first myth is that rules
    "The first myth is that rules of thumb are inappropriate; the second is that oversizing of equipment is disastrous."

    Perhaps it is you whom meant the opposite of what you stated. I have read your earlier installments (with much interest, thank you) and obviously agree with the vast majority of your various assertions. But I would argue that these two "myths" are in fact, not.

    If few "furnace guys" and Architects with a "German" education can't handle, then to whom shall we turn?

    The fact is; over-sizing condensing boilers is most certainly a disaster most often visited by the injudicious use of misleading "Rules of Thumb".

    "The vast majority of residential HVAC contractors never perform a Manual J calculation, and a customer who asks an HVAC contractor to perform one is (in many areas of the country) often met by a blank stare."

    In this regard, we are in complete agreement.

    I too live in a compact house and use point source cooling (mini-split) but prefer my radiant floors when the cold sets in.

    I do appreciate your work and the opportunity to disagree...respectfully.

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Morgan Audetat
    You wrote, " I have read your earlier installments (with much interest, thank you) and obviously agree with the vast majority of your various assertions." I'm glad that further reading has brought us closer to agreement.

    I think we both agree that right sizing is far preferable to oversizing. I hope to educate designers and HVAC contractors so that they know when to use a Manual J, and when to use an appropriate rule of thumb. (An appropriate rule of thumb is not the same as the ridiculous rules of thumb currently used by HVAC contractors -- the ones that result in gross oversizing of equipment.)

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