Who Can Perform My Load Calculations?

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Who Can Perform My Load Calculations?

Most HVAC contractors don’t know how to do heating load and cooling load calculations — so what’s a homeowner to do?

Posted on Mar 24 2017 by Martin Holladay

To design a residential heating or cooling system, the first step is to perform a load calculation. (A load calculation determines the size of a building’s heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. on one of the coldest nights of the year and the size of a building’s cooling load on one of the hottest afternoons of the year.) It’s important to know the size of these loads to determine the size of the required heating and cooling equipment.

The best way to determine residential heating and cooling loads is with Manual J, a load calculation method developed by the Air Conditioning Contractors of America (ACCA). Manual J calculations are made with a Manual J software program — for example, Adtek AccuLoads, Cool Calc, Elite RHVAC, EnergyGauge, or Wrightsoft RightSuite Universal.

Manual J software doesn’t just provide the heating and cooling load for the entire house; it provides the heating and cooling load of each room in the house. Room-by-room calculations are required, because without them, it’s impossible to design a home’s heating and cooling distribution system (ducts in the case of cooling, and either hydronic tubing or ducts in the case of heating).

Accurate load calculations aren’t just a good idea; they are required by most building codes. In the 2012 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code., the requirement can be found in Section M1401.3: “Heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies.”

Even though contractors are supposed to show a local code official proof that a load calculation was performed, enforcement of this requirement is spotty. When Allison Bailes was in Aspen, Colorado, a local official mentioned a conversation he had had with other Colorado code officials about Manual J paperwork. They said, “Yeah, we collect it, but we don’t know how to assess it.”

A recurring question

Here at GBAGreenBuildingAdvisor.com, owner-builders often post questions about heating system design or cooling system design. When this happens, another GBA reader often asks, “Have you performed a Manual J calculation yet?” Typically, the homeowner then posts a few more questions on this topic — maybe including, “What’s Manual J?”

Once everyone knows what a Manual J calculation is, and why it’s essential, the homeowner eventually asks another question: “Who should perform the Manual J calculation?”

That’s not an easy question to answer.

Should you trust your HVAC contractor?

Most load calculations are made by HVAC contractors who use inaccurate rules of thumb instead of Manual J. If you doubt this statement — if you’re inclined to trust the calculations made by your HVAC contractor — you might need a reality check.

Start by reading these GBA articles:

The bottom line is that any load calculation supplied by an HVAC contractor should be looked at with suspicion. If your HVAC contractor tells you that he or she has performed a Manual J calculation, ask to see the print-out. In many cases, the contractor can’t (or won’t) show it to you.

“There are a few HVAC contractors who can perform a Manual J,” Allison Bailes told me. “But the majority don’t. They use rules of thumb. I’ve even run into HVAC contractors who don’t know what Manual J is. Contractors are often engaged in a race to the bottom. There just isn’t any money in their budget for proper design.”

If a contractor shows a print-out showing the numbers used for a load calculation, examine the document closely. There may be a few items on the document that you can check — for example, the area of the house, the area of the windows, the U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. and SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. of the windows, the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the insulation, or the delta-TDifference in temperature across a divider; often used to refer to the difference between indoor and outdoor temperatures.. If you can, verify that the contractor’s inputs were accurate. A common trick used by contractors is to fudge the inputs to get the result that the contractor wants. (In almost all cases, the contractor wants to sell you a heating or cooling appliance with more capacity than you need.)

The DIY approach

If you decide (like most GBA readers) that you can’t trust the load calculations made by an HVAC contractor, what do you do?

One approach is to perform your own load calculations. If you want to give it a try, read these articles:

  • How to Perform a Heat-Loss Calculation — Part 1
  • How to Perform a Heat-Loss Calculation — Part 2
  • Calculating Cooling Loads
    • As these articles note, heating loads can be calculated with a pencil-and-paper method, but it’s much harder to use this approach to calculate cooling loads. When it comes to calculating cooling loads, it's best to use software.

      If you don't want to use software to calculate your cooling load, and you intend to use a rule of thumb, at least use a rule of thumb that makes sense. Allison Bailes proposes that a good rule of thumb is one ton per 1,000 square feet of conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. . If you want to know the reasoning behind his proposal, read An AC Sizing Benchmark for High-Performance Homes.

      Online calculators

      Another option for those who don’t want to pay for Manual J software, which is admittedly pricey, is to use an online calculator. The trouble with online calculators posted by well-meaning amateur enthusiasts is that it’s hard to evaluate the accuracy of the tools. (Be suspicious of any online cooling load calculator that doesn’t ask you to input the SHGC of your windows.)

      For what they are worth, here are links to two online tools:

      What if I want to hire an experienced professional?

      If you are building a new custom home that costs hundreds of thousands of dollars, it probably makes sense to hire an experienced professional to perform your Manual J calculations.

      When homeowners ask me who they should hire, my stock answer is, “You should contact a HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. rater, a mechanical engineer, or an energy consultant to perform your Manual J calculation.”

      This advice is OK, as far as it goes. But it’s imperfect advice. Let’s take a closer look at each of these three categories.

      Hiring a HERS rater to perform a Manual J

      Certified HERS raters have been trained in the rudiments of building science principles. They’ve also been trained to perform energy audits and to use REM/rate software. Those facts alone mean that HERS raters are much more likely to perform an accurate Manual J calculation than the average HVAC contractor.

      That said, the REM/rate software used by HERS raters only calculates the heating and cooling load of the entire building; it doesn’t perform room-by-room load calculations. So any HERS rater who wants to perform a Manual J calculation has to first buy the necessary software.

      Some, but not all, HERS raters have purchased Manual J software. So if you call up a few HERS raters to ask about Manual J calculations, don’t be surprised if the answer is, “We don’t offer that service.”

      Mechanical engineers

      Most single-family homes are designed without the help of a mechanical engineer. A corollary of this fact: few mechanical engineers have much experience with single-family homes.

      That said, most mechanical engineers are highly educated and capable of very accurate work. If you can find a mechanical engineer who is willing to perform your Manual J calculation, you’re likely to get a high-quality report.

      Curt M. Freedman, a senior mechanical engineer in Fall River, Massachusetts, told me that he would charge between $300 and $500 to perform a Manual J calculation for a 2,500-square-foot single-family home.

      Most mechanical engineers, including Freedman and several others I spoke with (Kristof Irwin of Positive Energy in Austin, Texas; Marc Rosenbaum of South Mountain Company in Climark, Massachusetts; and David Butler of Optimal Building in Sierra Vista, Arizona), prefer to provide HVAC system design services rather than just Manual J calculations. These services usually start at $3,000 for a single-family home, and can easily cost twice as much.

      When I recently called Positive Energy, the phone was answered by Miguel Walker, the company's head of business development. Walker described the services that Kristof Irwin provides. “Our typical client is an architect,” Walker told me. “Our process starts with helping the client to develop good enclosure details. We focus on doing a very thorough HVAC design, so we typically don't offer Manual J calculations.” According to the company's web site, the fee for these services “often comes in at less than 1% of total construction costs.”

      When Positive Energy gets calls from homeowners or builders looking for a Manual J calculation, they often refer these people to Fresh Air HVAC Sizing of Austin, Texas (512-814-7638). According to Fresh Air's web site, the company charges $299 for a Manual J calculation for a house that measures between 2,000 and 3,000 square feet.

      What about Marc Rosenbaum or David Butler?

      Another experienced engineer I spoke with, Marc Rosenbaum, has written many articles for GBA. Like Miguel Walker, Rosenbaum told me that he doesn't perform Manual J calculations for clients looking for that service.

      David Butler, the founder of Optimal Building, has specialized in heating and cooling system design for energy-efficient single-family homes for many years. While I wouldn't hesitate to recommend David Butler's services, he has posted a notice on his web site: “I'm no longer accepting new clients at this time.” In short, Butler is too busy to take on new work.

      When I contacted Butler recently with questions pertaining to Manual J calculations, he responded by email. “More and more homeowners are learning about the importance of proper design, and in particular the value of hiring a third-party designer. Even the best mechanical contractors often specify equipment that isn't necessarily the most appropriate for the home and location. The problem is that the residential HVAC market doesn't reward good design practice. I'm sure you know that. It takes several hours or more to do a proper load calculation and mechanical design, and mechanical contractors are expected to submit their proposals without compensation. So it's not surprising they don't spend the time and effort required to build an accurate Manual J model or run cost-benefit analyses on different equipment options.”

      Butler continued, “I now find myself frequently being asked by homeowners to refer them to someone who can do a load calculation and mechanical design. I don't see how the two can be separated. If someone just needs a ‘J’ to satisfy the building inspector, I politely tell them to look elsewhere.

      “I haven't developed a referral network beyond the handful of folks I've either trained or worked with, and most of those also have more business than they can handle. There's clearly demand, but I'm afraid most folks aren't willing to pay what it costs. One-off projects take a lot of time to come up to speed on all the particulars: envelope specs, builder and subcontractor capabilities, local energy rates, incentives, etc. All of those factors play into the optimal mechanical and envelope design. Several years ago, I had to start charging an initial plan review and consultation fee to new clients — typically three hours — in order to cover the cost of educating the client on my process and gather what I needed to prepare a design proposal. You can check my rates, which are still posted on my website.” (For a simple house with a simple HVAC system, Butler charges $140 plus 8¢ per square foot conditioned area, with a $299 minimum, for a load calculation.)

      Butler provided these final thoughts: "Given that most homeowners aren't willing to spend 4 figures for their mechanical design (nor does everyone need that level of design expertise), I think the best advice to a homeowner is to find a local HERS rater or Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. provider (or non-local if necessary). Many now do load calcs or know someone who does.”

      Energy consultants

      When I advise homeowners to hire an energy consultant to perform their Manual J calculation, I’m thinking of a certain category of energy consultant — people like Allison Bailes or Mike Duclos.

      The problem with this advice is that the term “energy consultant” means different things to different people, and that there are some areas of the country where knowledgeable energy consultants are hard to find. In other words, some energy consultants are likely to provide a very accurate Manual J calculation; others, not.

      Allison Bailes, a GBA blogger and energy consultant in the Atlanta area, is the principal of Energy Vanguard. His company charges 15 cents per conditioned square foot of floor area to provide a residential Manual J calculation. In other words, a Manual J calculation for a 2,500-square-foot house would cost $375.

      Mike Duclos (DEAP Energy Group, Newton, Massachusetts) told me that he charges in the range of $600 to $750 for a residential Manual J calculation.


      The businesses and people listed below are often available to perform Manual J calculations for designers, builders, and architects.

      DEAP Energy Group

      Energy Vanguard

      Fresh Air HVAC Sizing

      Duclos told me that he’s seen a lot of inaccurate load calculations. “Someone had calculated the heating load for a client we were working with — the block load for the whole building. This calculated the block load was 3 or 4 times higher than what we had figured with REM/rate. If a Manual J calculation is done correctly, there is usually a good correlation between the Manual J load and the REM/rate load, so something was clearly wrong here. I had the client speak to the person who calculated the block load. The guy bragged, ‘I know what I’m doing. I do 200 of these a week.’”

      Find a contractor who will work with you

      Before you hire a third-party consultant to perform a Manual J calculation and design your heating and cooling system, talk to your HVAC contractor.

      Allison Bailes noted, “Suppose a homeowner hires someone to perform a Manual J and to design the heating system. Then the homeowner gives the design to the contractor, and the contractor says, ‘I’m not doing that.’ Now what? So the first thing you should do, before you hire someone to do third-party design, is to ask the contractor if they are OK with that. Tell them you want the design you provide to be implemented.”

      Martin Holladay’s previous blog: “Installing Closed-Cell Spray Foam Between Studs is a Waste.”

      Click here to follow Martin Holladay on Twitter.

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  1. Martin Holladay

Mar 24, 2017 9:31 AM ET

Great start, but its not that simple!
by Armando Cobo

The code, building officials and the building industry have created a world where “all you have to do is perform a Manual J and everything will be alright”. Those of us who live in the real world know that we must perform Manuals J, D, S and T. A Manual J without ALL the other calculations mean NOTHING! Manual J is just the start… and yes, not only most building officials don’t know what to look for in a MJ8, but neither the big majority of the design and building community.
Also let’s be fair, I work with an HVAC contractor who does things right for my clients because he trusts my designs and the builders I work with. He knows that we require and perform 1ACH50, HERS40s in our houses, with systems inside the conditioned space and all metal ducts. He tells me he can’t do the same work with most of his builder clients because they don’t have the same performance standards.
To me it all starts with commitments from the homeowner, the builder and his subs, and the design community. I have chosen not to design houses that doesn’t meet my minimum efficiency standards. I have chosen not to work with homeowners and builders not committed to build same houses. I’ve chosen not to work with any subcontractor (HERS rater, Framer, Insulator, HVAC, etc.) that is not committed to do the work it needs to be done right. It’s just that simple… no fuss, no mess, no arguing and no headaches.
The bottom line, it’s a very complex problem, but if tackled correctly up front, the rest is easy.

Mar 24, 2017 5:21 PM ET

online calculator
by Michael Brackett

I am currently dealing with this. I am giving www.hvacloadcalculator.com/ a try. It is $39 and hopefully will be helpful. All the HVAC contractors are giving me calculations that are about 3 times what we need.

Mar 24, 2017 5:29 PM ET

Response to Michael Brackett
by Martin Holladay

From the information provided on the sample print-out, the inputs look simplified. It doesn't look like you can input the window SHGC or glazing U-factor.

It also appears that air leakage rate inputs are vague, with terms like "tightness is average."

-- Martin Holladay

Mar 28, 2017 4:46 PM ET

Response to Martin
by Michael Brackett

I think there is a place to input the window manufacturer's U-factor and SHGC, but I'm not sure that you can individually for different windows in the house. I haven't had much time to work on it lately but I will try to learn more about it. Also, it does have a place to input your blower door test results.

Mar 29, 2017 5:26 AM ET

Response to Michael Brackett
by Martin Holladay

I'm glad to hear that my hunches were wrong.

-- Martin Holladay

Mar 29, 2017 3:43 PM ET

make the effort to share the specs
by Leigha Dickens

As a homeowner or builder, if you do find a willing HVAC contractor, take the time to actually provide him or her with detailed envelope specs, and make sure that information is accurate (and like, actually figured out ahead of time) and not going to change wildly from design to reality! It seems obvious, but I definitely have seen this *not* happen. In a perfect world the HVAC contractor would ask about and pay attention the envelope design and not just make assumptions, but so should the builder or homeowner go ahead and make sure the envelope info is available and clear and final. For builders over time this can build trust and understanding between you and an HVAC contractor who is willing to do it right, with better buildings as a result.

I suspect a HERS rater or mechanical engineer is probably more likely to prompt a homeowner or builder to provide missing envelope specs or help those who haven't figured out some key aspect of the envelope that affects load calcs to get that figured out, rather than just make an assumption, as is probably more likely to suggest a wise course for said missing envelope spec, so for that reason alone one of those professionals may be worth the money. I would definitely agree to ask the HVAC contractor if they're ok with using a third party's calculations...I have run into those who aren't. (Or maybe it was just my approach that turned him off. Turns out "I know a lot about this trust me do what I say" isn't the most diplomatic way to go...who knew?) Good relationships require effort to build. The one with the HVAC contractor is a really important one.

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